Re: [Elecraft] dipole antenna efficiency

[David Woolley]

John Magliacane wrote:


http://books.google.com/books?id=SNJhVlHGgC&pg=PP1&sig=qdNrli_UaJsiZB_gq3cRqvjy4DY#PPT961,M1

http://www.jampro.com/index.php?page=c-circularly-polarized-spiral-tv-antenna


Interesting, although it doesn't seem to be a compact antenna (each turn 
of the helix is several wavelengths) and the multiple helices appear to 
be there to achieve the radiation pattern and polarization, not 
performance (you don't use stainless steel for low losses).

--
David Woolley
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Elecraft products and more general topics related ham radio"

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Re: [Elecraft] dipole antenna efficiency (PROGRAMS)

[ARDUJENSKI]

If you want to deal with specifics rather  than generalities let me suggest 
you look at one of the computer programs by the  late Reg G4FGQ and in 
particular DIPOLE 3. You can play with various  arrangements and get relative 
values. 
I often use this in conjunction with EZNEC  for comparison. His programs are 
relatively simple to use and I have found them  to be quite accurate when 
comparing to real-world   performance

http://www.zerobeat.net/G4FGQ/


Alan  KB7MBI
Woodinville, WA
FISTS: 5702   CC: 1885   ARS:  582
SKCC: 1988   NAQCC: 058   ARCI: 12141  




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Re: [Elecraft] dipole antenna efficiency

[John Magliacane]

> John Magliacane wrote:
> 
> > It's a traveling wave antenna where the helix is wound around a
> > supporting vertical mast that serves as a reflector.

David Woolley <[EMAIL PROTECTED]> wrote:

> None of the references I found (including university course
> notes), when researching this at the weekend, described anything
> other than a dielectric core.  They did suggest a spectrum of
> behaviour from basically linear polarized, to circular polarized,
> depending on the pitch.

Johnson's "Antenna Engineering Handbook" (McGraw-Hill) among others
describes the antenna design to which I am referring.  A preview of
chapter 28 where the antenna is described is available at:

http://books.google.com/books?id=xTSNJhVlHGgC&pg=PP1&sig=qdNrli_UaJsiZB_gq3cRqvjy4DY#PPT961,M1

Dielectric Communications (formerly RCA) manufactures such an antenna
(model TCL) for the purpose I originally described.  Their older
print catalog describes the antenna in more detail than what is
available on the web:

http://www.dielectric.com/broadcast/images/tcl1.jpg
http://www.dielectric.com/broadcast/vhf.asp

Jampro does a better job of describing and illustrating the antenna
on-line.  In particular, they state:

"The JTC Circularly Polarized Spiral Antenna consists of a supporting
pole around which are multiple stainless steel spirals wound at
specific pitch angles and spaced from the pipe by heavy-duty
fiberglass-reinforced low-loss insulators. The array acts as a
traveling wave antenna, with the main beam broadside to the support
pole and exceptionally low radiation along the axis of the pole. It
produces an omni-directional azimuth pattern with low VSWR."

Full description and photos are available here:

http://www.jampro.com/index.php?page=jtc-circularly-polarized-spiral-tv-antenna


73, de John, KD2BD


Visit John on the Web at:

http://kd2bd.ham.org/
.
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.
.


  

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Re: [Elecraft] dipole antenna efficiency

[David Woolley]

John Magliacane wrote:


It's a traveling wave antenna where the helix is wound around a
supporting vertical mast that serves as a reflector.


None of the references I found (including university course notes), when 
researching this at the weekend, described anything other than a 
dielectric core.  They did suggest a spectrum of behaviour from 
basically linear polarized, to circular polarized, depending on the pitch.


Also, travelling wave antennas normally radiate almost parallel to the 
length of antenna.



capacitively loaded, gamma matched dipoles.  Some implementations of
these antennas place a number of identical dipole rings in parallel
with one another to reduce ohmic losses and improve efficiency. 


I think I can produce a fairly good qualitative argument that that might 
not work for radiating elements, even if there is an advantage for 
lumped inductors.


If the individual wires strongly interact, which will be the limiting 
case as you bring them close together, they will behave like the 
individual elements of the cross section of a single conductor, and 
therefor suffer a skin effect which will exclude current from the inner 
ones and the insides of outer ones.


If they are positioned so that they don't interact with each other, each 
will act as an independent radiator and have the radiation resistance it 
would have had in isolation.  It will also have the the same ohmic loss 
resistance.  So, although you reduce the ohmic resistance by paralleling 
them, you also reduce the radiation resistance, in proportion.


For a non-qualitative proof, you would need to demonstrate that there 
isn't some peak in performance, between these two extremes


These antennas may not be made from Litz wire, but the underlying
concept of distributing RF current among closely placed parallel
conductors in an effort reduce resistive losses is still the same.


Note that a fundamental part of the concept of Litz wire is that it is 
intertwined, so that no one strand is always outside or always inside.



--
David Woolley
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Re: [Elecraft] dipole antenna efficiency

[John Magliacane]

> John Magliacane wrote:
> 
> > 
> > I believe the Normal Mode Helix, as is used in HI-VHF television
> > broadcasting, is a travelling wave antenna, rather than a
> > resonant standing wave antenna, such as a dipole.

David Woolley wrote:

> You are thinking of axial mode helices.  Normal mode helices are
> the most common antennas on VHF hand-helds, and are often called
> "rubber ducks".

I suppose various antennas go by the same name, even though their
operation is considerably different.

I was referring to a Normal Mode Helix also known as a "Side Fire
Helix".  It's described in the Antenna Engineering Handbook
(Johnson), and used in horizontally and circularly polarized
omni-directional applications (mostly VHF and UHF-TV broadcast). 
It's a traveling wave antenna where the helix is wound around a
supporting vertical mast that serves as a reflector.

Axial mode helices are a different beast.  They are typically used to
provide broadband, UNI-DIRECTIONAL, circular polarization (OSCAR
satellite communications).  Invented by John Kraus, W8JK.

I always considered a "rubber duck" to be nothing more than a
continuously loaded monopole, and is considerably different in
operation from the other two helical antennas described above.

> > When Skin Effect losses become an issue in RF conductors and 
> > inductors, Litz wire is typically used.  I see no reason why a 
>
> One has to ask why this isn't used in all the commercial
> "miracle" antennas.

Look at some of the older antennas used for FM broadcast, such as the
Collins or RCA ring dipole arrays.  These are physically short,
capacitively loaded, gamma matched dipoles.  Some implementations of
these antennas place a number of identical dipole rings in parallel
with one another to reduce ohmic losses and improve efficiency. 

These antennas may not be made from Litz wire, but the underlying
concept of distributing RF current among closely placed parallel
conductors in an effort reduce resistive losses is still the same.


73, de John, KD2BD


Visit John on the Web at:

http://kd2bd.ham.org/
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Re: [Elecraft] dipole antenna efficiency

[David Woolley]

John Magliacane wrote:


When Skin Effect losses become an issue in RF conductors and
inductors, Litz wire is typically used.  I see no reason why a


One has to ask why this isn't used in all the commercial "miracle" antennas.



I believe the Normal Mode Helix, as is used in HI-VHF television
broadcasting, is a travelling wave antenna, rather than a
resonant standing wave antenna, such as a dipole.


You are thinking of axial mode helices.  Normal mode helices are the 
most common antennas on VHF hand-helds, and are often called "rubber ducks".




--
David Woolley
Emails are not formal business letters, whatever businesses may want.
RFC1855 says there should be an address here, but, in a world of spam,
that is no longer good advice, as archive address hiding may not work.
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Re: [Elecraft] dipole antenna efficiency

[David Woolley]

bill KE5KWE wrote:

Let me bring this down to the practical world.  My QTH is a 42.5 ft. Travel
Trailer which is insulated from the ground by 6 Rubber Tires and Air.  It


Is it connected to the mains electricity supply?  If it is, you will 
probably need to consult someone familiar with your local electrical 
codes as, at least in the UK, rules for such structures are much 
stricter than for permanent buildings. At the very least, that will 
affect if and how you provide an RF ground.


You haven't actually specified what the constraints on your antenna are. 
 Are you constrained to not having an external antenna?  Are  you 
allowed one but it has to fit within the area of the trailer.   How high 
above the trailer can you go?  Are you allowed to install ground rods 
(electrical codes apart)?


Without either an external antenna or a ground connection, you are 
probably lost.


--
David Woolley
Emails are not formal business letters, whatever businesses may want.
RFC1855 says there should be an address here, but, in a world of spam,
that is no longer good advice, as archive address hiding may not work.
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Re: [Elecraft] dipole antenna efficiency

[WILLIS COOKE]

Bill, let me first state that operating QRP makes a
good antenna much more a requirement than operating
QRO.  Of course, the antenna does not need to handle
as much power, so some of the components can be
lighter weight.  For someone to give you a really good
suggestion we need to know a few things:

1.  Do you plan to operate mobil in motion?

2.  How long do you stay in one place?

3.  Do you usually park in commercial RV Parks, or on
private property?

4.  What sort of restrictions on antenna installation
do you anticipate?

5.  Are you reasonably physically fit so that you can
erect masts, etc.?

6.  Is the roof of your RV strong enough for you to be
on top of the RV to erect the antenna?

7.  Anything else that might give us clues about
problems that you may encounter?

8.  Are you thinking a K1, 10 watt K2 or just what for
a rig?

9.  What kind of operating and bands do you consider
required and what do you consider desirable?

Cookie, K5EWJ

--- bill KE5KWE <[EMAIL PROTECTED]> wrote:

> 
> Let me bring this down to the practical world.  My
> QTH is a 42.5 ft. Travel
> Trailer which is insulated from the ground by 6
> Rubber Tires and Air.  It
> has Metal, I presume Aluminum, siding and a Rubber
> over Wood Roof which is
> approx. 10 ft. off the ground.  My K2 is almost
> built and I will be
> operating CW exclusively QRP.  Are there any
> suggestions for either a
> commercially built antenna or a homebrew.
> (Recognizing that I am not a EE
> and have no formal electrical background!)
> 
> Thanks
> 
> Bill Fogel, KE5KWE
> "On the Road in the USA"
> 
> 
> This thread started with a person trying to find an
> antenna that would work reasonably well that does
> not
> take up much space. 
> -- 
> View this message in context:
>
http://www.nabble.com/dipole-antenna-efficiency-tp14609719p14650029.html
> Sent from the Elecraft mailing list archive at
> Nabble.com.
> 
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Re: [Elecraft] dipole antenna efficiency

[David Cutter]

I made this system primarily for its mechanical properties.  I will make 
some measurements one fine day


David
G3UNA


- Original Message - 
From: "Bob Cunnings" <[EMAIL PROTECTED]>

To: 
Sent: Sunday, January 06, 2008 11:06 PM
Subject: Re: [Elecraft] dipole antenna efficiency



The capacitance between individual strands in Litz wire limits it's
usefulness above 1 MHz or so. W8JI has an interesting discussion of
skin effect at:

http://www.w8ji.com/skindepth.htm

Bob NW8L

On Jan 6, 2008 3:54 PM, David Cutter <[EMAIL PROTECTED]> wrote:

I've used several strands of enamelled copper wire twisted together.  I
usually twist 3 sets of 3 of quite thin wire, whatever is on hand; 0.2 to
0.45mm dia is easy to use; after twisting I give the group a good pull 
the
stretch and stiffen it a bit but it still remains very flexible indeed 
and
does not have a memory effect.  I tie the wires to a fence post then fit 
the

other ends into an electric hand drill 25m, it's surprising how much
twisting is needed. You must not let the wires touch the ground.   It is
also very slippery and does not cling to other wires which I find a great
advantage when manipulating multi-wire structures.  I call it "multi 
twist".


David
G3UNA

> An "El Cheapo" approach might be to use multi-conductor cable
> instead of a single, fat conductor to minimize Skin Effect losses.
>

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Re: [Elecraft] dipole antenna efficiency

[Bob Cunnings]

The capacitance between individual strands in Litz wire limits it's
usefulness above 1 MHz or so. W8JI has an interesting discussion of
skin effect at:

http://www.w8ji.com/skindepth.htm

Bob NW8L

On Jan 6, 2008 3:54 PM, David Cutter <[EMAIL PROTECTED]> wrote:
> I've used several strands of enamelled copper wire twisted together.  I
> usually twist 3 sets of 3 of quite thin wire, whatever is on hand; 0.2 to
> 0.45mm dia is easy to use; after twisting I give the group a good pull the
> stretch and stiffen it a bit but it still remains very flexible indeed and
> does not have a memory effect.  I tie the wires to a fence post then fit the
> other ends into an electric hand drill 25m, it's surprising how much
> twisting is needed. You must not let the wires touch the ground.   It is
> also very slippery and does not cling to other wires which I find a great
> advantage when manipulating multi-wire structures.  I call it "multi twist".
>
> David
> G3UNA
>
> > An "El Cheapo" approach might be to use multi-conductor cable
> > instead of a single, fat conductor to minimize Skin Effect losses.
> >
>
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Re: [Elecraft] dipole antenna efficiency

[David Cutter]

I've used several strands of enamelled copper wire twisted together.  I 
usually twist 3 sets of 3 of quite thin wire, whatever is on hand; 0.2 to 
0.45mm dia is easy to use; after twisting I give the group a good pull the 
stretch and stiffen it a bit but it still remains very flexible indeed and 
does not have a memory effect.  I tie the wires to a fence post then fit the 
other ends into an electric hand drill 25m, it's surprising how much 
twisting is needed. You must not let the wires touch the ground.   It is 
also very slippery and does not cling to other wires which I find a great 
advantage when manipulating multi-wire structures.  I call it "multi twist".


David
G3UNA


An "El Cheapo" approach might be to use multi-conductor cable
instead of a single, fat conductor to minimize Skin Effect losses.


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RE: [Elecraft] dipole antenna efficiency

[John Magliacane]

Ron D'Eau Claire wrote:

> We simply do not have conductors that will handle RF with
> anything like the efficiency they will handle DC or low frequency
> AC. That's because all the RF current 'crowds' onto the very
> surface of a conductor.

When Skin Effect losses become an issue in RF conductors and
inductors, Litz wire is typically used.  I see no reason why a
similar approach cannot be used (one of using multiple conductors
tied in parallel with each insulated from one another).

An "El Cheapo" approach might be to use multi-conductor cable
instead of a single, fat conductor to minimize Skin Effect losses.

Cookie, K5EWJ wrote:

> The capacitive reactance will require 397 microhenrys
> to cancel out which in turn will require a coil 4
> inches in diameter and about 24 inches long with 160
> turns (about 168 feet of wire).  Then you would need
> an 8333/1 balun transformer with its associated wire
> resistance.

Capacitance hats can be used to reduce the amount of
inductive loading required to establish resonance.

I agree that the impedance transformations appear astronomical,
but if performed in manageable steps (instead of a single
transformation), efficiency might be improved, and the process
might not seem so formidable or lossy.

David Woolley wrote:

> For antennas in practical locations, at least one other thing
> will happen:
>
> d) The antenna will convert part of the AC power into (near field)
> electromagnetic energy which will induce currents in the ground,
> building structure, wiring, water and gas pipes, etc.  Much of that
> energy will be converted to heat after it has lost, although some
> will be re-radiated (I believe, in extreme cases, if the current
> is induced in something large enough and resonant, the re-radiator
> can become the actual antenna and the antenna act as a feed device,
> but, normally, for low, and indoor antennas, this is where most of
> the energy turns to heat).

I fully agree that the surrounding environment is part of the antenna
SYSTEM, and should be taken into consideration regardless of whether
the antenna is half-wave or an electrically shortened version of a
half-wave antenna.

> This reflection abstraction causes a lot of confusion.  It is
> possibly easier to see it as simply a bad match between the
> transmitter source impedance (which is usually rather different
> from the optimum load impedance) and the antenna impedance,
> causing most of the DC input to the transmitter to end up as
> heat in the output devices.

Provided measures to re-reflect the energy back to the antenna in
phase with the incident power aren't made.  (And the device that
does this very nicely is the antenna tuner.)

Actually, I mentioned reflected power to dispell the untrue but
widely held belief that reflected power "cancels" forward power
on a transmission line.  Forward and reflected power can peacefully
co-exist on a transmission line without interaction.  If this were
not true, repeater systems using duplexed antenna systems would
be impossible.

> As already pointed out, skin effect means that this is not true.
> People experimenting with small magnetic loops have to use large
> copper pipes to keep ohmic losses manageable.

The problem (in my mind) is that the copper pipe is really only a
single conductor, rather than the more efficient approach taken by
Litz wire where multiple conductors, each a skin depth in diameter,
are operated in parallel.

> For example, the KAT2 has a 10:1 SWR matching specification, but
> matching the antenna discussed here, at infinite height above the
> ground, needs a 250:1 range, or more.  They can also have power
> losses.

Agreed.  The impedance transformation has to (in my opinion) be
step-by-step process -- the same as a modern receiver design.
(In a receiver, we don't try to get all our gain and selectivity
in a single stage.  By the same token, we shouldn't try to make
a large impedance transformation in a single "stage", either.)

(Just my opinion...)

> > The penalties for using physically shortened antennas are:
> >
> > (a) Decreased operating bandwidth
>
> I'm not sure that is inevitably true.  My reference for normal mode
> helices included them in the section on broadband antennas.

I believe the Normal Mode Helix, as is used in HI-VHF television
broadcasting, is a travelling wave antenna, rather than a
resonant standing wave antenna, such as a dipole.

The bandwidth decreases in shortened resonant antennas as a
result of the antenna's increased Q.  The Q rises because the
RF energy must oscillate back and forth between feedpoint and
endpoint of the dipole many more times than it does in a
full-sized antenna before all the energy applied to the antenna
is finally radiated into space.

In travelling wave (non-resonant) antennas (rhombics, Beverages),
the RF energy travels down the length of the antenna just once.
Therefore, they must be very long in terms of wavelength to radiate
all the energy into space on the first shot.

Inc

Re: [Elecraft] dipole antenna efficiency

[N2EY]

In a message dated 1/5/08 11:40:39 PM Eastern Standard Time, [EMAIL PROTECTED] 
writes:


> We simply do not have conductors that will handle RF with anything like the
> efficiency they will handle DC or low frequency AC. That's because all the
> RF current 'crowds' onto the very surface of a conductor. 
> 
> As we make an antenna physically smaller, the impedance drops. As the
> impedance drops, the RF currents and resistive losses go up. Even silver or
> gold - the best electrical conductors known - are not nearly good enough for
> the sorts of currents we see in small antennas. 
> 
> As an antenna is made shorter the resistive losses far exceed any other
> losses in the system. Of course those resistive losses occur in the matching
> network too. It's not just the antenna itself that converts RF into heat
> better than it makes electromagnetic waves. 

This is so well said that it deserves repeating.

In *theory*, a dipole 1.3 feet long can be made to radiate 80 meter RF almost 
the same as one 130 feet long. But in *practice*, the resistive losses of 
real-world practical antenna systems of those sizes are very different.  

And what we're looking for are practical, real-world antennas that we can put 
up in the limited space we have available.

---

Another factor to remember is that under good conditions we can do pretty 
incredible stuff with very low power.100 watts into an antenna system that has 
1% 
efficiency will radiate the same amount of RF as 1 watt into an antenna 
system with 100% efficiency. So even a poor antenna will sometimes let you make 
a 
QSO.

---

It seems to me that one of the biggest stumbling blocks we amateurs have with 
antennas is seeing and understanding the entire antenna *system* - which 
includes the antenna itself, its surroundings, the feedline, matching networks, 
etc. We have to consider all of them together. And like the links of a chain, 
the weakest one will be the problem. A great antenna in poor surroundings will 
work poorly, etc.

73 de Jim, N2EY


**
Start the year off right.  Easy ways to stay in 
shape.
 
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RE: [Elecraft] dipole antenna efficiency

[Ron D'Eau Claire]

Absolutely pattern is of significance, Alan, but unless you have a lot of
space, there's not much one can do about the pattern of HF antennas and most
wire antennas will be omni directional at heights most Hams can achieve on
typical lots. 

Even a full-sized 1/2 wave horizontal antenna is essentially omni
directional unless it's at least 1/2 wavelength high in the air! So you need
a 66 foot run in the correct orientation with a 66 foot high supports to
plan a specific pattern with a simple 40 meter dipole. And, even then, it's
very, very broad with a maximum front/side ratio of perhaps one S-unit. Put
that dipole up at a more typical height of 30 feet and its maximum radiation
is absolutely unidirectional and straight up! 

You can squeeze some significant directivity out of lower, smaller antennas
with Yagi Ueda or cubical quad "beam" designs, but it's fairly minimal
unless you can get it up near 1/2 wavelength off of the ground. That's why
most Hams don't bother with "beams" - Yagis or Quads - except on 14 MHz and
above where they can get them up high enough to be effective. 

Perhaps the biggest pattern issue most Hams working at HF can reasonably
tackle is the question of vertical vs. horizontal polarization, and even
then the differences aren't all that great unless the horizontal antenna is
at least 1/3 to 1/2 wavelength above ground or the vertical is over an
excellent ground (e.g. over salt water) that extends for many wavelengths in
at least the important directions. 

The end result is the vertical vs. horizontal question is usually settled
based on the space limitations. That's why few horizontal antennas are found
in use on 160 and 80 meters compared to the other bands.

On the other hand, efficiency drops like a stone as an antenna is made
physically small. That's something we can address, at least in a small way,
by getting clever about making the antenna as large as possible, minimizing
resistive losses in the conductors and minimizing ground losses. 

Sometimes Hams are confused when they see patterns published for very small
antennas such as mobile whips mounted on a car. An azimuth plot will show a
significant pattern suggesting the signal is much stronger in certain
directions. That's true, but the important issue is that directivity is a
measure of inefficiency, not gain. That is, those patterns show the
directions where the losses are slightly lower than in other directions, so
the signal appears somewhat stronger in those directions! Those are exactly
the things we can address: how to make some small improvement in efficiency
that improves the signal. Sometimes we can make our small, low antennas less
INefficient in the most desirable direction. For example, a mobile at the
beach might discover it's easier to work DX across the ocean in that
direction than it is to work someone a few hundred miles away across land in
another direction. That's because the losses over the salt water are less
than over the land, but it's still not a very efficient antenna. 

Bottom line is that it comes down to tackling issues we can do something
about and not fretting over those we can't. We can often do something about
efficiency where we often cannot do something about the pattern. 

Ron AC7AC

 

-Original Message-
Interesting discussion. I have a question  regarding this topic I hope the 
more learned in the group cans help clarify.  Although efficiency is
important 
is determining an antenna selection, is antenna  lobe pattern just as 
important? If the lobe pattern did not allow you to say  work DX or to work
a 
particular direction then is efficiency all that important?  Should we look
at the 
desired pattern for a particular operating goal then  consider the antenna 
efficiency?

Alan KB7MBI
Woodinville, WA
FISTS:  5702   CC: 1885   ARS: 582
SKCC: 1988   NAQCC:  058   ARCI: 12141  


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Re: [Elecraft] dipole antenna efficiency

[ARDUJENSKI]

Interesting discussion. I have a question  regarding this topic I hope the 
more learned in the group cans help clarify.  Although efficiency is important 
is determining an antenna selection, is antenna  lobe pattern just as 
important? If the lobe pattern did not allow you to say  work DX or to work a 
particular direction then is efficiency all that important?  Should we look at 
the 
desired pattern for a particular operating goal then  consider the antenna 
efficiency?

Alan KB7MBI
Woodinville, WA
FISTS:  5702   CC: 1885   ARS: 582
SKCC: 1988   NAQCC:  058   ARCI: 12141  




**Start the year off right.  Easy ways to stay in shape. 
http://body.aol.com/fitness/winter-exercise?NCID=aolcmp0030002489
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RE: [Elecraft] dipole antenna efficiency

[Ron D'Eau Claire]

I'd go "mobile" Bill. Like an automobile (at least when they had significant
metal), the frame of your trailer will provide capacitive coupling to ground
and you can use a short, loaded  vertical "mobile antenna" such as sold for
use on automobiles. 

It will be a very inefficient antenna, compared to a half-wave long antenna
up 1/2 wavelength or more in the sky, or a vertical that's a full 1/4
wavelength long working against an ideal ground, but you will make contacts
- lots of them when band cdx are decent. Hams have made contacts all over
the world at QRP power levels for decades with just such setups, even on
phone.

I know many operators have excellent results with the "screwdriver" types of
mobile whips. Those have motors to adjust the loading coil according to the
frequency of operation so you can "tune" from inside the trailer. The better
your installation, the sharper the tuning. As others noted, a reasonably
efficient shortened antenna has high "Q" - that is the bandwidth between,
say, SWR of 2:1 is much, much smaller than a full sized antenna so you want
to be able to adjust it easily. I know one or two operators who routinely
use that type of antenna at their homes, where they have little space or
time to fiddle with other options.  

I would also plan to mount the antenna on the roof, even if that means
taking it down (unscrewing it from a mount affixed permanently and
water-tight on the roof) when traveling. After all, that should only require
a moment's work to erect or stow it and it's well worth the effort. The
majority of the radiation from such an antenna will be at its base. Keeping
the base away from "ground" (the metal parts of the  trailer) and as high as
possible will help your signal a lot. Putting it in the center will produce
the most uniform pattern. Putting it one end will produce a stronger signal
in the direction of the trailer. 

Once you do that and you are happily operating QRP from your trailer, you'll
find endless things you can "do" with your antenna installation, depending
upon your circumstances and interests. Some of them might be:

1) Improve the ground system. Instead of relying on the capacitive coupling
between the metal and ground, arrange to string some "radials" on the ground
or insulated slightly above the ground that are easy to deploy and not a
trip hazard. 

2) Improve the antenna. You might find you are in situations where you'll be
spending a few days with a handy tree nearby, etc., and you get the wire,
etc., needed to throw a longer antenna over a tree limb and connect it to
the mount on top of the trailer in place of the normal antenna. You'll want
an ATU at the rig to do that, and  that means your coaxial feeder will be
operating at a very high SWR (which produces loss) but those things should
be minimal because the coax shouldn't be so terribly long in a trailer! Some
mobile ops do this with a wire and a clip lead that they just clip onto
their regular whip for when they can add it. Every foot of wire will improve
your antenna's efficiency. 

3) Another way you might considering improving your antenna would be to
investigate surplus military "whip" antennas that show up on the market from
time to time. Some of these are 15 or 20 feet long! Over the years more than
one trailer traveler has arranged such a whip, mounting at the top of his
trailer but at one end. Then he could either tilt the mount or just bend the
tip over and tie it at the opposite end when in motion, making the whole rig
only a couple of feet higher than without the antenna. A number of hams have
bought a "full sized" loaded vertical intended for home use and arranged a
tilt-over mount for it on their trailer. After all, most of those antennas
are never more than 12 or 18 feet tall and the upper section can often be
disassembled if needed for transport. 

4) Consider *where* you go in your trailer! A trip to the seashore, mountain
or open desert can produce unexpected and amazing DX from even the simplest
locations by reducing the sort of losses to surrounding objects and the
earth itself for several wavelengths around us that most of us in fixed
locations cannot do anything about. 

Those things all depend upon how your interests and opportunities develop
over time. One thing for sure is that you'll have a lot of fun operating and
make lots of contacts even if you just use the basic mobile antenna
installation. 

Ron AC7AC





-Original Message-
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of bill KE5KWE
Sent: Sunday, January 06, 2008 7:53 AM
To: elecraft@mailman.qth.net
Subject: Re: [Elecraft] dipole antenna efficiency



Let me bring this down to the practical world.  My QTH is a 42.5 ft. Travel
Trailer which is insulated from the ground by 6 Rubber Tires and Air.  It
has Metal, I presume Aluminum, siding

Re: [Elecraft] dipole antenna efficiency

[Ken Kopp]

Bill,

My Lady and I are serious half-time RV'ers and I've had
very good results with my K2 into a modified High Sierra 
screwdriver on a 5th-wheel Holiday Rambler.


I'll send you photos.  It's also pictured on K0BG's mobile
site.

73! Ken Kopp - K0PP
[EMAIL PROTECTED]
or
[EMAIL PROTECTED]
 


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Re: [Elecraft] dipole antenna efficiency

[bill KE5KWE]

Let me bring this down to the practical world.  My QTH is a 42.5 ft. Travel
Trailer which is insulated from the ground by 6 Rubber Tires and Air.  It
has Metal, I presume Aluminum, siding and a Rubber over Wood Roof which is
approx. 10 ft. off the ground.  My K2 is almost built and I will be
operating CW exclusively QRP.  Are there any suggestions for either a
commercially built antenna or a homebrew. (Recognizing that I am not a EE
and have no formal electrical background!)

Thanks

Bill Fogel, KE5KWE
"On the Road in the USA"


This thread started with a person trying to find an
antenna that would work reasonably well that does not
take up much space. 
-- 
View this message in context: 
http://www.nabble.com/dipole-antenna-efficiency-tp14609719p14650029.html
Sent from the Elecraft mailing list archive at Nabble.com.

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Re: [Elecraft] dipole antenna efficiency

[David Woolley]

John Magliacane wrote:

When it comes to antenna efficiency, it is important to understand
that when RF energy is applied to any antenna, three things will
invariably happen:


For antennas in practical locations, at least one other thing will happen:

d) The antenna will convert part of the AC power into (near field) 
electromagnetic energy which will induce currents in the ground, 
building structure, wiring, water and gas pipes, etc.  Much of that 
energy will be converted to heat after it has lost, although some will 
be re-radiated (I believe, in extreme cases, if the current is induced 
in something large enough and resonant, the re-radiator can become the 
actual antenna and the antenna act as a feed device, but, normally, for 
low, and indoor antennas, this is where most of the energy turns to heat).


Normally, even without losses, induced currents in the ground below a, 
low, horizontal dipole will tend to cancel the far field signal, 
resulting in less power actually being radiated, and a lower radiation 
resistance, requiring even lower losses in all components from antenna 
wire back to and including the ATU.


(One way of considering the far field is that you need to create a 
relatively large electric or magnetic field far enough from the antenna 
that its phase lags that close to the antenna by a significant amount.)



c) The antenna will reflect a portion of the applied AC voltage and
current back to the transmitter as a result of an impedance mismatch
between the antenna and that of the source.


This reflection abstraction causes a lot of confusion.  It is possibly 
easier to see it as simply a bad match between the transmitter source 
impedance (which is usually rather different from the optimum load 
impedance) and the antenna impedance, causing most of the DC input to 
the transmitter to end up as heat in the output devices.




Unless steel or nichrome wire is used, or electrically poor
connections exist in the antenna structure, losses due to (b)
will be low.


As already pointed out, skin effect means that this is not true.  People 
experimenting with small magnetic loops have to use large copper pipes 
to keep ohmic losses manageable.  (In some cases I suspect they are 
still high compared to radiation resistance, but lower than the near 
field loss resistance.)



Effects of (c) can be reduced or eliminated by using intelligent,
low-loss impedance matching techniques and low-loss feedline.
(Technically, reflected power isn't a "loss" per se, since
energy isn't dissipated when a reflection occurs.)


Note that devices capable of doing this for the sort of antenna being 
considered in this thread are not easy to find, if they can be found at 
all.  For example, the KAT2 has a 10:1 SWR matching specification, but 
matching the antenna discussed here, at infinite height above the 
ground, needs a 250:1 range, or more.  They can also have power losses.



That leaves us with (a), the desired outcome of applying RF energy to
an antenna.  Since losses due to (b) and (c) are typically low and/or
easily corrected, it is very difficult NOT to achieve high antenna
system efficiency.


(b) and (c) are not typically low for the sort of antenna considered 
here, although (c) isn't really achievable, anyway.




Shortening the physical length of an antenna below that of a
half-wavelength DOES NOT reduce its efficiency provided the
necessary efforts to minimize resistive losses in the antenna
structure and the impedance matching networks are made.


But, apart from possibly cooling everything to near absolute zero, they 
cannot be made.




That last statement is so important and so often misunderstood,
it bears repeating:


I'd agree that there is a misunderstanding that is common.  It arises 
because people have difficulty with the idea that an antenna can have a 
capture area that is a lot bigger than the antenna, and because people 
don't understand that the real limitation on small antennas is power 
losses.  Large antennas have gains that equate to directivity, and 
people try to extrapolate these down to small antennas, whereas there 
are no Maxwellian reasons why a small antenna cannot be efficient, only 
materials science, engineering and environmental ones.


If we were to apply 100 watts to such an antenna, and we get zero
watts reflected back, and the antenna and matching networks remain
cool, then 100 watts of RF energy is being radiated from that one
foot dipole -- the same as if a full-sized dipole were used.


Radiation is normally used to refer to far field radiation, which is the 
only radiation useful for normal ham radio communications.  The antenna 
can remain cool even if all the power is going into heating up the 
ground, or the re-inforcing bars in your concrete building.




The penalties for using physically shortened antennas are:

(a) Decreased operating bandwidth


I'm not sure that is inevitably true.  My reference for normal mode 
helices included them in the sec

RE: [Elecraft] dipole antenna efficiency

[Ron D'Eau Claire]

John wrote:
b) The antenna will convert a portion of the applied AC voltage and current
into heat energy as a result of resistive losses in the antenna structure.

Unless steel or nichrome wire is used, or electrically poor connections
exist in the antenna structure, losses due to (b) will be low.

-

That's not correct John, and it's the Achilles heel in all small antennas.
We simply do not have conductors that will handle RF with anything like the
efficiency they will handle DC or low frequency AC. That's because all the
RF current 'crowds' onto the very surface of a conductor. 

As we make an antenna physically smaller, the impedance drops. As the
impedance drops, the RF currents and resistive losses go up. Even silver or
gold - the best electrical conductors known - are not nearly good enough for
the sorts of currents we see in small antennas. 

As an antenna is made shorter the resistive losses far exceed any other
losses in the system. Of course those resistive losses occur in the matching
network too. It's not just the antenna itself that converts RF into heat
better than it makes electromagnetic waves. 

Resistive losses are also the primary source of losses in coaxial feed lines
working at high SWRs, for example. If we had better conductors, we'd not
care about running 100 feet of 50 ohm coax at high SWR. Even with excellent
conductors, we know that coax used that way may consume virtually all of the
RF and convert it into heat.

To get around those losses we need a room-temperature RF "superconductor",
and one hasn't been invented yet. 

As much as we might like, Maxwell was right. His laws still stand. 

Ron AC7AC


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Re: [Elecraft] dipole antenna efficiency

[WILLIS COOKE]

This thread started with a person trying to find an
antenna that would work reasonably well that does not
take up much space.  While in theory, you are correct
the practical aspects of making a very small antenna
for low frequencies lead to impossible parameters. 
The radiation resistance of a dipole 2 meters long for
3.8 mHz is about 0.134 ohms.  Since placing such an
antenna in free space would require the assistance of
NASA we need to assume that it is a bit lower.  If we
assume that the person cannot or does not want to
erect a tall mast, probably a height of about 3 meters
would be reasonable to expect.  This would make for a
radiation resistance of about 0.006 ohms and need. 
The capacitive reactance will require 397 microhenrys
to cancel out which in turn will require a coil 4
inches in diameter and about 24 inches long with 160
turns (about 168 feet of wire).  Then you would need
an 8333/1 balun transformer with its associated wire
resistance.  Clearly not a very practical way to make
an antenna that would radiate straight up and would
have a bandwidth less than required for a single
sideband signal.  Now if you have theoretical
inductors and capacitors and  no resistance wire
available to you, maybe you can do better.
(Calculations by EZNEC)

A more practical solution for a ham that cannot afford
or is prohibited from putting up a large antenna is a
mobile antenna attached to a porch rail or rain gutter
or whatever metal that is available.  Better is the
common trap vertical.  If a person has a bit of space
and can erect a 30 or 40 foot mast all sort of
possibilities present themselves.

This has certainly been an interesting thread and can
go on forever if we choose.  Hams have been designing,
trying and debating antenna designs since Marconi
started it all and I don't think it will end soon.

73 to all, Cookie, K5EWJ

   
--- John Magliacane <[EMAIL PROTECTED]> wrote:

> When it comes to antenna efficiency, it is important
> to understand
> that when RF energy is applied to any antenna, three
> things will
> invariably happen:

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Re: [Elecraft] dipole antenna efficiency

[Kurt Pawlikowski]

David,

   Actually, if you follow some of the other related articles Lloyd
wrote, the effect of this design seems to approximate a full wave dipole
with a device about 2% of a wave length. That is, a 40 meter antenna
about 3 feel long. He also purports a kind of "folded" antenna for 80
meters at about the same length. Wow. The possibility of a 6 foot long
160 meter antenna that works. I am definitely intrigued!

   Regards,

   kurtt

   Kurt Pawlikowski, AKA WB9FMC
   The Pinrod Corporation
   [EMAIL PROTECTED]
   (773) 284-9500
   http://pinrod.com

David Cutter wrote:
Ooops.  I must apologise for my mistake:  I've read the article in 
more detail and Lloyd is not feeding a small antenna, he is using the 
"feeder" as the antenna by deliberately unbalancing it with a terminal 
unit to obtain maximum current at the far end of the feeder.  
Presumably this would then be elevated to a convenient point for 
maximum effect.


David
G3UNA

Here is an interesting antenna made by Lloyd Butler VK5BR.  It uses 
open wire feeder to a VERY small antenna.


http://users.tpg.com.au/users/ldbutler/ReverseFeedTopLoading.htm

David
G3UNA


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Re: [Elecraft] dipole antenna efficiency

[John Magliacane]

When it comes to antenna efficiency, it is important to understand
that when RF energy is applied to any antenna, three things will
invariably happen:

a) The antenna will convert a portion of the applied AC voltage
and current into electromagnetic energy and radiate it into space.

b) The antenna will convert a portion of the applied AC voltage
and current into heat energy as a result of resistive losses in the
antenna structure.

c) The antenna will reflect a portion of the applied AC voltage and
current back to the transmitter as a result of an impedance mismatch
between the antenna and that of the source.

Unless steel or nichrome wire is used, or electrically poor
connections exist in the antenna structure, losses due to (b)
will be low.

Effects of (c) can be reduced or eliminated by using intelligent,
low-loss impedance matching techniques and low-loss feedline.
(Technically, reflected power isn't a "loss" per se, since
energy isn't dissipated when a reflection occurs.)

That leaves us with (a), the desired outcome of applying RF energy to
an antenna.  Since losses due to (b) and (c) are typically low and/or
easily corrected, it is very difficult NOT to achieve high antenna
system efficiency.

Shortening the physical length of an antenna below that of a
half-wavelength DOES NOT reduce its efficiency provided the
necessary efforts to minimize resistive losses in the antenna
structure and the impedance matching networks are made.

That last statement is so important and so often misunderstood,
it bears repeating:

Shortening the physical length of an antenna below that of a
half-wavelength DOES NOT reduce its efficiency provided the
necessary efforts to minimize resistive losses in the antenna
structure and the impedance matching networks are made.

We could make an 80-meter dipole just one foot long and realize
high efficiency if the proper low-loss impedance matching techniques
are employed.

If we were to apply 100 watts to such an antenna, and we get zero
watts reflected back, and the antenna and matching networks remain
cool, then 100 watts of RF energy is being radiated from that one
foot dipole -- the same as if a full-sized dipole were used.

The penalties for using physically shortened antennas are:

(a) Decreased operating bandwidth
(b) Decreased directivity
(c) Somewhat difficult impedance matching

If we're wiling to adjust our impedance matching networks when we
QSY, then (a) isn't much of a problem since our signals are seldom
more than a few kHz wide, anyway.

Dipoles have 2.14 dB "gain" over isotropic radiators.  As we make
our dipole shorter and shorter (and keep resonating and impedance
matching it in the process), its directivity (b) approaches that
of an isotropic radiator.  If our radiating structure and impedance
matching networks are lossless, an extremely short dipole may be as
much as 2 dB weaker than the signal produced by a half-wave dipole
in the broadside direction.  That's less than half an S-unit!

So, the key to success lies in (c), the impedance matching network.
It needs to have extremely low loss, be capable of matching a very
wide range of impedances, and (unless you're rock-bound) be agile
so the antenna can be operated across a broad range of frequencies.
Ideally, it should be placed at the antenna feedpoint, itself.


73, de John, KD2BD


Visit John on the Web at:

http://kd2bd.ham.org/
.
.
.
.


  

Never miss a thing.  Make Yahoo your home page. 
http://www.yahoo.com/r/hs
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Re: [Elecraft] dipole antenna efficiency

[David Yarnes]

- Original Message - 
From: <[EMAIL PROTECTED]>

To: <[EMAIL PROTECTED]>; 
Sent: Saturday, January 05, 2008 7:28 AM
Subject: Re: [Elecraft] dipole antenna efficiency


In a message dated 1/4/08 4:34:29 PM Eastern Standard Time, 
[EMAIL PROTECTED]

writes:



I've never understood the popularity of the G5RV.  It's
achieved a sort of "cult" following.


It's an ingenious compromise antenna for several bands, that's all.
Actually, just a ~102 foot dipole with a matching system that gives
"low" (but not unity!) SWR on several HF bands, so that a simple
ATU can match it.





Well, it depends on what your definition of "low" is.  The SWR is below 2:1 
on only 1 band--20 meters.  On others it goes to 5:1 or more on most of the 
non-WARC bands, and is almost unusable on 30  and 17 meters.  That is, 
unless you use a tuner!  Use of a tuner is really the big bone of contention 
usually.  This antenna has been described as not requiring a tuner, when it 
really does need one for the most part.  Even Varney anticipated that as he 
has discussed in several articles.


Another problem is that, although Varney described his antenna sufficiently, 
so many variations have been born (but still are called "G5RV's"), in an 
attempt to improve the SWR on one band or another, that the "real G5RV" 
isn't even described that much anymore.  SWR can be improved on various 
bands, but usually at the expense of the SWR on another band.  Cebik 
describes a pretty good variation on his website.  As I understand it, 
Varney's objective was primarily to add 20 meters to an antenna that would 
also load acceptably on 80.  But he never intended for this to be a 
"tunerless" antenna from 80 through 10 meters.


Dave W7AQK



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Re: [Elecraft] dipole antenna efficiency

[David Wilburn]

Absolutely.  Even when you look at Joe's site, www.k1jek.com there is
enough info in what you see to make the antenna.  Rotor cable, ladder
line, a balun and coax.  I personally do not think what he asks for the
on the website is overly much.  He makes the products himself, and
stands behind them.  He actually answers the phone when you call and ask
questions.  My life is a bit complicated, but I could have easily made
one.  I was willing to make the trade of money vs. time and materials.

As an example, I had a quick QSO with AA4AK the other night, so I could
have Maine for WAS LOTW.  I found out the next morning, after finishing
my DL1 and going to play with it, that the rig was set to 8w.  The
antenna must be doing something right.
-  

David Wilburn
[EMAIL PROTECTED]
K4DGW
K2 S/N 5982


On Fri, 2008-01-04 at 18:11 -0500, Thom LaCosta wrote:
> On Fri, 4 Jan 2008, David Wilburn wrote:
> 
> > I understand many G5RV's are getting replaced with these antennas.  I
> > have the Grampy version, that is 100' long on each side, for a total of
> > 300' of wire on each side, all in a 100' on each side package.  Have had
> > good luck with it.  Their shortest is a bit over 70'.
> 
> And should you want to roll your own:
> 
> http://www.hamuniverse.com/cobraantenna.html
> 
> Thom,EIEIO
> Email, Internet, Electronic Information Officer
> 
> www.baltimorehon.com/Home of the Baltimore Lexicon
> www.tlchost.net/hosting/ Web Hosting as low as 3.49/month

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RE: [Elecraft] dipole antenna efficiency

[Ron D'Eau Claire]

Ha, ha! Check the date on the article. 

VK5BR has a parallel tuned circuit (two plates for the capacitor and coil
for the inductance) with the "feed" tapped across a couple of turns of the
coil. 

One could use any parallel coil/capacitor combination with adequate voltage
and current ratings to do that. 

It's all fed with RF from the unbalanced open wire line, which certainly
does radiate a bit. The coil/cap combination will add a tiny bit of
radiation too. 

It reminds me the famous "light bulb" antenna by, if I recall correctly,
Larson E. Rapp, aka Byron Goodman, W1DX (SK) in which a feed line was
connected to an incandescent bulb atop a fence post in a much earlier April
issue of a well-known Amateur radio journal.

VK5BR's antenna might radiate a bit more, due to none of the energy being
converted to photons at visible light frequencies, but Rapp's version
offered a clear visual indication that the antenna was getting RF power.

Rewriting Maxwell's laws is a long-standing, honored tradition in many
electronics journals every April, Amateur and Professional.  

Ron AC7AC 



-Original Message-
Ooops.  I must apologise for my mistake:  I've read the article in more 
detail and Lloyd is not feeding a small antenna, he is using the "feeder" as

the antenna by deliberately unbalancing it with a terminal unit to obtain 
maximum current at the far end of the feeder.  Presumably this would then be

elevated to a convenient point for maximum effect.

David
G3UNA

> Here is an interesting antenna made by Lloyd Butler VK5BR.  It uses
> open
> wire feeder to a VERY small antenna.
>
> http://users.tpg.com.au/users/ldbutler/ReverseFeedTopLoading.htm
>
> David
> G3UNA
> 

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Re: [Elecraft] dipole antenna efficiency

[Kurt Pawlikowski]

David,

   Actually, if you follow some of the other related articles Lloyd 
wrote, the effect of this design seems to approximate a full wave dipole 
with a device about 2% of a wave length. That is, a 40 meter antenna 
about 3 feel long. He also purports a kind of "folded" antenna for 80 
meters at about the same length. Wow. The possibility of a 6 foot long 
160 meter antenna that works. I am definitely intrigued!


   Regards,

   kurtt

   Kurt Pawlikowski, AKA WB9FMC
   The Pinrod Corporation
   [EMAIL PROTECTED]
   (773) 284-9500
   http://pinrod.com

David Cutter wrote:
Ooops.  I must apologise for my mistake:  I've read the article in 
more detail and Lloyd is not feeding a small antenna, he is using the 
"feeder" as the antenna by deliberately unbalancing it with a terminal 
unit to obtain maximum current at the far end of the feeder.  
Presumably this would then be elevated to a convenient point for 
maximum effect.


David
G3UNA

Here is an interesting antenna made by Lloyd Butler VK5BR.  It uses 
open wire feeder to a VERY small antenna.


http://users.tpg.com.au/users/ldbutler/ReverseFeedTopLoading.htm

David
G3UNA


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Re: [Elecraft] dipole antenna efficiency

[David Cutter]

Ooops.  I must apologise for my mistake:  I've read the article in more 
detail and Lloyd is not feeding a small antenna, he is using the "feeder" as 
the antenna by deliberately unbalancing it with a terminal unit to obtain 
maximum current at the far end of the feeder.  Presumably this would then be 
elevated to a convenient point for maximum effect.


David
G3UNA

Here is an interesting antenna made by Lloyd Butler VK5BR.  It uses open 
wire feeder to a VERY small antenna.


http://users.tpg.com.au/users/ldbutler/ReverseFeedTopLoading.htm

David
G3UNA


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Re: [Elecraft] dipole antenna efficiency

[N2EY]

In a message dated 1/4/08 4:31:49 PM Eastern Standard Time, [EMAIL PROTECTED] 
writes:


> The most expensive thing is ground to install
> them where antenna restrictions don't bring down the
> wrath of the taste police. 

AMEN!!

(I have been trying for
> 
> over 50 years to convince the world that antennas are
> beautiful, but without success) 

Me too. 

I've always found it odd that the very people who want the convenience of 
modern technologies often consider the technologies themselves to be 
unattractive, and want things to look like a time when life was much more 
difficult. 

73 de Jim, N2EY


**
Start the year off right.  Easy ways to stay in 
shape.
 
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Re: [Elecraft] dipole antenna efficiency

[N2EY]

In a message dated 1/4/08 4:34:29 PM Eastern Standard Time, [EMAIL PROTECTED] 
writes:


> I've never understood the popularity of the G5RV.  It's
> achieved a sort of "cult" following.

It's an ingenious compromise antenna for several bands, that's all.
Actually, just a ~102 foot dipole with a matching system that gives
"low" (but not unity!) SWR on several HF bands, so that a simple
ATU can match it.

  The original design
> 
> was for a 20M -ONLY- antenna, 

Not true!

I have PDF's of the original articles by G5RV himself, and from the very 
beginning it was a multiband design. He had a small garden ("back yard" to us 
Yanks) and wanted to get on the air easily, quickly and simply. For his 
application, it worked.

But it must be remembered that when G5RV designed the antenna, the ham bands 
were somewhat different than today. 30, 17 and 12 meters weren't ham bands at 
all. 9-/75 meters in G land was 3.5 to 3.8 MHz only, and 40 meters was 7.0 to 
7.1 MHz.

Most important of all, the rigs in use were capable of matching "reasonable 
levels of SWR" - meaning 3 or 4 to 1 wasn't considered to be worth worrying 
about for the bands and short lines being run.  

and somehow has
> 
> morphed into a do everything hoax.

Not a hoax, but there's a lot of misunderstanding about the antenna.

The biggest misunderstanding is that too many folks expect to put up a 
G5RV-like antenna and get 1:1 SWR on all parts of every HF band from 80 thru 10 
meters, automatically. And work the world with the same ease as folks with big 
aluminum. 

That's just not going to happen. It's just a dipole with an ingenious feed 
system, not magic.

> I do antenna talks at conventions and hamfests, and
> I always ask; "How many of you use a G5RV"? The
> hands go up and it's usually about 50% of the audience.
> I say; "Gosh, I'm sorry", and try to show them how much 
> easier and more efficient it would be to simply use the
> open wire feeder portion of the antenna and a balun at/in
> the (required) tuner.
> 

It's easier to do it that way *IF* you can make the feedline and antenna 
length such that the balun doesn't have to deal with very high, very low, or 
highly reactive impedances on the bands of interest. Or if you can run the 
balanced 
line all the way to a true balanced tuner that can handle the impedances 
presented to it. 

> Some years ago the "Carolina Windom" had the same
> sort of following
> 

And for the same reasons - with the same limitations.

The G5RV and Windom antennas can be useful solutions in many cases. The main 
thing is to understand how they work and what their limitations are.

---

Comparing HF antennas can be very misleading because of all the vagaries of 
propagation and expectations. For example, suppose two hams with identical 100W 
output rigs put up identical dipoles, but Ham A's dipole has a feedline/tuner 
system that is 88% efficient and Ham B's dipole has a feedline/tuner system 
that is 22% efficient.  

Ham A loses only 12 watts in the feedline/tuner system - that's about as good 
as it gets on HF. Ham B loses *78* watts in the feedline/tuner system - 
almost six times the loss of Ham A! 

Yet at the receiving end, the difference is only one S unit - 6 dB. Slight 
differences in propagation could easily mask that and make Ham B's antenna 
appear to be as good or better than Ham A's.

A lot of hams will say a particular antenna "works great" for them. But what 
does that really mean? I remember one multitransmitter Field Day, some years 
ago, when a variety of antennas were tried out by the various station teams. 
All reported their setups "worked great" when notes were compared a few hours 
into the contest.

But for one team, that meant they were able to average 40-60 QSOs/hour, and 
for another team, it meant 10-15 QSOs/hour!  Their expectations were completely 
different. (And compared to truly competitive setups, neither was a 
world-beater).

73 de Jim, N2EY


**
Start the year off right.  Easy ways to stay in 
shape.
 
http://body.aol.com/fitness/winter-exercise?NCID=aolcmp0030002489
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Re: [Elecraft] dipole antenna efficiency

[David Cutter]

Here is an interesting antenna made by Lloyd Butler VK5BR.  It uses open 
wire feeder to a VERY small antenna.


http://users.tpg.com.au/users/ldbutler/ReverseFeedTopLoading.htm

David
G3UNA 


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Re: [Elecraft] dipole antenna efficiency

[Shaun Oliver]

bloody 'ell,,, that was slightly more than I barganned for. oh well, 
every little bit helps, and ultimately confuses.

shaun.


On 4/01/2008 11:58 PM, the old scribe known as Dale Putnam was able to 
impart this pearl of wisdom:
What is needed is a good effecient, multiband antenna with a general 
coverage pattern.
The old saying applies... higher is better. Ok.. now that means up is 
good. Low isn't. Unless you are
intent upon working everyone, including the local tv, stereo, and 
sometimes even the  garage door opener, within a couple hundred miles. 
So.. high is good.
  Now.. lets take a little bite at a time. A finite amount of power out 
of a transmitter.. that doesn't change much, or shouldn't at least. So.. 
power out.. now it radiates best from an antenna... ok... now, it seems 
to me that the best way to get the power to the antenna is with a 
feedline, and the more power that can get into the antenna, the better 
it will get radiated. So.. best feedline, under the conditions... 
installation restrictions apply. The best feedline in the world... won't 
do a lotta good if it isn't installed correctly. Shorts.. poor 
connectors... bends too tight,... too close to metal... all apply.
How about asking the feedline to transfer power outside of the 
parameters that it was intended? Ok.. so what is best? We ask feedline 
to transfer power over a range of freq. into an antenna that is asked to 
radiate with high effectiveness over the same range. If you want to do 
the math... go for it. There are others that do that rather well... I'd 
drather spend time playing radio, but the math still applies. So.. what 
works? Everyone has their own "best". Generally speaking, the most 
effective is a rather high impedance transmission or feedline. Ok... 
open line.. window line.. homebrew... high price...  all are 
considerations.. even consideration of a single wire feeder known as a 
"G" transmission line will do rather well, if the previous 
considerations are taken. So.. now we have a feedline. Next the antenna. 
Oh boy, here it comes,  to where the rope meets leather. The antenna 
needs to radiate. Really well.. and that is generally measured at the 
other fellow's s meter or ears. Ok, so first, where do you want to talk? 
Lets break it down. Inside a 600 mile radius or outside that circle? Now 
we all want to talk all over the world... and have the strongest signal 
all the time.. That just isn't going to happen. If a good signal is to 
be expected within the country.. the antenna needs to be able to radiate 
in such a manner as to put a signal to right  "there"! Ok, we can all 
figure out which antenna has what radiation pattern on paper.
And that is a great start. Now... how about the ground effects... the 
extra bends and twists? All those can be considered, sure, but the 
overriding thought may well be that the antenna needs to accept the 
power being brought to it, and radiate it. Resonance, or being "cut to 
frequency" works well for the imedance match to the feedline, if that is 
a choice to apply. If it is a low impedance feedline.. a small change 
will be noticed, because a  5 ohm change referenced to 50, isn't the 
same as the same 5 ohm difference referenced to 600 ohm.  Bottom line? 
Ok.. here it is.. an antenna that works over the range that is needed, 
and radiates with the best chosen pattern, with the highest transfer of 
power from the feedline to the ether. Pick one... there are many. Many, 
many pages of very good information is available to help you choose. 
That is applicable.. right now. With the winter weather upon us.. now is 
the time for a bit of reading, consideration... and choose. And.. if you 
really want that antenna to stay up?... better consider putting it up in 
the winter too.. if you put it up in the winter, it will last longer.. 
and pretty much work well. Some of the best antennas I've used, have 
been put up in the dead of winter, and some during blizzards. Not 
necessarily recommended. Which would you rather do? Build it, or maybe 
read about it and choose wisely, get everything ready.. and the first 
warm day... get ready... get set... then build away!
 
See you in the pileups, foxhunts, and most importantly... in the log, 
and that's a nice QSL too!


--... ...--
Dale - WC7S in Wy




Make distant family not so distant with Windows Vista® + Windows Live™. 
Start now! 



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Re: [Elecraft] dipole antenna efficiency

[Don Wilhelm]

Folks,

Not to dispute the info that Ron has provided, but to add a bit to it ---
Let me throw in my $.02 worth in this.  Any antenna will radiate if you 
can feed power to it, and it will radiate all the power that is fed to 
it, but there is loss to consider too.  If the radiator and the 
transmission line have zero loss, then all your transmitter power will 
go into the antenna and be radiated - period, but the practical world is 
not quite so simple, real components have real loss - the challenge is 
to make the loss a minimum given the circumstances that exist for your 
installation.


OK, so zero loss is the ideal condition.  In practice, we have resistive 
and other losses to contend with.  When the load impedance of an antenna 
falls below 20 ohms or so, I begin to worry about losses because the 
radiation resistance is a significant fraction of the total resistance.  
A loss of 10 ohms is insignificant with a radiation resistance of 100 
ohms, but a loss of 10 ohms with a radiation resistance of 5 ohms is 
indeed significant.  - that is especially true of verticals where ground 
loss is a significant factor.  A higher feedpoint impedance can be dealt 
with by using a matching network of some sort, but low feedpoint 
impednces need to be given special attention so that the line resistance 
(and the ground resistance in the case of verticals) is minimized.  
Remember that low impedances mean high currents, and given a fixed 
resistance, the higher currents mean greater losses - it is all in the 
physics - you cannot fool Mother Nature.


As a side note - window line performs well and with low loss when dry, 
but when it is wet, the losses in window line can become severe.  If you 
can arrange true open wire feeders, the loss should be low, dry or wet.


73,
Don W3FPR
.
Ron D'Eau Claire wrote:

That's a "linearly loaded" doublet. The zig-zag provides a little more
inductive reactance to help offset the capacitive reactance caused by the
overall length being short. Studies suggest that the resistive losses in the
wire is somewhat less using the "zig zag" than using loading coils each side
of center. So the linear loaded antenna is at least somewhat more efficient.


Since that antenna is fed with a ladder line, the length is unimportant
anyway, except to choose a combination of radiator and feeder length that
provides a reasonable impedance on each band. If you have a wide-range
tuner, that's not important either. 


You can skip the linear loading and simply put up a center fed wire fed with
high-impedance (400 to 600 ohm), low loss line. There will be almost no loss
in "gain" over a 1/2 wave radiator as long as the radiator segment is at
least 1/4 wave long, end to end, plus another 1/8 wavelength in the feeders.
That adds up to a total of 1/2 wave of wire counting the two wires in the
feeder and the radiator. 

  

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Re: [Elecraft] dipole antenna efficiency

[Geoffrey Mackenzie-Kennedy]

WILLIS COOKE wrote:

>I have a 1973 and a 1977 Handbook and I don't see it in either.   What 
>matching method did they use? 

Sorry my error, I should have said the 13th (1974) edition of the ARRL 
*Antenna* Handbook - page 219.

They used an airwound transformer at the centre of the driven element, 19 turns 
of B&W 3025 stock connected between the two halves of the helical driven 
element with a 12 turn tightly coupled link primary. Looking into the link the 
impedance at antenna resonance was approximately 12 ohms. They used a broadband 
1 : 4 balun to raise the impedance to 48 ohms. To reduce antenna Q, short 
lengths of aluminum welding rod were attached to the free ends of the helical 
elements. The antenna was a yagi, driven element and director spaced 15ft 8 
inches.

>I don't think that EZNEC can model a helix very well.  EZNEC can only handle 
>straight wires and inductances can be entered either by inductance value or 
>impedance.  The closest representation >would be short lengths of wire with 
>several loading coils.

I agree, and I am not certain either that an EZNEC model consisting of several 
loading coils would provide useful answers because the helix could be viewed as 
a slow wave structure rather than a classic loaded wire.

73,
Geoff
GM4ESD
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RE: [Elecraft] dipole antenna efficiency

[Ron D'Eau Claire]

David wrote:
I'm also fairly sure that the amount of wire to wind on the helix is 
significantly different from the amount that would be used in a straight 
line.  My gut feeling is that it should be less, but I haven't 
researched that, either,

-

I have experimented with continuously-loaded antennas (those with the
radiating element a long space-wound coil) and found Doug DeMaw's estimates
quite close: a half wavelength of wire space wound with spaces equal to the
wire diameter on a small diameter form will resonate against ground like a
1/4 wave linear Marconi antenna. For example, I space-wound 135 feet of #12
wire on a 2X2 inch wooden form. The antenna was about 12 feet long.  Working
against ground, it was self-resonate in the 80 meter band. So it takes
roughly twice the amount of wire that would be needed if it was stretched
out in a straight line. 

Ron AC7AC

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Re: [Elecraft] dipole antenna efficiency

[David Woolley]

WILLIS COOKE wrote:

Shaun, it will decrease the efficiency drastically.  A
two meter long 80 meter dipole will be about 10% as
efficient as one 40 meters long at the same height. 


That's only conditionally true, although the conditions are probably 
going to apply in this case.  An antenna that is a long way from the 
ground (or over a low loss ground) will have a low radiation resistance 
if it is short, but could be efficient if the loss resistances in the 
antenna and ATU are small.  (I suspect that an antenna that is short 
compared with its height may also have lower ground losses, but I 
haven't researched this.)


As far as I know, a helically loaded antenna will have a higher 
radiation resistance than one with all the inductance lumped in the ATU, 
otherwise they wouldn't be so popular on hand-helds. On the other hand, 
it may well have rather higher ohmic losses.  If 
 is correct, one would expect an 
80m dipole, constructed from normal mode helices, with a span of 2 
metres, to have a radiation resistance of about 200 milli-ohms, whereas 
 seems to suggest 123 
milliohms for lumped loading in the ATU, The radiation resistances 
scales as the square of the length, providing the antennas are short.


I'm also fairly sure that the amount of wire to wind on the helix is 
significantly different from the amount that would be used in a straight 
line.  My gut feeling is that it should be less, but I haven't 
researched that, either, although another URL pulled in searching for 
the above ones suggests one gets the length by pruning back from a 
quarter wave, although it didn't say whether that allowed for end effects.


If this is being used indoors, coupling to the building structures and 
services may well increase the losses - or they could act as the real 
radiator!


Note that these radiation resistances are over an order of magnitude 
outside the specification range for the KAT2.


I do sympathize with the problem though, as I'm antenna restricted, and 
haven't really solved the problem yet.


--
David Woolley
Emails are not formal business letters, whatever businesses may want.
RFC1855 says there should be an address here, but, in a world of spam,
that is no longer good advice, as archive address hiding may not work.
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RE: [Elecraft] dipole antenna efficiency

[Ron D'Eau Claire]

That's a "linearly loaded" doublet. The zig-zag provides a little more
inductive reactance to help offset the capacitive reactance caused by the
overall length being short. Studies suggest that the resistive losses in the
wire is somewhat less using the "zig zag" than using loading coils each side
of center. So the linear loaded antenna is at least somewhat more efficient.


Since that antenna is fed with a ladder line, the length is unimportant
anyway, except to choose a combination of radiator and feeder length that
provides a reasonable impedance on each band. If you have a wide-range
tuner, that's not important either. 

You can skip the linear loading and simply put up a center fed wire fed with
high-impedance (400 to 600 ohm), low loss line. There will be almost no loss
in "gain" over a 1/2 wave radiator as long as the radiator segment is at
least 1/4 wave long, end to end, plus another 1/8 wavelength in the feeders.
That adds up to a total of 1/2 wave of wire counting the two wires in the
feeder and the radiator. 

Such an antenna operated where the top is  1/4 wave long is only about 2 dB
(1/3 of one S-unit) below a full size dipole in "gain". On bands were then
radiator is longer, the antenna will be more efficient, even showing
significant gain on the higher frequencies where the radiator is a
wavelength or more long. 

The high impedance line is important to keep the SWR at a reasonable level
to avoid unnecessary feeder losses. 

Note that the K1JEK antennas meet this criteria even without the linear
loading. 

The biggest things the prefab antennas have to offer is convenience. In the
case of a simple wire antenna like this, you don't need to dig around for
the wire, feeder and suitable insulators.

Ron AC7AC



-Original Message-
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of David Wilburn
Sent: Friday, January 04, 2008 3:05 PM
To: David Yarnes
Cc: Dohn; elecraft@mailman.qth.net
Subject: Re: [Elecraft] dipole antenna efficiency


I understand many G5RV's are getting replaced with these antennas.  I have
the Grampy version, that is 100' long on each side, for a total of 300' of
wire on each side, all in a 100' on each side package.  Have had good luck
with it.  Their shortest is a bit over 70'.

http://www.k1jek.com/ 

-  

David Wilburn
[EMAIL PROTECTED]
K4DGW
K2 S/N 5982


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Re: [Elecraft] dipole antenna efficiency

[Thom LaCosta]

On Fri, 4 Jan 2008, David Wilburn wrote:


I understand many G5RV's are getting replaced with these antennas.  I
have the Grampy version, that is 100' long on each side, for a total of
300' of wire on each side, all in a 100' on each side package.  Have had
good luck with it.  Their shortest is a bit over 70'.


And should you want to roll your own:

http://www.hamuniverse.com/cobraantenna.html

Thom,EIEIO
Email, Internet, Electronic Information Officer

www.baltimorehon.com/Home of the Baltimore Lexicon
www.tlchost.net/hosting/ Web Hosting as low as 3.49/month
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Re: [Elecraft] dipole antenna efficiency

[David Wilburn]

I understand many G5RV's are getting replaced with these antennas.  I
have the Grampy version, that is 100' long on each side, for a total of
300' of wire on each side, all in a 100' on each side package.  Have had
good luck with it.  Their shortest is a bit over 70'.

http://www.k1jek.com/ 

-  

David Wilburn
[EMAIL PROTECTED]
K4DGW
K2 S/N 5982


On Fri, 2008-01-04 at 09:33 -0700, David Yarnes wrote:
> Shaun, Dohn, and All,
> 
> Your proposal to wrap wire around pipe will probably end up being a "bust". 
> In essence, you would just be loading a coil, and it would be very 
> inefficient as a radiator.  Some have tried to use a "helical" approach in 
> order to shorten the overall length, but again that doesn't work all that 
> well either.  An example would be using the old "Slinkey" wire toys that we 
> all probably had at one time or another.  Those can work, after a fashion, 
> but are generally poor performers.
> 
> I disagree about the G5RV also.  That is not a very good antenna, 
> notwithstanding the huge amount of use those antennas get.  The problem is 
> that the G5RV was never intended to be an "all band" antenna, and it is a 
> performer on all but a couple of bands.  Unless you use a tuner, you will be 
> very unhappy with the results you get with a G5RV design, particularly with 
> modern rigs that don't have the advantage older tube-type transmitters had 
> with  pi-network coupling.   The warts that a G5RV has is well documented, 
> and the latest thing I remember reading about it was in a recent issue of 
> Sprat--the Spring 2007 issue.  Therein is a discussion by ex-ZS6BKW, who is 
> famous for originating much of the variations for the G5RV that are in use 
> today.  Paraphrasing his own words, don't expect much from a G5RV unless you 
> at least use a tuner!
> 
>  Unless you want to put up some sort of trap dipole, or a multi-band 
> vertical, your best option is to get a decent tuner and put up a dipole with 
> balanced line feeders.  The 88 foot antenna described by L.B. Cebik on his 
> website (www.cebik.com), is an excellent choice for a simple antenna that 
> will work well on 80 meters through 10 meter.  You can shorten that to 44 
> feet if you want to give up 80 meters.  That will fit in just about anyone's 
> backyard, unless you live in a condo.  The higher you get it, the better it 
> will work.  Just don't use coax, or the losses will eat you alive!  You can 
> coax feed an antenna that is resonant with good results.  But when you try 
> to use it on a higher frequency (like using an 80 meter coax fed antenna on 
> 40 meters), the results turn to mush.  The same antenna fed with balanced 
> line through a tuner will work substantially better at higher frequencies. 
> The resonant antenna (130 feet on 80) works better than the 88 foot antenna 
> does on 80, but the 88 foot version (or the 44 foot version) is intended to 
> standardize your radiation pattern, and is still a decent performer on 80 
> meters.
> 
> Another option would be a remotely tuned vertical on your roof.  Dr. 
> Megacycle (KK6MC), suggested this to me not long ago when he visited my 
> shack.  His advice was to put up a 22 foot vertical, remotely tuned, and 
> using balanced feedline.  This would give you pretty decent performance from 
> 40 meters through 10 meters.  You would need as many radials (random length) 
> as possible.  This was a suggestion as an alternative to my R7 vertical, 
> which is a pretty good performer, but perhaps not as good as the suggested 
> alternative.  I haven't tried that yet, but I'd like to.  I'm sure the 
> suggestion is a good one.
> 
> This may be more than you wanted to hear, but it seemed as if you were 
> struggling to come up with a solution.  Hopefully it helps some in that 
> process.  If wire antennas, or verticals, are just not practical for you, 
> you might want to try a mini-loop.  These aren't nearly as efficient as a 
> good wire antenna, but they are quite small, and the results can often 
> surprise you.  Besides, they are fun to build!
> 
> Dave W7AQK
> 
> 
> 
> - Original Message - 
> From: "Dohn" <[EMAIL PROTECTED]>
> To: "'Shaun Oliver'" <[EMAIL PROTECTED]>; 
> 
> Sent: Friday, January 04, 2008 2:50 AM
> Subject: RE: [Elecraft] dipole antenna efficiency
> 
> 
> > Shaun,
> >
> > For materials needed vs. radio theory, you just can't beat a G5RV antenna.
> > IMHO I've used mine with my K1, K2 and several home brew projs and it 
> > works
> > great time and again.  Yes, you want to get the center up there a ways. 
> > Say
> > ar

Re: [Elecraft] dipole antenna efficiency

[Geoffrey Mackenzie-Kennedy]

Shaun Oliver  <[EMAIL PROTECTED]> wrote:

I was thinking of using 18 mm electrical conduit. with 2 mm spacings for 
each turn of the wire.


-

The diameter is a bit on the small side for the low bands, but NM Helix 
antennas working as 'quarter wave' radiators on the low bands designed for 
mobile use, and whose diameter was similar, performed well when evaluated 
using a proper antenna range, better than a base loaded 8ft whip. The 
important consideration is that the antenna resonates within the range of 
frequencies used in the band for which the antenna is designed. I have not 
tried a single NM Helix with an external tuner for multiband operation nor 
tried to model one using EZNEC.


Good luck!

73,
Geoff
GM4ESD

PS  'Old scribe'  --- hmmmpf :-)

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Re: [Elecraft] dipole antenna efficiency

[Ken Kopp]

Dave's observations are "right on" and good reading.

I've never understood the popularity of the G5RV.  It's
achieved a sort of "cult" following.  The original design
was for a 20M -ONLY- antenna, and somehow has
morphed into a do everything hoax.

I do antenna talks at conventions and hamfests, and
I always ask; "How many of you use a G5RV"? The
hands go up and it's usually about 50% of the audience.
I say; "Gosh, I'm sorry", and try to show them how much 
easier and more efficient it would be to simply use the

open wire feeder portion of the antenna and a balun at/in
the (required) tuner.

Some years ago the "Carolina Windom" had the same
sort of following

73! Ken Kopp - K0PP
[EMAIL PROTECTED]
or
[EMAIL PROTECTED]

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Re: [Elecraft] dipole antenna efficiency

[WILLIS COOKE]

There are many forms of shortened antennas that work
to varying degrees.  Antenna selection and
installation is a very complicated situation and the
ARRL Antenna Handbook, as large as it is only has a
fair sampling of all the information that is
available.  Any antenna that completed the desired
communication worked.  Any antenna that could not
complete the desired communication did not work.  We
talk a lot on this reflector about how wonderful the
Elecraft products work, but even Elecraft transceivers
need to be connected to an effective antenna to
achieve the desired result.

I have a friend who has a 3 element SteppIR at 70 feet
on top of a mountain.  He thinks it is a poor antenna
because at his previous QTH he had a Hygain 5 element
full sized beam at 110 feet which worked much better. 
Several stations around the world have antenna systems
that cover many acres and cost 6 or 7 figures and they
work very well.  On the other hand, I knew a ham in
1976 who set out to work all states using a Weller
Soldering Gun laying on his hamshack floor for an
antenna.  He was however, driving it with a kilowatt
and an expensive antenna tuner.

The G5RV antenna and its variations is a clever design
and allows working most bands with the assistance of a
good antenna tuner.  My 160 meter Carolina Windom is a
different design that does the same and I have worked
DXCC with it in less than a year's time.  Both are
wire antennas and not very expensive, but not free
either.  The most expensive thing is ground to install
them where antenna restrictions don't bring down the
wrath of the taste police. (I have been trying for
over 50 years to convince the world that antennas are
beautiful, but without success) Neither can compete in
a match with a good tri-bander at 60 feet.

Helix antennas will work.  They will not work as well
as a full sized antenna.  The Outbacker and the
Hamstick come to mind as helix antennas that work. 
The shorter that you make them, the less efficient
they are but we all know that in some instances we can
reduce power to a watt or less and still maintain
communication.  The same goes for antennas.  The
rubber duck on your hand held will work a repeater if
you are close enough.  A quarter wave will work the
repeater from farther away.  Either one will work
better if you go to the roof of a 10 story building
instead of standing on the ground.  Either antenna
will work better outside and in the clear than in a
building.

Antenna selection, like most things in life, requires
a lot of compromising. You have to live with your
budget, your living circumstances, the XYL and your
local taste police.  But, there is no free lunch!  You
can move, you can get a new XYL, but you can't cheat
Mother Nature.

Cookie, K5EWJ 
--- David Cutter <[EMAIL PROTECTED]> wrote:

> I seem to remember a 40m beam made this way in one
> of the old ARRL books.
> 
> David
> G3UNA
> >
> > While Cookie's comments are true, there is a
> useful type of short antenna 
> > known as a Normal Mode Helix, which if built,
> tuned and matched correctly 
> 
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Re: [Elecraft] dipole antenna efficiency

[Geoffrey Mackenzie-Kennedy]

David, the beam was descibed in the 13th (1974) edition of the ARRL 
handbook, perhaps in an earlier edition also, it performed well..


73,
Geoff
GM4ESD

David Cutter  <[EMAIL PROTECTED]> wrote:


I seem to remember a 40m beam made this way in one of the old ARRL books.

David
G3UNA


While Cookie's comments are true, there is a useful type of short antenna 
known as a Normal Mode Helix, which if built, tuned and matched correctly


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RE: [Elecraft] dipole antenna efficiency

[Ron D'Eau Claire]

We're back to wanting our RF "superconductor" again .

First, to add a note to the original question, it's useful to think of the
RF field as flowing OVER the outside of the conductor, so a helix (coil) or
any form of zig-zag arrangement of wires simply look like fat conductors to
the RF field. In this case a very short, fat conductor. The normal rule of
thumb for fitting an antenna into a limited space is to avoid any bends
sharper than 90 degrees in the wires. Once the bend is greater than 90
degrees, the overall length of the wire - as "seen" by the RF field - is no
longer as long as the conductor. Instead it begins to look like a conductor
of varying thickness whose length is about equal to the gross distance from
one end of the antenna to the other, ignoring the bends or turns (as in a
coil). Current does flow through the conductor (wire) but the important
point here is that the field generated by the currents doesn't follow the
wire when it bends or turns too sharply. 

To transfer power to an antenna efficiently we need to balance the
capacitive reactance with the inductive reactance so the currents and
voltages are in phase (the condition we normally refer to as "resonance"). A
half wave wire - a "dipole" - is one in which the natural capacitive
reactance of the wire matches the inductive reactance of the wire at what we
call the 'resonant frequency'. A Marconi - 1/4 wave radiator that is
grounded at one end - achieves the same thing with the lower inductive
reactance of the 1/4 wave wire being matched by the higher capacitive
reactance between the wire and the earth or some artificial ground such as a
ground plane or counterpoise. 

A short radiator has lower capacitive reactance, so we need to add inductive
reactance to a short antenna to achieve resonance.

For an coil to show inductance it must have current flowing through it. The
more current, the more inductance for a given number of turns on the coil. A
coil also has loss resistance since we don't have the perfect
"superconductor" handy. That loss resistance is much, much greater than the
d-c resistance since RF currents flow only along the very surface of a
conductor. The ideal RF coil uses the best conductor material available and
it uses a very large conductor for maximum surface area (although it can be
a very thin tube, if desired, since no significant current flows below the
surface). That's why high-efficiency RF coils tend to be large and use large
sized conductors that are often silver-plated. 

For the highest current to flow to minimize the amount of inductance needed,
we might put the coil near a "current loop" or point of maximum current (the
center of a dipole or the ground end of a 1/4 wave "Marconi" antenna). The
problem with that is that higher current equals greater resistive losses. If
we move the coil farther out toward a voltage loop (minimum current) we need
more turns to reach resonance. More turns means more wire which means more
resistive losses. 

A popular compromise is to put the coil about half way in between the loops.
On a short "Marconi" radiator, such as a mobile "whip" used on a car, that
puts the coil in the center, halfway between the base and the tip. On a
short dipole, one needs two coils about half way between the current loop at
the center and the voltage loops (the electrostatic 'poles' in a dipole) at
the ends.

A third approach is to continuously load the radiator with a helical
conductor such as Shaun mentioned. To the RF field, it's still a short
radiator, so it needs inductive reactance to achieve resonance. That
reactance is provided by making a long coil the length of the radiator. 

In all of these approaches, the biggest problem is resistive losses in the
wire. 

If we only had that perfect RF superconductor, we could make short antennas
as efficient as full-sized antennas. But then our full sized antennas would
be more efficient too! 

Lacking our RF superconductor, some designs use "capacitive hats" at the
ends of the antenna. These are discs, or disc-shaped constructions of wire
or tubing that don't add to the physical length - presumably we can't make
the antenna any longer or we would - but they do increase the
self-capacitance of the radiator. Increasing that capacitance reduces the
inductance needed and hence the losses in the inductor needed to establish
resonance. 

Ron AC7AC


-Original Message-
I seem to remember a 40m beam made this way in one of the old ARRL books.

David
G3UNA
>
> While Cookie's comments are true, there is a useful type of short 
> antenna
> known as a Normal Mode Helix, which if built, tuned and matched correctly 

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_

Re: [Elecraft] dipole antenna efficiency

[David Yarnes]

Shaun, Dohn, and All,

Your proposal to wrap wire around pipe will probably end up being a "bust". 
In essence, you would just be loading a coil, and it would be very 
inefficient as a radiator.  Some have tried to use a "helical" approach in 
order to shorten the overall length, but again that doesn't work all that 
well either.  An example would be using the old "Slinkey" wire toys that we 
all probably had at one time or another.  Those can work, after a fashion, 
but are generally poor performers.


I disagree about the G5RV also.  That is not a very good antenna, 
notwithstanding the huge amount of use those antennas get.  The problem is 
that the G5RV was never intended to be an "all band" antenna, and it is a 
performer on all but a couple of bands.  Unless you use a tuner, you will be 
very unhappy with the results you get with a G5RV design, particularly with 
modern rigs that don't have the advantage older tube-type transmitters had 
with  pi-network coupling.   The warts that a G5RV has is well documented, 
and the latest thing I remember reading about it was in a recent issue of 
Sprat--the Spring 2007 issue.  Therein is a discussion by ex-ZS6BKW, who is 
famous for originating much of the variations for the G5RV that are in use 
today.  Paraphrasing his own words, don't expect much from a G5RV unless you 
at least use a tuner!


Unless you want to put up some sort of trap dipole, or a multi-band 
vertical, your best option is to get a decent tuner and put up a dipole with 
balanced line feeders.  The 88 foot antenna described by L.B. Cebik on his 
website (www.cebik.com), is an excellent choice for a simple antenna that 
will work well on 80 meters through 10 meter.  You can shorten that to 44 
feet if you want to give up 80 meters.  That will fit in just about anyone's 
backyard, unless you live in a condo.  The higher you get it, the better it 
will work.  Just don't use coax, or the losses will eat you alive!  You can 
coax feed an antenna that is resonant with good results.  But when you try 
to use it on a higher frequency (like using an 80 meter coax fed antenna on 
40 meters), the results turn to mush.  The same antenna fed with balanced 
line through a tuner will work substantially better at higher frequencies. 
The resonant antenna (130 feet on 80) works better than the 88 foot antenna 
does on 80, but the 88 foot version (or the 44 foot version) is intended to 
standardize your radiation pattern, and is still a decent performer on 80 
meters.


Another option would be a remotely tuned vertical on your roof.  Dr. 
Megacycle (KK6MC), suggested this to me not long ago when he visited my 
shack.  His advice was to put up a 22 foot vertical, remotely tuned, and 
using balanced feedline.  This would give you pretty decent performance from 
40 meters through 10 meters.  You would need as many radials (random length) 
as possible.  This was a suggestion as an alternative to my R7 vertical, 
which is a pretty good performer, but perhaps not as good as the suggested 
alternative.  I haven't tried that yet, but I'd like to.  I'm sure the 
suggestion is a good one.


This may be more than you wanted to hear, but it seemed as if you were 
struggling to come up with a solution.  Hopefully it helps some in that 
process.  If wire antennas, or verticals, are just not practical for you, 
you might want to try a mini-loop.  These aren't nearly as efficient as a 
good wire antenna, but they are quite small, and the results can often 
surprise you.  Besides, they are fun to build!


Dave W7AQK



- Original Message - 
From: "Dohn" <[EMAIL PROTECTED]>
To: "'Shaun Oliver'" <[EMAIL PROTECTED]>; 


Sent: Friday, January 04, 2008 2:50 AM
Subject: RE: [Elecraft] dipole antenna efficiency



Shaun,

For materials needed vs. radio theory, you just can't beat a G5RV antenna.
IMHO I've used mine with my K1, K2 and several home brew projs and it 
works
great time and again.  Yes, you want to get the center up there a ways. 
Say

around 35-40 feet, but the payoff was good for me.  There are a number of
sites out there that have good versions of a very old, classic antenna.
Very few parts (all of which can be made with junk around the house.  Very
little investment for a sound payoff.

Dohn   N8EWY

-Original Message-
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Shaun Oliver
Sent: Thursday, January 03, 2008 9:20 PM
To: elecraft@mailman.qth.net
Subject: [Elecraft] dipole antenna efficiency

Hi all,
if I were to make a dipole, would coiling it round a piece of plastic
piping decrease the efficiency of the thing? I intend on making one for
40 meters, one for 80 meters, and one for 10 meters. I've worked out
that1 meter of piping will suffice for each leg of each antenna.
thanks in advance.
shaun
___
E

Re: [Elecraft] dipole antenna efficiency

[David Cutter]

I seem to remember a 40m beam made this way in one of the old ARRL books.

David
G3UNA


While Cookie's comments are true, there is a useful type of short antenna 
known as a Normal Mode Helix, which if built, tuned and matched correctly 


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Re: [Elecraft] dipole antenna efficiency

[Geoffrey Mackenzie-Kennedy]

Shaun Oliver  <[EMAIL PROTECTED]>

if I were to make a dipole, would coiling it round a piece of plastic 
piping decrease the efficiency of the thing? I intend on making one for 40 
meters, one for 80 meters, and one for 10 meters. I've worked out that1 
meter of piping will suffice for each leg of each antenna.

thanks in advance.
shaun


Cookie, K5EWJ wrote:


Shaun, it will decrease the efficiency drastically.  A
two meter long 80 meter dipole will be about 10% as
efficient as one 40 meters long at the same height.



You
can't cheat the laws of physics, they work the same
even if no body is watching.  In
addition, the antenna you are considering will have an
impedance of only an ohm or two and be almost
impossible to match.


---

While Cookie's comments are true, there is a useful type of short antenna 
known as a Normal Mode Helix, which if built, tuned and matched correctly 
can work as well as a full size dipole at the same height - although its 
VSWR (say 2:1 for example) bandwidth is narrower than that of a full size 
dipole. It consists of wire wound around a fibreglass or plastic rod / pipe 
with the turns spaced, not closewound. If plastic pipe is used it must have 
good RF 'Insulating' characteristics i.e if a piece 'cooks' when placed in a 
microwave oven, when your XYL is not looking, do not use that material. For 
80m and 40m a pipe diameter of 2 1/2 inches is about right, for 10m 3/4 inch 
or thereabouts. The total length of wire in a NM Helix 'dipole' is 
approximately one wavelength, equations do exist which could be used to 
calculate the length but I have yet to find an equation which is exact 
enough to eliminate the need to tweak. The length of pipe for a 40m NM 
Helical 'dipole' is close to 6 metres, the length could be reduced by 
increasing the pipe's diameter but there is a limit beyond which the 
radiation pattern suffers. Very short and effective NM Helix antennas have 
been built using additional end loading such as hats, but their drive point 
impedance is very low and difficult to match efficiently and the VSWR 
bandwidth becomes narrower.


The 'Rubber Duck' antenna sometimes used on mobile phones is one type of NM 
Helix antenna.


73,
Geoff
GM4ESD



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RE: [Elecraft] dipole antenna efficiency

[Dohn]

Shaun,

For materials needed vs. radio theory, you just can't beat a G5RV antenna.
IMHO I've used mine with my K1, K2 and several home brew projs and it works
great time and again.  Yes, you want to get the center up there a ways.  Say
around 35-40 feet, but the payoff was good for me.  There are a number of
sites out there that have good versions of a very old, classic antenna.
Very few parts (all of which can be made with junk around the house.  Very
little investment for a sound payoff.

Dohn   N8EWY

-Original Message-
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Shaun Oliver
Sent: Thursday, January 03, 2008 9:20 PM
To: elecraft@mailman.qth.net
Subject: [Elecraft] dipole antenna efficiency

Hi all,
if I were to make a dipole, would coiling it round a piece of plastic 
piping decrease the efficiency of the thing? I intend on making one for 
40 meters, one for 80 meters, and one for 10 meters. I've worked out 
that1 meter of piping will suffice for each leg of each antenna.
thanks in advance.
shaun
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Re: [Elecraft] dipole antenna efficiency

[WILLIS COOKE]

Shaun, it will decrease the efficiency drastically.  A
two meter long 80 meter dipole will be about 10% as
efficient as one 40 meters long at the same height. 
Most texts quote dbi information for a full sized
dipole 100 feet high.  If you put your antenna near
the ground it may not be much more than 1% as
efficient as the full sized antenna at 100 feet.  You
can't cheat the laws of physics, they work the same
even if no body is watching.  There is no substitute
for high and big!

If you are trying to run QRP with 5 or 10 watts then
you will be doing well to work across the street.  In
addition, the antenna you are considering will have an
impedance of only an ohm or two and be almost
impossible to match.

Sorry, nice idea, but it will not work well at all.

Cookie, K5EWJ, BS Physics, PE/EE, 52 years a ham 
--- Shaun Oliver <[EMAIL PROTECTED]> wrote:

> Hi all,
> if I were to make a dipole, would coiling it round a
> piece of plastic 
> piping decrease the efficiency of the thing? I
> intend on making one for 
> 40 meters, one for 80 meters, and one for 10 meters.
> I've worked out 
> that1 meter of piping will suffice for each leg of
> each antenna.
> thanks in advance.
> shaun
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