Ethan,
I can't agree with your last paragraph. The usual problem with RF
in the shack is traceable to a couple of factors. First, your shack is
right there under your antenna and the RF in the shack is direct
radiation from the antenna. However, that is not the problem in most cases.
In most cases, RF in the shack can be traced to having RF on the
outside of the shield when using coax or an unbalance in the current on
open wire feeders. Usually the problem is manifest in the case of coax.
The really good news is that there are easy fixes for the problem. The
fix is to use a proper balun or line isolator. A proper balun is one the
prevents currents from flowing on the outside of the coax shield. I dare
say that most inexpensive baluns are of the voltage type. What that
means is, if the SWR gets too high or the input terminals don't see
equal but opposite currents, current WILL flow on the outside of the
shield; there are several books on this subject. A Windom or OCF dipole
will not present equal and opposite current to the balun input. The fix
is to use a balun type that will minimize current flow on the outside of
the shield, and that is a CURRENT balun.
The other thing you can do is to install a line isolator. This is a
device that is called a choke or 1:1 balun. It is nothing more than a
bunch of magnetic material, ferrite or powdered iron, on a section of
the coax below the feed point and balun, if used. Coiled up coax works
also, about 10 or so turns about 6 or so inches in diameter. Taking
these steps should make a dramatic improvement to the conditions you
reflect.
73,
Barry
K3NDM
On 1/11/2014 3:02 PM, Ethan Miller K8GU wrote:
All,
I don't think anybody was questioning the fact that the OCF dipole
"works." If you haven't read it, I encourage you to borrow or buy a
copy of the book Array of Light by Tom Schiller, N6BT, and read the
chapter "Everything Works." The point of the essay is that you can
make QSOs (even DX QSOs from W6) with a 100-watt lightbulb "antenna"
at the end of a piece of coax. Compared to that, the OCF will
certainly allow you to work stations, especially when conditions are
good!
However, the OCF dipole is in a class of antennas commonly used by
amateurs that is known to invite problems (e.g., RF in the shack,
common mode noise, and so forth) that are easily mitigated by using a
different type of antenna and feed arrangement. That does not mean
that people in urban areas with relatively high noise floors or those
who run low power might not be perfectly happy with them. It's all
about the design goals you have for your station. If someone notices
that they've been experiencing one of the symptoms of imbalance listed
above, it's only natural to suggest that the antenna might be the
culprit. That is all.
I should probably go play some more in the NAQP while my toddler is
still asleep...
73,
--Ethan, K8GU/3.
On Sat, Jan 11, 2014 at 2:10 PM, Stephen Roberts <[email protected]> wrote:
I also use the 7 band Buckmaster OCF antenna. My config is the long leg is
East/West and 80' on one end and about 50 on the other (also sloping property).
The short leg is at a right angle and is about 50' on one end and probably 30
on the other. It's fed with about 30' of coax.
It's a fantastic antenna and I can work coast to coast QRP. It also works DX
nicely and I've worked Antarctica, NZ, etc. I think I'm up to 185 countries now
QRP.
Steve
W1SFR
Kx3helper.com
Sent from my iPad
On Jan 11, 2014, at 1:50 PM, Barry LaZar <[email protected]> wrote:
Doug,
Nice write up.
Other Readers,
I'd like to make a comment about the antenna as I know more than just a
little about the things. An off center fed, OCF, dipole is not a magic antenna
as some hams have come to believe. They are just a good wire antenna that is
fed in a way to make them appear to have a 50 Ohm impedance. They are based on
the 1/2 wave dipole which only has a workable impedance at 1/2 wave. At even
multiples of a 1/2 wave, the antenna exhibits a high impendance which most
radios do not like. So, way back in radio history a smart guy said if I don't
feed the dipole in the center I can find a spot that looks like 300 Ohms and
use a transformer to make it look like 50 Ohms over several bands. When he
invented that antenna, the only odd band was 15 meters. What that meant was
that if you created an 80 meter OCF dipole, you could use it on 40, 20, and 10
meters, without a tuner.
The basic 1/2 wave dipole antenna can be awesome if hung up at some
reasonable height. When fed in the center it is going to have a very high SWR,
which means you need to be careful of transmission line loss and have some way
to deal with the SWR. Open wire feeders handle the loss issue and a very good
tuner does the rest. Or you go to the OCF dipole and use a 6:1 CURRENT balun or
balun and line isolator.
When a dipole is operated at multiples of a 1/2 wave you can actually
develops gain in the plane of the antenna. How much gain and lobe direction is
a function of how many 1/2 waves the antenna is.
Those of you who want a good basic antenna and don't want to spent a lot
and can get it up about 30-35 feet or higher go for the OCF dipole. I think
you'll be happy. I have one up in my yard at about 40' on one end and about 35
on the other( it has to do with the slope of the land) and, as Doug, I work out
everywhere. There are several sources for the basic OCF antenna and a source
for a variant that I run. However, building one is a snap. Just go to Home
Depot and buy 135' of #12 stranded insolated house wire ( color is unimportant
) have it cut such that the cut is about a 1/3 of the way off an end. Pick up
some insulators at a hamfest and order either a 4:1 or 6:1 CURRENT balun. feed
the beast with some low loss or reasonable loss cable, and then sit back and
have fun. I would expect DXCC in 30 days, band conditions being good.
Some final comments. I use #12 insulated, stranded wire just for the extra
strength. I don't want to be puting up a new antenna every time we have high
winds or ice. You could use #14 which most use, but like I said I don't want to
have to re-install my antenna. You will need an antenna tuner, but that is OK
as you will now be able to use the antenna on all bands 80-10. Lastly, if you
don't have 135' feed of horizontal space you could bend the ends down to fit if
it represents a minor portion of the antenna Or you could build a 40 meter
version using 66' of wire and just not work 80 effectively; I can actually get
mine to tune on 80, but it isn't great.
73,
Barry
K3NDM
On 1/11/2014 12:34 PM, Doug Ellmore Sr. wrote:
I saw some discussions on OCFS. I thought I would share my setup.
I worked 160+ countries with a Buckmaster 8 band OCF QRP. K3NDM and N3HU
have observed its operation with my former ICOM 703, K3 and KX3.
I think Buckmaster uses a 6:1 balun. I have my feedpoint at the peak of my
house about 34' fed with 60' LMR400. The feedline drops down the roof 45
deg then perpindicular to the ground. It is connected to a static arrestor
before being fed back up to the 2nd floor shack. The long leg is supported
by a 14' 4x4 support north of my house but far enough away with a rope the
the end is 18-20' above the ground. The short leg is supported by a tree
up ~20' up SSE of the house.
I used the internal tuner to tweak swr, but have also used the kx3 &
SB-200 to get 200 watts with no tuner with no problems.
Check the specs on the Buckmaster, it might help those trying the OCF setup.
A north to south orientation seems to help me work good dx from Maryland.
73,
Doug, NA1DX
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