Don suggested this would be worth posting...
When dealing with antennas, things are never as "precise" as we might
want them to be. Folks talk about antenna tuners as if they work
magic, then wonder why things aren't as good as they should be. An
ATU at the shack in line with a coax-fed antenna has one big purpose.
It provides a nice load for the transceiver. It does absolutely
nothing to help out the feedline or antenna itself. The very precious
RF gets burnt up in the coax (with its unknown, but potentially very
high SWR) between the tuner and the antenna. There are strange RF
currents all over the place, especially on the coax, which not only
heats the coax, but tend to screw up the antenna radiation pattern as
well.
My belief is that there are three solutions to the problem.
1) Place the ATU _AT_ the antenna feed point. The problem is that it
is really a pain to do in most cases.
2) Use matched antennas on all bands. Cebik points this out in a
great paper on his website that discusses open-wire vs coax fed
antennas (see "To Trap or Not To Trap",
http://www.cebik.com/wire/trapqq.html). This is also the solution
used by the larger (more serious, lots of money to spend) contesters.
3) Use open-wire feed. There we don't care about the mismatch since
the feedline loss is microscopic. The RF currents balance themselves
out so that the feedline doesn't radiate. But we then need a decent
tuner to feed the antenna & open-wire. Here I drop back to the old
tank-circuit tuners which make things "sing!" I don't like to use a
balun here - it's just another device to burn up the precious RF when
you hit a severe mis-match node.
Now if you don't have a swinging-link balanced tuner, but do have an
L-network tuner and are running low power, go ahead with a 1:1 balun.
At low power there is little chance you will saturate the balun core
unless you get really unlucky and hit a severe node at the operating
frequency.
I do use coax to feed my antennas, but only for those that are
matched for the band/frequency. For all others, especially ones that
are used on multiple bands, I use open-wire feed. The length of the
feed really doesn't matter because of the super low loss. What
happens is the feedline transposes the antenna feed impedance to a
value dependent on the feedline length and operating frequency. The
balanced antenna tuner then matches whatever impedance it is
presented with, making the transceiver very happy. And with the low-
loss characteristics of open-wire feed, just about everything the
K2/100 puts out gets radiated by the antenna!
On Oct 1, 2006, at 6:20 PM, Don Wilhelm wrote:
Zac,
Why guess?
You can find the antenna feedpoint impedance for various bands on L B
Cebik's website (at least it used to be there) www.cebik.com .
Armed with that information along with the known length of your
feedline,
use TLW which you can download from the ARRL website
http://www.arrl.org/notes/9043/TLW3B.zip and feed the information
into TLW
to find your shack end impedance for each band of interest. A bit
of cut
'n' try with TLW can find you the best compromise feedline length
to ues
with your antenna.
73,
Don W3FPR
-----Original Message-----
I'm sorry but I need to add this before Zac Brown does something
real stupid. When using 450 ohm feed line you must use this EXACT
length. The exact length is that which goes from the antenna to the
balun in your shack.
Any talk about matching the 450 ohms is not correct. The feed
line
runs with a very high SWR and since it is low loss there is just a
tiny
loss. It will correct to a 1:1 SWR between your tuner and the radio.
And about the balun. Seems if you have a 4:1 or 1:1 to choose
from I
would choose the 1:1 because it does a better job of keeping the
current
balanced in the balanced feed line.
This is pretty complex stuff but you Zac do not need to do the
math.
Just put it up and use it. I have been using it for 8 years now.
Had to
get feed line. A hail storm just beat up my feed line :-)
I have done the math. It takes several Smith Charts to plot the
current through the system.
73 Karl K5DI
Zac Brown wrote:
So what's the best way to add some more line to the middle of my
feedline? Is there a technique for joining two pieces of 450 ohm
ladder line that doesn't create an imbalance in the line? Can I
just
leave one "leg" straight and then wrap the corresponding
"leg" (on the
other piece) other around it, then solder them together? Or
should I
just leave both legs straight and then solder them together? Or how
about using those twist locks that you use for wiring up lights?
This is probably a silly question, but I took great care to route
the
line away from anything that would imbalance it, and would hate to
mess that up now. This is my first experience with balanced line :)
Thanks,
Zac
KD5IEF
K2# 4907
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