What were you running Pete, 50 KW? ;-)
Give us some more information on your setup there.
People should not be afraid of this antenna, just because Pete had bad
experience with it.
I have experienced the same type of problem with openwire fed dipoles at
some locations, but only when running about 1kW.
Actually what Pete described can be done with 500w but it's not gonna happen
with 100w.
That's why one of my questions was how much power is going to be run.
There are two ways to approach this problem.
You can engineer the heck out of it and calculate your impedance down to the
milliohm +/- j milliohm, but try to match these results with the published
specs of any matchbox!
In most cases, the antenna will only fit in one place and you can't move the
shack, so the fast road to success is to simply build the antenna, run the
feedline and then measure the impedance at the end of the feedline. You
don't know the exact length anyway until you build it.
If you have the kind of problem Pete speaks of, you will need to adjust the
feedline length. Usually 25' of (Wireman) window line for 80m or 13' for
40m will do the job. For openwire (not the Wireman stuff), add about 10%
more to those lengths. The problem occasionally is on the higher bands, but
most of the time I have only had it on the lower bands.
In addition, you would be better off going with a Link Coupled Matchbox,
such as the Johnson Viking Matchbox or the Annecke. These are superior to
other designs in dealing with common mode current on symmetrical feedlines.
Next best is a fully symmetrical matchbox such as the one Scott suggests in
his article, or the Palstar or MJF as tested by Joel Hallas (W1ZR) in QST
Sep 2004.
The worst solution is to use an asymmetrical tuner (such as the T-Network
tuner) and a balun. It is adequate for low power levels but can quickly get
problems with higher power levels, thus requiring external choke baluns as
Jim describes (see below).
I have used this antenna on various occasions at several locations, and I
have never experienced that kind of problem, but I have never run more than
500w with mine. I have also always used a link coupled tuner with it and
this is a big advantage. I don't think that kind of problem is more likely
to occur with this antenna than with others.
Common mode current can be a real beast and in really bad cases you need
some hefty choking - as Jim (K9YC) has pointed out in his excellent paper,
"RFI-Ham". In some cases you need up to 5 FT-240 cores in one choke and
you may need multiple chokes. In those cases, if you do less than that, you
will burn up the choke/balun.
It's never a bad idea to read the ARRL handbook, but it doesn't have very
much information on this specific antenna. There has been excellent, more
detailed work done by several people on this topic, dating back to the mid
70s - maybe earlier. The problem is to find it. Much of it was done before
the Internet. I have some good articles from the70s but all in German.
..Paul Carr, N4PC did a lot of experimenting with this type of antenna back
in the 1980s and wrote several articles on them.
..L.B. Cebik, W4RNL (SK) wrote an excellent paper on this antenna.
.. Guenther Schwarzbeck wrote an excellent article on this in a multi-part
article on antennas published in the German CQDL magazine.
.. Scott's QST article is a good place to start! Nov 2006 QST.
73 - Rick, DJ0IP
(Nr. Frankfurt am Main)
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Peter
Bertini
Sent: Tuesday, December 30, 2014 4:33 AM
To: tentec@contesting.com
Subject: Re: [TenTec] Antenna help
Wow, lots of questions...
But, you need to know the feedpoint impedance of the loop... and regarding
the feed line, you do realize that it is most likely going to work as a
matching (or MISmatching network) depending on its electrical length? It
would impossible to recommend either a 4:1 or 1:1 balun unless the antenna
has been modeled and these important parameters known.
There are some nice snap-on molded spreaders that are designed to be used
with THNN electrical wire--they work quite well.
By the way, some time back I tried a similar setup, and ended up with
extremely high RF currents--end up melting PL-259 and SO-239 fittings,
damaged a Dentron 3kW tuner, and blew the snot of out a 1:1 DX Engineering
balun--which was at a high voltage point.
Pete
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