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Re: [TowerTalk] Balun/Choke power ratings

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Balun/Choke power ratings
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Mon, 28 Apr 2014 10:03:09 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On 4/28/2014 5:40 AM, Jim Lux wrote:
And then
what would be a "real" objective power rating method? Seems that one of the key factors is the dissipation of whatever you're choking, although if the Z is high enough, maybe that's insignificant.

I can't think of a good one that covers all bases. My choke cookbook is aimed at US legal limit operation into coax-fed antennas near resonance that are reasonably well balanced. They are NOT designed for non-resonant "all band" dipoles nor off-center fed wires; these designs place far greater stress on a choke, and even a very good choke is likely to fry with high power.

A torture test that I've used is to use a choke as the end insulator of an end-fed vertical dipole, as described in two presentations I've done at various clubs and hamfests. One is in the slide show for Coax Chokes, the other is in the slide show about whether an HF vertical should roof-mounted or be on the ground. http://k9yc.com/publish.htm

Retired ARRL Antenna Book editor N6BV, is a huge fan of all-band dipoles fed with high impedance 2-wire line. At Pacificon last fall, Dean showed that if one of my choke designs were to be added at the feedpoint of a coax-fed half wave horizontal dipole cut for 80M, the DIFFERENTIAL dissipation in the choke would be several hundred watts when the antenna was used on 40M because of the extremely high mismatch on that band (standing waves). That's in addition to the dissipation due to whatever common mode voltage there might be across the choke.

Now, the antenna system that Dean was showing was what I call "an unnatural act" -- an antenna/feedline system that has a very high SWR on a band where it is being operated. In addition to the power that overheats the choke and fries it, there's a lot more loss along the line, so only a small fraction of the transmitter power gets to the antenna to be radiated.

73, Jim K9YC
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