Also, as I have mentioned off line to Tony, some fixed capacitors will drift
consirderably with temperature changes often caused by circulating current
in tuned / matching circuits. Possibly a compensating capacitor in the
balun - ?
Gene / W2LU
----- Original Message -----
From: "Tod - ID" <email@example.com>
To: "Tony" <firstname.lastname@example.org>
Sent: Wednesday, December 21, 2011 10:50 AM
Subject: Re: [TowerTalk] SWR Problem Balun or lightning protection?
> This is an interesting thread which has gotten difficult to follow.
> Let me see if I understand it correctly.
> 1. When you transmit at high power either with a continuous carrier [
> RTTY] or with a high transmit duty cycle the VSWR you observe changes and
> I have some questions that I would seek to answer if it happened at my
> 1. Is this something that I noticed only recently?
> 2. If the answer to (1) is yes, then how long ago do I think it was
> operating correctly?
> 3. What has changed since I think it was operating correctly?
> a. New mode or method of operating?
> b. New equipment - feed line, connectors, filters, relays, transformer
> balun , current balun, lightning protection, antenna tuner, amplifier,
> c. The outside temperature is different than it was when it last was
> working correctly and the last time the temperature was the same as it is
> now the system was working.
> If it was working and is now not working the cause should be associated
> with a change and that change should be somewhere in the collection of
> choices listed in (3) above.
> Two years ago I experienced a similar phenomenon when operating on 160
> meters. I mesured the change in VSWR as a function of time when I
> transmitted on air using a continuous RF carrier. I could observe the VSWR
> rising as the length of transmission increased. I could also see that the
> VSWR increased at a different rate when the temperature outside was 30
> degrees F than when the temperature was 10 degrees F.
> The rate of VSWR rise changing with temperature led me to conclude that
> the cause was something outside rather than inside the shack. The fact
> that I had recently placed a trap in a top loading wire of the vertical
> was another change. The trap had been fabricated to handle very high
> voltages, but it simply was inadequate. The problem was dielectric heating
> of the coil form. The heat would dissipate more rapidly when it was cold.
> I had to abandon that plan for my vertical.
> I know that ferrites will heat when they are used in some high current
> situations. If the ferrite in your balun is being heated because you are
> operating with a higher transmit duty cycle that might cause what you
> observe. If there is a defect in the manufacture of the balun it might
> have failed or might never have been capable of what you wish it too do.
> In my case I tried an Unadilla trap but it too experienced dielectric
> heating my particular application. [ my application was not the one it was
> designed to handle]'
> Maybe something in the above will help you resolve your problem .
> Tod, K0TO
> Sent from my iPad 2
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