[TowerTalk] SWR Problem Balun or lightning protection?

[Tod - ID]

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 increases.

I have some questions that I would seek to answer if it happened at my station.

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, etc.
  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



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