From: Bill <w...@billnjudy.com>
Date: Sun, 31 Oct 2010 Time: 08:39:08
Russ
A old timer taught me a trick many years ago.
I've passed it on more than once.
A friend of mine told me that there was something wrong with his amp.
He could not get it to load on 12 meters and it would trip out at
almost any power level.
I advised him to add a random length of coax to his antenna cable.
Walah, it worked. His amp now loaded perfectly to full power.
I 'think' I understand why. But not being so good at the math, I'm not
sure if I can explain it.
Bill,
What you're seeing here is the change in antenna system impedance, as
seen by the transmitter, when you change the length of feeder. Under
certain (usually unpredictable) circumstances, the change in impedance
seen by the TX is sufficient to cause the conditions you describe.
I have illustrated this very graphically in my AIM4170 Antenna Analyzer
presentation here:
http://homepage.ntlworld.com/wadei/aim4170.htm
Take a look at slides 85, 87 and 90.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
--- Slide 85 shows the impedance of the antenna system with its usual
feeder.
--- Slide 87 shows the impedance when the feeder is extended by 1.5m
(about 5 feet). See how the resonant frequency (where the purple trace
crosses the zero axis) and the other curves have shifted LF.
--- Slide 90 shows the impedance when the feeder is extended by a
further 1.5m. See how the curves have shifted even further LF.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In all three scenarios, the antenna itself remained unaltered; all that
changed was the length of the feeder.
[Of course, other people's comments about preventing stray RF getting
back into the shack are the key to getting to a stable situation. I just
thought you might be interested in seeing the effects of extending the
feeder as you suggested].
--
73
Ian, G3NRW
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