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Re: [TowerTalk] SWR shifts with power

Subject: Re: [TowerTalk] SWR shifts with power
From: Peter Chadwick <>
Date: Tue, 24 Jan 2006 10:24:09 +0100 (CET)
List-post: <>
I gather that there's a beam providing top loading? if so, you can have fun 
things happening in the rotator. I had a 205BA providing top loading and  a T2X 
rotator. The RF current through the capacity hat that the 205BA represented had 
a lower impedance path through the rotator than through the tower bearing. This 
led to the position indicator pot in the T2X burning up. $25 - ouch!
My T2X now has some 0.01 microfarad disc ceramics bypassing the ends of the 
position indicator pot to ground, and there's a strap around the stub mast with 
pigtail to the tower.
Peter SM/G3RZP (this week)

Message Received: Jan 24 2006, 05:01 AM
From: "Tom McDermott" 
Subject: Re: [TowerTalk] SWR shifts with power

Hi Bill,

You mention an L-network, but do you additionally have a
capacitor in series with the shunt-feeding arm? Or is your
L-network just one series-L from the coaxial feed point to
the arm, and one shunt-C from the arm to ground?

If you have just the two parts, (and no series-C feeding the
Shunt-arm), then you can get some really high voltages.
On 80 meters, and with a 15-m shunt arm, the input Z to the
Shunt-feeding-arm will look something like 50+j375. In parallel form,
that's 2860 ohms resistive in parallel with 382 ohms inductive.
That high-resistive number should ring alarm bells!

If the shunt-C of your tuning network cancels out a fair
amount of that parallel-L (forming an almost parallel resonant
circuit), then your network can have a really high voltage
point across the shunt-C but also across the series-L of your
tuning network. Thus you could have arcs either across the
shunt-C, or across the series-L. Especially so if there are tap
points on the inductor that go over to a switch (bringing those
tap points physically close to one another).

Using a series-C in the feed arm brings the impedance
down to a reasonable value, placing high-voltage stress just on that
one component - the series capacitor.

-- Tom, N5EG


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