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.
73
Peter SM/G3RZP (this week)
========================================
Message Received: Jan 24 2006, 05:01 AM
From: "Tom McDermott"
To: towertalk@contesting.com
Cc:
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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