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Re: [TowerTalk] SWR shifts with power

Subject: Re: [TowerTalk] SWR shifts with power
Date: Tue, 24 Jan 2006 10:26:43 EST
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Further to Peter's comments if there is a beam on top:
The driven element of the beam can add considerable top loading to the  shunt 
fed tower. The balun that connects the driven element to the system  probably 
adds some inductance as well. Depending on the balun type, especially  if 
there are ferrites involved, there can be some heating and inductance shift  
higher power low frequency energy is imposed on the balun.
The old KLM KT34 4:1 balun was vulnerable to this problem.
This can be tested by disconnecting the coax where it connects to the balun  
and watching for the swr change on the lower band. If this is the problem then 
a  different type of balun will be needed.
In a message dated 1/24/2006 3:26:57 A.M. Central Standard Time, writes:

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 
bearing.  This led to the position indicator pot in the T2X burning up. $25 -  
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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