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Re: [TowerTalk] SWR shifts with power

To: Tom McDermott <>
Subject: Re: [TowerTalk] SWR shifts with power
From: Bill Coleman <>
Date: Tue, 24 Jan 2006 21:20:23 -0500
List-post: <>
On Jan 24, 2006, at 12:00 AM, Tom McDermott wrote:

>       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).

I've considered this possibility, even to the point of possibly  
winding a new coil just for 80m.

I'm seeing about 200 pf on 80m on the shunt, and around 600 pf on 160m.

>       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.

What value series cap should I be using? What voltage rating should  
it be?

Bill Coleman, AA4LR, PP-ASEL        Mail:
Quote: "Not within a thousand years will man ever fly!"
             -- Wilbur Wright, 1901


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