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LF: SWR Bridge Diode Failures

To: "RSGB LF Group" <[email protected]>
Subject: LF: SWR Bridge Diode Failures
From: "Tom Boucher" <[email protected]>
Date: Tue, 19 Jun 2001 07:44:17 +0100
Reply-to: [email protected]
Sender: <[email protected]>
Hello LFers

A while ago I had an unexplained failure of the diodes in my LF SWR
bridge. These were high voltage, fast diodes and I put it down to some
kind of random failure. Last week end I noticed the SWR meter was again
not working and found that the diodes had failed again. Bit puzzled
until my wife mentioned there had been a thunderstorm while I was away!

My long wire antenna is not dc grounded and is resonated on 136 by a
simple series variometer. Also the T/R relay is wired such that there is
no DC path from antenna to ground when power to Tx is off. The
capacitive divider in the bridge had obviously arced across with static
build up.

I have now fitted a 300 VRMS varistor (capable of passing 8000 amps
peak!) across the antenna connector on the SWR bridge. Nothing will
protect against a direct strike but perhaps this will discharge some of
the static build up on my 1100 ft wire. I suppose a better idea would be
to get into the habit of grounding the antenna when I leave the shack!

73, Tom G3OLB

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