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Re: [TowerTalk] Pl-259 Flash over voltage

To: "K8RI on TowerTalk" <>,<>
Subject: Re: [TowerTalk] Pl-259 Flash over voltage
From: Jim Lux <>
Date: Fri, 01 Dec 2006 16:49:18 -0800
List-post: <>
At 04:19 PM 12/1/2006, K8RI on TowerTalk wrote:
>The talk about breakdown voltage which is basically air across the space
>between the inner and outer conductors at the PL-259. Given LMR-400 has
>roughly 34 pfd per foot (23.9) or 230 pfd per hundred feet that is not a lot
>of capacity.

Why would the capacitance for 100 ft not be 100*the C for 1 
foot?  Something is weird here.

But, even 1000 pF or so, when charged to 10kV, can make a fairly 
impressive spark.  Energy = 1/2*c*V^2.. if V in kV and c in uF, then 
E in joules.  50 mJoule in this case.

>The arc is initiated when the breakdown voltage across the gap is reached,
>but when the arc is a bright blue and extends out from the end of the
>connector by a 1/4 inch or more it has to consist of much more than a

yes.. the charging current is a microamp.  The discharge is probably 
several hundred amps  ( consider it as several kV being fed from a 50 
ohm source)

>  I'd guess the brightness, thickness, and amount the arc extends
>out from the tip are determined by the magnetic field induced due to rise
>time and current.

The diameter (and the brightness) are almost entirely determined by 
the current.

>So I wonder just how much current could be in some of these arcs.  I
>wouldn't think it'd be much due to the small capacitance, but the size of
>the arc, the brightness, the sound volume, and the poke they sometimes give
>indicates a substantial charge.  Where would that come from?  Remember we
>are not talking about induced voltages due to nearby lightning strikes.

You've got something akin to charging a Leyden jar with a 
electrostatic apparatus.  slow charging, fast dicharging. (actually, 
the Leyden jar discharges much faster.. very low inductance)

>Roger Halstead (K8RI and ARRL 40 year Life Member)
>N833R - World's oldest Debonair CD-2
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