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
>microamp.
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
>www.rogerhalstead.com
>
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