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Re: [TowerTalk] Rain Induced Voltage

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Subject: Re: [TowerTalk] Rain Induced Voltage
From: Jim Lux <>
Date: Fri, 01 Dec 2006 09:49:10 -0800
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At 08:08 AM 12/1/2006, you wrote:
>Do Polyphaser lightning arrestors drain static?  I thought they only shorted
>to ground after reaching a preset high voltage.  He was only measuring 7
>volts maximum on his coax.  If it did drain at 7 volts, what happens when
>you transmit with a KW into a less than perfect antenna?  Does your RF also
>drain and not reach the antenna?

7 volts into what impedance.  If it was the 10 Megs of a DMM, then 
you're looking at a microamp at most (which is the sort of magnitude 
of charging current one might expect).  If you're looking at a 
unterminated run of coax that's say, 100 ft long at 13 pF/ft, you're 
basically charging a 1.3 nF capacitor with that microamp or so.  Say 
your switch breaks down at a couple kilovolts. 1 uA into 1.3nF gets 
to 2.6 kV in 2 milliseconds.

Fortunately, there's not much energy (in a junction destroying sense) 
in that system (1/2*C*V^2 = about 4 mJ with 2 kV at 1.3nF)  Plenty of 
energy to put a nice wideband RF spike out, though (and heck, it's 
hooked up to antenna, so it will actually radiate!)  In terms of 
signal levels one might see...

If you discharge it, the pulse will last about 100-200 ns (the length 
of the transmission line), so it will have a bandwidth of a few tens 
of MHz.  The instantaneous power is 4E-3/1E-7 or about 40 kW.  RF 
wise, that would be spread over, say, 40 MHz bandwidth (to make the 
calculation easy), so the power spectral density is about 1 mW/Hz 
(0dBm/Hz), which is a pretty good "pop" in your 3 kHz bandwidth...

I might be off by a few orders of magnitude here (4mJ at 500Hz rep 
rate implies 2W, and that seems a bit high for rain charging.. but, 
it's not out of the question for a fairly small Van deGraaff 
generator, so maybe it really is that high).  There's also resistive 
losses in the cable, etc. (the current is quite high if it sparks 
over, so IR losses suck up a lot of energy)

Jim, W6RMK 


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