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Re: [TowerTalk] Simpler Version of Shunt Fed Tower - 80M?

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Subject: Re: [TowerTalk] Simpler Version of Shunt Fed Tower - 80M?
From: "Tom McDermott" <>
Date: Sun, 05 Feb 2006 10:14:10 -0600
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Brian, K1NW writes:
> If I am only looking for resonance on 80M can I just use high power  
> capacitors (recommendations welcome) and skip the vacuum variables?

Hi Brian, the issue is the voltage and current ratings of the capacitor. A
45-foot pipe will present about 15 uHy of reactance that has to be cancelled
with the capacitor. At 1500 watts, 5.4 amps of current needs to flow into
the gamma arm (50 ohm load). The voltage across the capacitor is V = IZ, or
about 1950 volts RMS (about 2700 volts peak). The capacitor needs to
withstand that voltage (plus some safety factor) while passing 5.4 amps.  My
experience has been that typical breadslicer type capacitors will arc over
at that RF voltage. Of the ones I could find in the junk box, those that
have enough plate spacing to withstand the voltage don't have enough
capacitance to get the job done. Fixed capacitors can work, but then you
can't tune the antenna (and it's pretty narrow!). Thus vacuum variables are
usually the most practical solution.

        -- Tom, N5EG


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