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LF: Re: RE: loops

To: [email protected]
Subject: LF: Re: RE: loops
From: "Dave Brown" <[email protected]>
Date: Sat, 30 Mar 2002 23:36:40 +1200
References: <[email protected]_wil08>
Reply-to: [email protected]
Sender: <[email protected]>
Bill A sed---
For those that are this crazy, the loop conductor will have to
use #6 or multiple runs of smaller size wire and the capacitor will have
to
be...... well, something exotic!
The 'Crazy' I agree with. But not necessarily 'exotic' in regard to loop
capacitors. Certainly it MUST be built so as to keep components operating
within their ratings.

For the usual 500 volt 4700 pf polystyrene ( or the current 'equivalent
material- polycarbonate??) capacitor this means enough wired -
a) in parallel --- to keep the current in each to a safe figure- 1 amp seems
to be about all the polystyrenes will sustain long term, and
b) in series---- to withstand the applied PEAK voltage.
So a large array of caps in series/parallel is what you wind up with. Not
pretty, but functional.

In my case I run up to about thirty amps in the loop so with around 15000pf
needed for resonance I have about 90 of the 4700pf/500 volt units in series
parallel. A 2000 pf air spaced variable serves as fine tuning. At 3kv rating
it's a bit under rated but hasn't failed yet--- in several years of 180 kHz
operation.
Loop conductor is two runs in parallel of 7/1.5mm aluminium-- your bog
standard 11 kV overhead drop-lead stuff.  Loop DC resistance of 34 milliohms
and effective resistance at 180kHz of 0.99 ohms - a bit on the high side,
but quite practical given we use SSB quite a bit down here in ZL and
anything lower would start seriously negating the need for a filter in the
SSB exciter!  Loop tx efficiency isn't always a prime consideration.

Dave, ZL3FJ



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