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Re: LF: Loading coil - suggestions

To: [email protected]
Subject: Re: LF: Loading coil - suggestions
From: "James Moritz" <[email protected]>
Date: Wed, 10 Nov 2004 19:51:08 -0000
References: <[email protected]>
Reply-to: [email protected]
Sender: [email protected]
Dear Chris, LF group,
 
When I first started running QRO on LF, I had problems with loading coils overheating. So I made a big loading coil using ex-Decca litz wire and a 500mm diameter plastic "sectional manhole" as a former. This has a Q of about 700 and significantly improved antenna current over the original, with a Q of about 200. Since it is outdoors, I built a shelter for it out of 2" x 2" timber covered in plastic sheet. I was alarmed to discover the Q had gone down to 200 again when I put the coil in the shelter. Some investigation showed that this was because of the timber frame - the wood was pressure-treated with preservative, and was quite damp inside. Replacing it with some kiln-dried timber restored the Q, but it seems to be slowly deteriorating again after a few years outdoors. The frame is quite well spaced away from the coil, perhaps 150mm or more at the nearest point, so obviously wood is not the best of dielectrics...
 
Having said that, GBR worked for decades with a loading coil wound on a wooden former - this with around 200kV and half a megawatt going through it - so some wood must be OK!
 
Cheers, Jim Moritz
73 de M0BMU
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