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From: "Talbot Andrew" <ACTALBOT@dera.gov.uk>
To: rsgb_lf_group@blacksheep.org
Subject: LF: RE: Bonfires
Date: Mon, 8 Jan 2001 15:10:36 -0000
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I've used a plastic dustbin for years, with no problems when it gets
wet, although a ceramic high voltage insulator would be a useful
addition in very wet weather to reduce surface leakage.  The dustbin
stands on four 100mm high legs made from plasic drain pipe to get the
bottom of the coil away from the ground an reduce losses.   May not be
big enough for some of the larger coils though.

Although a bit pricey, the polycarbonate twin walled sheeting (designed
for conservatory roofs) and available from most DIY outlets (B+Q are
reasonably priced) would make a suitable low loss housing.  Use L-shape
plastic moulding and suitable (low loss)  glue to join.   It is
available 600mm or wider,  and 2.4 / 3 / 5m m lengths.   This material
is very strong..

Andy  G4JNT


> -----Original Message-----
> From: James Moritz [mailto:j.r.moritz@herts.ac.uk]
> Sent: 2001-01-08 14:30
> To: rsgb_lf_group@blacksheep.org
> Subject: LF: Bonfires
> 
> 
> Dear Dave, LF Group,
> 
> Sorry to hear about G3YXM's slight overheating problem. This type 
> of thing is certainly a hazard with LF - I expect most people have 
> their tales to tell. It is one reason why I don't leave my station 
> running in beacon mode while I sleep; it is also a good reason for 
> having the loading coil located away from the house.
> 
> I have found that the biggest problem comes from damp timber. If in 
> a high voltage field, the losses are rather high, so it gets 
> hot. Little 
> white arcs form on the surface of the wood, and wander around in 
> a fascinating way, leaving charred trails, and setting fire 
> to things. 
> It is very impressive at night. Paradoxically, dry wood seems to be 
> much less prone to this. During the summer, I made a shelter for 
> my loading coil from 47 x 47 mm preservative treated timber 
> covered with plastic sheet. This doubled the loss resistance of the 
> antenna, and it was obvious that the wood had a high moisture 
> content. Replacing the original wood with thinner, kiln-dried stuff 
> solved the loss problem. Then the plastic sheet developed a leak, 
> one piece of timber got wet, and the result was a minor fire 
> - luckily 
> I caught it early. The rebuilt shelter uses heavier polythene 
> sheeting which is more carefully sealed, and has stood up well to 
> the wind and rain of the last few months (fingers crossed!). The 
> high voltage antenna connection passes via a ceramic feed-
> through mounted in the middle of a rigid plastic sheet which forms 
> the roof. The feed-through by itself did not have adequate 
> insulation. I have also installed a cheap 1kW fan heater, so the 
> whole lot can be dried out from time to time. I think thoroughly 
> varnishing the wood woud help a lot, but this is a job for next 
> summer.
> 
> I would certainly be interested to hear of a material for making 
> loading coil shelters which is at the same time fireproof, a good 
> dielectric, and easily worked.
> 
> Hope you are back on air soon, Dave,
> 
> Cheers, Jim Moritz
> 73 de M0BMU
> 


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