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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