I've carried out a few tests to see why the LF loading coil /
ATU housing caught alight and I'm now just about certain it was
caused by sparking from the aluminium foil used to seal the
joins in the B+Q Garden Store box and not RF being absorbed by
teh plastic housing.
Combustion:
Placing pieces of the plastic housing in a gas flame, they
caught alight almost immediately and continued to self-sustain
burn with a yellow flame. The fumes given off were almost
completel odourless so I'm pretty sure the material is
polycarbonate - which on a reasonable assumption based on its
name, contains only C, H and O, and explains why it burns
moderately well. I was rather surprised how quickly
the material caught alight, and then how slow but steady the
resulting flame was.
Incidently, I've tried burning clear polycarbonate roofing
sheet in the past, and it barely self-sustains - so I suspect
the latter has a fire retardant added. They're not going to
bother with that on a cheap garden storage shed..
In a microwave cooker:
After several minutes of exposure, the material wasn't even
warm. I tried pieces of the black base material and the green
plastic housing, so it looks as if this material is not going to
suffer any RF absorbtion effects.
The 600 Watt beacon transmission had been going for about 30
minutes (at 33% WSPR duty cycle) before I noticed anything, so
it may well have started burning right at the start to end up
with the result shown in those pictures.
An incident in the past lends weight to the sparking idea.
Just once, a few months ago, digital TV reception was suffering
intermittent interference and blocking and I realised the
interference was correlated with the transmission sessions of my
150 watt 503kHz beacon signal. Checking in the ATU showed
(smelled) nothing out of the ordinary, and I put the
interference down to "EMC Related issues" perhaps along the
mains. TV reception was OK the next day and ever afterwards.
On reflection, 500kHz signals have no real way of affecting UHF,
so I am wondering if minute sparking was going on just
occasionally, with the resulting harmonics enough to cause QRN
at UHF - with perhaps a session of dehanced propagation
contributing.
Conclusions.
Cheap polycarbonate (probably) garden buildings from DIY
centres are suitable for RF housings PROVIDED there is no
sparking / direct heat exposure.
Don't even think of using metal tape without soldering or
properly bonding all joints - especially the sort not designed
for electrical use.
Don't set light to garden sheds
