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

 

Andy
www.g4jnt.com