Jim, alec, thanks for the replies!
Ok, the box is extruded aluminium with two halves that slot together, so, aside from the end plates, they make
very good contact with each other. I couldn't find the exact model, but here's a couple that are similar (save dimensions):
https://www.electrostock.com/images/Bop/build_a_box.gif
**broken link removed**
The grooves that allow the two halves to slot together aren't just a rectangular cross-section, more of a teardrop, so it has to slide together, holding it in place - pretty damn good physical, and electrical contact - its a stupidly tight fit. The end plates are just flat alu with 4 screws, but there is no discernible gap.
In 'normal' operation, there is indeed a cable attached to the XLR connector, they use this as 'relay out'. The wires to this XLR inside the box are not screened, one is the negative terminal of the SLA battery, the other is the relay NO output (open). I think you are right about that XLR cable... I haven't seen how long it is (or even what they attach it to.. could be a bulb or something) but it looks like that is the *only* real way to get RF into this case. As to length of this cable, or what the load its attached to, I do not know - that could explain the wild inconsistencies in range... some cables 'just so happen' to have a reasonable length, along with loading, to allow some amount of signal to get in.
I'm always dubious about an external antenna of any kind, even just a wire, as this can be pulled off, lost, snapped, or even intentionally cut of (I have designed for 'public hire' and factories before.. if something can be broken, it will be in days). But of course, it *needs* a half decent antenna.
alec, the antenna does not come into contact with the enclosure, at all. It is soldered the radio module, which iself is mounted on a small PCB, that is bolted about 2mm off of the inside of one of the end plates (what a mouthful..).
As I seem to be typing lots here, I'll knock up a diagram later on, just with dimensions, and where the antenna is. As I said in my first post, these things work, and I wasn't asked to 'increase range' it really is just a curiosity.. I seriously doubt the manufacturer of these things has used some clever RF coupling techniques (ala apple..) because the tolerances of machining is quite slack.
Cheers!