Interesting, I've tried it with the standard CD size disc, old Peugeot 405 disc (which doesn't get blown off the roof!), a Cadbury Roses tin lid and also without any ground plane - no difference between them. There is no physical connection to the ground plane, so I'd assumed the magmount base worked as a capacitively coupled device. Having said all that, when I first received Glyn's spare standard antenna, it worked superbly for the first 10 days or so, then started losing performance, on dismantling the centre core was dry-jointed to antenna whip, and the screen lead was an interference fit. Once chemically cleaned and all soldered properly - fantastic performance and fully waterproof, just need the whip to be made from stainless/corrosion-proof material would be better.
