Thanks for your reply - apologies it took a while to respond but I haven't had much time to experiment. Attached are some pictures as I can't include them yet.

The transponder is hanging from a piece of string 18 inches from the table, where a large pickup loop of stranded wire is fixed. A scope probe (with GND lead shorted to tip) is hung around the inductor.

The yellow trace is off the large pickup loop, fed through a simple RC filter. The blue trace is from the probe. I don't get quite as high Vpp from a distance but it seems fairly clean - and 18 inches is much further than I'd hope for.

Right now I'm just running some C code on the PIC to toggle periodically, I haven't tried to match it to the required 4 cycles yet. I'm using a 12F1840 with 4x PLL though so this wont be hard.

You can see that I have switched to using a slightly different inductor, I wrapped a number of turns around a ferrite rod. This measures around 3.5uH using an LCR meter and because of this my capacitors are much larger than yours. I tried this as I'm hoping to use it on karts rather than RC cars and this seems to be how the kart transponders work - I assume the magnetic permeability of the ferrite material helps it to propagate further. I know that the AMB kart transponders operate on a different frequency but as I said previously I am not worried about compatibility.

Does the scope screenshot look roughly correct to you? My next step is to build your loop amplifier and see if I can get it to work nicely.