Howard, many thanks for all this information. I have been trying to make something similar for a while now, but with no access to AMB equipment (I am not interested in compatibility).

I have been playing with your transponder circuit this weekend, right now just the LC tank portion with a 47p, 62p and a square wound core using magnet wire which is slightly bigger than the one in your first post. Right now I'm feeding a square wave at 5VDC from a function generator, through two 100R resistors as if it were driven by the XOR gates.

Sweeping the tank shows it is resonant at around 5.22Mhz (I will adjust capacitors later). I measured directly across the inductor and got about ~13Vp-p.

One thing confuses me though, I am trying to pick this up with two loops of wire. This is fed straight into the scope.

One is a large square of thin stranded wire, roughly 30" x 12". I get a mostly clean 5.22Mhz sine wave from this, even at a fair distance.

The other is an 18AWG loop at 18" long. I get practically no 5.22Mhz, even right next to the loop. If I adjust the frequency to 12.2Mhz I get a sine wave at the loop, but scoping the tank itself is very messy. It isn't a resonant frequency of the tank. I am guessing here I have built a poorly coupled transformer!

I did not think that reception should depend on the size of the loop, have I missed something? Or is it just because my shorter loop so small that it stands no good chance of receiving the 5Mhz signal?