I stand corrected... The way it is set up at the track where I drive: they run the yellow AMB cable from the transponder receiver box a the loop that is buried which is standard low-grade coax cable that makes a loop and both ends come back together and are attached to a Deans connector. The AMB cable, having had the loop portion worn out years ago, ends past the first (larger) black box. Both smaller "loop wires" are connected to a Deans connector which attaches to the buried coax loop. But there is still no resistor.
I'll have to defer to Ed Anderson's post above for resistor info.
Oh, and I found this earlier today: aftermarket (cheaper) AMB compatible stuff: