Interesting and informative article Jon particularly if it describes the immobiliser system in the Plus 4. Thoughts are that the electromagnetic field created by the coil surrounding the ignition key barrel would require it's location to be critical to the magnetic core it interfaces with given the greater energy is transferred from that not the coil per se.
DaveW (
thread here) gained some improvements in his by effectively putting it closer to the key born RFID. It strikes me that it's proximity to what becomes the magnetic core is of greater relevance than distance when considering efficient transfer of electromagnetic energy to the capacitor within RFID.
Experience with induction hobs and cordless (induction) phone charging confirm positioning is critical, maybe explaining John's findings above.
