One of the projects I was asked to 'covertly' develop (one of those jobs where we weren't allowed to share any info with co-workers in the same room what I had been tasked with) was a system to transfer wireless power together with a low data rate (4800bits/s) communications channel to operate across the gap between a door frame and the door (which contained a solenoid lock).
Because it was 4 decades or so ago RF tech wasn't as it is today, so I decided to use cheap readily available parts (costs are always a main concern) which happen to be a couple of 6"ish ferrite rods (as used in LW/MW radios) because they would also easily fit inside a door frame and the door side by side (energy across the 1" gap), I decided on a frequency of 125kHz for the wireless power transfer and comms frequency - might give you a clue as to what it later turned into ;)
