What you don't go into in your question is orientation. A simple vertical antenna for instance will generate a uniform field that (apart from obstructions and reflections from nearby objects) spreads itself 360 degrees around the antenna. But if you go vertically too high or too low the field pattern has dramatically changed.
I'm not saying use RF, rather I'm trying to use the example of a radio antenna to demonstrate that you need to define the two moving objects better. If they are free to move with respect to each other totally then it's unlikely an electromagnetic antenna is going to work consistently.
However, you may have better luck with magnetic fields - the loop antennas can be "stacked" so that you get more "even" coverage and they are less effected by the normal things that affect radiowaves.
If you have power at both ends it makes things much easier. If you haven't then you might just about get enough power in the "powerless" unit from the field generated by the "powered" unit. I've got a system that couple power (2W) to a remote circuit and I can get 5cm out of it. You'll need nothing like 2W - probably less than 20mW. I think Mr. Tesla might have something to say about this if he were alive.
If there is power at both ends then both can measure their distance from each other.
Assuming it's like a transponder system i.e. power at one end then you'll need to design a loop antenna that is highly tuned/resonant. It doesn't need to work at VHF - a few hundred kHz or a few MHz will do. The "remote" device coming into the field will pick-up the magnetic field and, using a highly resonant coil/capacitor start to charge power into a bigger capacitor that can power a small "respond" circuit for a few milliseconds. The respond circuit could send a response possibly by retuning the power coil and this could be picked up by the powered unit. You could even "kick" an RF transmitter to send something and this could be picked up.
This solution does need engineering for it to work but gut-feeling tells me it should.