How about using a couple of standard reference occilators. you would feed them into the Antena. If you tune into just that frequency, you know the frequency of that point. With a couple of points you could extrapulate.

A simular technique is used to moniter dopler shift from stars to detect planets. It might be simular. Well, the shifts are rather small from the reference. But they still get fairly useful data.

Cavity resonators might work. But I don't know the resistance in the walls might spoil everthing.

Superconductors are becoming more accesble. You might be able to get ahold enoph for the cavity for about $500.