That might be done rather easily. An open ended transmition line is placed across the antenna output terminals. That is, one end of the transmition line has its two terminals hooked to nothing. The other end has its two terminals hooked to the transmitoin line wires going from antenna to receiver.

At first I would say that the method would not be exact. But maybee with refinement you can get +- 2%. I have tried this remarkable teqnique on a tv and 950 MHz occillator, successfully. At first when I tried it, I was a little disappointed. But it may have been occurring, but I didn't notice it. The shunt transmiton line teqniqe works basically by destructive interference of radio waves. You may have even wondered weather radio waves for fm, UHF, VHF, can destructivly interfere. They can. And you can even show that with a tv. Don't be alarmed by the complexity, I can advise you if you wish. In wich case chances of succsess will be about 90%.