Depends on what you need. A non-RF engineer shared a trick with me that was brilliant. Just a simple Germanium diode connected to an Oscope and the other to a pick-up loop. Useful for just sniffing to see if the TX is on or off.
Don't you have electricity in your walls and many radio and TV stations all around you?
Can you get very close to the RF transmitter's antenna that you are measuring?