You need a simple field strength meter . Just a detector diode and a meter would probably be sufficient . But do a search on field strength meter and have a read . But my worry is that you are testing a critical safety device without the legally required RF screenedroom , and without the necessary qualifications , technical skills or equipment . This could get you into serious trouble with the authorities . You shouldn ' t be relying on testing inside to prevent its signals showing up on military or civilian radar . In fact , your whole question doesn ' t make much sense . How are you triggering the transponder ? Do you have a radar test generator , or are you relying on outside signals ? If the latter , then it WILL show up on airport radar along with your location and the authorities will come looking for you .