Today, there is an inherent issue with ground testing of an aircraft fitted with an ADS-B transponder. Since an ADS-B transponder automatically reports its position, this presents a problem for TCAS or ADS-B In equipped aircraft when testing above field elevation, especially around congested airspace. As a result, the FAA issued SAFO 17002 in 2017 to follow proper procedures and use antenna shielding to prevent test signals propagation.
Although antenna couplers are used to reduce the strength of aircraft transmissions, their attenuation capabilities are limited and may not be adequate to prevent your test event from being picked up by ATC and rebroadcast. Antenna couplers typically only provide around 25dB+ signal attenuation, which was once adequate to eliminate interference with TCAS equipped aircraft, but now ADS-B ground stations are positioned all over the globe and have a much greater level of sensitivity (around -84dBm)—which can pick up RF leakage from your test event if not properly shielded and rebroadcast your test aircrafts position causing interference with TCAS and ADS-B In equipped aircraft.