Abstract: An RF coupler is used to measure transmitted power and receiver sensitivity of an avionics RF transceiving system (e.g., a secondary radar transponder). The coupler physically mates with (e.g., by conformally enveloping) an airborne type blade antenna mounted to the fuselage of an aircraft. The construction and theory of operation of such a coupling device for coupling RF energy into an antenna is disclosed. The coupling device provides receiver sensitivity measurements and produces rectified pulses or DC levels for the purpose of measuring transmitter power. The system measures such parameters in the extreme near-field (e.g., about 0.1 wavelength) to simplify the measurements and to isolate the measurements from errors due to standing waves and the presence of other nearby objects.