A microstrip probe that can be inserted into the mouth or radiating portion of a thick flared notch or open ridged waveguide radiator element within an array antenna aperture and coupled to the TEM feed circuit transition of that element. The probe can then be used to extract performance diagnostic data of the circuitry behind the array aperture without removing the aperture itself. The probe bypasses the radiating portion of the array element and the effects of mutual coupling associated with the array. The probe is a printed circuit board whose thickness matches the gap dimension of the flared notch feed circuit transition line at its feed launch point. A coaxial connector at one end provides a connection to a microstrip line formed on the circuit board. The ground plane of the microstrip narrows to transform the microstrip line into a broadside coupled strip transmission line, which runs to the insertion end of the probe. The strips are shorted together at the insertion end. A slot is formed at the insertion end which is one quarter wavelength in length to encourage coupling from the feed circuit into the probe.