The transmit–receive (T/R) operation of an ultra-thin organic antenna array is presented at a center frequency of 9.5 GHz. High transmit power is achieved while maintaining an ultra-low profile in a novel system-on-a-package scheme whereby 32 silicon–germanium (SiGe), transmit/receive integrated-circuit (TRIC) modules have been flip-chip bonded to the array board. Each SiGe TRIC drives a pair of slot-coupled microstrip patch antennas that form an 8 8 rectangular array, which is all packaged in an organic substrate stack of liquid crystal polymer and RT/Duroid 5880LZ. The organic package occupies an area of 30.5 cm 25.4 cm and has a total thickness of only 1.80 mm. The small-signal characterization of the array showed a , and a measured receive gain of 20.1 dB with a variation of 0.7 dB over a 1-GHz bandwidth (BW). Finally, far-field large-signal experiments showed a measured effective isotropically radiated power of 47.1 dBm with a variation of 2.36 dB over the same BW, and without the aid of additional thermal management components.