Spatial distribution of intermodulation products in active phased array antennas

In an active phased array (i.e., an array that has amplifiers in each element) transmitting multiple beams, the intermodulation products generated by the amplifier amplitude nonlinearities form beams that, in general, radiate in directions different from the principal beam directions. In effect, the phased array antenna spatially filters the intermodulation products formed by signals from different beams. In a communications application this effect can be used to reduce the signal-to-intermodulation interference ratio. A description of this phenomenon is given and examples are presented that illustrate the mainbeam directions of beams formed by third- and fifth-order intermodulation products. An experiment that was performed to demonstrate this effect is described. The spatial filtering effect for beams with amplitude modulated signals is also treated, with an improvement in carrier-to-intermodulation ratio theoretically demonstrated.