Pattern characteristics of harmonic and intermodulation products in broadband active transmit arrays

A three-term power series model for the nonlinearity in the power amplifiers of an active array is used to explore the radiation characteristics of the harmonics and intermodulation products. This model leads to simple equations for predicting the beam-pointing direction and the aperture amplitude taper of the spurious signals. Cases of a single transmit signal, dual transmit signals within a single beam and dual beam operations are analyzed. This model shows that the second and third harmonics are co-boresighted with the fundamental signal but have squared and cubed aperture amplitude distributions. The third-order intermodulation products in general have beam-pointing directions offset from the fundamental signal beam(s). Graphical methods of locating the intermodulation beams in (u, v) space are outlined and illustrated by example.