I. Introduction
Conventionally, adaptive processing is done at a single frequency to separate the signal of interest (SOI) from interferers and clutter. In this communication, this concept is extended to deal with adaptive processing at multiple frequencies using the same antenna array. The antenna array may contain elements of unequal lengths and nonuniformly spaced. The goal is then not only to simultaneously direct multiple beams along multiple directions but also these beams are formed at different frequencies. In addition, it is assumed that the array can be deployed in any realistic environment. For specificity, the environment is treated as an array operating over an imperfectly conducting ground plane in this communication.