Analytical method of radiation mode radar cross section (RCS) of low profile phased arrays

The Radar Cross Section (RCS) of a phased array comprises of two scattering modes, the antenna (or the radiation mode), and the structural mode [1]. The antenna mode RCS depends on the radiation behavior of the antenna array. The structural mode arises from the currents induced on the antenna and the platform. In case of phased array, the structural RCS also signifies the case when the antenna array is matched with the feed network. However, it is not practically possible to achieve such perfectly matched antenna array. The feed structure of the phased array itself contributes to most of the impedance mismatches and hence scattering. When the incident energy strikes the phased array, it traverses through the components of feed network, viz. phase shifters, couplers, and load impedances. Each feedline of a certain feed width acts as a transmission line having finite input and characteristic impedance. At every junction of impedance mismatch, a portion of incident wave is reflected and rest is transmitted further in to the feed network. The coherent sum of all these reflections within the array and its feed structure contributes to the total radiation mode RCS [2], which is significantly higher than the structural mode RCS. This makes the study of radiation mode RCS most important especially for a radiating phased array. This paper proposes a novel analytical method to estimate the radiation mode RCS of a low profile patch array. The incident wave is traced as it travels from the array aperture into the feed network. The scattered field contribution from each component of feed network is expressed in terms of the reflection and transmission coefficients. The individual contributions are then coherently superimposed to arrive at total array RCS, given by