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Improving the polarization characteristics of microstrip electronically scanned arrays with linear polarization


A. Yu. Grinev – Dr.Sc. (Eng.), Professor, Moscow Aviation Institute (National Research University)
E. V. Ilyin – Ph.D. (Eng.), Associate Professor, Moscow Aviation Institute (National Research University)
D. A. Evseev – Engineer, JSC “MIC Mashinostroyenia”

The polarization properties of microstrip electronically scanned arrays (ESAs) are often considered as their drawback which prevent use of patches in ESAs for linear polarization. This article discusses the reasons of cross-polarization appearance and measures for cross-polarization suppression in ESAs.
Traditionally, polarization characteristic is defined by the polarization ellipse, which is determined by the axial ratio r and the slope angle of its major axis. As a primary component Eco of a linearly polarized wave, that component of the electric field vector is chosen which is collinear to the major axis of the polarization ellipse. A cross-polarization component Ecr is understood to be orthogonal Eco. The object of study is ESA which is based on microstrip patch elements (MPEs): basic MPE, double-layer MPE and Y-slot MPE (the last one provides expanded frequency bandwidth). This article is focused on those values of axial ratio which are realized in the direction of ESA's scanning beam. Some ways are considered to reduce cross-polarization radiation.
The first way is providing symmetrical out-of-phase excitation of patch radiating elements of ESA. This is carried out by attaching two ports to the patch on its opposite sides, symmetrically relative to its center. The advantage of this solution is significant reduction of cross-polarization radiation in case of H-plane scanning, the benefit can reach 50 dB. The disadvantages are the relatively complicated feed circuitry as well as scanning sector width narrowing.
The second way is mirror arrangement of neighboring MPEs in array when each two adjacent elements are mirrored to each other. This solution also requires out-of-phase excitation circuitry.
The third way is introduction of defects into conductive screen of the substrate. This partially compensates the radiation of magnetic currents flowing along "non-irradiating" sides of patch and reduces cross-polarization by 6 to 12 dB. The problem is the backward radiation caused by defects in the conducting screen of the substrate.
The fourth way is introduction of a polarizer into the ESA aperture. The polarizer is the one-dimensional periodic structure of conductive strips. Simulation has shown 25–30 dB cross-polarization suppression in wide scanning sector, however the polarizer increases the length of ESA. As well, the interacting structure of the «ESA – polarizer» may suffer from resonance phenomena arising at certain frequencies and scanning angles. This phenomena causes blindness effect in ESA, i.e. increase of the reflection coefficient from radiating elements' inputs, which is accompanied with rise of cross-polarization radiation.
The article concludes with estimation of cross-polarization influence on monopulse ESA direction finding accuracy.

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