Radar corner reflectors for linear or circular polarization

Abstract

"Vhen a grid of parallel wires is put in front of a plane or corner reflector, interes ting e ffects of polarization conversion ca n be obtained. These effects m ay be used to construct trihedral corner reflectors which, contrary to the normal ones, are radarvisible even with circular polarization. 1. Introductory Remarks It is known that echoes from rain become very attenuated with circular-polarized radar. Circular polftrization can be obtained, e.g. , by placing in front of the antenna of a horizontally-polarized radar the microwave equivalent in optics of a quarterwave birefracting plate. This consists of a set of metal strips inclined at an angle of 45 °, of such dimen ions that the component of the electrical field which is parallel to the strips is accelerated by % period relative to the normal component. Upon leaving the polarizer, the two components differ in phase by a ;~ period instead of being in pha e. If a circular-polarized wave is incident upon a "nondepolarizing" target which reflect circular polarization without modifying it, the inverted " cir-cular" wave is changed to "linear," at 90 ° to the initial direction, upon passing through the "A/4 plate." The term "without modifying" doe not deny the existence of an inver ion in the directwn of rotation relative to the propagation vector, which always happens in a single plane reflection or, generally , in odd-nmnbered plane reflections. One fea-tme of this system is that the reflected wave does not pass into the waveguide feed of the antenna, nor does it activate the receiver. It thus follows that a circular-polarized radar echo from a sphere (e.g. , a raindrop), a Luneberg lens-reflector or a plane or trihedral corner reflector becomes completely, or at least practically, invisible. Actually, a circularly polarized radar is rigorously blind only for targets which are symmetrical with respect to the radar-target line of sight. For such targets, the two "orthogonal null polarizations" [Copeland, 1960] arc the two circular ones. However, the circular-polarized radar echo from a dihedral reflector becomes , instead, perfectly visible. It is because of double reflection, as well as depolarizing targets, that the echo from a ship becomes sufficiently visible even with circular polarization with an attenuation of only some db units, compared to linearly-polarized 1 This research b as bCP J1 sponsored by tbe Rome Air Development Center oC tbe Air l{esearch and D evelopment Command, throngb its European Office. ignals. Under these condiLions the echo from raiu is attenua ted by at lea t 20 db. Dihedral cornel' reHectors are, however, less practical than the trihedral, and their echo is very un-table when the situation involves rolling buo: r s or vessels. 2. Statement of Problem The problem can be proposed in this manncr : Radar with circular polarization is almo t completely free of echo due to rain. The latter, specially in the 3-em range, can mask signals from desired targets and create very hazardous situations. Unfortunately , with circular polarization, trihedral corner reflectors (and Luneberg lenses) arc ineffective and this may cause grave risks particularly in cloudy areas where such corner r eDectors arc indispensable to render radarvisible such targets as sDlall vessels on fire , barriers, and the like which arc otherwise incapable of ufFieient racial' rcspon e. What can be done to remedy such a situation ? Obviously, if the solution should involve a corner reflector, the solution could not amount to more than modifying the polarization of the reflected wavc, and adapting it suitably to radar use. This objective can be better achieved instrumentally with an arrangement whereby Lbe incident wave is not depolarized by absorption but by a true and appropriate conversion of polarization. Two types of conver ters can be considered in this connection: (A) Aj4-Converte r A converter w10ich wouJd be the equivalent in optics of a A/8 birefracting plate over a reflecting surface. The acceler ated component ("extraor-dinary component vibrating parallel to the optical axis) "gains" an eighth of a wavelength outgoing , and another eighth upon return, totaling ;"'/4. A conversion of polarization takes place from circul ar to linear (at 45° relative to tbe optical axis) or inversely from linear (at 45 ° to the optical axis) to circular .2 The microwave strip polarizers show the 2 '1"' 11e sense of t he circular polari7.ation is that which superposes the vector of the accelerated component npon the vector of the nonaccelerated one. 23