554 ANTENNAS AND RF COMPONENTS 



in other instances the noise power is introduced at reduced level through 

 a directional coupler. 



Components for Circular Polarization. Circularly polarized waves 

 are used in many applications from microwave. ferrite devices to radars 

 which discriminate against rain.'*^ There is nothing circular about a 

 circularly polarized wave, however. A circularly polarized wave is one in 

 which the electric polarization rotates at the carrier frequency, and may be 

 either right- or left-handed, depending on direction of rotation of the 

 electric vector. A circularly polarized wave is equivalent to two linearly 

 polarized waves differing in phase by 90° and polarized at right angles 

 to each other, and can be resolved into two such waves. Thus, two identical 

 TEii waves, differing in phase by 90° and orthogonally polarized in a 

 circular waveguide, will result in a circularly polarized wave. 



Quarter-wave plates are frequently used to generate circularly polarized 

 waves. These elements, also called A 90° plates, produce a phase delay in 

 one plane of polarization which is 90° greater than in the orthogonal plane 

 of polarization. Microwave quarter-wave plates take several forms. One 

 of the most popular, the dielectric quarter-wave plate, ^^ consists of a 

 longitudinal dielectric slab in a section of dominant mode circular (or 

 square) waveguide. The dielectric slab introduces a phase delay to a wave 

 in its plane which is just 90° greater than the phase delay undergone by a 

 wave polarized at right angles to the first. As shown in Fig. 10-26, a 

 dielectric quarter-wave plate oriented at 45° relative to an incident linearly 

 polarized wave will introduce a 90° differential phase delay between the 

 component in the plane of the dielectric plate and the component at right 



Circular ^^' 



Polarization Out 



Dielectric Slab 



Fig. 10-26 Dielectric Quarter-Wave Plate Used for Converting Between Linear 

 and Circularly Polarized Waves. 



«P. A. Crandell, "A Turnstile Polarizer for Rain Cancellation," IRE Tram. MTT-3, No. 1, 

 10-15 (January 1955). 



^2R. M. Brown and A. J. Simmons, Dielectric Quarter-Wave and Half-Jf'ave Plates in Circular 

 Waveguide, NRL Report 4218, Washington, D.C., November 1953. 



