556 



ANTENNAS AND RF COMPONENTS 



In such a phase shifter, phase change is a linear function of rotation angle of 

 the half-wave plate, and one revolution of the half-wave plate results in 

 720° of phase shift. A continuous phase shifter can also be used as a single- 

 sideband generator, the input carrier being shifted an amount equal to 

 twice the angular frequency of rotation of the half-wave plate. Whether 

 the carrier frequency is increased or decreased by this amount depends upon 

 the direction of rotation of the half-wave plate. The polarization of a 

 linearly polarized wave can be rotated by passing it through a half-wave 

 plate; the polarization will rotate in the direction of rotation of the half- 

 wave plate, and at twice the angular rate. 



Phase Shifters. Many methods have been used for introducing 

 controllable phase shift into microwave systems for one purpose or another. 

 The continuous phase shifter just described is one of the most valuable 

 types because it can be readily provided with a calibration which is inde- 

 pendent of frequency. Being a fairly sizable device, it often cannot be used 

 where a phase adjuster is required. Very few practical methods have been 

 developed for providing continuous phase shift. A number of methods for 

 producing limited ranges of phase shift are known, however. For example, 

 a dielectric slab moved from the side of a rectangular waveguide to the 



high-field region at the center of the 

 waveguide will increase the phase 

 delay and is used where only a 

 moderate amount of phase shift is 

 required. A more elaborate device, 

 but one capable of providing ap- 

 preciable phase variations, is il- 

 lustrated in Fig. 10-28; this employs 

 a short-slot hybrid junction and a 

 ganged pair of movable short cir- 

 cuits in a very compact, wide-range 

 component. Phase shift is controlled 

 by simultaneously moving both 

 short circuits. As indicated in Fig. 10-28,. the input wave is split by the 

 hybrid and, after reflection from the two short circuits, the two components 

 recombine as shown. This simple device is made possible because of 

 the quadrature phase characteristic of the slot hybrids^^; i.e. the slot hybrid 

 splits the input signal into two equal components which are 90° out of 

 phase. On being reflected back through the hybrid, these components 

 recombine in the output arm. The amount of phase shift is a function of 

 short-circuit motion, 360° of phase shift being obtained for a half-wave- 

 length motion of the short circuits. Methods for producing phase shift 

 electronically are discussed in Paragraph 10-16. 



Shorting Plungers 

 In Tandem 



Input 



Output 



Fig. 10-28 Principle of the Slot Hybrid 

 Phase Shifter. 



