42 



ANTENNAS 



of the two main lobes in opposite directions. An 

 adequate screening effect is produced by a set of 

 wires parallel to the direction of the radiating dipole 

 with spacings somewhat less than a tenth of a 

 wavelength. 



3.5.53 Corner-Reflector Antenna 



A simple directional device that gives an appre- 

 ciable power gain (of the order of 10 to 20) is a 

 corner reflector, which is essentially a combination 

 of two reflecting sheets and a dipole. In the case 

 shoAA'n in Figure 41 where the angle subtended bj' 



• DIPOLE 

 o IMAGES 



CROSS SECTION 



Figure 41. Corner reflector antenna. 



the corner is 90°, the corner is equivalent to the 

 combined radiation of three image antennas. The 

 reflector can also be made of wires j^arallel to the 

 direction of the radiating dipole. The reflecting 

 wires do not, however, act as parasitic antennas 

 but are taken so long that they are practicallj^ 

 equivalent to conductors of infinite length. 



These should not be confused with corner re- 

 flectors which are extensively used as targets and 

 consist then of three mutually perpendicular con- 

 ducting planes (see Section 9.2.4). 



3.6 



3.6.1 



PARABOLIC ELEMENTS 



Parabolic Reflectors 



These reflectors are the devices most commonly 

 used to produce highly directive radiation patterns 

 in the microwa^'e region. The three main types are 

 shown in Figure 42; they are the parabolic cj^linder, 

 the paraboloid of revolution, and the truncated 

 paraboloid, the latter being a rectangular section 

 cut from a paraboloid of revolution. If the parabolic 



cylinder is relatively short and provided «ith flat 

 metallic covers at the top and bottom, its shape and 



/I \ 





B PARABOLOID 

 OF REVOLUTION 



C TRUNCATED 

 RARABOLOID 



Figure 42. Types of parabolic reflectors. 



its electrical properties resemble those of a sectoral 

 horn (see Section 3.7.2). 



The directive action of the parabolic reflector 

 depends on two geometrical properties of the parab- 



DIRECTRIX 



Figure 43. Projierties of a parabola, 



ola (Figure 43). A ray coming from the focus is 

 reflected into a direction parallel to the axis of the 

 parabola, and the distance from any point P on the 

 parabola to the line called the directrix is eciual to 

 the distance from F to the focus. Consequently, the 

 effect of the parabola in the forward direction is 

 equivalent to that of a distribution of soiu'ces in the 

 directrix that all oscillate in phase (but usualh' have 

 varjdng intensities o^'er the directrix). 



The parabolic cylinder produces a directi^^e pattern 

 only in a plane perpendicular to the generating line 

 of the cylinder (horizontal plane in A of Figure 42). 

 In order to concentrate the beam in a plane parallel 

 to the generating line of the cylinder (vertical plane 

 in Figure 42), an additional directive device must be 

 employed. Usually this is a colinear array of dipoles, 

 as shown; the direction of polarization is parallel 

 to the focal axis. In microwave work this tj^pe of 

 antenna offers advantages over the two-dimensional 

 curtain of dipoles employed in VHF directional 

 antennas. 



