RADAR ANTENNAS 



261 



halves of a wavelength. Its phase velocity is determined by the plate 

 spacing in a manner given By the familiar waveguide formula 



Va = 



where 'c' is the velocity of light, e is the dielectric constant relative to 



free space of the medium between the plates, X is the wavelength in 



air and 'a' is the plate spacing. 



The TEM mode between parallel plates can be generated by extending 



the central conductor of a coaxial perpendicularly into or through the wave 



space and backing it up with a reflecting cylinder as indicated in Fig. 24. 



PARALLEL 

 PLATES 



REFLECTING 

 CYLINDER 



PARABOLIC 

 CYLINDER 



Fig. 24 — Parabolic Cylinder Bounded by Parallel Plates. Probe Feed. 



Alternatively this mode can be generated as indicated by Fig. 25 by a wave- 

 guide aperture with the proper polarization. 



The TEni mode, when used, is usually generated by a rectangular wave- 

 guide aperture set between the plates with proper polarization as indicated 

 in Fig. 25. Care must be taken that only the desired mode is produced. 

 The TEM mode will be unexcited if only the desired polarization is present 

 in the feed. The next parallel mode is unsymmetrical and therefore even 

 if it can be transmitted will be unexcited if the feed is placed symmetrically 

 with respect to the two plates. 



Parallel plate antennas as shown in figures 24 and 25 are useful where 

 particularly large aspect ratios are required. The aperture dimension per- 

 pendicular to the plates is equal to the plate spacing and therefore small. 



