44 



ANTENNAS 



3.7 



3.7.1 



HORNS 



3.7.2 



Sectoral Horn with TE^o Wave 



Types of Horns 



Many of the antennas pre\douslj^ described are 

 used in the high-frequency [HF] and very high- 

 frequency [VHF] bands of frequencies. Horns cannot 

 readily l^e used at these frequencies because the sizes 

 required would be excessive. 



But at the microwave frequencies, the size of the 

 horn is small and it is easy to feed energy to it 

 through a wave guide. In this arrangement the 

 horn acts as transition between the impedance of 

 the wave guide and the 377 ohms impedance of 

 free space and thus reduces to a minimum the 

 reflection of energy backward into the guide (such 

 as would occur if the wave guide ended in an open 

 pipe). 



Common types of horns are sectoral (discussed 

 in Section 3.7.2), pyramidal, conical, biconical, etc. 

 Onlj' the first type is discussed in this section. 



For this case the horn is flared only in width and 

 is an extension of the wave guide of width h and 

 depth a. For the TE^o wave the electric field is 

 parallel to the dimension a and varies in strength 

 cosinusoidally across the wave guide and horn open- 

 ing, as shown in Figure 44. The length of the horn 

 is R and the flare angle is (p. 



Figure 45 illustrates the pattern shapes in the 

 plane parallel to dimension b for various flare angles. 



For this type of wave, the cutoff frequency of the 

 wave guide is 



/. = 



3 X 1 0^ 



2b ' 



(43) 



with b in meters. The operating frequency should 

 be near but not greater than twice this value. 



The gain depends upon the length R and the flare 

 angle 4>, and is plotted in Figure 46. 



