526 ANTENNAS AND RF COMPONENTS 



the fan, while essentially transverse slots in the side wall of the guide result 

 in a polarization perpendicular to the plane of the fan. If the polarization 

 is not specified by the system requirements, it is preferred to use the 

 side-wall slots for several practical reasons. Side-wall slots are easier to 

 fabricate, and the vertical pattern in the fan is more readily controlled 

 with this polarization. Slots of these two types are shown in Fig. 10-7. 



Approximately 



Approximately 

 hXg/2H 



(b) 



Fig. 10-7 Waveguide Slotted in a Manner Suitable for Antenna Arrays with 



Reverse Phase Coupling in Adjacent Slots: (a) Top Wall Slots Which Radiate with 



Electric Polarization in the H Plane of the Guide, (b) Side Wall Slots with Electric 



Polarization in the H Plane of the Guide. 



Consider the slot spacing. There is a minimum slot spacing required to 

 prevent deterioration of the main beam, given by Silver ^^ 



^ < (1 - \/n)X (10-12) 



where n is the number of slots, X is the free space wavelength; and as all 

 slots must be driven in approximately the same phase, it is usual to space 

 the slots by essentially half a guide wavelength and arrange for the resulting 

 required phase reversal at alternate slots. For example, in using side-wall 

 slots, the direction of tilt in adjacent slots is reversed to provide this phase 

 reversal. The result is that £-plane components of the electric field in 

 adjacent slots tend to cancel, whereas //-plane components add. Thus, 

 an array of in-phase radiators has been obtained spaced at half a guide 

 wavelength, giving polarization parallel to the waveguide axis. 



It is not usual to space the slots at exactly half a guide wavelength, 

 however. This spacing of many small discontinuities gives a resonant 

 antenna, often with an impractically narrow bandwidth. A small change 

 in the slot spacing allows the many reflections to tend to cancel one another, 

 resulting in a good bandwidth. In this case, the fan beam is tilted a small 

 amount from the normal to the waveguide. For this reason, many arrays 

 of this type will be found with beam centers 1° to 5° from a perpendicular 

 position. 



"Silver, op. cit., p. 319. 



