276 BELL SYSTEM TECHNICAL JOURNAL 



nients occujnes one half of the final antenna aperture, and provides a Uni- 

 phase front across this half. If the two elements were excited with the same 

 phase the radiation maximum of the resulting antenna beam would occur 

 in a direction at right angles to the combined phase front. If the phase of 

 one element is made to lag behind that of the other by a small amount, 

 60° say, the phase of the combined aperture will of course be discontinuous 

 with a step in the middle. This discontinuous phase front will approximate 

 with a small error, a uniphase wave front which is tilted somewhat with 

 respect to the wave fronts of the elements. The phase shift will there- 

 fore result in a slight shift of the beam away from the normal direction. 

 When the phase shift is reversed the beam shift will be reversed. Two 

 properly designed elementary antennas in combination with a means for 

 rapidly changing the phase will therefore constitute a lobe switching an- 

 tenna. Such an antenna is described more in detail in Sec. 14.6. 



Another method of lobe switching is more natural for antennas based 

 on optical principles. In this method two identical feeds are placed side 

 by side in the focal region of the reflector. When one of these feeds illu- 

 minates the reflector a beam is produced which is slightly ofif the normal 

 axial direction. Illumination by the other feed produces a second beam 

 which is equally displaced in the opposite direction. The lobe of the an- 

 tenna switches rapidly when the two feeds are activated alternately in rapid 

 succession. The antenna must use some form of rapid switching appropri- 

 ate to the antenna feed line. In several applications switches are used 

 which depend on the rapid tuning and detuning of resonant cavities or 

 irises. 



11.2 Conical Lobiiig 



A conically lobing antenna j)roduces a beam which nutates rapidly about 

 a fixed axial direction. This is usually accomplished by rotating or nutat- 

 ing an antenna feed in a small circle about the focus in the focal plane of a 

 paraboloid or lens. This antenna feed can be a spinning asymmetrical 

 dipole or a rotating or nutating waveguide aperture. It can result in a 

 beam with linear polarization which rotates as the feed rotates, or prefer- 

 ably in a beam for which the polarization remains parallel to a tixed direction. 

 The beam itself must be nearly circularly symmetric so that the radar re- 

 sponse from a target in the axial direction will not vary with the lobing. 

 The reflector or lens aj)erture is consequently usually circular. 



When the antenna is small it is sometimes easier to leave the feed fixed 

 and to produce the lobing by moving the reflector. 



12. RAPID SCANNING 



A lobing radar can j)rovide range and angular information concerning 

 a single target rapidi)' and accurately but these things arc not always enough. 



