RADAR ANTENNAS 291 



mediately below the feed aperture. The E vector is in the plane of the 

 paper at this point. However, the ensuing vertical section is fitted with a 

 spiral septum. This gradually rotates the plane of polarization until at 

 the top of this pipe the E vector is perpendicular to the plane of the paper. 

 Thus, after transversing another 90° pipe bend, the energy emerges horizon- 

 tally polarized, to feed the main reflector. 



Specific electrical characteristics of the SL antenna are: 



Polarization — Horizontal 

 Horizontal Half Power Beamwidth — 6° 

 Vertical Half Power Beamwidth — 12° 

 Gain — about 22 db. 



14.3 The SJ Submarine Radar Antenna 



It had long been expected that one of the early offensive weapons of the 

 war would be the submarine. It was therefore natural that early in the 

 history of radar the need for practical submarine radars was felt. The 

 principal components of this need were twofold, to provide warning of ap- 

 proaching enemies and to obtain torpedo fire control data. The SJ Sub- 

 marine Radar was the first to be designed principally for the torpedo fire 

 control function. 



Work on the SJ system was under way considerably before Pearl Harbor. 

 When this work was initiated the advantages of lobing fire control systems 

 were clearly recognized, but no lobing antennas appropriate for submarine 

 use had been developed. Requirements on such an antenna were ob- 

 viously severe, for in addition to fulfiUing fairly stringent electrical con- 

 ditions, it would have to withstand very large forces due to water resistance 

 and pressure. 



The difficulties evident at the outset of the work were overcome by an 

 ingenious adaptation of the simple waveguide feed. It was recognized 

 that a shift of the feed in the focal plane of a reflector would cause a beam 

 shift. Why not, then, use two waveguide feeds side by side to produce the 

 two nearly coincident beams required in a lobing antenna? When this was 

 tried it was found to work as expected. 



It remained to devise a means of switching from one waveguide feed to 

 the other with the desired rapidity. This in itself was no simple problem, 

 but was solved by applying principles learned through work on waveguide 

 filters. The switch at first employed was essentially a branching filter 

 at the junction of the single antenna feed line and the line to each feed aper- 

 ture. Both branches of this filter were carefully tuned to the same fre- 

 quency, that of the radar. The switching was performed by the insertion 

 of small rapidly rotating pins successively into the resonant cavities of the 



