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BELL SYSTEM TECHNICAL JOURNAL 



ment hope to evaluate performance, to chose between alternatives or to 

 see the directions of improvement. Measuring technicjues employing 

 double detection receivers and intermediate frequency amplifiers had 

 long been in use at the Holmdel Radio Laboratory. By employing 

 these techniques radar engineers were able to make more sensitive and 

 accurate measurements than would have been possible with single de- 

 tection. 



Antennas are as old as radio. Radar antennas though different in 

 form are identical in principle with those used by Hertz and Marconi. 

 Consequently experience with communication antennas provided a 

 valuable background for radar antenna design. As an example of the 

 importance of this background it can be recalled that a series of experi- 



Fig. 1 — An Electromagnetic Horn. 



ments with short wave antennas for Transatlantic radio telephone 

 service had culminated in 1936 in a scanning array of rhombic antennas. 

 The essential principles of this array were later applied to shipborne 

 fire control antenna which was remarkable and valuable because of the 

 early date at which it incorporated modern rapid scanning features. 



In addition to the antenna arts which arose directly out of communi- 

 cation problems at lower frequencies some research specifically on micro- 

 wave antennas was under way before the war. Earl\- workers in wave- 

 guides noticed that an open ended waveguide will radiate directly into 

 space. It is not suri)rising therefore that these workers developed the 

 electromagnetic horn, which is essentially a waveguide tapered out to 

 an aperture (Fig. 1). 



One of the first used and simplest radio antennas is the dipole (Fig. 



