TRANSATLANTIC RADIO TELEPHONY 335 



for directional transmission and reception has become increasingly 

 important. Antenna arrays may be divided into two general classes: 

 (1) arrays of antennas having dissimilar directional characteristics, 

 and (2) arrays of antennas the directional characteristics of which are 

 identical. The array formed by the use of a loop and a vertical 

 antenna to form the familiar "cardioid" is representative of the first 

 class of antenna arrays. Foster^'' has pointed out that the ideal 

 wave-antenna may be considered as an array of an infinite number of 

 loop antennas, extending for the length of the wave-antenna, and 

 hence an antenna array of the second class. (An ideal wave-antenna 

 has no attenuation and a velocity of propagation equal to the velocity 

 of radio propagation in free space.) 



An important difference between arrays of dissimilar antennas and 

 arrays of identical antennas lies in the following peculiarity of these 

 two types. In general, the directivity of dissimilar antennas may 

 be increased with no loss in desired signal receptivity by combining 

 them in arrays with little or no separation between the individual 

 antennas. To obtain an increase in directivity by using several 

 identical antennas in an array, however, without too great a sacrifice 

 in desired signal receptivity, the array must cover a space comparable 

 to and of the same order of magnitude as the wave-length of the signals 

 for which it is designed. 



It has been stated earlier in this paper that the fundamental form 

 of wave-antenna consists of a single straight horizontal wire, terminated 

 to ground at each end in its characteristic impedance. If the input 

 circuit of a radio receiver be connected across the termination at the 

 end of the antenna most distant from the desired transmitter (the 

 far end of the antenna) this simple form of wave-antenna can be used 

 as a directional receiving system. If arrangements are made to bring 

 the output from the initial end of the wave-antenna to the radio 

 receiver as well as the output from the far end, the simple wave-antenna 

 immediately becomes available for use as two identical antennas in 

 an array. The ends of these two antennas from which the outputs 

 are taken are separated by the length of the antenna and their axes 

 are parallel but in the opposite sense. If before combining these two 

 output currents, that from the initial end of the antenna is changed 

 in phase and magnitude by the proper amount, it is possible to produce 

 a null point of reception in any desired direction. The name "com- 

 pensation" has been applied to the use of a single wave-antenna to 

 form this array." Since this null point is produced by balancing the 

 back-end current from one antenna of the array (relative to its direc- 

 tional diagram) against the front-end current from the other antenna, 



