HORIZONTAL RHOMBIC ANTENNAS 



151 



These substantial agreements between calculated and measured 

 values are an indication of the accuracy secured in theoretically 

 predicting antenna directivity when using the stated assumptions. 

 It is believed, therefore, that they establish the validity of the design 

 methods now being used for commercial applications of this type of 

 antenna. 



Standing Wave Suppression 



To obtain the uni-directional characteristics usually desired and to 

 provide a constant terminal impedance over a broad frequency range, 

 the end of the rhombic antenna, remote from the receiver, is terminated 

 in its equivalent characteristic impedance to prevent wave reflections. 



The termination was originally adjusted by determining the greatest 

 front-to-back signal reception ratios which could be obtained with the 

 terminating graphite resistors or the other circuits employed. This 

 method of testing proved both cumbersome and erratic for the following 

 reasons : 



Radiating field oscillators had to be placed at sizable distances both 

 in front and in the rear of the antenna. Manipulating these oscillators, 

 for frequency runs, proved inconvenient. Furthermore, properly 

 elevating the oscillators was impractical because of the large heights 

 required to simulate the actual average wave-angles usually en- 

 countered. When the oscillators were at more reasonable heights 

 above ground, the front-to-back signal ratios could rarely be reproduced 

 since the wave-direction was unstably situated on the lower steep edge 

 of the incident plane directive diagram. 



The procedure for testing the termination has been simplified through 

 the measurement, by a substitution method, of the antenna terminal 

 impedance as the frequency is varied over the required frequency band. 

 By readjusting the termination, until these measured values are 

 constant, the required value is obtained. 



The schematic diagram of the impedance measuring equipment 

 employed is shown in Fig. 16. In the actual apparatus, the individual 



BALANCED 



HIGH- 

 FREQUENCY 

 OSCILLATOR 



Hkr 



C| 



THERMO- 

 COUPLE 





A^ 



C2 



C3 



C3 



TO ANTENNA 



OR 



RESISTANCE 



STANDARD 



Fig. 16 — Schematic diagram of impedance measuring equipment. 



