HORIZONTAL RHOMBIC ANTENNAS 137 



For a receiving antenna, the dividing line between the optimum and 

 the compromise design methods is often defined by the relative 

 importance, as circuit Hmiting factors, of static as compared with set- 

 noise. The optimum design results in a large signal output, from the 

 antenna, which enables the over-riding of set-noise and it also results in 

 effective directional discrimination against static. The compromise 

 height arrangement maintains and sometimes improves upon the 

 directional discrimination against static but sacrifices a part of the 

 possible antenna output-signal level. If the frequency is sufficiently 

 low so that static rather than set-noise is practically always the circuit 

 limitation, no real harm will result from using the compromise height 

 design with its accompanying economic saving. 



In general, compromise in element length results in a loss in both the 

 antenna output-signal level and in static discrimination. This pro- 

 cedure is recommended only for exceptional cases where a restriction in 

 available land exists, or where a broad directional characteristic is 

 required because of a highly variable wave-direction. 



Directivity Equations 



In the appendix of this paper will be found the derivation, for the 

 stated assumptions, of the three dimensional directivity equation for a 

 horizontal rhombic receiving antenna terminated by its characteristic 

 impedance. The principal antenna dimensions will be apparent by 

 reference to Fig. 1. 



If the horizontal component /3, of the angle made by the wave- 

 direction with the principal antenna axis, is set equal to zero in the final 

 resulting equation in the appendix, the directivity equation for the 

 incident plane passing through the principal axis is as follows: 



rj,^k\^\ ^ae 



•lirl 



\ - e ^ , (1) 



where Ir = receiver current, 



k — proportionality factor, 

 // — height above ground, 

 / = wire element length, 

 <^ = one-half of side apex angle, 

 A = angle made with ground by wave-direction in incident 



plane, 

 a — amplitude ratio of ground reflected field to incident field, 

 a = apparent phase angle lag caused by ground reflection, 

 X = wave-length. 



