DEVELOPMENTS IN SHORT-WAVE DIRECTIVE ANTENNAS 673 



For long wires, the above readjustment is very small. As an exam- 

 ple, a ten-wave-length wire is properly tilted when it makes an angle 

 with the direction of wave propagation whose cosine is 0.950. Thus, 

 only a five per cent reduction in the termination from the characteristic 

 impedance value will give an infinite front-to-back ratio. 



In practice, we usually adjust a termination to a value which is a 

 compromise between the above value and the characteristic impedance. 

 This gives very favorable front-to-back ratios at any frequency within 

 the range of the antenna, particularly in the case of long wires. 



Theoretically infinite front-to-back ratios have been mentioned 

 several times in the preceding discussion. It is an experimental fact 

 that where very minute adjustments can be made in both the resistive 

 and reactive components of the termination impedance, the front-to- 

 back signal voltage ratio is only limited by the rigidity of the antenna 

 elements in space. Voltage ratios in excess of 1000 to 1 are readily ob- 

 tained, although such extremes are seldom warranted in practice. This 

 deep depression can be "steered" through a considerable range of 

 directions largely through changes in the reactive component of the ter- 

 mination impedance, the resistance alteration required being small. 

 This permits a high degree of discrimination against many specific 

 cases of interference in the rear quadrant of the antenna. 



The Diamond-Shaped Antenna 



In terminating inverted V antennas to ground, trouble has been ex- 

 perienced due to the instability of the ground contact resistance during 

 varying weather conditions. In addition, the signal "pick-up" in the 

 connecting leads was not always small compared with the antenna 

 signal response in directions of antenna minima. These difficulties 

 were avoided by terminating to the center point of a straight wire, 

 substantially a half wave-length in total length, lying perpendicular to 

 the favored wave direction. 



As is well known, a quarter wave-length open-ended element appears 

 to be a very low resistance when measured between its terminal and 

 ground or another similar element. Two such low resistance quarter 

 wave-length elements are effectively in parallel in the above arrange- 

 ment and the center-tapped symmetry substantially balances out the 

 effect of voltages induced in these elements. 



Variations of the above type of artificial ground have been used in 

 connection with inverted V antennas but, with few exceptions, they 

 have required readjustments as the frequency was altered. A more 

 satisfactory arrangement from several points of view is the double-V 

 or diamond-shaped antenna shown in Fig. 14. This provides a bal- 



