TRANSATLANTIC RADIO TELEPHONY 329 



Under this condition: 



r=tan-i-=^=- (21) 



Using Fig. 20 of Appendix 3, the value of 5 corresponding to this 

 value of T is determined (generally 5 = 7r/4). Second, T is revaluated 

 from (19) using the value of 8 so obtained. 



The ground resistivity is evaluated from the value of the quasi-tilt 

 angle by using Fig. 20 of Appendix 3. 



5. Directional Characteristics. The measurement of the directional 

 characteristics of a wave-antenna or a wave-antenna system consists 

 entirely of measuring the effective height of the antenna for several 

 directions of wave propagation, and determining the relative direc- 

 tional receptivity of the antenna in these directions by dividing the 

 effective height for each direction by the value obtained for the 

 direction of the axis of the antenna. For this purpose, the effective 

 height at the output of the antenna system is most convenient to 

 measure and use. This constant is defined as the ratio of the voltage 

 at the input of the radio receiver to the field strength producing this 

 voltage. It is exactly related to the effective height referred to the 

 characteristic impedance (defined in the preceding subsection of this 

 paper) by the real part of the total transfer constant between the 

 termination at the initial end of the antenna and the input terminals 

 of the radio receiver, and an additional factor of one-half because the 

 voltage at the radio receiver is measured across the proper termination. 



In certain receiving station locations, it is possible to utilize for 

 determining the relative directional receptivity the regular transmis- 

 sion from existing radio transmitters operating at or very close to the 

 frequency for which the directional characteristic is desired. At sites 

 less favorably located with regard to existing transmitters, the direc- 

 tional characteristic may be measured by transmitting test signals 

 from a portable transmitter, located successively in the several direc- 

 tions for which data are desired, and at least 15 wave-lengths from 

 the antenna system. 



A distinctly different method of measuring the directional char- 

 acteristics of an antenna is based on a statistical study of the reduction 

 of noise obtained by its use. While it is difficult to evaluate the direc- 

 tional characteristic exactly by this method, data showing the com- 

 parative decrease in noise with the wave-antenna as against a loop or 

 a vertical antenna are of great value in predicting the improvement 

 in a radio circuit to be obtained by its use. As a converse to these 

 results, the statistical combination of the improvement given by the 

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