ULTRA-SHORT WAVE PROPAGATION 129 



the point of low potential with respect to the shield. From the reading 

 of the meter and the dimensions of the loop, the field at nearby points 

 may be computed. ^^ This, therefore, provides a field strength standard 

 by comparison with which the unknown field can readily be ob- 

 tained. 



Signals were received by means of a simple half-wave antenna 

 supported on a portable mast at heights up to 12 meters above the 

 ground. For calibrating, however, the center of the antenna was 

 located about 4 meters above the ground and the standard field 

 generator was placed in operation at the same height one half wave- 

 length away. It has been determined experimentally that this avoided 

 serious complications due to the proximity of the ground. There are 

 certain other refinements which may or may not be important de- 

 pending upon the accuracy required. Such, for example, is the effect 

 of the finite length of the receiving antenna. It is beyond the scope 

 of the present paper to enter into this matter. It is sufficient to say 

 that the error so produced is less than one decibel. Field strengths 

 of the order of two or three microvolts per meter could be measured in 

 this way. This might be improved by increasing the sensitivity of 

 the set or by using directional receiving antennas. 



The meter in the transmitting antenna was calibrated by means of 

 this same standard field generator. The signal from the transmitting 

 antenna was measured at some nearby receiving point. The antenna 

 was then lowered to the ground, the standard field generator was 

 hoisted into the same position and the signal from it measured at the 

 same receiving point. Thus the field radiated from the transmitting 

 antenna was known in terms of the field from the standard field 

 generator. The meter-amperes in the transmitting antenna could 

 then be calculated in terms of the standard field generator. 



It is important that both transmitting and receiving equipments 

 were calibrated in terms of the same standards, namely, the dimensions 

 of the loop of the standard field generator and the current in it as 



^- The field from a radiating loop in free space is given by 

 ^ nOir'-NAI ( ^ . X \ 



where E = electric field strength in volts per meter 

 N = number of turns in loop 

 A = area of loop in square meters 

 / = current in loop in amperes 

 D = distance between loop and antenna in meters 

 X = wave-length in meters 



When the distance between the loop and the receiving antenna is a half wave-length 

 the terms in the parentheses become (1 —JO. 318) which has an amplitude of 1.05 

 (0.4 db above unity). Hence the second term increases the total field to 0.4 db 

 above the "radiation" field at this distance. 



