270 BELL SYSTEM TECHNICAL JOURNAL 



above the ground as the center of the receiving antenna, was obtained 

 by a standard method of field strength measurement.^" This equip- 

 ment then was removed and the truck placed in such a way that its 

 antenna was always at the point where the field strength had been 

 determined. By pivoting the truck about this point the horizontal 

 polar diagram of the mobile receiving equipment was obtained. It is 

 shown in Fig. 12. As the field was known at this point in the absence 



Fig. 12 — Directional characteristic in the horizontal plane of the mobile receiver. 



of the truck the attenuator settings gave a calibration of the receiving 

 equipment for all directions. With this calibration as a standard, a 

 comparison field generator which employed a loop radiator attached 

 to the opposite side of the truck was calibrated also. The comparison 

 field generator then was used throughout the test to check the calibra- 

 tion of the receiver. Since the polar diagram was fairly constant on 

 the right side of the truck, care always was taken to orient the truck 

 in such a way that the direction of the transmitter was at the right 

 side rather than the left. 



2" This method consists of comparing the unknown field with a known field 

 produced by a "Standard Field Generator," which is a small compact self-contained 

 oscillator. It is very carefully shielded except for a small balanced loop extending 

 in a vertical plane above the shield. A thermomilliamnieter is located in the loop 

 at 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. 

 See Proc. L K. R., Vol. 21, pp. 430-431, March, 1933. 



