644 BELL SYSTEM TECHNICAL JOURNAL 



employing apparatus of the type described by Englund and Friis' 

 and observations on the relative strength of the received signals were 

 made by inserting a sensitive microammeter in the plate circuit of the 

 third detector of the television receiver. These data indicated that the 

 lower frequency band suffered considerably less attenuation and also 

 afforded much more stable transmission. In spite of the compara- 

 tively short distance (approximately 22 miles), marked fading was 

 experienced beginning with the sunset period and increasing in ampli- 

 tude as the night advanced. The high frequency band proved to 

 be particularly disadvantageous in this respect. It was decided, there- 

 fore, to fix upon the lower frequency band and to confine the demon- 

 stration to the afternoon when reasonably stable transmission condi- 

 tions prevailed. 



Following the choice of a definite operating frequency, a number of 

 modifications were made in the transmitting antenna to improve its 

 efficiency and increase the field strength at the receiver. This work 

 finally resulted in a measured field strength of approximately 2500 

 microvolts per meter for an antenna input of 5 kilowatts. 



Further consideration of the available data on transmission and 

 traffic conditions and the performance characteristics of the apparatus 

 units involved lead to a choice of 1450 kilocycles for the speech channel. 

 In spite of an antenna input of approximately 30 kilowatts, the initial 

 tests at this frequency were very unsatisfactory due to inadequate 

 field strength at the receiver which necessarily resulted in an 

 abnormally high noise level. The height of the antenna was, there- 

 fore, increased from 60 to 100 feet by installing iron pipe topmasts. 

 This change brought the field strength at the receiver to approxi- 

 mately the same value as that obtained for the television channel 

 (2500 microvolts per meter) which was considered to be satisfactory 

 for the purpose. 



In order to insure that the reproduction of the picture might not 

 suffer from serious discrimination against essential frequencies at 

 some point in the radio system, very careful tests were made on the 

 individual units and on the system as a whole. 



The frequency characteristic of the transmitter was determined by 

 connecting a vacuum tube oscillator producing a relatively pure wave 

 to its input terminals through a suitable network involving a thermal 

 milliammeter and an adjustable artificial line. A rectifier of known 

 characteristics and a second thermal meter protected against radio- 

 frequency currents by means of a low-pass filter were coupled to the 



^"Methods for Measurements of Radio Field Strengths," C. R. Englund and 

 H. T. Friis. Presented to the Spring Convention A. I. E. E. at Pittsfield, Mass., 

 May 25, 1927. 



