Some Studies in Radio Broadcast Transmission' 



By RALPH BOWN, DeLOSS K. MARTIN 

 and RALPH K. POTTER 



Synopsis: The paper is based on radio transmission tests from station 

 2XB in New York City to two outlying field stations. It is a detailed study 

 of fading and distortion of radio signals under night time conditions in a par- 

 ticular region which may or may not be typical. 



Night time fading tests using constant single frequencies and bands of 

 frequencies in which the receiving observations were recorded by oscillograph 

 show that the fading is selective. By selective fading it is meant that differ- 

 ent frequencies do not fade together. From the regularity of the frequency 

 relation between the frequencies which fade together it is concluded that 

 the selective fading is caused by wave interference. The signals appear to 

 reach the receiving point by at least two paths of different lengths. The 

 paths change slowly with reference to each other so that at different times 

 the component waves add or neutralize, going through these conditions 

 progressivelv. The two major paths by which the interfering waves travel 

 are calculated to have a difference in length of the order of 135 kilometers for 

 the conditions of the tests. Since this difference is greater than the distance 

 directly from transmitter to receiver it is assumed that one path at least 

 must follow a circuitous route, probably reaching upward through higher 

 atmospheric regions. Various theories to explain this are briefly reviewed. 



The territory about one of the receiving test stations in Connecticut is 

 found under day time conditions to be the seat of a gigantic fixed wave inter- 

 ference or diffraction pattern caused in part by the shadowing of a group of 

 high buildings in New York City. The influence of this pattern on night 

 time fading is discussed. It is considered a contributing but not the controll- 

 ing effect. 



Tests using transmission from an ordinary type of broadcasting trans- 

 mitter show that such transmitters have a dynamic frequency instability or 

 frequency modulation combined with the amplitude modulation. At night 

 the wave interference effects which produce selective fading result in dis- 

 tortion of the signals when frequency modulation is present. It is shown 

 that stabilizing the transmitter frequency eliminates this distortion. A 

 theory explaining the action is given. The distortions predicted by the theory 

 check with the actual distortions observed. 



A discussion of ordinary modulated carrier transmission, carrier sup- 

 pression, and single side band transmission is given in relation to selective 

 fading It is shown that the use of a carrier suppression system should 

 reduce fading. 



o 



NE of the factors which inust be given increasing attention, if the 

 technique of radio telephone broadcasting is to consoHdate and 

 continue its remarkable progress, is the mechanism of the transmission 

 of radio signals through space. In many receiving situations the 

 largest apparent defects present in the reproduced signal are those 

 suffered not in the terminal apparatus but in transit through space, 

 and in these cases better methods of utilizing the transmitting medium 

 must precede any major betterment in overall results. In the present 

 paper we are reporting some investigations in this field of radio trans- 

 mission which have uncovered a number of interesting facts and have 

 led to at least one conclusion which is of practical utility. 

 1 Presented before the Institute of Radio Engineers, New York. Nov. 4, 1925 



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