Some Studies in Radio Broadcast Transmission1

Abstract

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 particular 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 different 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 progressively. 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 interference 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 controlling effect. Tests using transmission from an ordinary type of broadcasting transmitter 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 distortion 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 suppression, 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.