Some Studies in Radio Broadcast Transmission

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.