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 t h a t the fading is selective. By selective fading it is meant t h a t different frequencies do not fade together. From the regularity of the frequency relation between the frequencies which fade together it is concluded t h a t 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 t h a t 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 t h a t 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.