Appendix VIII

Author: Edmund A. Laport

Example of the Use of Directive Antennas for Minimizing Interference between Cochannel Medium-frequency Broadcasting Stations

The map shows the broadcasting stations on 620 kilocycles as they existed in North America in 1949. Under the North American Regional Broadcasting Agreement (NARBA) 620 kilocycles is a regional channel. The directive antenna pattern for each station on this channel is shown. Those in broken lines are used during local daylight hours, while those in solid lines are for night hours. In some cases a station is nondirective during daylight. It is seen that WROL and KCOM, for example, use different directive patterns day and night. Stations like WDNC and WHJB use the same pattern day and night but with different power.

The purpose of directive antennas is to provide a local service and, at the same time, to protect the service areas of stations that were already on the channel before a new station is installed. It will be noted that protections are not mutual in all cases, This is because existing stations are not required to change their antenna systems when a new station enters the channel, but the new station must protect the existing stations to rigidly prescribed limits. Directivity is also used to protect stations on adjacent channels when necessary.

The patterns shown indicate their shape only, and the size of the pattern as drawn does not in any way indicate a station's coverage. Each pattern is marked with the station call letters and its licensed power day and night. The symbol 5.U indicates 5 kilowatts unlimited (day and night); 0.5-1 LS means 1 kilowatt during daytime until local sunset and 0.5 kilowatt after local sunset. The symbol CP means that at the time the map was prepared the station had a construction permit to install the antenna system and the transmitter power using the pattern shown. (Map supplied through courtesy of Mutual Broadcasting System.)