ABSTRACT
The history of combining amplitude modulated (AM) transmitter sites go as far back as the 1930s. The performance to be expected through the filters that must be used to isolate the transmitter outputs from each other is an important consideration. Filters work to filter because they have different characteristics at different frequencies. In general, filtering for diplexing involves tower base series pass/reject filters to isolate the multiple feeds from the stations' antenna matching networks and the base series filters to sufficiently block the unwanted signals and prevent intermodulation in the transmitters' final amplifiers. Owners needing to relocate stations sometimes sought more economical solutions than developing new transmitter sites, and diplexed AM transmitter sites began to be developed in significant numbers. The 1996 common ownership rule change that once again allowed a single licensee to own more than one AM station in a market resulted in increased interest in diplexing, as owners sought to combine the facilities of their stations.
