How close apart can radio stations be?
Regulatory agencies, like the FCC (which has jurisdiction over broadcasting in the USA) have complicated sets of rules to determine which frequencies are available for use in which locations. These rules make assumptions about the following:
- Receiver selectivity
- How signal strength degrades with distance from the transmitter
- RF bandwidth occupied by radio stations
- How much interference listeners will tolerate
These rules analyze both how a proposed station would interfere with established stations and how much interference a proposed station would receive from established stations. As a result, these rules are complex, and the job of finding open frequencies is a task for engineers with very specialized training.
As very general examples, in the USA, AM radio stations that are in close geographic proximity to one another will never be closer than 40 kHz on the dial. Full-powered FM stations that are geographically close together will never be closer than 800 kHz, although it is now becoming common to find low powered stations and translators (250 W or less) within 400 kHz of full-powered stations.
The very conservative rule followed here on AM station spacing is a holdover from the era before the 1990s, when AM stations were not required to low-pass filter their audio. Then, AM stations produced significant sideband energy up to 15 kHz or so from the carrier, especially when playing music.
Other countries may pack stations (particularly AM stations) more closely, especially in large cities.