Encroachment is what happens when a full power station interferes with the signal of another radio station. Encroachment usually occurs when a full power station moves its transmitter, or when a new full power station is built. Since low power radio stations have secondary status with respect to full power stations, they have little protection from encroachment interference, even when the full power station threatens to obscure their entire signal.

On January 17, 2007, the FCC loosened the process for full power stations to change their community of license from a "major modification" to a "minor modification," prompting many full power stations to change their transmitter sites (often called a move-in). Since then, over 400 low power radio stations—roughly half of LPFMs on the air—have faced some form of encroachment.

Prometheus has won a number of procedural improvements, providing more options and protections for LPFMs facing fatal encroachment, in some cases even allowing for the denial of the full power move-ins. These changes were challenged by the National Association of Broadcasters and upheld by the DC Circuit Court of Appeals. Currently, Prometheus is working for stronger protections for LPFM stations from the degradation of their signal due to encroachment.

Background Reading