What creates intermodulation interference between two repeaters in close proximity?

Intermodulation interference (often called 'intermod') occurs when two or more signals mix together to produce new, unwanted frequencies. For repeaters in close proximity, a strong transmission from one repeater can travel down the antenna of a neighboring repeater and reach its transmitter circuitry. Once inside the second transmitter, this foreign signal encounters the final amplifier. Because amplifiers are inherently non-linear devices, they act as a mixer. The incoming signal mixes with the transmitter's own generated signal, radiating a completely new set of interfering frequencies back out of the antenna. Distractors involving 'harmonically related' frequencies describe harmonic interference, not intermodulation. Furthermore, 'feedback' typically causes oscillation or squealing, whereas true intermod relies on the mathematical mixing of discrete frequencies inside a non-linear stage.