Two mobile stations are communicating through a repeater. As they arrive in close proximity to each other, they begin to have difficulty communicating. What is the most likely cause?

When two mobile stations are physically very close, the RF energy emitted by one transmitter is overwhelmingly strong at the other station's receiving antenna. This massive influx of RF energy forces the receiving radio's front-end circuitry to drastically reduce its sensitivity to protect itself or simply overloads the amplifier stages. Because the receiver is now artificially deafened by the nearby transmitter, it can no longer hear the much weaker, distant signal coming from the repeater. This phenomenon is known as front-end desensitization (or simply 'desense'). This issue is not caused by entering an antenna null area, as mere proximity does not alter antenna radiation patterns. It is also not intermodulation, because a single overwhelming local signal is enough to desensitize the receiver without needing to mix with a second signal to create new frequencies.