During a club Field Day outing, reception on the 20-metre SSB station is compromised every time the 20-metre CW station is on the air. What might cause such interference?

Receiver desensitization (or 'desense') occurs when a receiver is exposed to a very strong, nearby RF signal on a closely related frequency. The strong signal from the CW station triggers the SSB receiver's Automatic Gain Control (AGC) or pushes its front-end amplifier into non-linearity, effectively lowering the receiver's sensitivity and wiping out weaker signals. Since both stations are operating on the same band (20 metres), harmonic emission is not the culprit, as harmonics would appear on higher frequency bands (like 10 metres). Grounding and generator issues would typically manifest as electrical noise or hum, not RF interference specifically tied to a simultaneous transmission.