Question 3-56G1

Intermodulation interference (IMD) occurs when two or more signals mix in a non-linear device, generating new spurious frequencies. When two transmitters are close, one transmitter's strong signal can enter the output stage of the other, causing mixing and IMD. A **terminated circulator or ferrite isolator** (B) is a unidirectional device installed in the feed line. It allows the transmitter's signal to go out to the antenna but shunts any signals coming *back* from the antenna (like the nearby transmitter's signal or reflected power) into a dummy load. This prevents the external strong signal from reaching and mixing in the non-linear final amplifier stage of the transmitter, effectively reducing or eliminating IMD generation. Using a Class C final amplifier (A) would *increase* intermodulation, as Class C amplifiers are inherently non-linear and designed for constant-envelope modes, not for linearity. A band-pass filter (C) in the antenna feed line primarily helps suppress out-of-band emissions from the transmitter or protect a receiver from out-of-band signals. While it might attenuate strong *out-of-band* signals from a nearby transmitter, it doesn't directly prevent in-band signals from entering the transmitter's output stage and causing IMD like an isolator does. A low-pass filter (D) primarily suppresses harmonics, which are integer multiples of the fundamental frequency, not intermodulation products generated by mixing different signals.