Intermodulation interference between two transmitters in close proximity can be reduced by which method?

• A. By using a Class C final amplifier with high driving power.
• B. By installing a band-pass filter in the antenna feed line.
• C. By installing a terminated circulator or ferrite isolator in the feed line to the transmitter and duplexer.
• D. By installing a low-pass filter in the antenna feed line.

Answer: (C)

Intermodulation interference from two nearby transmitters happens when strong signals mix in non-linear parts of the system, such as the feed lines and duplexer, producing unwanted frequencies that can affect both transmitters. A terminated circulator or ferrite isolator placed in the feed line between a transmitter and the duplexer provides directional isolation. It routes energy from each transmitter toward the antenna while preventing energy from one transmitter from leaking back into the other transmitter’s path or into the duplexer in a way that could create intermod products. This isolation dramatically reduces the cross-coupling that causes intermodulation.

Filters in the antenna feed line, like band-pass or low-pass filters, can help suppress certain unwanted frequencies, but they don’t address the root cause: energy from one transmitter being able to couple into the other path and mix nonlinearly. Using a Class C final amplifier with high power would actually increase nonlinearity and intermod, not reduce it.