VHF Frequency Confusion

Tom Kamlowsky of Salt Lake City, Utah, asks:

“While sailing in San Francisco Bay last month, I contacted the Coast Guard on VHF channel 16. On receiving my call, the Coast Guard radio operator asked me to change frequency to channel 22. I changed both my Icom M304 and M34 radios to channel 22 and quickly found that I could not use the M34 because of interference from a local FM station broadcasting on 106.9 Mhz.

I switched off the M34, finished the Coast Guard call on the M304, and later determined that channel 22 on the M34 can’t be used in San Francisco because of the FM broadcast interference. I suspect an IMD problem with the radio. Other channels could be affected, but I don’t have any way to test them. Now I worry the radio might not work properly in other ports of call because of additional, yet to be discovered interference issues with local FM stations. Other than replacing the radio, which is out of warranty, is there something I can do to resolve the problem?”

Gordon West replies:

I may have an easy answer to this problem. IMD (intermodulation distortion) can’t be the culprit; it’s too far a spread from 106.9 MHz to 157.100 MHz. So let’s do some math: 157.100 MHz (coast guard output and your input) + 4.600 MHz (international duplex VHF channel 22, receive). This adds up to 161.700 MHz, which is assigned by the FCC to FM and TV audio remote links.

Your Icom M34 radio is in international mode, where our U.S. “A” channels are domestic simplex mode, transmitting and receiving on the same frequency. The coast guard transmission on 22 alpha was simplex, domestic receive, not international duplex receive on 4.6 MHz and up.

Put your Icom M34 radio in “domestic” or “USA” mode. Then you will hear the coast guard on channel 22A, not the FM remote link on 22 international at 161.7 MHz!

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