Listening to Marine Band radio
by David Harvey VK2DMH on Thursday, July 16th, 2009 | 1 Comment
I have a marine band radio, and I like to listen in to boats and ships at sea.
I have discovered that I can hear Coast Guard stations well enough, but when a vessel responds to the shore station either I cannot hear it at all, or else the remote signal is too weak or garbled for me to understand.
I have been wondering if the Coastguard were scrambling their signals or something.
Any web sites with any info on marine band listening would be helpful.
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You haven’t specified that you are listening to shortwave signals (not VHF), but it sounds that way to me from what you said. So I’m assuming you are listening in to marine broadcasts in the MF and HF bands. They have channels all over the place between 500 KHz (0.500 MHz) and about 20 or 30 MHz tops.
Many of the channels use one frequency to transmit and a different one to receive. So unless you have the same kind of set (very expensive) you will only ever hear one channel.
The shore stations are coming in strongly because they are probably closer to you and have larger, more efficient antennas than the boats or ships can fit on their craft. Also, those are single-sideband radios, not AM (amplitude modulated or old-fashioned double-sideband). So if you listen with a shortwave set that has only AM reception, you have the right channel but you aren’t getting the whole signal… It sounds like Donald Duck talk, not normal speech.
Most HF radios use USB or upper sideband to transmit. You can listen to these signals if your radio receiver has a BFO or beat-frequency oscillator. It will inject the missing noise back into the received sideband audio so you can hear it properly. It takes a bit of practice to be able to adjust it quite right. But it isn’t scrambling or encryption.
VHF marine radios use FM signals and work between 156.050 and 162.025 MHz.
You can start reading up on Marine Band radio here: http://www.navcen.uscg.gov/marcomms/