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Old 08-31-2000
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VHF Duplex Channels

On a recent trip to the Bahamas, I had trouble talking to other people on the VHF because of something called duplex channels. What are these?

Tom Wood responds:

Your problem wasn’t entirely caused by duplex channel use—we’ll come back to that later. The problem in the Bahamas was probably due to the settings on your radio. You, and the people you were trying to communicate with, more than likely have the ability to choose USA, CAN (Canadian), or INT (International) on your radios. On many radios, this will default back to one setting or another every time the radio is turned off, and has to be reset if a different feature is wanted each time it is turned back on. The USA, CAN, and INT standards are different, sometimes using entirely different frequencies for the same channel number, and sometimes using duplex frequencies instead of simplex.

So now, let’s talk about simplex and duplex. On simplex channels, the sending and receiving frequencies are the same. Channel 16 is universally designated simplex channel on 156.800 MHz. On duplex channels, however, the radios actually transmit on one frequency and receive on another. For instance USA Channel 24 (a radiotelephone Operator’s channel)transmits on 157.250 MHz and receives on 161.850 MHz.

What I suggest was probably happening to you in the Bahamas was that you were set to USA configuration while your buddies were set for INT, or vice versa. The receive frequencies may have been the same so you could hear each other, but their transmit frequencies were different from yours. You don’t have to do anything special to use duplex channels – the radio does all the work for you. But you and the person you’re trying to talk to both have to be set to the same standard.

 

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