VHF CH 80

[mikenda - delete]

Can someone please explain to me how VHF Channel 80 works? I understand that it is a duplex channel that has a freq. for receive and a separate Tx freq. I can’t get my head around how this operates ,does the shore based receiver have a different configuration to the vessel? Can you call another ship on a standard receiver?

 

[rogerm]

Well actually I though channel 80 was Simplex ie transmit and recieve on the same channel. However you are right about Duplex (which I thought were only the channels used for telephone connection), the shorebased end is the opposite configuration to the boat end.
Roger

 

[Rob_Webb]

Ch80 is duplex which means the shore-based station is configured in the opposite direction to your vessel's radio for this channel. So you can speak to the marina and he can speak to other yachts - but the frequency 'swap' means that you can't use this channel for communicating to another vessel. Hence it will often sound as though the marina is having a conversation with no-one - many people assume that the boat he is talking to is simply out of range - not so, you just cannot hear him on this ch!

You need to use a ship-to-ship channel instead.

Rob

 

[gtmoore]

I wonder - is there any advantage to this set up? Or is it just the way the channels were originally configured?

 

[Bergman]

I can se no advantage in duplex, or more correctly semi-duplex.

Originally it was used where the shore station connected to telephone line or possibly relayed through separate transmitter.

Don't know why 80 is used for marinas 37 seems a better channel to me. Useful to hear who else is moving about, no confusion over who is talking to whom.

 

[VMALLOWS]

Ch 80 was originally allocated for ship-to-shore telephony, where Duplex (or even semi-duplex) give 'two-way' telephone-like communication. Landlubbers had difficulty getting used to 'one-way' OVER type communications. Now these channels are 'spare' they are being reallocated to other uses. ( The channel allocation as duplex or simplex is subject to international agreement and is pre-programmed into the set...hence has to remain).

Incidentally, many channels that are duplex in Europe (and most of the world) are simplex channels in USA. This leads to the interresting fact that if you have a radio with a USA/INT facillity (often found on hand-helds) you can listen to both sides of many duplex channels (inc Ch80) by switching between USA/INT!

On several occaisons I've had to explain to boats trying to use duplex channels why they couldn't hear each other!

 

[oldsaltoz]

G’day Mikenda.
Duplex operation was explained to me as follows and might help you understand it better, I just hope it’s correct. Many duplex set ups are used / created, in areas that signal strength is a problem; put simply, your fixed radio puts out a signal at around 5 watts, a hand held less than half that. The base station picks up the signal and re transmits it at around 25 watts giving it a much bigger range, this re transmission is on another frequency to avoid talk over.
Hope this helps.

Avagoodweekend Old Salt Oz……

 

[Chris771]

Sorry Old Salt, what you are taking duplex to be is use of a transponder to convert frequency and re-radiate at a higher power output.

In this system everyone transmits on the same frequency. This is received by a "transponder" which amplifies and re-radiates the signal on a different frequency, so you transmit on one frequency and receive on a different frequency (in the same way as duplex, BUT everyone transmits on the same frequency and everyone listens to the same frequency, so the effect is "Simplex"). We use this system in Malabo where the transponder is on top of the mountain and a 5 watt handheld can be used from an offshore platform some 40 miles offshore and one can talk to another person on a handheld in Douala over 100 miles away all on UHF handhelds.

Duplex operation, as correctly stated in the posts above, is where the offshore station transmits on one frequency and the shore station transmits on a different frequency. So shore stations can only hear offshore stations and offshore stations can only hear shore stations on that channel, (unless you use some of the US channel allocations in Europe where as also correctly stated these channels are allocated as simplex). On large ship installations the vhf uses two quite widely spaced aerials so that one can keep the microphone button pressed all the time during a "link" telephone call to a shore subscriber and the remote receiver aerial is far enough away not to be affected by the local transmission and so a normal telephone conversation can be held with both parties able to talk at the same time.

Whilst the vhf transceivers that we use on small craft have only one aerial we still have to use semi duplex mode, where we press the microphone switch to transmit on the ship frequency and when we release the PTT switch the receiver receives the shore frequency. If you had a second vhf with the receive aerial at the opposite end of a largish (50ft or so) craft, you would be able to have full duplex conversations by transmitting on one vhf and listening on the other simultaneously. On smaller craft the close proximity of transmit and receive aerials would cause feedback, cross coupling and howl problems, hence why we have to use semi duplex.


Chris

www.impact-computers.net/boat/cruiser.htm

 

[alex_rogers]

How do you configure your radio as the shore side station? Do modern VHF have an option for this somewhere or if you are setting up a shore side station, do you have to buy a dedicated radio?

 

[Rob_Webb]

I think it's more a case of ch80 being a conveniently free channel to allocate for Marina use rather than because it's duplex. But at least this frees up ch37 for clubs/racing use where I suppose it is arguably more important for other boats to be able to hear each other than prospective bertholders.