What is “alpha” and why doesn’t my older VHF have it?

[Scorpius]

Originally Posted by Scorpius

My understanding is:
Historically some channels are "simplex" (both participating stations send and receive on the same frequency) and therefore
a) only one at a time can transmit ("speak"), and
b) anyone listening in can here BOTH sides of the conversation.

Most channels, including ALL ship-to-ship are/were simplex - so that anyone could talk to anyone else.

However most ship-to-shore stations were DUPLEX! That is, the ship transmitted on one frequency and the shore station on another. That gave two benefits:
a) If a vessel were appropriately equipped (and the vast majority of small vessels weren't), both parties could talk and listen at the same same time - as on a regular phone or, now, on a cell phone (which operate in duplex mode), and
b) Anyone listening in could only hear one side of the conversation. Typically anyone on another boat could only hear the shore station side as the other end (the boat involved in the conversation) would be transmitting on a frequency only the shore station was equipped to receive.

Here in Canada we had quite a few duplex channels as, before cell phones, the Telcos had extensive facilities so that the maritime community (mostly commercial fishermen, those on tugs and in remote coastal communities) could connect into ("patch") the land telephone system to call loved ones, conduct business, etc. - all at a price of course.

Duplex channels were also use d by various government departments (e.g. Fisheries) and commercially in enterprises such as the big fishing companies like Canadian Fish Company and BC Packers ito provide a modicum of security and privacy

cy.

Carrying on:

The US, Canada, and the rest of the world agreed on all the channel designations and the PRIMARY frequency (usually the ship transmit frequency) for each - but disagreed on the number (and which) of channels that should be simplex and which should be duplex. In the end they agreed to disagree. Hence the same channels (e.g. 22 and 83) were designated simplex in the US and duplex in Canada (I'm leaving out the rest of the world for simplicity and out ignorance

With the advent of cell and satellite phones, etc. the Telcos have ceased providing phone "patches" via marine VHF radios - as have government departments and commercial interests - thus freeing up a bunch of channels. Many of these channels (such as 22 and 83)have been re-purposed. However, since they were duplex in Canada (and I think internationally) it is necessary to specify, if you are going to use them, that you be in US (simplex - alpha) mode - so that both ends are transmitting and receiving on the same frequency - and everybody listening in can hear both ends of the conversation.

Sorry to go on so long - but this topic is both technical and cultural and so, I believe, needs a full explanation - in context.

[StuM]

Originally Posted by Scorpius

However most ship-to-shore stations were DUPLEX! That is, the ship transmitted on one frequency and the shore station on another. That gave two benefits:
a) If a vessel were appropriately equipped (and the vast majority of small vessels weren't), both parties could talk and listen at the same same time - as on a regular phone or, now, on a cell phone (which operate in duplex mode), and
b) Anyone listening in could only hear one side of the conversation. Typically anyone on another boat could only hear the shore station side as the other end (the boat involved in the conversation) would be transmitting on a frequency only the shore station was equipped to receive.

Not always. A frequent implementation is for the shore station to have one or more repeaters on suitable high features (hill or tower) to extend range. The repeater receives both ship and shore station transmissions on the transmit frequency and re-broadcasts them on the receive frequency. That means that anyone within the coverage of the repeater can hear all transmissions to/from any other vessel in the same coverage area as well as to/from the shore station. The advantage is that this can extend VHF communications out to 20 nm or so from the repeater allowing vesssels 40 NM apart along a coast to communicate with each other as opposed to 10 NM or so typical for line-of-sight simplex communications.

[deblen]

Originally Posted by Scorpius

Carrying on:

The US, Canada, and the rest of the world agreed on all the channel designations and the PRIMARY frequency (usually the ship transmit frequency) for each - but disagreed on the number (and which) of channels that should be simplex and which should be duplex. In the end they agreed to disagree. Hence the same channels (e.g. 22 and 83) were designated simplex in the US and duplex in Canada (I'm leaving out the rest of the world for simplicity and out ignorance

With the advent of cell and satellite phones, etc. the Telcos have ceased providing phone "patches" via marine VHF radios - as have government departments and commercial interests - thus freeing up a bunch of channels. Many of these channels (such as 22 and 83)have been re-purposed. However, since they were duplex in Canada (and I think internationally) it is necessary to specify, if you are going to use them, that you be in US (simplex - alpha) mode - so that both ends are transmitting and receiving on the same frequency - and everybody listening in can hear both ends of the conversation.

Sorry to go on so long - but this topic is both technical and cultural and so, I believe, needs a full explanation - in context.

Thanks for the detail.My only point of possible disagreement regards Canada-east coast.
To my knowledge,the typical "all channel" VHF transceiver that we are familiar with today, became available on the east coast of Canada in the late 1970's. I worked on just about every brand available over the years.
99% of them came with a US & Intl mode switch. In later years,a few came with a Can. position on that switch.
If you left the switch in US mode,all the Alpha channels operated correctly as simplex(alpha) channels. This included 22A & others.
There was only one reason to switch to Intl mode. There were a couple of CCG wx broadcasts,that were not always available on WX1-9. You could not transmit on these wx frequencies.


I cannot imagine that there are many pre 1980 VHF txcvrs left operating in Canada that would not have a US mode position.
Leave it set to US mode-problem solved / Len