VHF, OK, BE HONEST, DO YOU KNOW...

Do you know that your VHF frequencies are simplex or duplex? I felt pretty dumb a few years ago when I thought my new hand held was stuffed because it didn't seem to work on all freguencies. I felt better when I found out almost no one else did either! Not even the distributor for the radio!

Are American sailors better informed? I wonder. Check this link out and let me know. But be honest!! http://www.thecoastalpassage.com/vhf.html

The link contains a chart you may wish to save whether or not you know...

Cheers

[CaptainK]

Simplex or Duplex?

Hell Bob!! You deffinately opened a can of worms there ole' buddy!!

I would be interested in knowing more about this topic as well!!

Thanks for bringing this topic to the forum, Bob.

To the best of my knowledge, US marine VHF is primarily simplex. Duplex operation refers to radios that transmit on one frequency, and receive on another. This enables the use of repeaters. Since standard marine VHF radios are unit to unit, duplex is not necessary.

Greetings

Hey K... I was wondering. This subject was reserected by a local chandlery that had taped the article to a counter top. I was there the other day and they commented how many times they had lifted the tape up to photo copy the article and chart of freq's for everyone from charter skippers to the guys running the local Volunteer Marine Rescue!! (Airlie Beach) A small thing that isn't commonly know is all, even by sailors that have many years experience and it can be a significant safety issue.

Kai Nui.. according to my info, 9 of the freq's on the USA scale on your VHF are duplex.

I read the article after I posted. I was surprised to find that out. Duplex won't do much good offshore. Actually, You should add to the article, the correct use of those frequencies.

Here is a link to the frequency allocations http://www.navcen.uscg.gov/marcomms/vhf.htm
Most of the duplex freq's are for marine radio telephone.

Hey Kai Nui... Thanks for the link, i will include that on the web site for American visitors. The freq's aren't alocated so much here in OZ, I assume due to much less trafffic. BUT... I should find out where they are and post that info. Around Sydney harbour or some such, there must be a strict protocol for which is reserved for commercial shipping, etc. I admit it is sloppy up here. Once you get a few miles away from Airlie Beach there isn't likely to be much of anything around except one or two other dumb yachties! And I get away with murder cause I just use my thickest yank accent!. Then nobody expects me to know nothin!!

I just worked out the time difference.... U B nite owl!

Cheers

this is the second try.... the program screwed me out of a more lengthy reply. thanks for the link! I will use it.

Cheers

[btrayfors]

All U.S. VHF marine frequencies are SIMPLEX (same transmit/receive frequency) EXCEPT for:

(1) public correspondence (marine operator) channels 24-28 and 84-86; and

(2) channel 20 which is for port operations (duplex).

The two AIS channels are also simplex.

Much more use is made of duplex (different transmit/receive frequencies) outside the U.S.

By contrast, ham radio makes extensive use of duplex operation on VHF and UHF channels. There are thousands of "repeaters" located all over the U.S. and in other countries which extend the useful range of VHF to 30-100 miles or more. In Maine, and in other locations, these repeaters are "linked" so that with a VHF handheld you can communicate with another amateur radio station located several hundred miles away.

Bill
WA6CCA
S/V Born Free

Bill, you are correct. Not to mention, many of the newer uhf/vhf ham rigs have the ability to convert to a repeater and will alow you to have an instant repeater on the boat for your HT's while exploring. My old Alinco DJ590, before it was stolen, had that ability.
Bob, yes, I rarely sleep. It is interesting how few people know the proper use of all of the channels. Most know 16,9,22, and 68, but that's it. It is also interesting that most of the marine radios that are sold in the US do not include any reference to that information.

[Talbot]

As has been said already we make much more use of duplex for port operations etc.
A good table for all the channels is here

As a leisure user I have use of 16 plus the following as working channels:

06 (also designated as a maritime/air channel for distress working)
08
09
10
13
15
17
67 (reserved for coastguard working)
69
72
73
77

plus of course those port operation channels that are relevant to my area

plus 2 channels for marina use 37 and 80 (sometimes designated M1 and M2)

Greetings Bill

Many thanks for the response. The info you gave in addition to the list that Kai nui gave suggests that the info I had on ch 20 may be in error. The list I got was from the manufactorer of the radio! It may be possible that radios made for non-US import may not match exactly to US standard? Who knows but in any case I will copy paste the material from the links provided from Talbot and Kai Nui (thanks!!) onto the web site now.

Cheers

[hellosailor]

Bob-
"It may be possible that radios made for non-US import may not match exactly to US standard? " Quite. My antique Standard Horizon has a secret button sequence that will change it from US to international frequencies and pairs, some channels are even marked with an "A" suffix to designate them as using the "A"merican frequency assignment rather than global.

With only one radio--the traditional limit of equipment on small craft--duplex is simply useless unless your radio can handle "real" duplex like a cell phone does. In VHF radios that's possible...but AFAIK it doesn't happen in marine handhelds. At least, not in the US market.

And since, agian AFAIK, the use of repeaters is not authorized for civilian marine VHF in the US...the whole subject is something most US sailors would never need to know of. That's why our marine VHF radios have channels instead of frequencies and offsets and sidetones. (All of which make good ways to ensure a total communications failure.<G>)

[coot]

Originally Posted by hellosailor

My antique Standard Horizon has a secret button sequence that will change it from US to international frequencies and pairs, some channels are even marked with an "A" suffix to designate them as using the "A"merican frequency assignment rather than global.

When the US channel is simplex and the international allocation is duplex, the ship-transmit frequency is named A and the shore-transmit frequency is named B. So, for example, channel 22 is a pair of frequencies, 22A and 22B. The radio doesn't have 22B because the B channels are not used in the US and you use A/B together when following the international standard.

In the US, all the channels have designated uses. As a recreational boat, you are only supposed to use 69, 71, 72, and 78A for regular communications, 16 or 9 for calling, and 13 for bridge-to-bridge traffic.

You can use other channels if you are talking to somebody who is using those channels for the designated purpose. e.g. You can use 22A to talk to the coast guard, but you aren't supposed to use that channel for anything else.

Since you mostly use the designated non-commercial channels, the question of duplex channels doesn't really come up.

[rsn48]

In answer to your initial question, did I know about simplex and duplex channels, I would say, "yes" I did know about it for about 10 minutes then flushed it from my brain as useless knowledge. In Canada a license is required to run a VHF radio. I just took the course about a year ago, so I grabbed my course book - Maritime Radio Course, Canadian Power & Sail Squadrons, 1st edition 2004 - and tell you what it says. The reason I flushed the info out of my brain was that my Standard Horizon Quest is simplex and the majority of VHF radios are simplex so I decided it was a waste of time to remember it: (I now quote the manual)

"2.7.2 Channel types and designations

A marine VHF radio channel may be a simplex channel or a duplex channel. A simplex channel is a channel on which a radio transmits and receives on the same frequency. If two stations attempt to transmit simultaneously, the frequency will be jammed for all users.

Simplex channels may be used for both ship-to-ship and ship-to-shore communications subject to the restrictions identified by the regulatory body governing channel usage in the area of operation.

Some channels are designated for duplex operation. A duplex channel is a channel on which a radio transmits on one frequency but receives on another frequency. For example Channel 24 (Canadian setting) is a duplex channel. When tuned to Channel 24, a vessel's marine VHF transmits on a frequency of 157.200 MHz, but it receives on a frequency of 16.800 MHz. It is convenient to think of the receive frquency as being offset or shifted 4.6 MHz above the transmit frequency on any duplx channel.

Duplex channels may only be used for ship-to-shore communications with authorized coast stations whose transmit and receive frequencies will be opposite to those of the vessel. On a duplex channel, an authorized coast station will recived on a vessel's transmit frequency, and transmit on the vessel's receive frequency. It should be readily apparent that duplex channels cannot be used for ship-to-ship communications.

Why duplex channels? The marine VHF radios typically used by recreational boaters are only capable of "semi-duplex" opration. They are able to transmit on one frequency and receive on another, but they are "incapable of doing both at the same time."

A channel containing the letter "A" as a suffix (sometimes called an Alpha channel) is the simplex counterpart of the duplex channel without the "A" suffix. For example, 22A is the simplex counterpart of duplex Channel 22. The duplex Channel 22 and simplex channel 22A have the same transmit frequency, but Channel 22 has a shifted receive frequency while 22A does not.

Simplex "A" channels are generally only used in Canada and the United States, and their used is normally not recognized or permitted outside of North America.

Unique to Canada channel group are four channel numbers followed by the letter "B" on which marine VHF radios do not have a transmit capability. These channels are used by CCG for "continuous marine broadcasts."