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Old 03-08-2014, 11:07   #16
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Re: Three Hand-Held VHF's None Work in Duplex?

Skipmac -- working on a response for you - will take a few minutes.
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Old 03-08-2014, 11:40   #17
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Re: Three Hand-Held VHF's None Work in Duplex?

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Originally Posted by skipmac View Post
If you are on a channel that is duplex how can you transmit on one frequency and then other stations on the same channel receive that transmission on a different frequency, unless the transmitted signal goes to a shore station to be retransmitted on the receive frequency.

But I thought there were HF bands on HAM or SSB that used duplex channels. How does that work or am I mistaken?
Let's start with some vocabulary so we are all on the same page.

A frequency is a specific "place" in the radio spectrum. It is measured in cycles per second or Hertz (Hz). Kilohertz (kHz) are thousands of Hertz; megahertz (MHz) are millions of Hertz; gigahertz (GHz) are billions of Hertz. For example 156.8 MHz is always the same place in the spectrum, regardless of what else you may call it.

Bands are ranges of frequencies that are used as a convenient description. Marine SSB bands are actually a number of discrete ranges of frequencies, as are ham bands, weather fax bands, air bands, etc. If there is any question just look at the bounding upper and lower frequencies for a "band."

Channels are another convenience label. There is nothing magic about the numbers. They are all made up and agreed for convenience. Marine VHF channel 16 on 156.8 MHz has nothing to do with CB channel 16 on 27.155 MHz which has nothing to do with television channel 16 on 483.25 MHz (for the video carrier). It's all just a convenience. If it helps you can think of channels like house numbers. 12 Main St has nothing to do with 12 Oak St even though both house numbers are the same.

Are we having fun yet? *grin*

Now lets talk about simplex and duplex.

In the context of radio communications, simplex just means that everyone talks and listens on the same frequency. Only one person can talk at a time; everyone else listens. Depending on the technology and relative signal strengths, two stations talking at the same time will either result in garbling that no one can understand or the strongest signal "winning." Regardless, keying up on top of someone is a fruitless exercise.

For purposes of this discussion I'm going to inject some more vocabulary: ship stations (in our case boats) and coast stations (which may be either shore stations or repeaters). In a duplex station the ship stations transmit on one frequency (we can call that 'A' since that is what the USCG and US FCC call it) and listen on another nearby frequency (which we can call 'B'). We have all heard reference to and may have used marine VHF channel 22A for communication with the USCG. That is the ship transmit side of a duplex channel; in the US 22A is used as a simplex channel and 22B is ignored.

With a lot of complex equipment (circulators, cavities, and filters) you can build a radio station that can simultaneously transmit on one frequency and receive on a second nearby frequency. You aren't going to see this gear on a boat but it is fundamental to the operation of a repeater and to high-end shore stations.

The intent of marine duplex channels is entirely for the communication of individual ships at sea and coast stations. There was no intent for ships at sea to talk to other ships at sea. A ship coming into harbor could call the harbormaster or pilot on the A side (frequency) of the channel and would listen on the B side (frequency) of the same channel. The coast station would listen on A and transmit on B. This had several positive benefits including the ability of the coast station to hear the ship even if the coast station was transmitting (if properly setup) and better support for ship-to-shore telephone calls, where supported.

Repeaters are automated coast stations that use circulators, cavities, and filters to allow them to continuously receive one frequency ('A') and transmit in near-real-time (there are some milliseconds of latency) everything heard on 'A' on a second frequency ('B').

As noted there are a few locations around the world where repeaters are installed that do mean everyone (ship and coast) transmits on A and receives on B. The repeater receives on A and retransmits everything on B. A mountaintop or tall tower location for the repeater greatly extends range for everyone.

Marine VHF radios manage all the duplex issues for you. The U/I/C (US/International/Canada) settings manage all the complexities for you. For example, in International mode marine VHF channel 22 is a duplex channel. In the United States marine VHF channel 22A is used as a simplex channel and 22B is ignored.

On HF duplex is more complex on the shore side. Circulators, cavities, and filters aren't very practical due to size so remote receive sites are used. AT&T High Seas radio had a huge infrastructure. To my knowledge Shipcom and other modern day successors use simplex.

I hope this helps.
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Old 03-08-2014, 12:25   #18
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Re: Three hand-held VHF's none work in duplex?

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Originally Posted by StuM View Post
They are a number of changes coming:
]So, the new channels will be numbered:[/I]
1078 2078 1019 2019 1079 2079 1020 2020
So will we need to buy new VHFs?


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Old 03-08-2014, 12:43   #19
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Re: Three hand-held VHF's none work in duplex?

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So will we need to buy new VHFs?
Probably not.

First ITU agreements need to be legislated in country law or regulation. Manufacturers need to catch up.

Some radios will be able to be updated with new firmware.

I think the biggest issue is going to be display space for 4 digit channel numbers.

Why couldn't they keep using A and B? *sigh*
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Old 03-08-2014, 13:04   #20
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Re: Three Hand-Held VHF's None Work in Duplex?

Skip,
Dave gave you a good explanation of the how's and why's of simplex/duplex, etc....and while he left out some details, I fear adding much more would just complicate things for you (and others here)....


But, as someone that built/owns/operates/maintains VHF repeaters myself, my 2m ham repeater is KA4WJA/R on 146.970/146.370mhz (and mine and Mario's 220 repeater is N4TSV/R on 224.1/222.5mhz and we maintains KA2MBE on 444.325/449.325mhz as well), I'd like to add that it isn't just the "complex equipment" that Dave mentioned, but a significant amount of actual engineering that goes into these systems...so much so that dozens and dozens books have been written, and many classes taught at universities, on just these subjects (duplex radio operations)....
Quote:
Originally Posted by Auspicious View Post
With a lot of complex equipment (circulators, cavities, and filters) you can build a radio station that can simultaneously transmit on one frequency and receive on a second nearby frequency. You aren't going to see this gear on a boat but it is fundamental to the operation of a repeater and to high-end shore stations.

Repeaters are automated coast stations that use circulators, cavities, and filters to allow them to continuously receive one frequency ('A') and transmit in near-real-time (there are some milliseconds of latency) everything heard on 'A' on a second frequency ('B').
No measureable latency at all in analog systems!!! (but those talking on modern digital cellular phones, understand the dreaded digital latency!!)
FYI, these are the exact same principles, procedures, engineering and equipment that go into ALL mobile phone systems, ALL cellular/3G/4G/LTE/GSM, etc. since the 1940's....
Yes, that is correct, not a typo.....since the 1940's!!!

Although the exact frequencies, modes, etc. have changed....and recently we've done away with all consumer analog mobile phone systems, there are still some commercial/business analog mobile radio/mobile phone systems still in operation, and many 1000's of ham radio "autopatch" interfaced vhf/uhf repeaters (including mine) operating 24/7/365, allowing users within range (typically 50 - 75 miles), to place non-business public phones calls (and in some systems, receive calls as well)....

Understand that ALL cellular/mobile phones, smart phones, etc. of ALL types, ALL brands, ALL systems, worldwide.... are all handheld duplex radios and work with a fixed base-station / repeater to interface with the Public Switched Telephone Network (PSTN), and/or connect to other cellular/mobile phones....
(yes, they are all handheld duplex radios...)







On HF, actually it's not more complex at all....it's just bigger and takes up acres of space!!
Quote:
Originally Posted by Auspicious View Post
On HF duplex is more complex on the shore side. Circulators, cavities, and filters aren't very practical due to size so remote receive sites are used. AT&T High Seas radio had a huge infrastructure. To my knowledge Shipcom and other modern day successors use simplex.
Shipcom / WLO does use duplex frequencies and has "full-duplex" filtering and antenna separation....(although not quite as sophisticated as the old. defunct AT&T systems, with dozens of antennas and multi-couplers at receive sites, located miles apart from their transmit sites...)
I cannot speak for Brunei Bay Radio, Monaco Radio, etc...but it's possible that Brunei uses "semi-duplex" (separate tx/rx freqs, but does not have the ability to receive at the same time they are transmitting)???





Bottom line here regarding maritime "duplex" channels, both HF and VHF, is that the rapid growth and expansion of "consumer electronics" (cellular phones, etc.), increase use of "text"/"data" communications and decrease in Voice comms, decrease in costs/complexity of satellite communications, etc. have all combined to render many/most of these "duplex" channels vacant / unused...
Yes, there are still some "public correspondence" Marine-VHF stations in Europe, etc. and yes, there are the Sea-Tow's automated radio-check system, both on VHF......and yes on HF, there are still some users of these duplex channels (WLO, KLB, Monaco, Brunei, Aus/NZ, etc. as well as the USCG for their weather broadcasts and working channels...)...
But, with dozens and dozens of unused HF duplex channels worldwide, (and many unused VHF duplex channels in most coastal locations), I see changes coming in the WARC....



I hope this helps a bit...

Fair winds..

John
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Old 03-08-2014, 14:00   #21
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Re: Three Hand-Held VHF's None Work in Duplex?

Skipmac,

One question you asked was about duplex operation and how others could hear the conversation. The simple answer is that when a ship is talking to the shore station other ships don't hear that part of the conversation. Because the ship radio is listening to the shore frequency they only hear the shore station part of the conversation. That is a normal scenario for duplex transmissions. Duplex allows both sides to talk at the same time like a telephone call. In simplex we can hear both sides of the conversation but because both use the same frequency only one side can talk at a time.
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Old 05-08-2014, 08:17   #22
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Re: Three hand-held VHF's none work in duplex?

prof_mariner,
I know you've solved the "mystery", but just saw your comment here and thought someone should highlight this, as it might be helpful to many here when discussing electronics of all types...
Quote:
Originally Posted by prof_mariner View Post
Testing duplex with two ocean based radios won't work. I suspected I was forgetting something. Funny how the electronics expert at West Marine didn't think of it either.
Thanks, mystery solved.
--- FYI, in the electronics / communications industry (where I've made my living for the past 30+ years), we used to say the definition of an "expert" was "an a***hole with a brief case"....

But now-a-days, we say it's "an a**hole with a smart phone"....

--- The point I'm making is this....ANYONE can be an "expert"....and especially when they're trying to sell 'ya something!!



--- MY second point is, there are NO "electronics experts" at West Maine....at least none that I'm aware of, anywhere in any store....(perhaps the guy that can beat the others when playing video games on their smart phones, is the one they call and "expert", but in fact there are none...)

But if you have questions, have an issue/problem, need advice, etc....my best recommendation is to post your queries on the SSCA Discussion Boards, and if you are interested in learning how some of the new marine electronics gear actually works on-board, check Panbo.com....

These sites have some fairly knowledgeable folks who won't BS you, and their egos rarely get in the way of the facts....
SSCA Forum • Index page

SSCA Forum • View forum - Communications

Panbo: The Marine Electronics Hub



--- And finally, in all good humor, with a name like "prof_mariner", you should've known this...


Fair winds...

John
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Old 30-08-2014, 15:41   #23
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Re: Three Hand-Held VHF's None Work in Duplex?

Just to help make it a little easier to understand....

A shore based radio has it's duplex channels reversed. It works upside down.

So, it transmits on your listening frequency and you listen on its transmit frequency and vice a versa.
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