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Author Topic:   VHF Marine Band Channels
jimh posted 12-17-2011 07:30 AM ET (US)   Profile for jimh   Send Email to jimh  
Use this thread to discuss the REFERENCE section article on VHF Marine Band Channels. The channel designators never made any sense to me, so I undertook a brief study of the channelization of the Marine Radio band in order that I could at least have a fundamental understanding of how it was carved into such a crazy array of channels. Perhaps others will also find my article adds clarity to the channel number designations.
dfmcintyre posted 12-18-2011 08:06 PM ET (US)     Profile for dfmcintyre  Send Email to dfmcintyre     
Jim--interesting. Aside from the split freq channels, I always wondered why they couldn't have just used the aviation frequency use concept instead of assigned a multitude of "channels." Perhaps the person or group that came up with the channel concept decided that boaters couldn't remember 156.8 instead of Channel 16. Pilots don't seem to have any problem with two more numbers and a period.--Regards - Don
jimh posted 12-18-2011 10:52 PM ET (US)     Profile for jimh  Send Email to jimh     
Don--That aviation uses frequency to designate channels is interesting. However, they also use amplitude modulation, which is a bit of step backwards compared to frequency modulation. Use of AM is historical in aviation. Ships often used CW, in part perhaps to the longer range to the home office and bandwidth restrictions in medium frequency ranges.

In the Medium Frequency Marine Band I believe frequency is also used to designate operation. For example, everyone knows that 2,182-kHz is the primary calling channel for MF Marine Band. I don't think channel designators are used there.

The channelization of VHF [and here I should clarify that I mean the reference to the operating frequencies by their channel designators not by their actual frequency in megahertz] might be related to the earlier technology used in the equipment. Most early radios were crystal controlled, and a particular vessel might only have a radio with a few channels available, say the primary channel (16) and its preferred ship-to-shore channel for contacting its home office. Having a radio that was agile enough to be able to tune to all the channels is a relatively modern development. I remember the sailboat radio we had c.1986 was only able to hit about 12-channels. [Early radios just had a few channel positions, and you wrote in what the channel number was.]

David Pendleton posted 12-19-2011 11:48 AM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
I was under the impression that VHF "channels" were intended to keep people from transmitting all over the band.
jimh posted 12-19-2011 04:57 PM ET (US)     Profile for jimh  Send Email to jimh     
Dave--Channelization is always for that purpose: to coordinate use of the spectrum to minimize interference. I think the only spectrum not channelized is the allocations for radio amateurs, although those allocations do have strict band plans regarding use and emission type.
jimh posted 12-19-2011 05:13 PM ET (US)     Profile for jimh  Send Email to jimh     
Many services use channels. The one most people are aware of is the Television Broadcast Service. The channelization of television was in 6-MHz channels, beginning at 48-MHz. However, 50-MHz to 54-MHz, which would have been part of Channel-1 of the TV band, was already allocated to another service in the United State of America, so Channel-1 was immediately dropped in the USA band plan. Channel-2 is 56 to 60-MHz, but no one ever refers to television channels by their frequency; the channel designator is used.

With the recent shift to a new standard for television transmission, all stations on Channels 2 to Channel 6 (or VHF TV Lo-band) had to move out, as that spectrum was being vacated by television to be sold off by the FCC to other services.

Now television broadcast stations refer to themselves with their virtual channel identities which are derived from their old frequency channel assignments, and any station on "Channel 2" through "Channel 6" is now actually transmitting on another frequency. For example, in my area, "TV-2" moved from actual channel-2 to actual channel-7 digital, but still identifies as "TV-2". There was a station operating on channel-7, but they chose to move to UHF for their new signal, and now TV-7 operates on channel 41 digital.

I am certain televisions viewers would be totally confused if they had to tune to a TV broadcast by frequency instead of by channel, and now by virtual channel identification.

Another problem for aviation was the huge size of the allocation for their band. There are something like 700 channels allocated. With so many channels the use of channel numbers would be almost as cumbrous as the frequency itself, and there would be a risk of incompatible numbering among radio sets.

Tom Hemphill posted 12-19-2011 06:04 PM ET (US)     Profile for Tom Hemphill  Send Email to Tom Hemphill     
I was unaware "narrowbanding" of the marine channels is under consideration. If that were to occur, would all existing radios become obsolete?
jimh posted 12-19-2011 06:12 PM ET (US)     Profile for jimh  Send Email to jimh     
Tom--I have no answer for you. Check the link I give in my article to the USCG NavCen web page where there is some information. My impression is that the International Telecommunication Union (ITU), which is the global regulating body of frequency coordination, may have authorized a new band plan, but I don't have any information about a schedule for implementation of it.

There are some anecdotal reports that some current production VHF Marine Band radios already have the 12.5-kHz channel spacing band plan stored in their firmware, and that users who possess specialized knowledge can unlock that feature. However, I know nothing about that myself.

My main interest in the article is just to understand the very odd channel numbering organization of the current band plan.

David Pendleton posted 12-19-2011 07:27 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
Slightly off the topic, but do aviation radios allow any frequency (within their allocation) to be selected (and transmitted on) as Don suggests?

jimh posted 12-20-2011 11:24 AM ET (US)     Profile for jimh  Send Email to jimh     
For use in most radio services other than Amateur Radio, the transmitters must be either type-approved or type-accepted by the FCC. I suspect that for use in any radio service where operation is defined for specific channels, that any transmitter sold for that service that was type-approved or type-accepted would only be able to transmit on the specified channels. The only continuously-variable tuning radios I know of are usually for the Amateur Radio service. In the Amateur Radio service the transmitters do not have to be type-accepted or type-approved by the FCC, but they must meet emission purity and stability requirements, even if home-made.

On that basis, I suspect that radio approved for use in the Aviation Service will only be able to tune to the frequencies of the channels used in the band plan. I don't know that for a fact. Maybe Don--an avid aviator--can comment.

dfmcintyre posted 12-29-2011 07:14 PM ET (US)     Profile for dfmcintyre  Send Email to dfmcintyre     
Dave--Within their allocated aviation band, yes. Some of the frequencies used are used exclusively for navigation; the VOR (VHF Omnidirectional Radio) and ILS (Instrument Landing System) for example (there are more devices) are essentially a one way transmission from the ground to the aircraft.

I've never tried tuning a aircraft radio to transmit on one of those frequencies. Thinking back, I don't believe you can. Most a/c radios are Comm or Nav or in most cases a combination of the two (NavComm) The lower part of the band is from 108 to 117.975 containing 200 channels. This band is the one I mentioned in the previous para. For talking between approach, tower, departure, clearance, ground, uncontrolled airports, aircraft to aircraft, there are an additional 720 channels from 118.000 to 137.000.

The military uses UHF-AM band from 220.0 - 399.95 MHz for the same.

For example, prior to liftoff from Port Huron to fly to Pontiac, I'll check the current weather on 118.375. I'll announce departing on a particular runway on 123.050. Then switch to the local radar (119.600) facility for assistance in alerting me of other aircraft in the vicinity. When getting close to the Detroit radar coverage, I'll switch to 127.500.

When I'm getting within 15 miles of Pontiac, I'll check their weather and what runways are active for the day on 125.025. Then announce on 120.500 who I am, where I'm at, inbound and that I have the current weather. The tower will sequence me among other aircraft they are controlling in their airspace. Once landed, I'll switch to ground control (121.900) for directions to where I want to go.

Seven different frequencies for my little 20 - 25 minute trip. Sounds confusing with placed on paper (or screen...), but in reality, very easy.

Regards - Don

David Pendleton posted 12-30-2011 03:51 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
Thanks, Don. Interesting stuff. I have a VHF/UHF radio in my truck; I eavesdrop on the airport all the time.
dfmcintyre posted 12-30-2011 08:38 PM ET (US)     Profile for dfmcintyre  Send Email to dfmcintyre     
What airport? And, what type of [radio]? VHF/UHF? Ham band? Tuneable? If your reception is somewhat "fuzzy", keep in mind that the aviation radios are still using AM as opposed to FM modulation. Best - Don
David Pendleton posted 12-30-2011 08:50 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
The Minneapolis/St. Paul (MSP) is only a half-dozen miles or so from me. I have a Kenwood TM-V71A dual-band radio in my truck. It has a wideband receiver which allows me to snoop on the airport, but I can't transmit on aviation bands. I am a licensed Amateur (technician).
jimh posted 12-31-2011 11:11 AM ET (US)     Profile for jimh  Send Email to jimh     
Thanks for the long sidebar on Aviation Radio Service. Perhaps we could return to the topic: VHF Marine Band Channels.
jimh posted 12-31-2011 11:29 AM ET (US)     Profile for jimh  Send Email to jimh     
In a modern radio it is not particularly difficult to display the frequency of the transmitter because the frequency control mechanism is most likely a frequency synthesizer. The control system of synthesizer can compute the frequency and display it. However, this is really a recent evolution in radio design for VHF Marine Band service devices. Historically the frequencies were controlled by crystals, and the crystals were selected by a rotary switch. It was much simpler to just write a channel number on the switch legend than to write out the exact frequency. Also, many sets provided only a few positions for crystals, and the panel legends were left blank, to be filled in when the radio was set up with crystals for the desired channels. The switch legend might say A-B-C-D-E, and then the corresponding VHF Marine Band channel numbers would be denoted on a write-in area or on a separate label. We used to have an old Motorola crystal-controlled VHF Marine Band radio that was set up like that. You only had a few channels available, typically CH-16 and a few others of your choice.

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