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Old 10-10-2006, 02:24   #31
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Excerpted from the FCC’s “Ship Radio Stations”:
http://wireless.fcc.gov/marine/fctsht14.html

<quote>
MAY I USE MY RADIO ON MORE THAN ONE SHIP?
If you can provide justification for the use of a single transmitter from two or more ships, a portable ship station license may be issued. This could authorize various types of marine radio equipment to be carried from ship to ship.

MAY I USE MY HAND-HELD MARINE VHF RADIO ON LAND?
You must have a special license, called a marine utility station license, to operate a hand-held marine radio from land -- a ship station license IS NOT sufficient. You may apply for this license by filing FCC Form 601 with the FCC. To be eligible for a marine utility station license, you must generally provide some sort of service to ships or have control over a bridge or waterway. Additionally, you must show a need to communicate using hand-held portable equipment from both a ship and from coast locations. Each unit must be capable of operation while being hand-carried by an individual. The station operates under the rules applicable to ship stations when the unit is aboard a ship, and under the rules applicable to private coast stations when the unit is on land.
<end quote>
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Old 10-10-2006, 22:54   #32
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In the US, regulations mostly apply to transmitting only. On land, it is illegal to transmit on the marine VHF frequencies unless you have that special license, but you can use your marine radio to listen all you want.

I've seen handhelds that contain both a marine VHF transmitter and an FRS transmitter. In the US, the radio just has to be certified (or is it "certificated"? - the new name that replaced "type-accepted") for both services.

I have not seen any radio that has both marine and air band transmitters.
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Old 10-10-2006, 23:37   #33
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A little radio geekery:

The way you make a multi-band radio is to design a radio for each band and work out a way for them to share subsystems as much as possible. Without going too deeply into radio theory, I can only say that you can share quite a lot, even when one band is UHF and the other is VHF and one is AM and the other is FM.

Quote:
Originally Posted by hellosailor
James-
There are a number of radios (not necessarily HT's) that do both AM and FM. I've got an HT that is rx-only on AM, but rx+tx on FM. Covers air and marine bands, but will only tx in fm.

I've made no attempt to confirm this, but have been told some of the fm schemes (reactance/vs/phase/vs "true" fm, I forget which) will produce intelligible signal on AM under certain conditions. Any idea about that?
It sounds like you're thinking of "slope detection". You tune your receiver slightly off the center frequency of the FM signal. The greater the deviation of the transmitted signal, the further the signal is from the center frequency of the tank circuit. The result is that the received signal is attenuated by different amounts at different deviations. This imposes some amplitude variation on the received signal, which you then detect as for any AM signal. If I remember correctly, this was an early design for FM receivers.
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Old 11-10-2006, 09:04   #34
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Thanks, Mark, slope detection sounds like what I was thinking of.

Certification, type acceptance..I know, I should remember that kind of thing but since I'm not in the business of issuing them, "Not My Job".<G>

FRS in a marine VHF is a neat idea, since the frequencies are similar enough that it doesn't really need to stretch the equipment a lot, and FRS are popular for families ashore. Or, "foredeck versus the helm" shouting matches on bigger boats. Also neatly solves that "I'm on the dock and legally can't call the boat on my marine radio but I'd really like to come home now!" problem.<G>

Ah, the overkill of cell phones and free weekend air time. One of these days, they'll build cell phones with real walkie-talkie features in them, so they can talk to each other (instead of pretending to do that) directly even when there are no towers and service around.
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Old 01-01-2007, 21:29   #35
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duplex/simplex

I seem to recall reading in the user's manual that came with my VHF radio that the channels therein were "Half Duplex".

As an example, it cited the ordinary telephone as being a duplex instrument because it could transmit and receive at the same time.

Half duplex, it explained, means that although the VHF instrument can transmit and receive, it cannot do both at the same time. Pressing the transmit button engages the transmission function and broadcasts one's voice. While transmitting, the instrument cannot receive. Releasing the transmit button stops the transmission process and enables the receive function so that one may hear what another operator is transmitting.

Makes sense to me.
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Old 02-01-2007, 02:53   #36
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Quote:
Originally Posted by thefantasea
I seem to recall reading in the user's manual that came with my VHF radio that the channels therein were "Half Duplex".

As an example, it cited the ordinary telephone as being a duplex instrument because it could transmit and receive at the same time.

Half duplex, it explained, means that although the VHF instrument can transmit and receive, it cannot do both at the same time. Pressing the transmit button engages the transmission function and broadcasts one's voice. While transmitting, the instrument cannot receive. Releasing the transmit button stops the transmission process and enables the receive function so that one may hear what another operator is transmitting.

Makes sense to me.
It is also a load of horese manure.

The difference between simplex and duplex has been explained earlier on this thread.
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Old 02-01-2007, 03:30   #37
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Talbot:
Please don’t be quite so quick to dismiss Augie’s comments.
The terms simplex, half-duplex, and duplex are subject to differing definitions (ANSI, ITU, et al).

Half-Duplex has been described as send & receive, but not simultaneously - what we typically call Simplex. Under that definition, Simplex is one-way only, such as in Television (you’re either a broadcaster or a receiver).
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Old 02-01-2007, 03:54   #38
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Typical some other standards agency unilaterally changing the definition of terms that have been in use for aeons and are well understood by the majority of practitioners in this field.
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Old 02-01-2007, 11:33   #39
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Telephone conversations on marine VHF

One REAL reason that there are half-duplex versus simplex alloted channels on marine VHF radios is so that it is more difficult for someone to evesdrop in on commercial telephone calls made over those channels assigned for that purpose.

The half-duplex channels use different frequencies for transmit and receive so someone listening in with another radio only hears the shore transmision and not the ship transmision. Obviously the shore transmitter has "reversed" frequency assignments compared to those radios that we carry on board. They also have more stringent frequency accuracy and stability requirements.
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Old 02-01-2007, 18:33   #40
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Duplex/Half Duplex

Talbot,

No need to get your knickers in a twist.

Another reference:



The Difference Between Half and Full Duplex Explained
.
by Paul Wotel
.
"Duplex" simply means you're able to send and receive data (most often the human voice) from the same device whether that be with your phone, 2-way radio, or PC.

Half-duplex devices let you send and receive, but only one-way at a time. If you've ever used a walkie-talkie, then you know what half-duplex conversations sound like. You have to push the TALK button to send your message. But as long as you are holding the TALK key, you can't hear what anyone else is saying. You must release the button to receive.

Source: The Difference Between Half and Full Duplex Explained
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Old 26-02-2008, 17:54   #41
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Icom 504

So if I buy a VHF radio by mail order (say the Icom 504 - much cheaper) from the USA will it work in Oz?
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Old 26-02-2008, 18:12   #42
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Quote:
Originally Posted by Boracay View Post
So if I buy a VHF radio by mail order (say the Icom 504 - much cheaper) from the USA will it work in Oz?
Only if you install it upside down :-)
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Old 26-02-2008, 18:16   #43
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Talbot-
Assuming the IC-M504 sold here meets all the Australian regulations (emissions limits, power limits, frequencies available) bear in mind that you can't buy a radio from "Icom" in the US. You can only buy it from "IcomAmerica" and the warranty service is, as far as I can see, provided only if you ship the radio back to IcomAmerica in the US. So whatever you save, factor in zero warranty, plus any import duties or tariffs they may levy on it.

Other than that, if you can find a web site that shows the Australian requirements, or one from Icom-Oz that shows the ratings for the IC-M504 that is sold there, you can compare it directly with the product manual from the IcomAmerica web site, where all the specs are listed for the models sold here.
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Old 27-02-2008, 07:38   #44
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Boracay, Boracay, Boracay. This is ...

Ok I am getting a bit carried away here. I am (among other things) an Icom agent in Perth. I wasn't aware of a M-504 (but Icom USA lists them), in Aussie, Icom sell a M-505. Not sure what the difference is but can find out for you if you want. At the lower other end of the market, the M-304 is sold in both countries. Again if you want, I can PM you with info on prices.

At the risk of upsetting some regarding simplex, half duplex, duplex - much earlier discussion on this thread - In my professional opinion simplex means transmit and receive on the same frequency (and by definition, this can't occur at the same time); duplex means transmitting on one frequency while receiving on another frequency (at the same time) with the frequencies "reversed" for the other station. Half duplex is the same except the station can only transmit OR receive; not both together at the same time. Because almost all radios in common retail use are (by their design) limited to simplex or half duplex operation, the term half duplex has dropped out of fashion and the term duplex has come to replace it in common usage especially among general users. I am not saying I am 100% right here as after 37 years in the radio and communication industry, I am still learning the trade. From what I have seen on this forum, there are many more experienced radio folk around here than me.

BTW, I haven't yet installed a USA com upside down in Oz but it might work. I once took a simple receiver and reversed the battery leads and it became a transmitter - thought I might patent the idea but never got a round tuit.
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Old 27-02-2008, 09:48   #45
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Wotname,

Yes, Round Tuit's are getting very hard to find these days :-)

Many modern VHF rigs are already programmed with U.S., International, and other VHF channels. You just have to do a short programming sequence to select which you want.

For example, the Icom M-504 has U.S., Canadian, and International channels in memory. To choose which you want to use, here's the sequence:

Quote
U.S.A., international and Canadian channels
The IC-M504 is pre-programmed with 57 U.S.A., 57 international
and 61 Canadian channels. These channel groups may
be specified for the operating area.

Push [CH/WX•DUAL•U/I/C] to select a regular channel.
• If a weather channel appears, push [CH/WX•DUAL•U/I/C] again.
While pushing and holding [H/L], push [CH/WX•DUAL•
U/I/C] to change the channel group, if necessary.

• U.S.A., International and Canadian channel groups can be selected
in sequence.
Rotate [DIAL] to select a channel.
• “DUP” appears for duplex channels.

Unquote

Warning: don't try this upside down, without a couple of drinks in yer, mate :-)

Bill
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