"Scott Downey" wrote in message
...
Ok that makes sense that other boaters would only hear the coast guard
shore
station, BUT what about the guy on the boat the CG shore station is
talking
to?

It is common for the shore station to rebroadcast the ships signal so that
all can hear it. This technique is often referred to as a "repeater".

I don't know about any other country, but here is a link to the US Coast
Guard's web page that provides information regarding what channels are used
for what:

http://www.navcen.uscg.gov/marcomms/vhf.htm


There are 51 channels in this list. One of these is a "receive only" (with
respect to the boat) channel. Ten of these are "duplex" channels, where the
transmit and receive frequencies are different. Of these 10, 8 are
designated as "Public Correspondence (Marine Operator)". One is designated
"Automatic Identification System duplex repeater". Only one, channel 20, is
used for official use: "Port Operations (duplex)"

I haven't listened in on the port operations much, but I suspect that they
would operate in the "repeater" mode. On this channel, the ship transmits on
a frequency of 157.000 MHz. The port office has a radio that is listening
on 157.000 MHz, and transmits on 161.600 MHz. The port office can transmit
directly, or it can take the signal it received and transmit it again on
161.600. If it is in repeat mode, all the ships within range can hear the
entire conversation. This would be very helpful in coordinating traffic
between ships that might be out of range with each other but within range of
the port (which would have the benefit of a much taller antenna and possibly
a better receiver and more powerful transmitter).

It has been a number of years since I have heard any Marine Operator traffic
on VHF. Before cell phones, marine operator traffic was fairly common. If
a ship wanted to place a call, they would contact the marine operator on
channel 16. The operator would instruct them to go to a specific duplex
channel where the call would be made. Let's say that the operator directed
you to channel 28. The boat would transmit on frequency 157.400 and receive
on 162.000. The marine operator would do the opposite. Since the person at
the other end of the phone line can't "push to talk" the marine operators
transmitter is going all the time, transmitting whatever it gets from the
phone line.

When the call was connected, the very nature of the telephone line "repeats"
the boats signal. Just like you hear your own voice in the earpiece when
you talk on the phone, the marine operators transmitter will hear the
skipper's voice after it has looped through the telephone central office. A
third party eavesdropping on the channel will be able to hear the entire
conversation.

Rod McInnis