In US boarder areas with Canada B could be in use. B channels are usually duplex. The US to get more freqs took a lot of the Marine duplex freqs and gave the other half to rail roads along with changing the bandwidth requirements for marine in US waters.nd5y said:This is from the Coast Guard communications web site
Note that the letter "A" indicates simplex use of the ship station transmit side of an international duplex channel, and that operations are different than international operations on that channel. Some VHF transceivers are equipped with an "International - U.S." switch for that purpose. "A" channels are generally only used in the United States, and use is normally not recognized or allowed outside the U.S. The letter "B" indicates simplex use of the coast station transmit side of an international duplex channel. The U.S. does not currently use "B" channels for simplex communications in this band.
There is duplex coms on marine, cell phones have almost replaced Ship to Shore phone calls, ie ch's 26 and 28 in the Detroit area. The ALPHA channels that the USCG uses came from taking the second freq of a duplex channel and making it simplex, ie 21A simplex or 21B duplex.sean_kd4adv said:.
Ship-to-Shore means exactly that.
*There is no such repeater.*
Used for telephone interconnect/ "phone patches"
It is a "cross-talk" method that is built into the VHF Marine Mobile transceivers.
Simplex ship to ship comms and ship to shore comms mean exactly that!
157.1000 - 22A for example :
When a vessel has established comms on VHF CH.16 with the US COAST GUARD CAMSLANT - Hampton Roads Group (NMN) on the East Coast near Virginia then they move to 22A and that is Direct Comms.