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ContinuousWave: Small Boat Electrical
VHF Marine Channel Variations
|Author||Topic: VHF Marine Channel Variations|
posted 12-12-2008 12:30 AM ET (US)
Variations in VHF Marine channel designations among US, Canadian, and International band plans appear to affect the following channels:
1, 3, 4, 5, 7, 20, 21, 22, 23, 61, 62, 63, 64, 65, 66, 67, 77, 78, 79, 80, 81, 82, 83, 88.
Most of those channels are designated for Public Correspondence, Port Operations, Commercial-only, or Government-only.
For the most part, these changes do not affect the normal channels used by recreational boaters for either boat-to-boat or boat-to-shore traffic. Those channels are typically:
6, 9, 13, 16, 17, 68, 69, 70 (DSC), 71, 72,
posted 12-13-2008 09:14 AM ET (US)
The changes to the channel plan noted above are generally a change in the channel assignment from a simplex channel to split channel. These terms may not be familiar to all mariners, and I will elaborate.
In a simplex channel, the ship radio transmits and listens on the same frequency, and all stations using the channel use that same frequency for both transmit and receive. Thus ships can communicate with other ships or with shore stations using that channel.
On a split channel, the ship transmits on one frequency and listens on a different frequency. All ship radios are set up with the same configuration, and thus such a split channel cannot be used for ship to ship communication. Shore stations are set up with the opposite configuration, that is, their radios listen on the frequency the ships are transmitting, and transmit on the frequency the ships are listening. Such a split channel is therefore useful only for communication with the shore station.
The term "simplex" refers to radio communications in which a station cannot receive while it is transmitting. Because the same frequency is used for both transmitting and receiving, the local transmitter signal blankets any incoming signal from being received. In simplex radio communication, the transmitting station is unable to receive while transmitting. When two stations communicate using simplex, they generally exchange turns in transmitting. A token (the word "over") is used to indicate the end of a transmission and to signal permission for the other station to transmit. If there is confusion or misunderstanding between the stations, both may transmit simultaneously. This is known as "doubling," and it results in neither station hearing the other's transmission. Consistent use of a token word ("over") helps avoid this.
When a split channel is used, it is possible for a station to be configured so that it can simultaneously receive and transmit. Use of different frequencies for the receive and transmit channel allow this. In the VHF Marine Band, the frequency separation is not great, and generally a separate antenna is used for receive and transmit. It is possible to share a common antenna for receive and transmit, but this requires very elaborate filtering and wider frequency separation. These factors tend to make it difficult to implement simultaneous receive and transmit on a small recreational vessel. However, simultaneous receive and transmit may be implemented at a shore station, where the two antennas can be widely separated or even located at different sites and joined by radio links.
Stations which can simultaneously transmit and receive are said to be capable of duplex operation. If two stations that are fully duplex capable communicate, the circuit works like a normal telephone circuit, and both parties can hear and speak simultaneously. There is no need to alternate transmission or signal when the other station is to transmit.
When one station (e.g., the ship station) is only able to achieve simplex communication, but it communicates with a station that is capable of duplex (e.g. the shore station), a half-duplex communication takes place. This allows the shore to maintain constant reception on the ship. The shore station is generally operated by experienced personnel, and they are aware that their transmission cannot be heard by the ship while the ship is transmitting. The shore station thus operates as if it were in simplex mode. If the ship station becomes confused about whose turn it is to transmit, the shore station will hear the ship. This arrangement reduces confusion. It is particularly useful in emergency communications where the ship may be in distress and the radio operator may misinterpret the token for exchange of transmissions.
Half-duplex circuits are also useful in connecting a ship to a shore telephone circuit. Prior to the widespread use of cellular telephones, this was the most common method of establishing a telephone call from a vessel.
The half-duplex arrangement also affords some privacy to the communication, as other ships cannot hear the ship station in the circuit. For commercial operations, this could be important. Commercial ships may communicate with their home office base station this way. If the base station operator is aware of the radio circuit, certain confidentiality can be maintained.
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