Navigation and Seamanship

6.5.1. VHF-Marine communication system

                 

                                                       

                                    

Marine radio systems are similar to having a conversation on the telephone, but with significant differences that the marine radio conversations could be listen by many people. An electronic device that performs in receiving radio signals called a radio receiver or a radio. There are several types of marine radios with different frequencies MF, HF and VHF, and, are identified by the mode of transmission.

· Very High Frequency (VHF) is transmitted within the frequency range 156‑162 MHz,used for local, short-range marine communications. Clarity in the communication and effective range depends mainly on the height of antennas of both the receiving and transmitting stations and power output of the transmitting station. VHF equipment is called“line-of-sight radio” because its radio waves travel in nearly a straight line,meaning, if one antenna can “see” another antenna, communications between the two is possible.

· Medium Frequency (MF) is used to communicate for long distances. The MF band uses low frequencies, so the ground wave travels along the surface of the earth, permitting communications at distances up to 200 miles during daylight hours. The low frequency also makes communications at much greater distances at night easier. In general, MF and HF radios will always have greater range than VHF.

Very High Frequency(VHF) marine radio is essential communication equipment for fishing vessels operating in coastal waters. It allows instant communication between boats of in that area and port stations. T he VHF is the most valuable equipment for safety communication and any call for assistance of nearby vessels and coast stations . VHF is not a telephone and radio conversations are not private. When you talk on the VHF, everyone within range tuned to that channel is listening. The users of VHF should not use the equipment for private talk sand to chat. VHF has the advantage that equipment is relatively simple, and can therefore be compact and low cost.

VHF sets occupying the 156-162MKz band and generally considered the most suitable for short range (25-40 miles) marine communication. In the VHF system, different channels are set aside for ship-to-ship, ship-to-shore, safety, distress and calling and other specialized uses.

Simplex/duplex mode of communication

Duplex In the duplex system, the transmitter and the receiver are tuned to two different (or paired) frequencies permitting communication in both directions simultaneously.

Simplex In this mode, one person cannot be a talker and listener at the same time during a communication process. When a person at one end talks, the person at the other end listens and vice versa.