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Very High Frequency





A Radio frequency spectrum has been established to control the use of frequencies to gain an idea of the approximate range over which radio waves may be received. This spectrum of frequencies has been divided into usable bands to designate the anticipated radio range, for which, VHF is effectively that of 'line of sight'. VHF transmissions are primarily space waves that that depend on the height of both transmitting and receiving antennas. Large objects and transient atmospheric conditions may alter the direction of space waves creating blind spots, making reception extremely difficult. There are several advantages to VHF over lower frequencies. For starters, the radio noise is less, the band is less crowded, and there is a little interference. At lower frequencies radio signals sometimes bounce off the ionosphere and skip long distances and then reappear on the surface of the water to cause interference. This happens often with VHF. Common uses for Very High Frequency in a Marine environment include digital selective calling DSC and automated identification system AIS. DSC radio can be referred to as a combination of a telephone and a marine VHF radio. DSC acts like the keypad and bell of a telephone, allowing you to call direct and ring other DSC radios, or allow others to call you, without having to monitor a speaker for voice. If the call isn't answered, the details are stored in the received calls log. The DSC radio allows you to send a formatted distress alert to any other DSC radios in your local area, and eventually to the coast guard. DSC radios center around short range VHF instead of the long range single sideband transceivers. The range of VHF transmission is less than at lower frequencies and is limited, depending upon transmitter power, to about 100 miles. This is a great distance that is unnecessary for most situations, seeing that ships do not need to contact each other that far off on a bridge to bridge radio. Signals, which cause interference are classified as one of the following:

1.)    Co-channel interference is another station on the same frequency.

2.)    Adjacent channel interference is created by a transmission on another channel whose frequency is close to the desired frequency but outside the band acceptance of the receiver.

3.)    Spurious responses are signals that are on other frequencies than the desired channel but that the receiver cannot reject.

Along with the maritime industry, VHF radio is also used in the trucking industry, railroads, highways, maintenance vehicles, police and fire departments, and even civil defense.

AIS Channel 16 or EMER is the primary channel for calling the coast guard and passing distress and urgency messages by voice. It is also used for calling other vessels and coast station in your local area. When the Ch 16 bottom is pressed the radio will automatically tune to Ch 16. This bottom may be labeled '16/9', in which case it accesses Ch16 and Ch9. Channel 9 can be an alternative calling channel for recreational craft.

AIS

VHF is used in a variety of different electronics that aid in nautical navigation. An important piece of equipment commonly found on the steering console of most vessels is the Automated Identification System which assists in Identify passing vessels. It is most commonly integrated with navigating systems like RADAR and ECDIS. "DSC"