VHF Short Range Radio

Description

VHF (Very High Frequency) marine radios are the most popular form of radio equipment on vessels. The benefit of a VHF radio is due to it being standard equipment on all commercial vessels, channel 16 is monitored for distress and contact calls, it has the clearest transmission of all types of marine radio, it is cheap to buy and easy to install.

Worldwide, the VHF channel 16 is reserved for calls from vessels in distress and as a first contact calling ship to shore and ship to ship channel. Once contact has been made it is mandatory for the two stations to move to any one of a number of other channels known as "working channels" in order that the distress frequency can remain clear in case of an emergency.

Operators of VHF (and MF/HF radios) must be holders of a marine radio operators certificate of proficiency or equivalent. There are two grades of qualification, one for VHF radio operation only and the other for operating VHF and the long range MF/HF radio.

VHF working channels are:

• Commercial vessels - 6, 8, 72, 74, 78
• Professional fishermen - 71, 72, 77
• Pleasure craft and Rescue Groups - 72, 73, 77
• Port Operations - 6, 8, 10, 11, 12, 14, 72

You will note that Ch 72 is a common working channels for all categories of stations. Ch 6 is also used for ship to air search and rescue purposes.

Here is a list of all VHF frequencies and the VHF Channel changes planned to commence from 1 January 2017.

VHF Marine Repeaters

In Australia, marine repeater stations use VHF Channels 20, 21, 80, 81 and 82. All these channels operate in duplex mode, that is the radio transmits on one frequency and receives on another. For a repeater station to operate and retransmit a ship message the repeater has its frequencies reversed.

Locations and details of the VHF Marine Repeaters in Tasmania can be found on the Marine and Safety Tasmania (MAST) website.

The purpose of unmanned repeater stations is to extend the range of the VHF transmission. Usually located on high ground the range of a repeater can be up to 50 nautical miles, a distance usually unobtainable from vessel to vessel that have low height antennas. The range of transmission of a vessel such as a yacht with the VHF antenna situated on top of its mast will be greater than a motor boat with a lower mounted antenna. In the latter case the motor boat might have a VHF range of less than 20 miles.

A common problem with using repeaters is that operators have not set their VHF radio to International or Australian frequencies. If your VHF radio indicates that it is set for USA frequencies it must be changed to INT or AUS for it to operate correctly.

Is it useful to request a relay using a repeater?

In situations where a vessel's transmission is difficult to hear via a repeater channel it is of no use to ask another station to assist with a relay. This is because all stations are hearing the transmission through the repeater not from the transmitting station.

What is the usual VHF transmission distance?

A rough calculation can be made using the known heights of the transmitting and receiving antennas. The calculation is done for each station and the sum of the two calculations is the approximate VHF transmission distance.

Use the formula -

1.17 times the square root of your height of eye in feet (or antenna)
= Distance to horizon in nautical miles.

For example, if you are aboard a motor cruiser and your height of eye (or antenna) is 9 feet then the calculation is -

1.17 * 3 = 3.51 nautical miles

If the second vessel also has an antenna set at 9 feet above sea level then the theoretical transmitting distance is 3.51 + 3.51 = 7 nautical miles. In practice you will find that you can probably considerably better this distance but do not rely on any extended transmission range.

Navigators with access to Norie's Nautical Tables can look up the table "Distance of Sea Horizon " to save the arithmetic.

Another way to calculate the theoretical transmission distance is here.