Marine Radio in Tasmania

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.

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.

VHF Marine Repeaters

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.

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 (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 probably can 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 "For Finding the Distance of Sea Horizon for Given Heights" to save the arithmetic.