Understanding VHF Radio Transmission Distances

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Written by Alastair Douglas
November 2010

Some recent events have prompted this article in the interests of safety and to give VHF radio operators a better understanding of some of the limitations of this means of communication. There is an indication that there are many radio users who do not fully understand how to use their VHF radios to best advantage and what the limitations are with VHF radios.

VHF (Very High Frequency) marine radios are a most useful addition to vessel's safety equipment. They are an excellent means for communication and to receive marine weather forecasts from the RYCT's own member operated limited coast station Tascoast Radio and from Tas Maritime Radio and other marine stations. VHF radios provide a very clear signal that is not affected by atmospheric interference and very rarely affected by local external electrical interference such as engines. They are cheap to buy, relatively easy to install and straightforward to operate.

If being straightforward to operate then why do they not perform as well as expected when communication is required over relatively short distances?

A VHF radio signal distance can be considered to be close to the combined lines of sight (LOS) from both vessels. A rough guide of the transmission distance over a clear, across water, path with no obstructions of headlands, vegetation etc can be calculated using the nautical text book Norie's Nautical Tables and referring to the Distance of Sea Horizon table. For example when standing at sea level your height of eye is about five feet and the sea horizon is 2.6 nautical miles away. If there is another person standing in a dinghy also with a height of eye of five feet giving a visual distance to the horizon of 2.6 miles and both persons have VHF handheld radios, transmission should be satisfactory over the combined distance of 5.2 miles.

To increase the distance of transmission then it is essential to elevate the radio antenna as high as possible. That is why most yachts have their VHF antenna on top of the mast and shore stations like to use headlands, lighthouses, mountains or very tall masts to maximise their transmission range. Motor vessels tend to mount their antennas on a Targa bar or the fly bridge to maximise their transmission range.

One way to overcome some of these distance limitations is the use of VHF repeater stations. These stations are located on high ground around the coast and can considerably increased transmission distances between stations (vessels). An example here is if the yacht antenna is mast mounted at a height of 50 feet and the other station is the RYCT repeater at Mt Raoul that is 1515 ft (462 metres) high then the theoretical transmission range is about 50 miles. This is fine for unobstructed transmissions across water but landforms such as mountains, cliffs, headlands or even wet forest can create obstructions to the effective transmission range. We are also assuming that the radio installations are good, batteries are fully charged and the radio antennas are vertical. A yacht heeling over and side on to the other station could be transmitting into space or the sea.

Recently VHF radio communication was necessary between Hobart and a yacht at Wedge Island near Nubeena on the Tasman Peninsula. The communication was not the best as the signal varied considerably from being reasonable to broken to indistinguishable. The direct line transmission path was across some not very high ground on the South Arm peninsular but the distance was more than 20 nautical miles, both factors causing reception problems. The use of the VHF repeater station on the 462 metre high Mt Raoul would possibly have allowed easy communication with the yacht as long as the yacht was not shielded under cliffs or other obstructions.

So the answer for good communication boils down to operator experience and understanding of the terrain over the transmission path. While there can be no hard and fast rule of thumb to transmit using VHF to various locations around our shores because of the varying topography, experience and consideration of the locations will assist the radio operator in selecting a simplex frequency direct to the vessel or a duplex frequency via a repeater station that will extend the radio transmission range.

A simplex frequency is when transmission and reception are on the same frequency. The distress, urgency and first call VHF Channel 16 has a single frequency of 156.8 MHz. A duplex channel has different frequencies for transmission and reception. The Mt Raoul repeater on the Tasman Peninsula operates on channel 81 and receives on 161.675 MHz and transmits on 157.075 MHz. Channel 16 is the prime VHF channel and should be monitored at all times. But being a direct station to station frequency it has limitations in terms of transmission distance.

As radios have a dual watch capability that allows another channel and Ch 16 to be monitored simultaneously this feature gains importance allowing the station to listen to a selected working channel and still maintain a watch on Ch 16. Many VHF radios have a scan facility that allows numerous channels to be monitored but some care needs to be made of their selection as the radio will stop on a busy but otherwise unwanted channel until it is clear before continuing its scan.

How VHF repeater stations actually work will be the subject of another article. For the purposes of this article a repeater station does as the name implies, retransmits the received signal extending the radio transmission range that could not be achieved by direct transmission due to being blocked by high terrain or by the distance being too great between the vessels.

Often I hear complaints that transmissions of Tascoast Radio's weather details have been difficult to hear. Enquiry usually reveals that the vessel has been in an anchorage on the west coast of Bruny Island where there are hills between them and the CH 81 repeater at Mt Raoul. Some local places known for reception difficulty include Tinpot Bay, Mickeys Bay and Cloudy Bay on Bruny Island, the Huon River, Port Cygnet and Recherche Bay. On the east coast there can be difficulties if you are close in under the Tasman Peninsular cliffs and north of Maria Island. Vessels with low mounted antennas will find they have greater difficulties in being shielded from good transmissions. But like all radio transmissions there can and will be variations so rather than exact textbook theory, the acquired experience of operators is most valuable for radio operation.

The accompanying map illustrates Mt Raoul VHF Ch 81 radio coverage, the shaded areas indicating marginal or no coverage. For the technically minded, the prediction is based on Ch 81 transmitter power of 25 watts, the receiving vessel antenna height of 3 metres and a one microvolt received signal. Map created courtesy of Andrew Boon and Brian Muir.