Understanding Marine Radio Communications.      

These notes are mainly concerned with the VHF frequency range, as applicable to the recreational boating community. More concise details are available from the Marine Radio Operators Handbook (available from VMRG for $25.00) or on the | Australian Communications Authority | web site.

The Radio Spectrum.
MF and HF. The transmission characteristics of radio signals alters considerably as the frequency or wavelength changes. At the lower end of the marine band, medium frequency (MF) transmissions travel near the Earth's surface, by what are called ground waves. Range is limited to about 300 miles. The higher frequencies that make up the HF band have their waves bent by the ionosphere - a layer of ionised gases about 50 miles above the Earth's surface - and reflected back down. This ' Skip' can happen more than once with the effect that high frequency (HF) transmissions can travel considerable distances. However, changing atmospheric conditions, solar activity and even man-made influences can effect these transmissions to cause different degrees of interference. The end result is that effective long-range communications depend greatly upon the correct choice of frequency for the time of transmission.

27 MHz, and VHF 156 mhz marine bands...
At 27 MHz frequencies and above, propagation starts to change significantly. A wave travels directly from transmitting antenna to receiving antenna. This ' line of sight' transmission is called ' direct' wave. Under normal conditions, there is no reflection of VHF radio energy from the ionosphere. Range is greatly reduced, but atmospheric conditions have minimal effect. As a general rule, VHF communications between two stations over an all water path, are possible over a maximum range approx. equal to the combined line of sight distance of each station. Consequently, the greater the height of the transmitting and receiving antennas, the greater the range.

The frequency bands of VHF and 27 MHz are divided into channels and unlike HF, one channel is usually as good as another no matter what time of day transmission takes place. As the wavelengths continue to shorten even further, voice and data transmissions give way to radar and microwave applications.

A radio wave can be referred to by either its frequency or wavelength. The frequency is the number of cycles per second, measured in ' hertz' (Hz), while the wavelength is the distance between crests, measured in metres. Frequency increases as wavelength decreases.
One cycle per second is called a Hertz (Hz).
One thousand Hertz is called a kiloHertz (kHz).
One million Hertz is called a MegaHertz (MHz).
One thousand million Hertz is called a GigaHertz (GHz).

Use this simple formula if you need to know the wavelength of a radio wave :-
Wavelength (in metres) =      300     
                                    frequency in MHz

Types of Frequencies.
There are two types of frequencies used in all marine communications, except 27 MHz.

• Simplex. uses a single frequency for both receive and transmit. Other stations on the same frequency and within range will hear both conversations. For example, VHF channel 73 is a simplex channel. Simplex channels are not used by VHF repeater stations, and so will generally be limited to ' line of sight' for ship to ship or ship to shore communications.
  
• Duplex uses two different, but paired frequencies, one for receive and another for transmit.
  For example, during communications with a coast station, a vessel transmits on frequency A and receives on frequency B. The coast station receives on frequency A and transmits on frequency B. Normal recreational boating VHF sets switch from receive frequency to transmit frequency when the handpiece button is pressed. e.g. VMRG VHF Ch. 82 repeater utilises frequencies 157.125 Mhz to transmit and 161.725 Mhz.to receive, and all vessel based sets have complimentry frequencies on Ch. 82.

VHF Repeaters.
The normal limited range ( 20 - 40 n.m.) of VHF transmission can be extended significantly by the strategic placement of coastal repeaters. They are designed to transmit and receive signals simultaneously, and can extend ship/shore range up to 140 n.m. when two stations are on either side of a repeater. Repeaters activate automatically as soon as the receiver detects a signal. This retransmitted signal can be received by any radio on that channel. In most cases they are installed and maintained by VMR organisations to aid sea safety, and as such, should not be used as chat channels.
Usual repeater channels in Australia are 21, 22, 80, 81,and 82. To determine your ability to access a repeater, momentatily depress the transmit ( t/x) button. If a brief ( one second) burst or tail of noise is heard when the button is released, the repeater is being activated. If not heard, you may be out of range, or your set has insufficient transmit power. ( low batterey or bad power supply connections.)

Operating Protocol.
It is most desirable for all concerned that recreational mariners log on with the local VMR group prior to undertaking a voyage. It is not an invasion of your privacy, but an insurance for your safety. At least obtain a radio check if working offshore. Remember that correct function of your set in port is no guarantee of its operation offshore.
A marine radio is not a C.B. radio, and slang terms are neither legal nor acceptable.
Always use proper call signs or vessel name, and procedures, eg. "Gladstone zzz, ( 3 times ) this is Gladstone xxx on 82, ( 3 times)."
When calling VMR Gladstone, use the same procedure, and identify the channel on which you are calling, as the radio operator has a number of radios to monitor. Be patient during times of heavy radio traffic. The VMR operator has to record all incoming calls, so may not be able to respond to non emergency calls immediately.

VMR Gladstone prefers users to stand by, and call initially on Ch. 82. If calling another vessel, arrange to change channels to chat after contact is established, to avoid tying up our working channel 82.

Channel 80 is the professional fishermen's repeater and they don't mind others using this channel, so long as the call is kept short. If you want to keep your favourite spots a secret, then after calling on Ch. 82, go to a simplex channel such as 72 to chat on. Then both vessels should return to channel 82 for stand by and calling.

It is a requirement of law for marine radio operators to be licenced. VMRG conducts regular courses on behalf of the | Australian Maritime College. |

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