Maritime mobile amateur radio

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Most countries' amateur radio licences allow licensed operators to install and use radio transmission equipment while at sea. Such operation is known a Maritime Mobile amateur radio. In most cases the operator's call sign needs to be extended by adding the suffix '/MM' when transmitting at sea.

1 Maritime Mobile nets
2 Technical considerations
- 2.1 Antenna design and installation
3 See also

Many long-standing and sophisticated radio nets are regularly operated by shore-based amateur volunteers for seafaring operators.

Maritime mobile amateur radio nets
	Frequency (MHz)	Time(s) (UTC)	Operator(s)	Notes
Transatlantic maritime mobile net	21.400	1300	Trudi (8P6QM)	Based in Barbados, Atlantic crossing
Worldwide weather net	21.303	1300	Neville (G3LMO), Richard (KT4UW), Don (6Y5DA)
UK maritime mobile net	14.303	0800 1800	Bill (G4FRN), Bruce (G4YZH), Tony (G0IAD) and others
Mississauga maritime net	14.121	1145	Doug (VE3NBL), Ernie (VE3EGM)	Based in Canada, Atlantic crossing
Caribbean maritime mobile net	7.241	1100	Lou (KV4JH)	Caribbean cruising
Caribbean weather net	7.086	1120	George (KP2G)	Caribbean weather information
INTERMAR German maritim mobile service net	14.313	1630	Rolf (DL0IMA)	daily all Oceans
Note: Information dated 2000 - Mistweb entries South, East-west, West-east; and www.intermar-ev.de

There are some special considerations when installing and using amateur radio transmitters and receivers afloat. These include power supply, RF earthing, antenna design and EMC (Electro-Magnetic Compatibility) with other electronic equipment aboard.

For MF and HF use, the most common antenna design is to add two RF insulators into the backstay of the mast and feed it from the transceiver using a sintered bronze earthing plate, bolted to the outside of the hull, well under the waterline, as an earth. On metal hulled boats the earthing plate can be dispensed with, and the whole hull used as a ground. In this case, the thickness of any paint layer is entirely negligible at RF.

On a yacht with twin backstays, if insulators are placed in both of them and they are fed from the mastead, they may be usable as an 'inverted vee' avoiding the need to feed the antenna against 'ground'.

Either format will require the use of an ATU (Antenna Tuning Unit) to achieve resonance for the HF frequency in use, as the physical length of the antenna will almost invariably be incorrect at the frequency of choice.

A few twin-masted sailing vessels have the space to erect a "Tee" antenna or an inverted "L" between masts. These antenna configurations are more common on merchant ships.

For VHF and UHF operation, one option is to mount a small yagi antenna to a pole 1-2 m (3-6 ft) long and haul this to the masthead using a flag halyard. If the halyard is correctly knotted to the middle and bottom of the pole, it is easy enough to make the antenna project above the clutter at the masthead into clear air. The problem is in rotating it - it usually needs to be lowered and re-raised to alter the direction of its beam. For the safety of masthead fittings and lights it is better if these yagis are light in weight and made largely of, for example, plastic tubes supporing internal wire conductors. Operating in this way is best reserved for when in harbour or at anchor, to avoid interfering with the operation of the boat. Repeated loss of signal due to rolling and pitching would make it impractical for useful communication at sea anyway.

For FM operation on the 2 m band, the masthead vertical whip that is normally installed for marine VHF operation will provide good omni-directional vertically polarised signals. The frequency of operation around 145 MHz is close enough to the antenna's design frequency of 156 MHz that most amateur tranceivers will not need an ATU and will not suffer unduly from the SWR.