Boats that venture off-shore will quickly move beyond the normal communication range of VHF radio. Although the 406 MHz EPIRB system provides remarkable emergency signaling capability it is an emergency one-way system. Maintaining communication, for both routine and emergency purposes requires the use of either satellite or high frequency radio.

The same electronic developments that have so remarkably influenced VHF radio transceivers have made HF radios capable of world-wide communication practical for even small boats. Today's HF transceivers transmit and receive signals in a format called single sideband, SSB.
This method of radio communication differs from both the narrow-band FM system used for marine VHF radio and from the double sideband amplitude modulation system employed for conventional AM broadcast radios. Single sideband modulation provides the advantage of using all of the available transmitter power to convey information using voice or digital means. Typical uses include communication with commercial high frequency shore stations that can connect to the world's telephone system, communication with other vessels and amateur (ham) radio.

Many cruising sailors find that the SSB "Nets", informal groups of ham operators who communicate on specific frequencies at preset times are an invaluable resource. The use of SSB frequencies allows much wider area coverage than is possible on VHF. The exchange of information includes weather, notice to mariner information of possible value to those transiting the area, arrangements for meetings of friends and so long as no commercial value is involved, arrangements for obtaining repair parts when sailing in out of the way places. Obtaining a ham license is now much easier than ever before. The FCC deleted the Morse Code requirement from the Amateur Radio Service Part 97 rules on 23 February, 2007. Unlike other radio station licenses, there is no charge for a ham license

While marine VHF radio transmitter power is generally limited by law to 25 watts, HF-SSB sets are generally capable of emitting between 100 and 150 watts of peak envelope power (PEP). These power levels, when delivered to a properly designed antenna system and using the appropriate frequencies, can provide communication over distances of hundreds to tens of thousands of miles. The wiring that supplies DC power for the SSB set must be properly sized for the radio's peak current drain and the distance from the boat's battery. Although the average transmitting power drain of a 150 watt PEP radio won't exceed 5 amperes at 12 volts, the momentary peak drain can reach 25-30 amperes.

Short wave signals do not travel far along the earth's surface. Communication over very long distances occurs because transmitted signals proceeding upward in the atmosphere may encounter a layer of air molecules that have been electrically modified by energy from the sun. These layers can act as mirrors, reflecting the radio energy downward toward the surface of the earth. In effect, the transmitted energy bounces off the high altitude layer and can then be received at a point far distant from its origin. The altitude and the reflecting quality of the ionized reflecting layers in the atmosphere is influenced by the season of the year and the time of day at both the transmitting and receiving stations. Establishing communication over long distances therefore requires choosing frequencies that will return to the earth's surface at the location of the listener. Numerous aids are available to guide the choice of appropriate frequency.

The microphone used with the HF set and the way it is used can determine the ability of the remote listener to understand your message. As with VHF sets, the microphone is designed to be held close to the user's mouth, no more than an inch or two away. It is best to use the mic in as quiet an environment as possible. Since most HF communications can occur from a below deck position wind noise won't normally pose much of a problem.

HF-SSB sets can be used with various types of modems (modulator-demodulator) to provide digital communication capability. Some modems allow a computer to be used as a form of teletype, others allow the computer to send and receive e mail via the radio. The e mail services available include commercial providers such as WLO, Mobile, Alabama and PinOak, in New Jersey. Licensed amateur radio operators can use e mail services provided on a voluntary basis by some ham organizations such as Sail Mail andWinLink. HF-SSB radios also have great value in receiving weather reports and notices to mariners and can be connected to computers to decode weatherfax transmissions.