Coastal navigators will be aware of the value of ship-to-ship and ship-to-shore radio communication and that there are very few small craft venturing offshore today without a VHF radiotelephone. Ocean cruising mariners also need a reliable means of communication but over much greater distances, sometimes across half the circumference of the Earth. VHF radiotelephony, being essentially a line-of-sight form of radio, has far too short a range for ocean and worldwide communication. For long-distance ocean cruising some form of long-range radio communication is strongly recommended. These chapters provide information about the various options available.

It is not necessary for the small craft mariner to have a detailed understanding of the principles of radio transmission, but knowledge of the meaning of, and relationship between, the various terms used is desirable.

The transmission of a message (information) by radio can take two basic forms. One, wireless telegraphy (usually abbreviated to W/T) uses the transmission of Morse Code at high speed and was used mainly by large ocean-going vessels possessing powerful equipment and skilled and certificated radio operators depressing a telegraphic key. The other, radiotelephony (usually abbreviated to R/T) uses normal voice (speech) transmissions and is available to all vessels, large or small under licence (in the UK) from British Telecom. /T has largely replaced W/T.

Wavelength is a measure of the distance travelled by a radio wave during one alternating cycle – peak to peak. Conversely, the number of alternating cycles per second is a measure of the frequency.

Frequencies are measured in cycles per second (c/s) or Hertz (Hz) in honour of Dr. Heinrich Hertz, an early German radio pioneer (kHz = kc/s). Frequencies greater than 1000 Hz are expressed in kilohertz (1000 Hz = I kHz) For frequencies higher than 30,000 kHz, the term megahertz is used (1000 kHz = I MHz).

Electromagnetic radio waves in the range 10 kHz to 300,000 kHz form the usable radio frequency spectrum, parts of which are used for broadcasting, communications and radio navigation systems throughout the world. The velocity of electromagnetic radio waves is approximately 300 x 10’ metres per second. This figure is important because it enables the wavelength of the transmitted frequency to be calculated, thus:

Wavelength (metres)   300 x 106

                                      Frequency (kHz)

From this formula it can be seen, for example that 1515 metres is equivalent to 198 kHz (BBC Radio 4) and that 198 metres equals 1515 kHz.

Radio waves are transmitted (propagated) in one or more of three main waveforms: ground (surface) wave, sky wave, and space (direct) wave, as shown in fig. 64-1. Intelligence is carried on one or more of these waves depending on the carrier frequency, which is used.

The MF (Medium Frequency) Band extends from 300 to 3000 kHz and includes most of the land-based broadcasting stations as well as the frequencies used by coast stations throughout the world. HF communication is mainly by ground wave which suffers greater attenuation (thinning and weakening) as the frequency is increased. In the band below 1500 kHz sky-waves are returned both by day and night although communication using these waves is unreliable. Above 1500 kHz the returned sky-wave has greater reliability but is affected by diurnal and seasonal changes in the ionosphere and by sunspots. This band has a range of approximately 200 miles by day (more by night) depending on aerials and power output.

The HF (High Frequency) Band extends from 3 to 30 kHz and is widely used for terrestrial global broadcasting and communications. Its ground-wave range is insignificant and the use of this band depends on sky-waves bounced off the ionosphere.

The VHF (Very High Frequency) Band lies next to the HF band in the radio spectrum and extends from 30 to 300 Mhz but, despite this, VHF radio-waves behave in an entirely different manner to HF waves. This is because VHF communication is via the space wave, which may be ground reflected. Space waves effectively produce line of sight transmission, i.e., a maximum of between 40 and 60 miles depending on the respective elevations of the transmitting and receiving aerials. Large objects in the path of a space wave will produce a blind spot in which reception is extremely difficult.

Fig. 64-2 shows the frequency allocations at a glance, including the positions in which HF radio, MF radio and the TV channels lie in the radio spectrum.

Intelligence (the ‘message’ ) is impressed upon a radio emission by modulation. The amplitude of the radio emission can be fluctuated at a rate and to a degree corresponding to a sound wave to produce speech or music. 

The oldest system was called amplitude modulation (AM) and this is still used for long wave, short wave and medium wave broadcasting but not for marine communication.

When a band of audio frequencies is added to a radio frequency carrier, two sidebands are created, one on either side of the carrier, the upper sideband (USB) and the lower sideband (LSB). The resultant is commonly called AM but its full title is amplitude modulated double sideband (AMDSB) or an A3E signal (see fig. 64-2). Like most low-tech systems it is simple, effective, reliable and cheap, but grossly inefficient. It is inefficient because the human voice has a bandwidth of 27 kHz (3000 Hz – 300 Hz) but, when used to amplitude modulate a transmitter, the resultant signal (shown in fig. 64-3) is 6 kHz wide, so that channels must be spaced at least 6 kHz intervals. Additionally, two-thirds of the transmitted power is contained in the carrier, which does not, in itself, convey any information. For example, a 150-watt All transmitter actually transmits only 25 watts of information.

Clearly it would be better if the whole transmitter power of, say, 150 watts were concentrated into one of the sidebands. Not only would it be more efficient, but the transmitted bandwidth would be only 2.7 kHz, enabling the number of channels to be doubled, and no power would be wasted in transmitting the carrier.

Most communication services now transmit a single sideband (SSB) without a carrier, called J3E emission. The transmitter filters out the unwanted sideband and the carrier, leaving the single sideband to be amplified and passed to the aerial (antenna) for transmission. Although technically immaterial which sideband is used, it is in fact the upper sideband (USB) which is transmitted (see fig. 64-3).

When the marine radio world changed from HF to SSB on I Jan 1972 it immediately doubled the number of available MF and HF channels by decreasing the channel spacing from 6 kHz to 3 kHz thus allowing much more effective use of transmitter power.

The single sideband is able to ‘support’ itself without a carrier because it is a band of radio frequencies. Although it is still necessary to generate a carrier within the transmitter for modulation purposes, it is not necessary to transmit the carrier. Having converted a band of audio frequencies into a band of radio frequencies by frequency addition, it would be a waste of power to actually transmit the carrier.

A second way of impressing intelligence on a radio wave is to fluctuate the frequency with the amplitude staying constant. This is called frequency modulation (FM) and is used in high quality broadcasting and in VHF radiotelephones (see fig. 64-4).

In order to understand the use and allocation of VHF and SSB Channels, it should be remembered that there are two separate circuits within any radiotelephone set. One is used for transmission and the other for reception, and in most of the equipment used by small craft, the two circuits are never alive together.

However, commercial ships and Coast radio Stations use equipment in which both circuits are in operation at the same time so that the operator can speak and listen at the same time, as on a land-line telephone. This is called Duplex operation, and not only are Duplex sets themselves considerably more expensive, but it is necessary to have two separate aerials. or highly sophisticated Duplex filters.

 For Simplex operation, the set can be used only on those channels, which use the same frequency both for transmission and reception. Normally the set’s receive circuit only is in operation, and the transmission circuit is activated by a spring-loaded ‘press-to-speak’ switch in the handset. In practice this mode is highly restrictive because ‘Simplex only’ sets can never be used for traffic to Coast Radio Stations.

A compromise used on almost all small craft is radiotelephone equipment using Semi-Duplex operation.

These sets can be used on all channels, both Simplex and Duplex, but speech can travel in only one direction at any one time. Duplex channels can be used with Semi Duplex equipment because, although the small craft (Simplex) end of a radiotelephony exchange still has to press to speak as with ordinary Simplex channels, the switching at the Coast Radio Station is automatic. Semi-Duplex equipment is considerably less expensive than full Duplex radiotelephones.



There are six distinct types of radio communication systems of interest to small craft mariners listed below. Of these, those marked with an asterisk are of particular interest to ocean cruising navigators for long-distance communication.

Very High Frequency (VHF) sets have a range a little more than line of sight between aerials involved, but are much cheaper than MF and HF, simple to install and relatively free from interference. It is the VHF frequencies which account for practically all small craft communication in coastal waters. [see fig. 64-6(a).

Medium Frequency (MF) SSB sets with a range of 200-300 miles (320-480 km) and operating in the 2 MHz band were fitted to some yachts before VHF became common but are now very expensive. Nevertheless MF R/T is essential for mariners who must keep in touch with the shore when outside the 30-50 mile range of VHF R/T [see fig. 64-6(b)].

*Long Range High Frequency (HF) SSB sets providing world-wide communication through Portishead Radio, but these are large and costly so that they are appropriate only for yachts making extended ocean cruises. These sets usually incorporate the MF band.  SSB is described in 53 and 54 of this manual (see 52-6(c).

*Standard C’ and ‘M’ Satcom sets providing an SES (Ship Earth Station) under the INMARSAT system of world-wide satellite communication (described in 55 of this Manual), also limited to yachts making ocean cruises.

Citizens Band (CB) radio is not a real substitute for VHF R/T afloat because it has less range and because the CB emergency channel 09 is not monitored in the same way as the VHF emergency channel 16, although it can be useful for club purposes (organising events) and for social chat.

*Amateur (‘Ham’) SSB Radio sets can be purchased for sometimes less than half the price of a Marine SSB set and are popular on ocean cruising boats because of the many marine nets (networks) around the world. However, operators are required to take the Radio Amateurs Exam (RAE), which is technical and difficult. The system is described later in this chapter.


A complete marine SSB station consists of a transceiver (combined transmitter and receiver), an aerial (antenna) and an aerial tuning unit (sometimes called an antenna coupler) between the transceiver and the antenna. Both the transceiver and the coupler must be electronically bonded to a proper ground (earth). Without a good ground, an SSB station simply will not function adequately.

Before recent advances in integrated circuit technology, virtually all marine radios were crystal controlled. Advanced sets nowadays use modern digital circuitry to synthesise operating frequencies. Crystals continue to be used in even the newest synthesised radios, but only one crystal is required. The single crystal serves as a frequency reference, which is converted into all other frequencies. Advantages include more frequencies, increased reliability and the ability to add features like dual frequency watch or frequency scanning.

Crystal-controlled SSB radios may cost less initially than those which are fully synthesised but in the long run a synthesised set generally proves to be more economical on account of its increased flexibility and reliability. The labour cost of adding channels to a crystal-controlled set may well exceed the amount saved by not buying a synthesised set in the first place.

With a modern synthesised SSB radio (see fig. 65-3) it is only necessary to keyboard enter the channels and frequencies required. [,lost mariners need about five frequencies in each band, so that an overall total of 30 memorised frequencies is usually adequate. The older-style crystal sets offering 11 channels simply do not provide enough frequency capability to satisfy the needs of most cruising yachtsmen.

A synthesised set, which will ‘memorise’ more than 30 channels allows the operator to pre-program not only marine frequencies, but also some of these receive-only world-wide general coverage frequencies which provide ocean weather services, international time signals, navigational warning and general broadcasting facilities giving world-wide news and entertainment.

Most SSB transceivers offer at least 100 watts output and this is adequate for long-distance HF communication. However, if the set is required for only short to medium range transmissions in the lower (MF) frequency bands then 20 watts output may prove sufficient. Higher output means a stronger, more solid signal provided that the onboard installation is a good one.

There are many options on the types of marine SSB transceivers available. The best cover all the MF and HF frequencies, but some cheaper versions cover only the lower two or three frequency bands, thus severely limiting the set’s long-range potential. Since VHF radio is needed in any case for short-range coastal work, it is worth considering a combined VHF/SSB set if you haven’t already got VHF.

Some manufacturers offer channel scanning capability. his can be a helpful feature to avoid missing calls. canners permit operators to listen for traffic on several channels at once because the transceiver switches sequentially from channel to channel until an incoming signal is detected, then stops scanning for the operator to monitor the call. Scanners permit the operator to monitor call and distress frequencies while handling communications on other bands.

Some units are sold with an option called a speech processor or speech compressor. SSB radios without speech processors depend on the strength of the operator’s voice to activate the modulation circuits that inject ‘talk power’ into the transmitted signal. If the operator speaks softly into the microphone, there is little modulation, but if the operator speaks too loudly, the signal may be over-modulated, causing distortion that can render the transmitted speech unintelligible. A speech processor is a special circuit that will amplify weak voices and attenuate powerful voices, thus optimising the modulation to just the right level for the transmitted signal.

One of the latest developments in maritime communications is the automatic radiotelephone for VHF, MF and HF called Auto link RT. This operates through the yacht’s existing equipment with an inexpensive, easily-fitted modem/handset (see fig. 65-4) manufactured by Climat UK Ltd. There is a choice of models available from your local dealer. Autolink RT provides much faster connecting times bypassing the Coast Station operator (thus removing language barriers) and giving direct access to the UK’s and 99% of the world’s telephones. It also handles Distress, Urgency and Safety traffic on separate channels so that other valuable radio paths are not tied up. Unlike cellular systems, there is no rental for Autolink RT with a one-minute minimum charge, you simply ‘pay as you say’. More details of Auto link RT can be obtained from Maritime Radio Customer Services, BT Radio Station, Highbridge, Somerset, TA9 3JY (Tel. 0278 772253).

The installation of transceivers, aerials, couplers and an adequate grounding ‘earthing’ system is discussed in a later section of this Study.


The general principles of radio wave propagation have already been described, but this description can be expanded as follows.  Low and medium frequencies follow the curvature of the Earth so that, for any given frequency, the range is determined by the amount of power transmitted.  For a given amount of power, range is inversely proportional to frequency but, for a given frequency, range is proportional to power. Thus the Marine MF band gives a maximum range of 300 miles with the maximum allowed power of 400 watts.

Frequencies above 3 MHz do not follow the curvature of the Earth (ground wave) and short waves between 3 MHz and 30 MHz (the HF band) are ‘reflected’ off the ionosphere high above the Earth to give ranges of several thousand miles, and it can be said that, within the HF band, range increases with increasing frequency.

However, the situation is not quite as straightforward as depicted in fig. 65-6(c), where the ionosphere is shown as one neat layer for simplicity.  At times there can be four layers designated D. E, FI and F2 working outwards from the Earth as shown in fig. 65-5. These layers each have slightly different characteristics.

The D layer is the lowest layer at something in the order of 30-50 miles above the Earth’s surface, and it exists only during the warmest part of the day.  It does not refract (bend) radio waves and it absorbs all energy below 3 MHz.  In temperate latitudes during the winter it exists between about one hour after sunrise and one hour before sunset, while in summer it lasts about half an hour longer. 

The E layer is strongly ionised during the day and remains weakly ionised at night.  Its height, at between 60 and 90 miles above the Earth, is almost twice that of the D layer and it refracts radio waves of up to about 8 MHz during the day and about 4 MHz at night. 

The F1 Layer is also strongly ionised during the day and refracts radio waves of between about 8-16 1,1Hz at a height of between about 90 and 150 miles. 

The F2 Layer, too, is strongly ionised by day at a height between 150 and 250 miles in summer (a little lower in winter).  It refracts radio waves between 16-30 MHz. 

At night the ionosphere situation is very different (fig. 65-6) because the layers are formed by intense ultra violet radiation from the Sun which disappears at night.  However, the layers do not disappear completely at sunset.  The E layer remains weakly ionised and the two F layers combine to form one F layer roughly midway between the two daytime layers.

At night, since the D layer (which absorbs radio waves below 3 MHz) has disappeared, the E layer can refract radio waves in the MF band , thus increasing the range of MF R/T to 1000 miles or more by virtue of the sky wave. The HF (short wave) band is also affected at night because of the weakly ionised F layer.  The maximum frequency which can be refracted is reduced by a factor of about 2, so that whatever frequency has been found to be best by day must be reduced to about half that at night.  For example, if the 16 MHz band was being used by day, the 8 Mhz band will be found best at night.

Although in general it may be said that the higher the frequency, the greater the range, reference to fig. 65-6 shows that between the end of the fairly short ground wave coverage and the start of the sky wave coverage, there exists a dead zone or zone of silence. Thus, the frequency and therefore the achieved range, could be too great.  Taking all the variable factors (time of day or night, the season, sunspot activity, etc) into consideration it is up to the skill of the operator to use the best frequency for the distance covered – the OWF (optimum working frequency). The greatest distance that can be covered in one ‘hop’ is about 2,500 miles (4000 km).  However, the signal can be reflected back into the ionosphere to be refracted and reflected several times, eventually to encircle the world.

The unpredictable effect of sudden ionospheric disturbances (SIDS) and magnetic storms (products of an unstable Sun) is to cause a radio ‘blackout’ for a few hours and sometimes for a day or two

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