257. THE RADIOTELEPHONE OPTIONS AVAILABLE FOR SMALL CRAFT

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.

259. HF MARINE SSB EQUIPMENT

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.

260. MODES OF PROPAGATION AND FREQUENCY SELECTION

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