65 MARINE SINGLE SIDEBAND MF AND HF COMMUNICATIONS

258. OVERVIEW OF MARINE SSB RADIO

Marine single sideband (SSB) radio is a long-range communications system designed to reach beyond the range limits of VHF. As explained previously, VHF communication is primarily line of sight in which the radio waves travel between transmitter and receiver in virtually a straight line. It is essentially a short-range system, seldom permitting contact at distances beyond 40 miles. On the other hand, SSB radio is propagated by means of sky-waves, radio waves which travel from place to place by bouncing off the ionosphere, so that radio contact can be made between stations separated by many thousands of miles (see fig. 65-1).

The maximum possible communications distance with SSB radio is determined by the interplay of many factors, of which the time of day, the choice of marine SSB frequencies and the quality of the shipboard installation are generally the most important. In favourable circumstances, communications around the world are possible.

Marine SSB equipment is more expensive than VHF radiotelephone gear, but compact SSB radiotelephones are now available at substantially lower cost than they were only a few years ago. An SSB set should never be used when VHF radio would suffice, since this could cause needless crowding on the marine SSB frequencies. SSB should only be used for extended range communication outside the limits of radiotelephony.

A marine SSB transceiver broadcasts on one or more of six allocated frequency bands: 2 MHz, 4 MHz, 8 MHz, 12 MHz, 16 MHz and 22 MHz. The 2 MHz band is known as the medium frequency (MF) or ‘coastal communications’ band because frequencies in this band (between 2 and 3 MHz) are typically used for coastal communications just beyond VHF range. These frequencies are referred to by their actual values in kilohertz; for example 2182 kHz is the calling and distress frequency for [HF-SSB communications. The other five frequency bands between 4 MHz and 22 MHz inclusive are known collectively as the high frequency (HF) or Short Wave bands and are used for long-distance communication. Individual frequencies within the HF spectrum are usually given channel designations.

When a transmission is made on any of the SSB bands, one component of the radio signal hugs the Earth’s surface, is called the ground-wave and will travel approximately 100-200 miles. This ground-wave is the functional component of SSB transmissions in the 2 Mhz band between a vessel at sea and a shore station or between two vessels not more than 100 miles apart.

Ground-wave propagation is consistent day or night, depending for the most part only on a good, strong transmitted signal, although it can be interrupted by thunderstorms or other atmospheric disturbances.

The sky-wave component of the transmitted radio signal enables the long-distance communication for which SSB radio is noted. HF transmissions make use of the reflecting properties of the ionosphere to extend the transmission range. The signal bounces between the Earth’s surface and the ionosphere as it travels round the globe and this can give long-range coverage up to 6000 miles. Transmission and reception can suffer from the problem that the height of the ionosphere is not constant, ranging from 30 to 300 miles above the Earth’s surface. The height and intensity of this reflecting layer varies on a daily and seasonal basis as well as with the l-year solar cycle and this makes the frequency selected critical in order to obtain good communication. Raute errors can occur due to the signal skipping, and these ‘blank’ zones are usually between 50 and 750 miles from the transmitting station.

As a result, the operating frequency has to be selected to match the conditions and the positions of both transmitter and receiver. HF radio is primarily for making telephone and telex links and once established these can be quite satisfactory, but an element of patience is needed to make the connection. Links are made through international radio shore stations (such as Portishead Radio in the UK) and thence to the land telephone system.

In general, lower frequencies bounce back to Earth close in, while frequencies in the 12 MHz band reflect over fairly long distances, typically 3000 miles. In the 22 MHz band it is possible to communicate from, say, the Mediterranean to the W. Coast of the USA while, if the ionosphere is strong enough, the sky-wave will often bound a second time, doubling the maximum possible communications distance. In such circumstances it is sometimes possible to talk all the way round the world on 22 MHz.

During daylight hours the ionosphere rises and its ion density increases. The range available on the higher frequencies increases accordingly but the sky-wave components of the lower frequency transmissions are absorbed by the ionosphere. During the night, the ionosphere gradually lowers and its ion density decreases so that sky-waves in the 16 and 22 MHz bands may not be reflected at all after the evening hours. Sky-waves are unaffected by local weather conditions.

After a few weeks of experimenting with an SSB radiotelephone, a user will begin to get a feel for the expected range on any particular band of frequencies. The ground-wave range for ship-to-ship contact varies from 150 miles in the 2 MHz band to about 50 miles in the 16 Mhz band. The table in fig. 65-2 gives an idea of the ranges to expect from the sky-wave component of SSB transmissions.

Generally speaking, going to a higher frequency increases the maximum communication range but, if the signal is literally skipping over the desired receiving station, the user should switch to a lower frequency. The greater the maximum communication range afforded by a particular band at a particular time of day, the broader the intervening skip zone.

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

261. WORLD-WIDE COMMUNICATION WITH SSB: FREQUENCIES AVAILABLE

There are Coast Radio Stations (CRS) at strategic points around the coastlines of most of the world’s maritime nations.  For vessels beyond the VHF and HF range of Coast Stations and for world-wide communication with the UK there is a special international CRS at Portishead (a small seaside resort between Avonmouth and Clevedon at the mouth of the River Severn).

The long-range world-wide (HF) service provided by Portishead Radio forms the centre of a radio traffic handling system which controls transmitting facilities at Rugby and reception facilities at Somerton (Somerset).  Portishead Radio is a communications centre which can handle any aspect of long range communications for mobile and fixed stations.

Radiotelephone, radiotelex and wireless telegraphy are the three main forms of radio communication.  Of these, the small craft mariner is principally interested in radiotelephony.

The radiotelephone frequencies for Portishead Radio are shown in fig. 65-7.  Details of the frequencies and services of all CRS worldwide are contained in the Admiralty list of Radio Signals, Volume 1. Those of NW Europe are given in Macmillan’s N.A..

In addition to controlling communications in their respective areas and serving as a link between ship stations and the inland telephone network, CRS have several other functions: (1) to transmit traffic lists at fixed times, so that ships may be aware of traffic (i.e. messages) on hand for them (2) to transmit navigational warnings and weather bulletins at fixed times and (3) to control the distress, urgency, safety and medical traffic in their particular area.

Most of the major CRS in the UK, Europe and USA operate on both VHF and MF, while international stations such as Portishead operate on HF.

The primary purpose of radio communication is for safety and distress.  In order for this to be effective, and so that she may pick up any messages specifically for her, a craft fitted with ship/shore radio must keep a radio watch.  Small craft, for which radio is not compulsory, may keep either an intermittent or a continuous radio watch and the Coast Station in whose area they are sailing must be informed of their intentions.  On a comparatively short coastal voyage, where there will in any case always be someone on navigational watch, it is obviously preferable to keep a continuous radio watch.  A radio watch is kept by having the receiver tuned-in to the International R/T distress frequency of 2182 kHz (for MF), 4125 kHz or 6215 kHz (for HF) or Channel 16 (for VHF), which is the frequency which all ships, aircraft and survival craft will use to transmit a distress call.

Transmission on a distress frequency is limited to a maximum of two minutes (other than for distress), and, if possible, should be considerably less so as not to interfere with a distress call or prevent it being heard.

On departure from port, a ship station should notify the nearest Coast Station of her departure, her destination and the type of radio watch she intends to keep.  This is for the purpose of traffic-routeing and such a message is known as a TR message.  A further TR message should be sent to the new Coast Station when the vessel passes from the area of one Coast Station to another (including Portishead Radio) and this should include the name of the vessel, the approximate distance in nautical miles and bearing of the $hip from the Coast Station (or a known geographical location). together with the next port of call.  On arrival at her destination, or an intermediate port of call where she intends to suspend her radio watch, a ship must notify the nearest Coastal Station and, if appropriate, any other Coast Station (such as Portishead Radio) with which she generally communicates.

There are four types of messages which are normally transmitted between ship and Coast Stations: (a) distress, urgency or safety messages, which include calls for medical advice or assistance and for which no charge is made (b) public correspondence, which is the transmission of a telegram or a telephone conversation to or from an address ashore, via the inland Telecom channels and which are charged accordingly (c) service messages, which include TR’s and any queries about traffic-routeing, wording of messages, errors in messages, charges for public correspondence, etc. and which are free of charge and (d) port operations messages which concern the safety of navigation within a port or harbour, and the routeing, berthing, sailing and manoeuvring of ships therein, and which are normally free of charge.  In addition, a ship station may call another ship station on 2182 kHz (for MF), Ch 16 (for VHF) or one of the following frequencies for HF: 4125 kHz, 6215 kHz, 8255 kHz, 12290 kHz, 16420 kHz or 22060 kHz.The ship called should reply on the same frequency, unless reply on another frequency is indicated by the calling ship.  Once contact is established transfer should be made immediately to one of the authorised inter-ship frequencies or Channels for the exchange of traffic.  The intership (simplex) frequencies are given in fig. 65-8.

The R/T distress signal is the word “MAYDAY” (derived from the French “Maidez”), indicating that the ship or aircraft is in grave or imminent danger and requests immediate assistance.  This call may or may not be preceded by the alarm signal, which is a two-tone audio signal giving a distinctive warbling sound, for between 30 and 60 seconds, the purpose of which is to attract the attention of the person on watch, or to actuate automatic alarm devices.  Both HF and IAF distress frequencies have associated auto-alarm systems with which a particular signal triggers an alarm when a radio watch is not being kept, but VHF has no such system.

The content of a distress message and R/T distress/emergency procedure on SSB radio is identical to that used for VHF with which students should be familiar from their studies in Coastal Navigation.