64 MODERN OCEAN COMMUNICATIONS

256. THE BASIC PRINCIPLES OF RADIO COMMUNICATION

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.

 

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.

  1. 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).
  2. 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)].
  3. *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).
  4. *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.
  5. 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.
  6. *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.

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