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.
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:
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.
MF (Medium Frequency) Band
HF (High Frequency) Band
VHF (Very High Frequency) Band
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.
amplitude modulation (AM)
two sidebandsupper sideband (USB) lower sideband (LSB)amplitude modulated double sideband (AMDSB)
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.
single sideband (SSB)
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.
frequency modulation (FM)
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.
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.