It is not necessary to be an electronics
expert to use radio communication equipment, but an understanding of the
fundamentals will help to make optimal use of your marine radio.
Radio communication consists of carrying information (e.g. voice or digital data) through the air by means of radio waves. For this purpose a radio set consists of a chain of components each with a special task of capturing, processing, transporting and reproducing the information that must be conveyed:
In the following discussion mainly marine radio equipment will be considered. The marine radios include mounted VHF, portable VHF, and single-sideband (SSB) transceivers. Non-marine communication equipment such as amateur radio (HAM), the citizen's band (CB) and cellular telephones will not be considered here.
The basics of modern radio communication have already
been conceived in the beginning of the 20th century.
At that time radio was developed to broadcast basically voice and music.
But very soon also the possibility to transfer more complex information
such as text (in the form of telegrams) over radio waves was pursued.
This led to the Morse telegraphy.
Morse code was based on an audible tone, which was switched according
to fixed patterns, representing the characters of the alphabet.
Common to voice, music or Morse telegraphy is that the basic information
is an audible signal.
The Radio Frequency Spectrum
The frequency of signals is generally expressed in Hertz (Hz), but for radio signals the numbers are so large that the units used are kilohertz (kHz), megahertz (MHz) and gigahertz (GHz), for respective values of a thousand, a million and a billion times a Hertz.
The radio frequency spectrum is the range of frequencies that will radiate efficiently from the appropriate antenna. Since this is a very broad range, the radio frequency spectrum is divided into different frequency bands. Each band has specific characteristics with respect to radiation and attenuation in the atmosphere. As a consequence, each band has become allocated to specific communication applications as summarized in the tables below.
Microwave frequency bands:
Within these radio bands certain frequency ranges are allocated for marine radio communication. Other ranges are allocated for radio and television broadcasting and several other telecommunication applications. There is no international standard for the allocation of these different services to specific radio frequencies. However, the ITU has adopted worldwide fixed frequencies for marine radio communications. Here is a rough summary of the marine radio channels:
These marine channels have typically 25kHz channel width for VHF and
5kHz channel width for MF/HF-SSB.
Radio Wave Propagation
There are two ways radio energy travels from transmitter to
receiver: by means of ground waves or by sky waves.
The ground waves travel along the surface of the Earth whereas sky waves first travel into
the ionosphere where they may be reflected and returned to the surface of the Earth.
Depending on the inclination with which radio waves are transmitted, large skips
(several thousand Km) can be made between the location of the transmitter and the
location where the reflected sky wave reached the surface of the Earth again.
Ground waves reach out to the horizon and to distances
beyond the horizon that vary with their frequency. In general, the lower
the frequency the more they "bend" over the horizon and the farther they can
From the nature of wave propagation, MF and HF equipment
may be well suited for long range communication (1000 nautical miles and more)
- through "sky waves" - but performance will depend on the conditions
of the ionosphere and communication will not always be reliable.
Modulation is the process of bringing the information
such as voice, music or data (base-band) that must be transmitted by
radio, onto the basic carrier wave that will be able to travel from the
transmit antenna to the receiver antenna.
In the lower frequency MF and HF bands, the audio base-band information is transmitted by amplitude modulation. With this method, the frequency of the carrier signal does not vary. Speech or other audio information is communicated by changing the amplitude of the transmitted radio signal. Basically, amplitude of the information signal is translated into a proportional amplitude variation of the radio signal.
Simple AM signals have half of their power
in a central carrier, with the other half divided equally between two side
bands, on either side of the carrier in - double-sideband transmission.
Technically, either the upper or lower side band can be used but on the MF/HF marine channels only Upper Side Band modulation is used.
On the MF distress channel (2182 kHz) a modulation technique called H3E is still in use. This modulation format is also Single Side-Band but it contains the full carrier signal. In distress situations, this carrier signal may help a Search-and-Rescue vessels to find the transmitting radio station.
The band most used for marine communications, VHF, uses frequency modulation. The carrier wave is steady in amplitude level but is varied in frequency around it's center frequency. The audio frequencies that are being transmitted determine the rate at which the radio frequency varies. The extend to which the radio frequency varies, or deviates, is determined by the loudness level of the audio information. So basically, amplitude of the information signal is translated into frequency deviations of the radio signal.
Modes of Operation
An understanding of the two different modes of radio operation used in marine communications is essential for the proper use of radio channels. On the VHF band, most communications are in the simplex mode, where only one station can transmit at a time. The other can only listen and await its turn to transmit. You cannot interrupt the person speaking as you can in an ordinary telephone call - a mode called duplex communications.
Normally, the same frequency is used by both, or all, stations for transmitting and receiving. But even when two separate frequencies are used for transmitting and receiving, as may be feasible with SSB equipment, it is still not possible to transmit and receive at the same moment, since this would also require two different antennae.
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