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Radio Principles

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.

In 1887, Heinrich Hertz demonstrated that electromagnetic energy could be sent out into space in the form of radio waves, which travel through the atmosphere at approximately the speed of light. A radio wave is an electromagnetic wave characterized by the frequency at which the energy is oscillating between electric and magnetic fields. The frequency expressed in Hertz (Hz) or cycles per second and is related to the wavelength λ, which is the distance a radio wave travels during one oscillation: λ = c/f   (with c equal to the speed of light).

Radio Communication

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:

  • a device to capture the information into an electrical signal (e.g. a microphone). This signal is called the base-band signal and it is normally in the audio range.
  • a modulator will put the information of the base-band signal on the selected radio frequency.
  • the transmitter is a power RF amplifier, which will put the modulated radio frequency on the antenna
  • the antenna is an electrically conducting rod of a specified length, which will radiate an electromagnetic radio wave when an electrical radio signal is applied to it. The frequency of the applied radio signal must be tuned to the length of the antenna.
    At the receiver, an incident electromagnetic radio wave will produce an electrical radio signal at the antenna.
  • the RF amplifier on the receiver side will amplify the small RF signal captured by the receiver antenna.
  • the demodulator will recover the original base-band information from the received radio signal.
  • finally, a device to reproduce the conveyed information from the electrical base-band signal is needed (e.g. a loud speaker or head phone).

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.

Radio Waves

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.

Signals are basically characterized by their amplitude ("strength") and their frequency ("pitch"). The frequency of signals is expressed in Hertz (Hz) or cycles per second. Humans can hear "tones" in the range of 30 Hz, which is a very "low" tone, up to about 12 000 Hz, which is a very "high" tone. This range is called the audio spectrum. Principally, radio systems are designed to transport only these audible tones.

Since signals from the audio range have such a low frequency that they will not radiate from an antenna, a high frequency radio signal has to be used as a "carrier" signal to transport the information through the air. This principle takes advantage of the fact that - in contrast to low-frequency signals - high-frequency radio signals will "radiate" easily from an appropriate antenna into the atmosphere.

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.

Radio-frequency Bands:

LF-band 30to300 kHz its main use is for aircraft beacons, navigation (LORAN), information, and weather systems
MF-band 300to3 000 kHz Navtex uses 518 kHz and 490 kHz for important digital text broadcasts
HF-band 3 000to30 000 kHz this range is extensively used for medium and long range terrestrial radio communication
VHF-band 30 000to300 000 kHz VHF has some specific ranges used for terrestrial radio broadcast (87-108MHz), for aircraft communications (108-136MHz), for amateur radio (144-146MHz) and for marine communications (156-174 MHz)
UHF-band 300 000to3 000 000 kHz satellite-based positioning systems as well as mobile phones use the UHF spectrum

Microwave frequency bands:

L-band 950to1 450 MHz The carriers of the GPS, Glonas and Galileo positioning systems are in the L band
S-band 1 500to5 200 MHzThe S band is used by weather radar and some communications satellites (e.g. Inmarsat)
C-band 3 900to6 200 MHz The C band is primarily used for open satellite communications
X-band 5 200to10 900 MHz The X band is used by some communications satellites and for radars systems

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:
  • MF/HF-SSB: about 160 channels in the different MF/HF bands of 2 , 4, 6, 8, 12, 16, 20 and 25 MHz
  • VHF: 57 channels in the range 156.0 MHz - 164.0 MHz

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.

From a transmitting antenna, radio waves are normally radiated in all horizontal directions. There are directional antennae, but except for satellites these are not used in marine communications.
In the vertical plane, radio energy is radiated both horizontally and in several lobes upward at various angles. The horizontal radiation is transmitted as ground wave and the rest of the energy is radiated in the form of sky wave (see also details on Ionospheric Radio Propagation).

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 be received.

What happens to the energy radiated upward mainly depends on the frequency and varying conditions of the ionosphere. The sky waves may be reflected back to the Earth's surface by one of several layers of ionization in the ionosphere, or they may penetrate these layers and dissipate in outer space. These reflecting layers in the ionosphere, form and dissolve at various times of the day, in different seasons and are influenced by changing sunspot conditions.
As a consequence, the range and quality performance of radio equipment may strongly depend on the conditions of the ionosphere. This is especially the case for the lower frequency bands (MF/HF), which are reflected better than the higher frequency ranges (VHF/UHF).

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.
VHF equipment on the other hand will be used mainly for short range communication (typically 10 nautical miles) because VHF radio signals are poorly reflected in the ionosphere. Communication between VHF radio stations is based on "line-of-sight" and is primary determined by the height of the antennas above the sea level.


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.
The basic carrier signal has two distinct parameter that may be altered to carry the base-band information: the frequency and the amplitude. Hence, in voice or audio communication two types of modulation are commonly applied: amplitude modulation (AM) and frequency modulation (FM).

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.
This type of modulation is simple, but it wastes three quarters of the transmitter power in a redundant side band and in the carrier signal. Advancing technology developed single-sideband radio (SSB), a system of modulation in which the carrier and one side band are eliminated in the transmitter without diminishing the effectiveness of the communications. Although the system is still AM, it is referred to as SSB and more specific as J3E.

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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