Radio and Terrestrial Microwave
In 1865 James Clerk Maxwell, a British Physicist, predicted that moving electrons would create electromagnetic waves that could propagate through free space. His theory was confirmed in 1887 by German physicist Heinrich Hertz, who first produced and observed this phenomena. The number of times per second that an electromagnetic wave oscillates is called its frequency (ƒ), and is measured in units called Hertz (in honour of Heinrich Hertz). The distance between two consecutive peaks (or troughs) in the wave is called the wavelength, which is designated by the Greek letter Lambda (λ). All electromagnetic waves travel at the speed of light (3 x 108 metres per second) in a vacuum, whatever their frequency. The relationship between frequency, wavelength and the speed of light (c) in a vacuum is given by:
λƒ = c
The electromagnetic spectrum is shown below.
The electromagnetic spectrum
The parts of the electromagnetic spectrum which can be used to transmit information include radio, microwave, infrared and visible light. The higher frequencies (ultra violet, x-rays and gamma rays) are difficult to produce and modulate, do not propogate well through solid objects, and are harmful to living things. The amount of information that can be carried by an electromagnetic wave is largely dependent on its frequency. The higher the frequency, the higher the data rates that can be achieved. By attaching an antenna of the appropriate size to an electrical circuit, electromagnetic waves can be broadcast efficiently and picked up by a receiver some distance away. In 1895 Guglielmo Marconi developed the first radio system, and transmitted a message in Morse code. He subsequently came to England, and in 1897 managed to send messages in Morse code across the Bristol Channel. In 1901, he established the first transatlantic radio communications link between Poldhu in Cornwall and St. John's in Newfoundland, a distance of some 3,200 kilometres. These achievements encouraged Marconi to set up the Marconi Wireless Telegraph & Signal Company, which grew into a major global enterprise. The Marconi company was one of six wireless manufacturing companies that founded the British Broadcasting Company (BBC) in 1922.
Radio frequency (RF) waves are widely used for both indoor and outdoor communication because they are easy to generate, can travel over long distances, and can penetrate buildings easily. Because they travel in all directions from the transmitter (i.e. they are omni-directional), the transmitter and receiver do not need to be carefully aligned. Most radio transmissions are narrow band transmissions, in which most of the transmitted power is concentrated within a small range of frequencies, and are thus said to have a high spectral density.. Radio transmissions that distribute the transmitted power over a wide range of frequencies are called spread spectrum transmissions, and are said to have a low spectral density. Spread spectrum radio transmissions have been used for many years by law enforcement and military organisations because they are difficult both to intercept and to jam. The properties of radio waves are dependent on frequency. At low frequencies, they pass through obstacles easily, but the power falls off sharply as the distance from the transmitter increases. At high frequencies, radio waves tend to travel in straight lines and bounce off obstacles. They are also absorbed by rain. At all frequencies, they are subject to electromagnetic interference from electrical equipment such as electric motors. Their ability to propagate over large distances means that radio transmissions can also interfere with each other, which is one of the main reasons why the use of radio frequencies is tightly controlled by governments. Attempts to standardise the use of radio frequencies worldwide, however, have met with only limited success, and the bandwidth allocation for devices such as cellular phones can vary from one country to another.
In the very low to medium frequency bands, radio waves follow the ground and can be detected at distances of up to about 1000 kilometres. Radio waves at these frequencies easily pass through buildings, and as a consequence are widely used by terrestrial radio stations. The relatively low bandwidth, however, means that they are not suitable for data communication. High (HF) and very high (VHF) frequency radio waves that reach the ionosphere, which is a layer of charged particles approximately 100-500 km above the earth's surface, are refracted by it and sent back to earth. These bands are used by amateur radio operators to talk over long distances, and are also used for military radio communications. The main radio frequency bands are shown in the table below, together with some of the common applications for each band.
|Frequency band||Name||Principal applications|
|30 - 300 kHz||Low Frequency (LF)||Navigation|
|300 - 3000 kHz||Medium Frequency (MF)||Commercial AM radio|
|3 - 30 MHz||High Frequency (HF)||Short wave radio, CB radio|
|30 - 300 MHz||Very High Frequency (VHF)||VHF television, FM radio|
|300 - 3000 MHz||Ultra High Frequency (UHF)||UHF television, terrestrial microwave|
At frequencies of 100 MHz and above, electromagnetic waves travel in straight lines and can be narrowly focused. A parabolic dish antenna can be used to focus the transmitted power into a narrow beam to give a high signal to noise ratio. The antenna is fixed rigidly, and focuses a narrow beam to achieve line-of-sight transmission to the receiving antenna. Before the advent of optical fibre, some long distance telephone transmission systems were heavily dependent on the use of a series of microwave towers. Because microwaves travel in a straight line, the curvature of the earth limits the maximum distance over which microwave towers can transmit, so repeaters are needed to compensate for this limitation. As a general rule, the higher the towers are, the further apart they can be.
BBC Radio York's 103.7 FM transmitter mast
The range of frequencies from 300 MHz up to 300 GHz forms the microwave part of the electromagnetic spectrum. The microwave frequencies commonly used for communications are in the range 2 - 40 GHz. The higher the frequency used, the higher the potential bandwidth, and therefore the higher the potential data rate. Microwave signals do not easily pass through buildings. In addition, however well focused the transmitter may be, stray signals will often arrive at their destination somewhat later than more direct signals from the same source, having been refracted by low-lying atmospheric layers. These refracted signals will arrive out of phase with the more direct signals, and the overlapping signals can have a tendency to partially cancel each other out - an effect known as multi-path fading. Rain can also be a problem, as frequencies around 8 GHz and above are absorbed by water. At such high frequencies, more expensive electronic receiver circuitry is needed, and transmissions can be subject to interference from radar installations, and even domestic microwave ovens.
Microwave does, however, have several advantages over optical fibre. Microwave can support high data rates over long distances. Obstacles such as roads, railways and rivers may make laying cables difficult, whereas these problems do not exist for microwave, and rights of way are not an issue. Erecting simple towers, or mounting antenna on the tops of tall buildings, is usually far cheaper than laying several kilometres of cable. Microwave links also remove the need for reliance on telephone companies, and microwave is often used for point-to-point links between buildings. Governments worldwide have set aside the frequency band from 2.400 GHz to 2.484 GHz for unlicensed transmissions, so use of these frequencies is popular for various forms of short-range wireless networking. The most commonly used frequencies for common-carrier long-haul communications are the 4 GHz, 6 GHz and 11 GHz bands. The 12 GHz band is used as a component of the cable TV system. Microwave links are used to provide TV signals to local CATV installations, and the signals are then distributed to individual subscribers via coaxial cable. For short point-to-point links between buildings, the 22 GHz band is typically used. The higher frequencies are less useful for longer distances because of attenuation, but are adequate for short distances. In addition, antennae are smaller and cheaper for the higher frequencies.