16.1 The Crowded Party.
The need to communicate information over long distances and to mobile destinations lead to the development of Radio Communications. This uses electromagnetic waves radiating in space to carry information. Modern communications systems are often based upon the use of microwaves, millimetre-waves, or visible light. Although the frequencies of these waves differ from conventional radio waves the techniques used are essentially the same. We can therefore concentrate on the example of radio waves and remember that equivalent processes can be carried out using electromagnetic waves at other frequencies.
Perhaps the simplest and most primitive way of communicating with electromagnetic waves is illustrated in figure 16.1. This shows a system based upon the use of an Inductive Loop. The system is designed so that the current, I, flowing around the TX coil at any time is proportional to the sound pressure on the microphone. This will induce a corresponding magnetic field in and around the loop. As a result, provided that the receiver is somewhere inside (or nearby) the TX coil, a smaller current, i, will be induced in its RX coil. This current, which varies in proportion with the sound at the microphone can then be amplified and heard on the receiver's loudspeaker.
The most obvious limitation of this system is that it only works properly when the RX coil is inside (or near to) the TX loop. This is because the system acts more like a transformer than the kind of radio we're used to. The electromagnetic field produced by the transmitter is concentrated inside the loop. The reason for this is that the transmitting antenna (the loop) is much smaller than the wavelength we're trying to radiate (audio frequencies). This isn't a problem is some situations — and can even be an advantage when we don't want messages to travel very far. For example, we can use inductive loop systems as ‘pagers’ inside places like hospital buildings. The TX loop is laid around the outside of the buildings we need to cover. Provided that the person we wish to talk to is inside the building, or only a little way outside, their receiver will be able to pick up the message.
Another problem with this approach is what can be called the ‘crowded party’ problem. The system shown in figure 16.1 can only cope with one message at a time. Paging systems can partly get around this problem by transmitting a code signal at the start of each message. This allows us to have a number of receivers in the building, each of which listens for its own, unique, code. When a pager recognises that its code has been received it can switch on its output so that the message is only heard by the person its meant for. Although this improves the usefulness of the system we can still only use it to send one message at a time.
Depending upon where you live, if you try tuning a radio or TV you will discover that there are dozens — or even thousands! — of organisations all transmitting signals simultaneously. If your radio worked like the system we've just described you'd always hear them all on top of one another. The effect would be just like trying to hear what someone is saying at a crowded, noisy party. The loudest, closest signals would tend to win. Fainter signals would be impossible to listen to, and even the stronger ones would be set against an annoying background of gabble.
To transmit signals efficiently over long distances we need to arrange for the radiated fields to fluctuate at higher frequencies — i.e. use shorter wavelengths. This allows us to build and use more effective antennas. By changing the frequency of the electromagnetic field fluctuations used to communicate information we can also overcome the crowded party problem. This is achieved by choosing a different frequency for each transmitter and putting a tunable filter in the receivers to select just the required signal.
Figures 16.2 a and b illustrate typical radio transmission and reception arrangements. The transmitter and receiver employ a combination of two techniques — Frequency Conversion and Modulation/Demodulation — to overcome the problems mentioned earlier. This approach is adopted by almost every system which uses radiated electromagnetic fields to communicate information. For the sake of example, the illustration shows a system being used to communicate audio information (speech and music). Similar arrangements can, however, be used for many purposes ranging from TV broadcasts to satellite navigation systems.
To understand how this system works we can start by looking at the transmitter. Unlike the system we considered earlier, the transmitted electromagnetic field is produced by an oscillator (shown in the diagram as the ‘transmitter local oscillator’) whose frequency can be high enough to be efficiently radiated by the transmitter (TX) antenna. Information can then be transmitted by modulating — altering in a controlled manner — the output produced by this oscillator.
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University of St. Andrews, St Andrews, Fife KY16 9SS, Scotland.