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5.1 Radio and microwaves in communications
Radio waves are a very useful communications medium because, unlike electrical cables, there does not have to be a direct connection between the sender and the receiver, and unlike light, radio waves will penatrate buildings. These facts make radio very useful for broadcast communications, where there are many receivers for each transmitter. However, there is increasing use of radio for personal communications, such as radio telephones.
Microwaves are used mostly for very long distance (e.g., intercontinental) communications, particularly with a satellite in the middle.
5.2 Properties of radio waves
Radio waves are a type of electromagnetic wave. That is, they are a combination of electrical and magnetic signals travelling in space. A radio wave is generated whenever an electrical current changes quickly in a wire. However, for the radio wave to travel a useful distance it must contain a significant amount of energy. This energy comes from the transmitter. An electrical conductor will only transfer energy to a radio wave when the rate of change is fast enough, and when it is formed into a suitable antenna.
Most commercial radio and television services use frequencies of 10-500 MHz. However, useful transmission can be obtained from frequencies as low as 100 kHz (100 kilohertz, or 100,000 hertz). At this low frequency close attention must be paid to the design of the antenna to achieve effecient broadcasting.
As frequencies of electrical signals increase, it becomes increasingly difficult to prevent the generation of radio waves. In high-speed computers and communications equipment, it is a significant engineering challenge to restrict the electricity to the wires. Failure to do so results in crosstalk: interference between signals on adjacent wires.
Radio waves propagate through air and space at about the speed of light.
5.3 Properties of microwaves
Microwaves are also a type of electromagnetic wave, but with a higher frequency than radio waves. The higher frequency means that they can be more tightly focussed. By focussing the energy can be constrained to a narrow beam, so the signal will travel greater distances. This makes microwaves useful for satellite communications. In principle the higher bandwidth also allows a greater signal bandwidth to be covered. However, microwaves do not penetrate buildings as readily. Moreover, the tight beam pattern limits the coverage that can be obtained with a single beam, which is disadvantageous in broadcasting.
Microwaves propagate through air and space at about the speed of light. They can also be confined to a conductor (called a waveguide) in which case the speed of propagation may be about 30% slower.
5.4 Microwave and radio apparatus
Both are lauched transmitted from, and received by, antennae. The different shapes of the antennae are dictated by the different frequencies used. Careful design of antenna can significantly increase the strength of signal that a receiver has to work with. Also, the receiving antenna usually works best wisth a particular orientation with respect to the transmitter. This is particular apparent with microwave reception using very small aperture terminals (`dishes') which focus the very weak distant signals onto the detector.
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