Microwaves do not follow the curvature of the earth and therefore require line-of-sight transmission and reception equipment. The distance coverable by a line-of-sight signal depends to a large extent on the height of the antenna: the taller the antennas, the longer the sight distance. Height allows the signal to travel farther without being stopped by the curvature of the planet and raises the signal above many surface obstacles, such as low hills and tall buildings that would otherwise block transmission. Typically, antennas are mounted on towers that are in turn often mounted on hills or mountains.
Microwave signals propagate in one direction at a time, which means that two frequencies are necessary for two-way communication such as a telephone conversation. One frequency is reserved for transmission in one direction and the other for transmission in the other. Each frequency requires its own transmitter and receiver. Today, both pieces of equipment usually are combined in a single piece of equipment called a transceiver, which allows a single antenna to serve both frequencies and functions.
To increase the distance served by terrestrial microwave, a system of repeaters can be installed with each antenna. A signal received by one antenna can be converted back into transmittable form and relayed to the next antenna (see Figure 5.2-11). The distance required between repeaters varies with the frequency of the signal and the environment in which the antennas are found. A repeater may broadcast the regenerated signal either at the original frequency or at a new frequency, depending on the system.
Terrestrial microwave with repeaters provides the basis for most contemporary telephone systems worldwide.
Two types of antennas are used for terrestrial microwave communications: parabolic dish and horn.
A parabolic dish is based on the geometry of a parabola: every line parallel to the line of symmetry (line of sight) reflects off the curve at angles such that they intersect in a common point
called the focus (see Figure 5.2-12). The parabolic dish works like a funnel, catching a wide range of waves and directing them to a common point. In this way, more of the signal is recovered than would be possible with a single-point receiver.
Outgoing transmissions are broadcast through a horn aimed at the dish. The microwaves hit the dish and are deflected outward in a reversal of the receipt path.
A horn antenna looks like a gigantic scoop. Outgoing transmissions are broadcast up a stem (resembling a handle) and deflected outward in a series of narrow parallel beams by the curved head (see Figure 5.2-13). Received transmissions are collected by the scoped shape of the horn, in a manner similar to parabolic dish, and are deflected down into the stem.
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