Terrestrial microwave

Terrestrial microwave

Physical Description The most common type of microw~e antenna is the parabolic
"dish." A typical size is about 3 m in diameter. The antenna is fixed rigidly and
focuses a narrow beam to achieve line-of-sight transmission to the receiving antenna.
Microwave antennas are usually located at substantial heights above ground level to
extend the range between antennas and to be able to transmit over intervening obstacles.
To achieve long-distance transmission, a series of microwave relay towers is used,
and point-to-point microwave links are strung together over the desired distance.
Applications A primary use for terrestrial microwave systems is in long-haul
telecommunications service, as an alternative to coaxial cable or optical fiber. The
microwave facility requires far fewer amplifiers or repeaters than coaxial cable over
the same distance but requires line-of-sight transmission. Microwave is commonly
used for both voice and television transmission.
Another increasingly common use of microwave is for short point-to-point links
between buildings. This can be used for closed-circuit TV or as a data link between
local area networks. Short-haul microwave can also be used for the so-called bypass
application. A business can establish a microwave link to a long-distance telecommunications
facility in the same city, bypassing the local telephone company.
Two other important uses of microwave are examined in some detail in Part
Three: cellular systems and fixed wireless access.
Translnission Characteristics Microwave transmission covers a substantial
portion of the electromagnetic spectrum. Common frequencies used for transmission
are in the range 2 to 40 GHz. The higher the frequency used, the higher the
potential bandwidth and therefore the higher the potential data rate. Table 2.2 indicates
bandwidth and data rate for some typical systems.

The most common bands for long-haul telecommunications are the 4-GHz to
6-GHz bands. With increasing congestion at these frequencies, the ll-GHz band is
now coming into use. The 12-GHz band is used as a component of cable TV systems.
Microwave links are used to provide TV signals to local CATV installations; the
signals are then distributed to individual subscribers via coaxial cable. Higherfrequency
microwave is being used for short point-to-point links between buildings;
typically, the 22-GHz band is used. The higher microwave frequencies are less useful
for longer distances because of increased attenuation but are quite adequate for
shorter distances. In addition, at the higher frequencies, the antennas are smaller
and cheaper.