Note: Cellular base stations are usually depicted as being at the center of their
coverage area (cell). This is normal for open country, but in urban areas, a
single base station may be used to cover several smaller cells by having a
number of directional antennae aligned with each cell to be covered. This
provides a more cost-effective way of covering urban areas. The placement of
an antenna can be very important in order to achieve the best performance. If
the antenna is placed close to other objects that are electrically conductive, then
these can either screen the antenna from the signal or reflect the signal causing
multipath interference. Objects which can cause problems are anything made
from metal and the human body. When a dipole antenna is mounted on a
vehicle, the antenna must be mounted as near to the center of the vehicle body
as possible to ensure an adequate groundplane all round the antenna position.
See Figure 8 on page 30 for clarification.
The tolerances get more critical with higher frequencies and smaller
components.
Any antenna system must be matched to the transmitter and receiver, as well as
being designed to work at a particular frequency. As in any transmission line,
an antenna system will have a defined impedance, and the feed to the
transceiver together with the transceiver itself must have the same impedance.
If the impedances are different, significant amounts of RF signal will be lost. The
higher the frequency and the lower the power, the more critical the impedance
matching requirements. With higher frequencies, the design and quality of all
components such as connectors becomes very critical. If the impedance
between the antenna system and the transmitter are significantly different, it is
possible to cause permanent damage to the electronics of the transmitter.
2.1.3 Wireless LAN Frequencies
The two most widely used methods for wireless LAN usage are Infrared (IR) and
Radio Frequency (RF) channels.
Because most IR receivers detect the power (amplitude) of optical signals (not
their frequency or phase), the systems that use them are simple in design, have
no frequency conversions or precision components, and are readily available. It
is possible to use phase or frequency modulation and there are some existing
products, but they are not in common use. The cost efficiency of an infrared
design can be enhanced if it can share components or subsystems with such
mass-produced devices as television remote controls. Because infrared systems
are not regulated (other than with limits on the permissible optical power
densities), there are no regulatory constraints on a design. In comparison to
radio systems, this freedom from regulatory constraints can help to keep costs
down.
The worldwide demand for applications of RF technology has created extreme
competition for frequency spectrum, causing regulatory agencies to place tight
specifications on the use of an allocated band. The signal transmitted must be
kept within the permitted frequency band to tight tolerances adding significantly
to the cost of the transmitter. For some applications, the receiver must be able
to select bands in order to be able to operate with transmitters at different
frequencies, also adding to the cost. Selectivity is a measure of the ability of a
radio to detect and receive a signal while rejecting signals at adjacent
frequencies. Higher selectivity becomes more important in busy frequency
bands with many transmitters. For a receiver to exhibit high selectivity, the
34 An Introduction to Wireless Technology

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