Antenna

Antenna
The antenna radiates the modulated signal through the air so that the destination can receive it.
Antennas come in many shapes and sizes and have the following specific electrical characteristics:
• Propagation pattern
• Gain
• Transmit power
• Bandwidth
The propagation pattern of an antenna defines its coverage. A truly omnidirectional antenna
transmits its power in all directions, whereas a directional antenna concentrates most of its
power in one direction. Figure 2.12 illustrates the differences.
A directional antenna has more gain (degree of amplification) than the omnidirectional type
and is capable of propagating the modulated signal farther because it focuses the power in a
single direction. The amount of gain depends on the directivity of the antenna. An omnidirectional
antenna has a gain equal to one; that is, it doesn’t focus the power in any particular
direction. Omnidirectional antennas are best for indoor wireless networks because of relatively
shorter-range requirements and less susceptibility to outward interference.
Directional antennas will best satisfy a need for interconnecting buildings within metropolitan
areas because of greater range and the desire to minimize interference with other systems
The combination of transmit power and gain of an antenna defines the distance the signal will
propagate. Long-distance transmissions require higher power and directive radiation patterns,
whereas shorter-distance transmissions can get by with less power and gain. With wireless networks,
the transmission power is relatively low, typically one watt or less. Bandwidth is the effective part of the frequency spectrum that the signal propagates. For example,
the telephone system operates in a bandwidth roughly from 0KHz to 4KHz. This is
enough bandwidth to accommodate most of the frequency components within our voices.
Radio wave systems have more bandwidths located at much higher frequencies. Data rates and
bandwidth are directly proportional: The higher the data rate, the more bandwidth you’ll need. Some companies within the wireless LAN industry are developing what’s referred to as a
“smart antenna” technology. A smart antenna makes it possible to electronically and automatically
concentrate transmitted signal power in directions where end users will be operating the
wireless LAN (not in directions that end up bouncing off walls and other obstacles). This technology
extends the range of a wireless LAN and reduces the number of access points needed.