| DIRECTIONAL ANTENNA
A directional antenna is one in which there is more antenna radiated
power in some directions than in others. For efficiency, all
antennas are compared to what we call an
isotropic antenna - an antenna which theoretically broadcasts power
evenly exactly the same in all directions. The amount of
antenna gain that we say that an antenna
has, is actually a measure of how directional it is and how much it can
concentrate its total power in just the right directions.
Of course it often makes sense for antennas to be directional - such
that they can concentrate their power in just the direction
needed. Some examples of directional antennas:
Cellular antenna sites - Cellular antenna sites (cell-sites)
will often handle three cells. The manner that the cell sites
transmits to each of the three is to have three directional antennas all
up on the same antenna mast. Think of the area around the cell
site as a 360 degree radius. Each antenna will focus on 120
degrees of that 360 degrees. Note that if you look at some
cellular installation sites you will see 3 antennas on the same mast.
Some of these cellular antennas will be obvious since they may have
metal sheet reflectors in back of them to ensure that they just radiate
all of their energy forward.
| At the right is a simulation of a cellular tower
- looking down from the sky above it. This tower is
similar to the above discussion of a tower that has 3 antennas
and can therefore service three cells. In the picture, the
round black dots represent the three antennas. The gray
rectangle behind each is a "reflector" that helps direct the
energy in only one direction. The red circles that flow
out from the antennas represent the wireless transmit energy
going out to the mobiles. Three Cellular antennas sharing
the same mast is common. It is also common that all three
are very directional antennas - each taking 120 degrees of the
outward pattern. |
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Still other cellular antennas may be directional along a highway -
such that they just cover the road traffic itself and not the land next
to it that may not be well populated.
Satellite TV - note that satellite TV reception often uses a
parabolic dish antenna. The
purpose of this antenna is to concentrate all of the antenna energy for
transmit or receive into a single beam - that of course is then pointed
at the satellite. Concentrating all of the energy in a single beam
makes parabolic dish antennas the highest gain of all.
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