Page 139 - DCAP311_DCAP607_WIRELESS_NETWORKS

Unit 8: Wireless MAN

Usage Semi-directional antennas are ideally suited for short and medium range bridging. For Notes
example, two office buildings that are across the street from one another and need to
share a network connection would be a good scenario in which to implement semidirectional
antennas. In a large indoor space, if the transmitter must be located in the corner or at the end
of a building, a corridor, or a large room, a semi-directional antenna would be a good choice
to provide the proper coverage. Figure 8.6 illustrates a link between two buildings using semi-
directional antennas.
Figure 8.6: Two Buildings using Semi-directional Antennas

Point-to-point link using-directional Antennas

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Many times, during an indoor site survey, engineers will constantly be thinking of how to best
locate omni-directional antennas. In some cases, semi-directional antennas provide such long-
range coverage that they may eliminate the need for multiple access points in a building.

Example: In a long hallway, several access points with omni antennas may be used or
perhaps only one or two access points with properly placed semi-directional antennas - saving
the customer a significant amount of money.
In some cases, semi directional antennas have back and side lobes that, if used effectively, may
further reduce the need for additional access points.

8.2.7 Highly Directional Antenna

Highly directional antennas are of two kinds, the grid style seen on top of television sets and the
dish-shaped ones commonly used to receive satellite TV stations. A highly directional antenna
offers a number of advantages over the other two types, which are omni-directional and semi-
directional antennas. As its name suggests, the design of the highly directional antenna allows it
to hone in on one direction more efficiently

Types of Highly Directional Antennas

Grid antennas are flat with slightly curved edges. They look rather like barbecue grills with
a device in a central point to collect the signals as they are relayed off a mesh of bars which
make up the grid. Dish antennas are more properly known as parabolic antennas, from their
inverted-bowl shape. They work by concentrating all the signals into a center point, usually a
raised receiver or transmitter located a distance above the surface of the bowl, which is the point
where the signal is at its strongest.

How Highly Directional Antennas Work

To explain how this kind of antenna offers better reception or transmission, imagine one hand
cupped around one of your ears and pointed in one direction, while the other ear is covered
by the other hand. You have in effect transformed yourself into something similar to a highly
directional antenna. Sounds from the direction at the open end of the cupped hand reach the ear
more loudly and more clearly, while noises from the direction of the blocked ear are muted or
inaudible. This is how a highly directional antenna works.

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