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There is a wide choice of antennae available.
For long distances, a more directional antenna
is preferred, and for wide areas of coverage, a sectoral or omni
directional antenna may be a better choice. The application should
drive the choice of antenna. DTNet sells and installs antennae from
many different different manufacturers for this reason.
The antennas in a Wireless network serve a similar
purpose to the speakers in a stereo system. That is, they take the
radio frequency signals generated by the transmitter and broadcast
them through space, just like speakers take the sound signals from
the stereo and broadcast them through the air. Of course, the antenna
is also used to receive signals as well as transmit them.
Several types of antennas are used in Wireless
networks. Each one has characteristics that make it suitable for
different applications. The most important characteristic is the
radiation pattern, or in which direction the antenna concentrates
or focuses the Wireless signal. Different radiation patterns allow
you to provide Wireless coverage either to a wide area, to “point”
the signal at a particular location, or to cover a swath of varying
width. The tradeoff is that if you’re starting with the same
amount of signal power, having it all go in one direction will increase
the range at the expense of coverage area. Since there’s no
such thing as a "free lunch", focusing the power in one
direction means that the antenna will transmit very little in the
other directions. This can be a good thing, if you’re trying
to minimize picking up of stray signals from other directions. To
learn more about antenna signals, click here.
It is possible to turn some directional antennas
on their sides. This affects what radio engineers call the “polarization”
of the signal. Basically, as long as the sending and receiving antennas
are oriented the same way, the polarization matches, and they work
fine. However, if the polarization is not matched, the transfer
of signal from one to the other may be severely degraded. This may
be used to advantage, where two nearby systems can use different
polarizations to avoid interference from each other.
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Omnidirectional
(Omni)
This antenna looks like a
vertical pole, and radiates its signal in all directions in
a horizontal plane. It’s used for the central site in
a star network, since it puts out signal in all directions.
It doesn’t really have a beam width, but the vertical
radiation pattern can be taller or shorter, resulting in more
or less gain in the horizontal plane (remember, that power
has to come from somewhere; more power into the horizontal
plane means less everywhere else).
Radiates horizontally in all directions
Horizontal Beam width:
360 degrees
Gain:
3-15 dB
Cost:
usually cheapest, since it’s basically a carefully designed
metal pole
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Yagi
This is a fairly compact directional
antenna, often you can see these inside a plastic tube to
protect them from the elements such as the one shown below.
These antennas are often used in
point-to-point links where two fixed systems are talking to
each other.
Directional:
radiates mostly in one direction.
Horizontal Beam width:
25-45 degrees
Gain:
8-16 dB
Cost:
cheaper than parabolic, but still pricey, since they’re
usually welded |
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Parabolic
(Dish or Grid)
The parabolic antenna consists of
a dish-shaped reflector which “collects” the RF
signal and concentrates it on a small receiving element. Since
the dish can be large, it can potentially pick up weaker signals.
Since it works similar to a concave mirror, the receiving
element is positioned at the focal point of the dish, and
the dish is pointed in the direction you want to send the
signal. These can be made with very narrow beamwidths, so
are often the best choice for long links.
Directional, available in a variety
of beamwidths. Can be made with very narrow beamwidths for
long links.
Horizontal
Beam width:typically 7-20 degrees
Gain:
typically 9-25 dB, depending on beam width
Cost:
most expensive
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Patch
The patch antenna is a low
profile directional type which is often used where the larger
Yagi or Parabolic antennas would be too unsightly. Can be
made with a wide range of vertical and horizontal beamwidths.
Directional, available in a wide variety of beamwidths.
Horizontal
Beam width: 10-100 degrees
Gain:
typically 10-25 dB, depending on beamwidth
Cost:
moderate
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Sensory
Antenna
These antennas do not generally
work well for wireless networks.
Horizontal
Beam width:typically .125 -
.25 degrees
Gain:
typically -01 dB, with almost no beam width
Cost:
inexpensive and plentiful, but not functional. |
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