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Hack 64 Antenna Guide

How do you know which antenna is the best for
the job? This guide will explain.

The single most effective way to extend the
range of your access point or client radio is to add an external
antenna. Contrary to popular belief, antennas do
not give you more signal than you started with
(that's what
amplifiers are
for). They focus the available signal in a
particular direction, much like what happens when you turn the focus
head of a flashlight. It doesn't make the bulb any
brighter, it just focuses what you have into a tighter space.
Focusing a flashlight gives you a brighter beam that covers a smaller
total area, and likewise, more directional antennas give you a
stronger perceived signal in a smaller area. All antennas are
somewhat directional, and the measure of their directionality is
referred to as
gain.
Typically, the higher the gain, the better the range (in the
direction that the antenna radiates best in).

Another important characteristic of

antennas is the phenomenon of
polarization [Hack #84].

There are a few general types of antennas suitable for use at
microwave frequencies. Each works well for its own application, and
no single antenna works best for every application. When actually
shopping for antennas, be sure to look at the actual

radiation pattern of your antenna to
be sure that it fits your needs.

Plan your goals ahead of time, and configure your network to meet
those goals. The following sections describe the most common types of
antennas, listed in rough order of increasing gain.

Omni

Omnidirectional
antennas
(or omnis)
radiate outward in horizontal directions in roughly an equal manner.
Imagine putting an enormous donut around the center pole of an omni:
that's what the radiation pattern looks like. Omnis
are good for covering a large area when you don't
know which direction your clients might come from. The downside is
that they also receive noise from every direction, and so typically
aren't as efficient as more directional antennas.

As you can see in Figure 4-24, they look like tall,
thin poles (anywhere from a few inches to several feet long), and
tend to be expensive. The longer they are, the more elements they
have (and usually the more gain and the higher the price.) Small
"rubber
ducky" antennas ship standard with many access
points, such as the Linksys WAP11 or the Cisco 350. Omni antennas are
mounted vertically, like a popsicle stick reaching skyward. They
gain in the horizontal, at the
expense of the vertical. This means that the worst place to be in
relation to an omni is directly beneath (or above) it. The vertical
response improves dramatically as you move away from the antenna.

Sector (or Sectoral)

Sectors antennas come in a variety of
packages, from flattened omnis (tall, thin, and rectangular) to small
flat squares or circles (Figure 4-25). A close
cousin of the sector is the
patch antenna, which shares most of the same
properties. Some are only a few inches across, and mount flat against
a vertical wall or on a swivel mount. They can also be ceiling
mounted to provide access to a single room, such as a meeting room,
classroom, or tradeshow floor. As with omnis, cost is usually
proportional to gain.

Picture an omni with a mirror behind it, and you'll
have the radiation pattern of a sector antenna. Sectors radiate best
in one direction, with a beam as wide as 180 degrees or as narrow as
you like. They excel in point-to-multipoint applications, where
several clients will be accessing the wireless network, all coming
from the same general direction.

Yagi

As you can see in Figure 4-26, a
yagi
looks
something like an old television aerial. Some yagis are simply bare,
like a flat Christmas tree, and are pointed vaguely in the direction
of communications. Others are mounted in long horizontal PVC cans.
They can work well in point-to-point or point-to-multipoint
applications, and can usually achieve higher gain than sectors.

The typical beam width can vary from 15 degrees to as much as 60,
depending on the type of antenna. As with omnis, adding more elements
means more gain, a longer antenna, and higher
cost.

Waveguides and "Cantennas"

An increasingly popular antenna design is the

waveguide. The so-called
"cantennas"
are simple antennas for home-brew designers to build, which offer
very high gain for relatively little effort. Waveguides most closely
resemble plumbing, in that they are boxes or cans with nothing in
them but a tiny radiator. Figure 4-27 shows an
ambitious design, made from extruded and milled aluminum.

The Pringles can and coffee can antennas are examples of simple (but
effective) home-brew cylindrical waveguide antennas. A
rectangular waveguide can behave like a
sector or an omni, depending on how it is constructed. We will look
at these antennas in detail in Chapter 5.

Parabolic Dish

In some ways, a dish is the opposite of
an omni. Rather than try to cover a wide area, a dish focuses on a
very tight space. Dishes typically have the highest
gain and most directionality of any
antenna. They are ideal for a point-to-point link, and nearly useless
for anything else.

Figure 4-28 shows a mesh antenna, although solid
variations exist. Dishes come as small as 18" across or as big as you
like (a 30-foot dish is possible, but probably not very convenient).
A dish that can send an 802.11b signal more than 20 miles can be as
small as a few feet across. In terms of gain for the buck, dishes are
probably the cheapest type of antenna. Some people have been
successful in converting old
satellite and DSS
dishes into 2.4 GHz dishes; see Chapter 5 for two
of these designs.
Generally speaking, the difference
between a mesh reflector and a solid
reflector has little to do with gain, but is a consideration when
mounting your dish, as solid dishes tend to pick up more load from
the wind.

Putting It All Together

When trying to estimate what
antennas will be required for a particular installation, I find it
helpful to draw an overhead picture of the project site. Sketch out
where you intend to install your equipment and where you expect your
network clients to come from. Also include any
obstacles (such as
walls containing metal, or intervening buildings and trees on a long
distance shot.) This should help to determine your desired coverage
area, as well as help suggest optimal
placement of your access points and
antennas.

For example, if you are trying to cover a large office with obstacles
in the center (such as an elevator shaft or bathrooms), access points
at opposite ends of the room with sector antennas pointed inward
might make more sense than a single AP in the center with an omni. On
a long distance shot, it might make more sense to go around an
intervening tree or building by adding an extra hop, rather than
trying to shoot through it with high gain dishes. Knowing the
approximate
radiation pattern and gain of your
antennas ahead of time will help to focus your energy in the
direction that you intend to use it, so you can design the most
efficient network possible.