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Hack 79 Determining Antenna Gain

Figure out the approximate gain of your home
brew antennawithout a spectrum analyzer.

After building one of the
many antennas in this chapter, or perhaps designing one of your own,
you will inevitably wonder just how much gain your antenna provides.
While an ideal testing rig would include a spectrum analyzer and lab
conditions, most people can't afford to bring such
resources to bear on their little antenna project. Fortunately,
informal gain tests are simple to perform, given some simple tools
and a little patience.

Here is one method for estimating gain. While your results might not
be as accurate as those provided by a
"real" radio lab, it can give you a
fair estimate of how well your equipment performs, for very little
cost.

What you'll need:

Two radio cards of the same manufacturer and firmware revision, as
well as external antenna connectors (Lucent/Orinoco/Proxim cards, or
Prism II cards like the Senao/EnGenius work well)

Two laptops

The antenna to be tested

Two antennas of known gain (preferably low gain and somewhat
directional, like sector antennas)

Two tripods, mounts, and pigtails for the above antennas

A large, flat outdoor space free of obstacles

A notebook

A friend, and a means of communicating with that friend (such as cell
phones or FRS radios)

Connect an antenna to one of the cards, and using a program like
NetStumbler [Hack #21], run a simple
site survey. Walk around the area a bit, and look for an unused (or
lightly used) channel. Once you decide on a channel, quit NetStumbler
and return to the other laptop. With the two laptops close to each
other, set up an Ad-Hoc network on that channel.
Don't worry about your IP configuration, just set
both machines to the same ESSID and channel. If you are using a Prism
II card, you might prefer to use one laptop in Host AP mode [Hack #57]
instead.

If you are using an Orinoco card on a
Windows
machine, open the "Site Monitor"
utility in the client driver, on both machines. If you are running
Linux, I recommend using Wavemon
[Hack #33] on both machines.
Otherwise, open the client monitoring tool that came with your card.
In my experience, the Orinoco Site Monitor or wavemon on Linux are
the preferred tools for monitoring signal strength in real time, as
they update quickly and keep a history for you.
Don't use a network scanner like NetStumbler, as it
has been known to get confused when performing simple signal strength
tests. While still close together, verify that the two laptops can
monitor each other with no problems. It is much easier to debug
configuration problems now than when you are far away from your
friend later.

When you are satisfied that everything is working properly, you are
ready to head out into the field. Set up the tripods and mounts about
300 feet or so apart. Be sure that your tripods are at least 5 feet
high, to clear the 0.6 Fresnel zone. The
Fresnel (pronounced
"fray-NELL")
zone refers to the shape of a wave as it leaves
the antenna, expanding in a circular direction as it travels. It is
well illustrated by the diagram at http://www.ydi.com/deployinfo/ad-fresnel-zone.php.
Using antennas of known gain on both sides, plug in your laptop and
see what kind of signal you can find. With your friend keeping his
end steady, slowly rotate your antenna until you achieve the highest
possible gain between the two points. Now, lock down your side, and
let your friend rotate his end until he achieves the highest gain.
Work slowly, and keep in constant communication with your friend at
the other end, until you agree on the best possible position for both
antennas. Be sure that both of you take your hands off of the antenna
before taking a reading.

Once you are satisfied that the antennas are well aligned, make a
note of the received signal and noise from both sides. Let the entire
rig rest a couple of minutes, and see if the signal fluctuates at
all. If it does, you might be encountering unexpected noise on the
band, so you might try a different channel.

When you are happy with your link, it is time to try out your new
antenna. Without moving the other end, carefully replace one antenna
with the antenna to be tested. Ideally, you should use the same
pigtail and feed line to eliminate the possibility of variations in
the cabling. While watching the signal strength meter, slowly rotate
the antenna until the highest possible gain is achieved. Again, let
the entire system rest for a moment or two. When the link looks
stable, record the received signal and nose from both sides.

The difference between both readings, plus the gain of the antenna
that was traded out, is the approximate gain of your home brew
antenna. For example, suppose you first measured a signal of -56 dBm
using a 10 dBi sector. When you swapped it out for a circular
waveguide, your signal strength jumped to -50 dBm. The difference
between the readings (6 dBm) plus the gain of the known antenna (10
dBi) equals the approximate gain of the waveguide, which is
approximately 16 dBi.

You can also compute the difference in
noise readings to
see an approximate estimation of how well the antenna rejects noise
from the path. With noise, a lower signal is better. Remember that
you are dealing with negative numbers, so a noise reading of -100dBm
is actually better than -90dBm. Likewise, since
you want more signal, a reading of -50 dBm is
much better than -56 dBm.

One critical point that isn't measured in this sort
of test is Standing Wave
Ratio (SWR). This is a measurement of how much
signal is being reflected back into your radio from the antenna, and
tells whether your antenna is well matched to the
frequency your radio is transmitting
on. Unfortunately, I know of no good method for determining the SWR
of very low power cards at 2.4 GHz without using expensive equipment.
Fortunately, as these radios put out only a few milliwatts, there is
little chance that your radio will actually be damaged by a
mismatched antenna. It just won't work very well.

Once your antennas are aligned in a setup like this, you can test as
many home brew designs as you like. Just be sure to keep the other
end steady, and take everything one step at a time. Make a note of
everything you observe as you go, and keep the number of variables to
a minimum. While this method might not be as accurate as a spectrum
analyzer, it is a very cost-effective way of getting an estimation of
how your antenna design actually performs.