On 21 Feb 2007 16:20:44 -0800, "seaweedsteve" <seaweedsteve@gmail.com>
wrote:

>Question: I understand that doing a walk test at fixed connection
>speed is best, but still, is this approach totally worthless or is it
>reasonable for me to make comparisons in this manner?

Not totally worthless, but close. The problem is reflections and
multipath. It's very difficult to generate real antenna patterns and
gain measurements without either an RF anechoic test chamber, or a
proper test range. The last time I did it for 2.4Ghz, I had to put
both ends of the link on top of a 15ft high fiberglass pole and 12ft
ladder to keep the ground bounce out of the picture. This particular
test was done in a parking lot. We soon found that moving cars around
would create huge variations in apparent gain. You could see people
moving around.

For example, here's a good example of how to do it all wrong:
<http://martybugs.net/wireless/antennacomp.cgi>
The problem is with the 200 meter test range. The antennas at both
ends were well within the fresnel zone. Ground bounce was also
evident. There were also some obvious obstructions (trees, utility
box) in the path. Judging by the photos, it was done with at least
one end at ground level.

Here's a somewhat better way:
<http://www.olotwireless.net/catala/discono.htm>
See the sections "Measuring Radiation Pattern". Note that the other
end of the link uses a high gain yagi antenna to reduce the amount of
RF that hits the ground and bounces. They would have done better if
they had used something higher than a camera tripod, but good enough.

Try this simple experiment. Take a fiberglass pole and put any
antenna on top. Doesn't matter what type as long as the gain isn't
too high (<12dBi). Connect to some remote access point with line of
sight. Now, move the antenna up and down the pole slowly taking
readings at various altitudes. What you'll see are peaks and nulls
caused by a combination of reflections and knife edge refraction. The
variations will probably be substantial. As you get higher and
higher, the effects will be less and less.

Another clue is to notice the wide variations in signal strength
produced by Netstumbler graphs, even when tested from a fixed
location.

Also, please don't use the xx% signal strength numbers when you have
dB (decibels) available in Netstumbler.

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558 jeffl@comix.santa-cruz.ca.us
# http://802.11junk.com jeffl@cruzio.com
# http://www.LearnByDestroying.com AE6KS