On 22 Feb 2007 08:45:15 -0800, "seaweedsteve" <seaweedsteve@gmail.com>
wrote:

>If fact, my test was from an elevated base antenna (12dbi yagi) but
>the client end was on my tabletop and certainly included all the
>variables you mentioned.

Well, that's better than usual. Here's a fun test. Setup a link
between any two machines and continuously record the signal strength
over an extended period of time. Use SNMP in DD-WRT and MRTG to make
it easy. Instructions on request. Don't change or move anything on
either end. What you'll find are radical variations over time. It
can be weather related, reflections from moving objects, or just water
condensing on the antenna. Lots of sources of errors. Typical is at
least 4dB variations in signal strength. If you have bad luck and end
up with multiple reflections, probably 10dB overall variations. Kinda
hard to use such numbers as some kind of reference to measure antenna
gain.

>I guess the implication is that even though various antennas are
>compared in the same spot, unless they are up high, then just by
>having different patterns, one is inviting an often severe variation
>in reflections and refractions.

Exactly. That's what the anechoic chamber offers (besides a great
place to have lunch without the cell phone ringing). By eliminating
reflections, you eliminate most of the major sources of errors and
variations. Incidentally, when measuring antenna gain, it's
considered good form to use a reference antenna with known gain. You
make the initial measurement with the reference antenna, switch
antennas, and do it again. The difference in measurments is the
relative gain.

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

>I know, sorry. My bad. I didn't see a way to use netstumbler on a
>client router (or would it work?) but DD-WRT does give dB readings.
>My eye got seduced by the bar graph and I didn't realize I'd be
>"publishing" numbers.

Netstumbler gives BOTH the client and the access point's indicated
signal strength and signal to noise ratio (SNR). Scroll horizontally
with the numerical tables to see them. The column headings are not
very clearly labelled so you'll need to do some digging to figure out
what is indicated, peak reading, and in what direction. I tried to
find a Netstumbler reference page that defined the column headings,
but couldn't. You really need all 4 numbers to do a decent job of
determining what's happening. For example, an overpowered transmitter
will yield a much higher signal strength in one direction than in the
other. Any interference might show up only at one end.

>Next time I'll at least put it up on a pole and use dB readings.

--
# 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