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From: Skavenger on 2 Feb 2006 08:35
Hello;

I have been gathering information on Homebrew Wireless Antennas and have
made a couple as an experiment.
Basically its for accessing my wireless network from my garage which is some
way off.
My latest one out of a Mini Cheddars Can seemed to work well, I think.
I have been measuring the signal strength using Net Stumbler but I am a bit
confused and need some quick pointers.

I understand that signal strength and Noise is measured in dBm and think
from what I have read on the Antenna Shoot out that the lower the dBm the
better.

Netstumbler shows a graph with a left hand Axis that shows the Signal
Strength and Noise according to the help file the signal strength should be
in green( The higher up the graph the better, higher up the scale the lower
the dBm reading) and noise in red if available.
My graph only shows a green bar so I guess thats signal strength.
Last night while testing my PCMCIA card only I got a signal strength of
about 60 dBm when I attached the Antenna I got about 40 dBm (Wrote it down
somewhere) if that is the case I just increased my signal dramatically.

I know it got better because when accessing my Network the wireless networks
Icon increased from 36Mbps to 54Mbps.

Does anyone know of a software tool for bechmarking or measuring wireless
signal strengths in order to accurately record results.

Thanks

BH



From: Jeff Liebermann on 2 Feb 2006 19:10
On Thu, 2 Feb 2006 13:35:29 +0000 (UTC), "Skavenger"
<bilharvs(a)btinternet.com> wrote:

>I have been gathering information on Homebrew Wireless Antennas and have
>made a couple as an experiment.
>Basically its for accessing my wireless network from my garage which is some
>way off.
>My latest one out of a Mini Cheddars Can seemed to work well, I think.
>I have been measuring the signal strength using Net Stumbler but I am a bit
>confused and need some quick pointers.
>
>I understand that signal strength and Noise is measured in dBm and think
>from what I have read on the Antenna Shoot out that the lower the dBm the
>better.
>
>Netstumbler shows a graph with a left hand Axis that shows the Signal
>Strength and Noise according to the help file the signal strength should be
>in green( The higher up the graph the better, higher up the scale the lower
>the dBm reading) and noise in red if available.
>My graph only shows a green bar so I guess thats signal strength.
>Last night while testing my PCMCIA card only I got a signal strength of
>about 60 dBm when I attached the Antenna I got about 40 dBm (Wrote it down
>somewhere) if that is the case I just increased my signal dramatically.
>
>I know it got better because when accessing my Network the wireless networks
>Icon increased from 36Mbps to 54Mbps.
>
>Does anyone know of a software tool for bechmarking or measuring wireless
>signal strengths in order to accurately record results.

Mind if I not answer what you're doing wrong and simply tell you the
right way to measure gain with Netstumbler? The problem is that most
wireless cards simply do not have enough dynamic range to do a decent
antenna pattern or gain measurement. The granularity also sucks in
that the signal and noise levels are derived from an 8 bit (256
levels) value.

There right way(tm) is to use a reference antenna and an adjustable
attenuator. The reference antenna can be any antenna with a known
gain. It should be something with similar gain to the test antenna,
but it really can be almost anything. The adjustable attenuator goes
between the antenna and the receiver.

The test setup is important. It's important to eliminate any obvious
sources of reflections and multipath. The worst offender is a
roadway. There are web sites showing antennas mounted on tripods
about 3ft above the roadway. The test result are fairly erratic,
probably due to ground reflections. In addition, the midpoint Fresnel
Zone hits the ground, which causes additional path problems.

It is also very important to set the over the air data speed to a
fixed value. It can be just about any speed that gives a useable
measurement, but it should change. That's because the receiver
sensitivity varies with the speed. If the speed varies, the pattern
and reference levels will be ruined.

What I do is setup my test antenna on my flat rooftop and aim it at a
mountaintop about 4 miles away. The ground in between is a rather
large forested valley, which offers few sources of reflections.

Once you have a repeatable test setup, just setup the attenuator for
some arbitrary attenuation. I use about 10dB. Make VERY sure that
the test radio does not pickup or belch RF directly. All of it should
go through the antenna connector, not leak through the plastic box. If
you're stuck with a plastic box test radio, wrap it in aluminium foil.

Setup a reference level using Netstumbler or preferably a passive
sniffer such as Kismet. Record the level. Then, remove the reference
antenna and replace it with the test antenna. Adjust the attenuator
until Kismet or Netstumbler reads exactly the same value as with the
reference antenna. The difference in attenuator readings is the gain
difference between the two antennas. For example, if my reference
antenna has a gain of 8dBi and the attenuator showed a difference of
+2dB gain, therefore the test antenna has a gain of 10dBi.

Measuring the antenna pattern is done exactly the same way. Rotate
the antenna perhaps 15 degrees. Make the same measurement and record
the attenuator reading necessary to get the same measured level with
Kismet. Keep doing it all the way around with the test antenna. You
should get something like the theoretical antenna pattern. The only
added complications is that since the antenna is now pointing in all
kinds of different directions, the opertunities for reflections are
greatly increased. One test I helped run acted weired until we move
the cars parked behind the antenna.

If you have any furthur questions, it would nice to know:
1. What are you trying to accomplish?
2. What do you have to work with?

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558 jeffl(a)comix.santa-cruz.ca.us
# http://802.11junk.com jeffl(a)cruzio.com
# http://www.LearnByDestroying.com AE6KS
From: Moe Trin on 2 Feb 2006 19:17
On Thu, 2 Feb 2006, in the Usenet newsgroup alt.internet.wireless, in article
<drt1r1$d43$1(a)nwrdmz02.dmz.ncs.ea.ibs-infra.bt.com>, Skavenger wrote:

>I understand that signal strength and Noise is measured in dBm and think
>from what I have read on the Antenna Shoot out that the lower the dBm the
>better.

dB or decibels is a ratio. The 'm' on the end references the ratio to 1
milliwatt of power at the characteristic impedance. This ratio is actually
expressed as ten times the logarithm of the power ratio - and a positive
number means the ratio is larger than 1. A negative number would mean
that the ratio is less than 1. As far as received signal (and noise)
levels are concerned, it's rare to find signals larger than 1 milliwatt
coming out of a receiving antenna, so the levels would be negative dBm.
Now it seems that you aren't seeing that negative sign (trust me, it's
there), but normal (usable) signal levels would be between say -90 dBm
and perhaps -20 dBm. To put those numbers into absolute power levels, -90
dBm is 0.000 000 001 milliwatts, while -20 dBm is 0.01 milliwatts. Thus,
a smaller negative number is a stronger signal.

>Netstumbler shows a graph with a left hand Axis that shows the Signal
>Strength and Noise according to the help file the signal strength should be
>in green( The higher up the graph the better, higher up the scale the lower
>the dBm reading) and noise in red if available.

Take those numbers with 10 kilos of salt, but yeah. Again, those are
supposed to be negative dBm.

>Last night while testing my PCMCIA card only I got a signal strength of
>about 60 dBm when I attached the Antenna I got about 40 dBm (Wrote it down
>somewhere) if that is the case I just increased my signal dramatically.

OK, so you went from -60 dBm to -40 dBm, which is a 20 dB improvement.
Where those numbers reported with a calibrated device, that would be
quite good. In the direction the antenna is pointing, that's the
equivalent of a 10:1 increase in range capability in free space (which
really means a path free of reflections, trees, dogs or what-ever).

>Does anyone know of a software tool for bechmarking or measuring wireless
>signal strengths in order to accurately record results.

A software tool is only reporting what the perceived signal strength is,
based on what the hardware is saying. You're assuming that the hardware
reports bear some resemblance to truth. This is HIGHLY unlikely. The
numbers you see are a best guess - and may be absolute fables. You
could connect the receiver directly to a calibrated signal generator,
and record the indications while varying the signal strength (in effect
calibrating the receiver). Another way would be to connect a calibrated
variable attenuator to the receiver set things up with strong signals,
and then vary the attenuator setting while recording the indications. In
either case, you are only going to be as accurate as the calibration of
the signal generator or attenuator, and that is only true at the same
temperature, supply voltage, and may vary as under the influence of the
color of the moon, and the number of days since you last paid your
bookmaker.

Old guy
From: Rich on 2 Feb 2006 20:26
Can you recommend a good 802.11 antenna?
For a card to give me a mile.



From: Jeff Liebermann on 2 Feb 2006 20:34
On Fri, 03 Feb 2006 00:10:35 GMT, Jeff Liebermann
<jeffl(a)comix.santa-cruz.ca.us> wrote:

>There are web sites showing antennas mounted on tripods
>about 3ft above the roadway. The test result are fairly erratic,
>probably due to ground reflections. In addition, the midpoint Fresnel
>Zone hits the ground, which causes additional path problems.

Found one:
http://martybugs.net/wireless/antennacomp.cgi
The erratic results are from roadway reflections.

The test range is about 600ft long. At 2.4GHz, the Fresnel Zone is
about a 7ft radius at midpoint about the center line. For this test
range to work, all the test antennas would need to be elevated at
least 7ft off the ground. That obviously wasn't done.
http://www.terabeam.com/support/calculations/fresnel-zone.php


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