I have a microwave leakage tester. It is calibrated for 2450 mHz, so it should
be perfect for testing WiFi Network output power. It is calibrated in mw/cm2
(miliwatts per square centimeter). It is also ham related as I am limited to
100mw EIRP on the 2.4gHz ham band.

So I figured that I could use it by measuring the field strength at a specific
distance from the antenna, and then applying a conversion factor.

Anyone have any ideas?

Thanks,

Geoff.

--
Geoffrey S. Mendelson, Jerusalem, Israel N3OWJ/4X1GM

Geoffrey S. Mendelson wrote:
I have a microwave leakage tester. It is calibrated for 2450 mHz, so it should
be perfect for testing WiFi Network output power. It is calibrated in mw/cm2
(miliwatts per square centimeter). It is also ham related as I am limited to
100mw EIRP on the 2.4gHz ham band.

So I figured that I could use it by measuring the field strength at a specific
distance from the antenna, and then applying a conversion factor.

Anyone have any ideas?

Thanks,

Geoff.

The "near field" analysis feature of EZNEC or other modeling program
will tell you what field strength in V/m you should expect from an
antenna at any specific point in space, if you're able to model the
antenna with the program. Power density (mW/cm^2) will be meaningful
only if the observation position is in the far field, which is very
little distance away at that frequency, but the "near field" analysis is
valid also in the far field. For the conversion, use the relationship
that the power in watts = V^2/377 where V is the field strength voltage
in volts, or mW/cm^2 = (V/m)^2/3770. Again, this measurement is made in
the far field, which is typically any distance greater than a fraction
of a wavelength from the antenna. If your antenna is fairly simple, the
free demo version of EZNEC is adequate to do the modeling.

Roy Lewallen, W7EL

On Sat, 20 Jun 2009 17:54:04 +0000 (UTC), "Geoffrey S. Mendelson"
wrote:

I have a microwave leakage tester. It is calibrated for 2450 mHz, so it should
be perfect for testing WiFi Network output power. It is calibrated in mw/cm2
(miliwatts per square centimeter). It is also ham related as I am limited to
100mw EIRP on the 2.4gHz ham band.

So I figured that I could use it by measuring the field strength at a specific
distance from the antenna, and then applying a conversion factor.

See if you can get your money back. I had the same idea a few years
ago and purchased a Safeea Inc MD-2000 tester:
http://www.allproducts.com/ee/safeea/md-2000.html
Of course, the first thing I did was rip it apart:
http://802.11junk.com/jeffl/pics/Microwave%20Leakage%20Detector/index.html
The diode D1 (labelled C2) is the detector. The apparently random
circuit board traces nearby is the "antenna".

The sensitivity sucks. I'm only able to get an indication when it's a
few cm from the antenna of my access point. Worse, the indication
varies radically from zilch to tilt (9.9mw/cm^2). There's no stable
indication. At the microwave oven leak test distance of 2 inches, it
barely indicates anything, even when I generate continuous traffic
with my iPod Touch. If there were some way to generate a CW carrier
from the access point, perhaps it might deliver a better and more
stable indication.

The problem is that the duty cycle of Wi-Fi transmissions are not
constant. The MD-2000 microwave leak detector works nicely with the
constant duty cycle of the oven controller, but doesn't do as well
with the widely varying duty cycle of Wi-Fi. Also, the duty cycle of
just the beacon transmissions is very low, resulting in a very low
indication.

I also have several analog oven leak detectors, which produce similar
disgusting results.

Topic Drift: Think your microwave oven is well shielded? Put your
2.4GHz cordless phone handset inside the microwave oven. Close the
door. Hit the "page" button on the phone base and see if the phone
rings. It probably will. So much for shielding.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Jeff Liebermann wrote:


See if you can get your money back. I had the same idea a few years
ago and purchased a Safeea Inc MD-2000 tester:

I bought it about 10 years ago. I doubt they would do any thing now.

The sensitivity sucks. I'm only able to get an indication when it's a
few cm from the antenna of my access point. Worse, the indication
varies radically from zilch to tilt (9.9mw/cm^2).


Mine also read close up, but was relatively stable. I was downloading a large
file at the time, so there was plenty of traffic.



Topic Drift: Think your microwave oven is well shielded? Put your
2.4GHz cordless phone handset inside the microwave oven. Close the
door. Hit the "page" button on the phone base and see if the phone
rings. It probably will. So much for shielding.

Same here, except mine are 1.8gHz.

Geoff.


--
Geoffrey S. Mendelson, Jerusalem, Israel N3OWJ/4X1GM

"Geoffrey S. Mendelson" wrote in
:

I have a microwave leakage tester. It is calibrated for 2450 mHz, so
it should be perfect for testing WiFi Network output power. It is
calibrated in mw/cm2 (miliwatts per square centimeter). It is also ham
related as I am limited to 100mw EIRP on the 2.4gHz ham band.

Is the power specified as pX, pY, or pZ?

What does the 'instrument' indicate? At best it is probably pX, pY, and pZ
on an unmodulated carrier, but none of them on a WiFi signal.

Owen

On Jun 20, 2:47*pm, Owen Duffy wrote:
"Geoffrey S. Mendelson" wrote :

I have a microwave leakage tester. It is calibrated for 2450 mHz, so
it should be perfect for testing WiFi Network output power. It is
calibrated in mw/cm2 (miliwatts per square centimeter). It is also ham
related as I am limited to 100mw EIRP on the 2.4gHz ham band.


There's probably some standard orientation and test conditions for
these oven leak testers. They're not a "Narda ball" with carefully
designed broadband sensors for all three axes. I'd guess it's
basically a dipole across the end of the business end of the sensor (E
field horizontal, if the meter is laying on the table) with a single
diode and meter. Why do anything more sophisticated.



The shielding in an oven doesn't have to be all that wonderful to meet
the 2mW/cm2 level, either.

For a ballpark, let's say that you've got 1kW spread evenly out on the
inside of the oven. About 30x40x40 cm, or 48,000 cm2. So, in round
numbers 1E6 mW/50E3 cm2 or 20 mW/cm2.. all you need is 10 dB of
attenuation to get down to 2mW/cm2 Obviously, the power density
really isn't even, and you actually worry more about seams, but the
point is that you don't need exotic gaskets and such.


They do make a nifty $100-200 or so WiFi spectrum analyzer which has a
calibrated receiver. And, if you have a wifi card with an exernal
antenna, you can do even better.

Jim Lux wrote:
They do make a nifty $100-200 or so WiFi spectrum analyzer which has a
calibrated receiver. And, if you have a wifi card with an exernal
antenna, you can do even better.


First of all, a $100 item in the US is $200-$300 here (or more, depending
upon the regulation required to get it in, the rarity, etc). For example,
cheap MP3 players are around 25% more expensive, an iPod is double.

Second, I have never seen a real meaningfull number from a computer. Whether it
is MacOS, Linux, or Windows XP, they all give some sort of relative number,
for signal strength.

In some cases it has been useful, for example using Kismac, I found that the
signal strength as measured by my WiFi card in my Mac, was in the mid 50's
with a bluetooth dongle on the computer, and in the 60's without one.

No matter what the number really is 50 is about where it gives up completely.

However, it does not really help me. What I need is to be able to place a
device at a specific distance from a 2.4 gHz antenna and read the EIRP.

I can assume that a 100mW signal into the little whip on the back of a router,
produces and EIRP of 100mW, which makes all gain antennas illegal. Or I can
make an attempt at measuring it. It does not have to be 100% accurate, 10%
or even 25% off would be good enough.

For example, I found that placing my meter at about 1 inch from a WiFi router
running "full tilt" produced a reading of .36mv/cm2 on the meter. However the
only real information I can get from that is yes, the router is transmitting
a signal, and yes, I can read it, but no, I have absolutely no clue as to
what it means.

I may be asking the impossible, but what I would like to get is a number
for example, if I hold the meter an inch from the antenna, 100mW EIRP is
1 mv/cm2. Then I can do something useful with it, as I already know how
to compensate for distance. :-)

Geoff.

--
Geoffrey S. Mendelson, Jerusalem, Israel N3OWJ/4X1GM

On Sun, 21 Jun 2009 06:19:03 +0000 (UTC), "Geoffrey S. Mendelson"
wrote:

What I need is to be able to place a
device at a specific distance from a 2.4 gHz antenna and read the EIRP.

Take a leadless 366 Ohm resistor, and coat it with temperature
sensitive dichroic.

73's
Richard Clark, KB7QHC

Geoffrey S. Mendelson wrote:
Jim Lux wrote:
They do make a nifty $100-200 or so WiFi spectrum analyzer which has a
calibrated receiver. And, if you have a wifi card with an exernal
antenna, you can do even better.


First of all, a $100 item in the US is $200-$300 here (or more, depending
upon the regulation required to get it in, the rarity, etc). For example,
cheap MP3 players are around 25% more expensive, an iPod is double.

Ok.. so the Wi-Spy is going to be a bit on the pricey side for you.



Second, I have never seen a real meaningfull number from a computer. Whether it
is MacOS, Linux, or Windows XP, they all give some sort of relative number,
for signal strength.

What software are you using to get the signal strength from the WiFi
card? AND, is your WiFi interface one that actually supports scanning
and measuring signal strengths (not all do.. it's less and less common)

However, if you have a card that supports it, you can probably get a
quantitative (but not necessarily calibrated) measurement.


In some cases it has been useful, for example using Kismac, I found that the
signal strength as measured by my WiFi card in my Mac, was in the mid 50's
with a bluetooth dongle on the computer, and in the 60's without one.

No matter what the number really is 50 is about where it gives up completely.

However, it does not really help me. What I need is to be able to place a
device at a specific distance from a 2.4 gHz antenna and read the EIRP.



I can assume that a 100mW signal into the little whip on the back of a router,
produces and EIRP of 100mW, which makes all gain antennas illegal.


That's not a valid assumption. You'd have to look at the FCC
registration data for the device (assuming the configuration is the same
in the US as you have) and see what their analysis is. Gain antennas
aren't necessarily a problem, even under Part 15.

Or I can
make an attempt at measuring it. It does not have to be 100%
accurate, 10% or even 25% off would be good enough.

Your problem is going to be finding a way to calibrate your measurement.
Do you have ANY calibrated signal source around? If so, you can build
a standard gain antenna, and generate a known field to measure with your
measurement device.





For example, I found that placing my meter at about 1 inch from a WiFi router
running "full tilt" produced a reading of .36mv/cm2 on the meter. However the
only real information I can get from that is yes, the router is transmitting
a signal, and yes, I can read it, but no, I have absolutely no clue as to
what it means.

I assume you mean 0.36 mW/cm2... That's probably an average power (with
the averaging done by the meter movement).



I may be asking the impossible, but what I would like to get is a number
for example, if I hold the meter an inch from the antenna, 100mW EIRP is
1 mv/cm2. Then I can do something useful with it, as I already know how
to compensate for distance. :-)



If you hold the meter an inch from the antenna, the meter is going to
alter the performance of the antenna.


You need something with some gain (or better sensitivity), so you can
put the probe farther away (e.g. more than a meter), and still make your
measurement. If your device under test is radiating 100 mW, the
surface area of a 1 meter radius sphere is about 130,000 cm^2, so 8E-4
mW/cm2. (call it -30dBm/cm^2) At 2.4 GHz, a wavelength is 12.5cm, so
a dipole will be half that, or 6cm. The effective aperture will be
around 10-20cm^2 (I can't remember off hand), so a receiver hooked to a
dipole probe needs to measure -20dBm sorts of power levels, which isn't
too bad. There's a bunch of inexpensive power detector ICs around that
have sensitivities spanning this (like the AD 8314.. goes -45dBm to
0dBm, 2.7GHz, $1 each in quantity)

You could probably get a sample or two for free. Build a tiny board that
combines the AD log detector and some sort of display or IR telemetry
and a small battery, with the dipole sticking out.





Geoff.

I hope I don't sound to ignorant here, too often thats the
case.
But If you have a known working Wifi AP with a high end
reputation like Cisco Airnet model 2400 for instance.
They advertize 1 watt, it's one watt no funny bones about it
or busnesses wouldn't respect their higher prices and pop for
it.
Take an EMF meter like the one on Ghost Hunters TV show,
radio shack has them, and set it ten feet LOS from the AP
[Cisco] wile downloading a big video file or something large,
and set that as your "One Watt" point, now you have a
comparason, of course notice the Antenna "Type" and location.
Heck you can use a cheap NetBook with built in WiFi and
NetStumbler software to measure the IF strenth. All you have
to be sure of is the output of your "Sample" source.
My Alfa USB WiFi unit LIES!!! they claim a half a watt, NO
WAY!!! I bet if Cisco built it they wouldn't lie about that
stuff, and of course it would cost more than 60 bucks.

"Geoffrey S. Mendelson" wrote in
:

Jim Lux wrote:
They do make a nifty $100-200 or so WiFi spectrum analyzer
which has a calibrated receiver. And, if you have a wifi
card with an exernal antenna, you can do even better.

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