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.

----------snip2save------------------


--
Quote "Get SSL VPN services now, KEEP Government OUT of your
business... "

On 6/2/2011 8:25 AM, moronsbegone wrote:
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.


Uhh.. with the test scheme you describe, there's a ton of things that
could be perturbing your results: mostly because you're measuring
radiated power in a single direction, which is connected to RF transmit
power, but also affected (greatly) by the antenna.

Different Wi-Fi implementations may have the same RF transmitter power,
but have different transmit duty cycles. Your meter is doing some sort
of time averaging.

The antenna performance on your two units under test is probably
radically different. The little USB pods often have a small antenna
which has lower efficiency.

I doubt your inexpensive meter has uniform response in all directions
and all polarizations.

You could start to compensate for a lot of the latter by making multiple
measurements at different positions separated by, say, 5cm (2") moving
the meter by, say, 2 or 3 feet vertically, and 2 or 3 feet horizontally.

Anyway, measuring transmit power in the far field with an meter with
unknown spatial and polarization response is trickier than it seems.

On 6/2/2011 12:22 PM, Jim Lux wrote:
On 6/2/2011 8:25 AM, moronsbegone wrote:
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.


Uhh.. with the test scheme you describe, there's a ton of things that
could be perturbing your results: mostly because you're measuring
radiated power in a single direction, which is connected to RF transmit
power, but also affected (greatly) by the antenna.

Different Wi-Fi implementations may have the same RF transmitter power,
but have different transmit duty cycles. Your meter is doing some sort
of time averaging.

The antenna performance on your two units under test is probably
radically different. The little USB pods often have a small antenna
which has lower efficiency.

I doubt your inexpensive meter has uniform response in all directions
and all polarizations.

You could start to compensate for a lot of the latter by making multiple
measurements at different positions separated by, say, 5cm (2") moving
the meter by, say, 2 or 3 feet vertically, and 2 or 3 feet horizontally.

Anyway, measuring transmit power in the far field with an meter with
unknown spatial and polarization response is trickier than it seems.

In addition to that, I think the emission is frequency-hopping and may
have a duty factor associated with the scheme. I think he may need a
peak-power capture scheme.

Also, how is a field strength meter calibrated for emitted power with
all the variables involved?

On 6/2/2011 11:07 AM, John S wrote:


In addition to that, I think the emission is frequency-hopping and may
have a duty factor associated with the scheme. I think he may need a
peak-power capture scheme.

It doesn't hop, per se, but it does pulse.


Also, how is a field strength meter calibrated for emitted power with
all the variables involved?


It isn't..

The field strength meter measures power density (W/sq meter) or field
(V/m), and you have to figure out how that relates back to transmitted
power.

The OP wasn't actually reading it in power, he was using the meter as a
sort of transfer standard. i.e. say it reads linearly in relative power
from 0-100. You put a known 1 W source at 10 feet, and it reads, say, 88.

You put your unknown at 10 feet, and it reads, say, 44, so you calculate
that the source must have been 1/2 Watt.

The problem is that you're really making more of an ERP (Tx power +
antenna effects) measurement assuming an isotropic source/meter, which
can easily have 10 dB of error from a variety of factors.

On 6/2/2011 2:43 PM, Jim Lux wrote:
On 6/2/2011 11:07 AM, John S wrote:


In addition to that, I think the emission is frequency-hopping and may
have a duty factor associated with the scheme. I think he may need a
peak-power capture scheme.

It doesn't hop, per se, but it does pulse.


Also, how is a field strength meter calibrated for emitted power with
all the variables involved?


It isn't..

The field strength meter measures power density (W/sq meter) or field
(V/m), and you have to figure out how that relates back to transmitted
power.

The OP wasn't actually reading it in power, he was using the meter as a
sort of transfer standard. i.e. say it reads linearly in relative power
from 0-100. You put a known 1 W source at 10 feet, and it reads, say, 88.

You put your unknown at 10 feet, and it reads, say, 44, so you calculate
that the source must have been 1/2 Watt.

The problem is that you're really making more of an ERP (Tx power +
antenna effects) measurement assuming an isotropic source/meter, which
can easily have 10 dB of error from a variety of factors.

That makes sense. But, what about reflections, exact positioning, etc?
For example, in a low-signal location, moving my cell phone a fraction
of an inch can change the signal from "no connection" to two bars. In a
room with light fixtures, power wiring in the walls, picture frames,
computers, etc, I doubt the reading can be of much value. It might be
better to move the detector all around while recording readings to get
an average.

But, you know all that.

On 6/2/2011 2:43 PM, Jim Lux wrote:
On 6/2/2011 11:07 AM, John S wrote:


In addition to that, I think the emission is frequency-hopping and may
have a duty factor associated with the scheme. I think he may need a
peak-power capture scheme.

It doesn't hop, per se, but it does pulse.


Also, how is a field strength meter calibrated for emitted power with
all the variables involved?


It isn't..

The field strength meter measures power density (W/sq meter) or field
(V/m), and you have to figure out how that relates back to transmitted
power.

The OP wasn't actually reading it in power, he was using the meter as a
sort of transfer standard. i.e. say it reads linearly in relative power
from 0-100. You put a known 1 W source at 10 feet, and it reads, say, 88.

You put your unknown at 10 feet, and it reads, say, 44, so you calculate
that the source must have been 1/2 Watt.

The problem is that you're really making more of an ERP (Tx power +
antenna effects) measurement assuming an isotropic source/meter, which
can easily have 10 dB of error from a variety of factors.

I tried to cancel my last post but it may not have done so.

In thinking more about it, I have to agree. Especially in light of your
allowance of the 10 dB.

So, in summary, excellent reply.

73,
John

On 6/2/2011 12:22 PM, Jim Lux wrote:
On 6/2/2011 8:25 AM, moronsbegone wrote:
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.


Uhh.. with the test scheme you describe, there's a ton of things that
could be perturbing your results: mostly because you're measuring
radiated power in a single direction, which is connected to RF transmit
power, but also affected (greatly) by the antenna.

Different Wi-Fi implementations may have the same RF transmitter power,
but have different transmit duty cycles. Your meter is doing some sort
of time averaging.

The antenna performance on your two units under test is probably
radically different. The little USB pods often have a small antenna
which has lower efficiency.

I doubt your inexpensive meter has uniform response in all directions
and all polarizations.

You could start to compensate for a lot of the latter by making multiple
measurements at different positions separated by, say, 5cm (2") moving
the meter by, say, 2 or 3 feet vertically, and 2 or 3 feet horizontally.

Anyway, measuring transmit power in the far field with an meter with
unknown spatial and polarization response is trickier than it seems.


And even the cheapest units on the market have MIMO. 59USD is the
cheapest I have seen, but they could be cheaper.

For those that have never heard of MIMO it's a way of encoding more
information on the same frequency by having multiple antennas spatially
separated on 1 or both ends.

The upshot here is that once the unit has multiple antennas it not only
can spatially encode it, it can also beam it.

tom
K0TAR