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