Cheaply measuring wireless signal strength

802.11b wireless networks run on 14 channels from 2.412GHz to 2.484GHz. I want to be able to test the signal strength in different areas of a building cheaply, but the professional meters are rather expensive. It would be nice to be able to test the channel strengths individually, but a measure of signal strength over that whole range of channels would be better than nothing.

I was wondering if something clever could be done rather cheaply. I'm out of my depth here, but I was thinking something like:

(a) Can I just attach a little antenna to oscilloscope probes? Can I get a fast enough second-hand oscilloscope fairly cheaply? 2.4GHz is pretty fast! Or, do things just not work like that?

(b) Can I make simple little circuits, that have a little antenna, that resonate fairly specifically with the 802.11b signal (roughly over the right range, or even for individual channels) enough to build up a steadier signal that I can measure more easily with a digital voltmeter or something? (I've made plenty of digital circuits, but with analog I know little more than how do simple things with RC oscillators, op amps, etc.) Or, really, anything that responds to the high-frequency signal in a manner that's easier to detect with cheaper equipment?

Just looking for ideas. (-: Tiny GHz signals are not what I'm used to dealing with! I'm not even sure how tuned to the application the antenna needs to be so long as it's roughly the right length; it's probably relevant that I'm interested much more in relative signal strength than being able to accurately measure absolute power, so some losses in the signal path may be quite acceptable if they're consistent.

-- Mark

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Mark Carroll
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In message , Mark Carroll writes

13 I think.. Varies country to country.. 11 in the US I think.

Indeed, and everyone wants one. If it were as easy and cheap as you think there would be dozens on the market but there are (AFAIK) only two and those aren't very well reviewed.

High frequency oscilloscopes are *very* expensive and AFAIK there aren't any available at anywhere near the GHz range.

Spectrum analysers for the GHz range (at about £3,000) are the nearest thing, and they mix a frequency to the signal and display the difference.

RF design at GHz frequencies is bordering on a black art. And, since we're dealing here with low-level signals noise is a real issue. Cross-coupling another (thing suddenly bursts into oscillation jamming the very thing you're trying to detect/measure ) :-)

That is tuning where RF is concerned. Aerials are (or should be) resonant at the centre frequency. Look up "Q" in the text books. Q is synonymous with gain. And the thicker the aerial the greater the bandwidth but the lower the Q. Aerial tuning is based on a half-wave dipole - at 2.45GHs that's about 6.25cm.

The only thing that occurs to me is that all 802.11 devices have a receiver in them. And receivers generally use AGC (automatic gain control) using a derived voltage level corresponding to signal strength. Since AGC voltage is effectively a varying DC level proportional to signal strength, it might be possible to drive an inexpensive meter via an op-amp (for isolation). PCMIA/Cardbus and USB (better [1]) wireless devices for laptops are about £30 to £40 and (I imagine) it wouldn't be too difficult to "break-out" the AGC line for your op-amp/meter giving you a relatively accurate indication of signal strength..

Just a thought :-)

[1] A USB device like the Linksys WUSB11 seems better because it is easy to provide (battery) power to the device via the power pins on the USB plug. The WUSB11also has a real vertically polarised aerial which means you're extracting more signal from the ether to play with.

Anyway, best of luck - let me know if you have any success.

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Tony Morgan
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Tony Morgan

If you have an IPAQ, perhaps look at the YellowJacket series from...

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eg

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(slow site)

Reply to
CWatters

Oscilloscopes don't (thy will display the sum of all signals around, and will have a very low sensitivity compared to RF signals). Moreover I'm afraid that a cheap 2.4GHz oscilloscope doesn't exist... The general-purpose test equipment you really need exists and is a called a spectrum analyzer. It displays the signal amplitude over frequency and not over time, and has a very high sensitivity due to log amplifiers. Unfortunatly that isn't what you I think you will call a cheap equipment... Old units going to 3GHz could be found may be for $1000 (look for HP141T/HP8555, etc), new units starts at $4-5K

It will be difficult. It will be nearly impossible to find a narrow band pass filter in the 2.4GHz range that will be selective enough to get only the Wifi bands, except if you knows an expert in cavity designs. The usual scheme is to use a larger band pass filter, then a downconverter, then a narrow band pass filter, then a level detector, etc, but that's not an easy design.

I've seen some adds from commpanies using actual 802.11 access points and just providing a specific software that let see the average power level on each channel. That is probably the most cost effective solution for what you are looking for, even if the precision will probably not be high...

Good luck,

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Robert Lacoste - ALCIOM : The mixed signals experts
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Reply to
Robert Lacoste

I must be missing something here. If you put netstumbler on a laptop with a wireless card it will give you a list of all the signals it finds and their strength, s/n ratio etc. Won't that do what you want? mike

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Reply to
mike

O'scope will not work for what you want. A reasonable fast o'scope has a bandwidth of about 100 mhz. Something for 2.4 ghz would be very pricey.

As far as cheap a used spectrum analyzer that will do at least 2.5 ghz will also be very expensive. Best bet is to buy a card that can monitor in promiscous mode such as an Orinoco/Proxim Gold Card or thier dual 802.11 A/G card. This way you can run an application like netstumbler and determine what channels are in use, the relative signal strength etc. Total cost is about $100 for the card plus a laptop computer of your choice.

Also by using a laptop you will have real world results that a user is likely to experience.

Reply to
K Bloch

O'scope will not work for what you want. A reasonable fast o'scope has a bandwidth of about 100 mhz. Something for 2.4 ghz would be very pricey.

As far as cheap a used spectrum analyzer that will do at least 2.5 ghz will also be very expensive. Best bet is to buy a card that can monitor in promiscous mode such as an Orinoco/Proxim Gold Card or thier dual 802.11 A/G card. This way you can run an application like netstumbler and determine what channels are in use, the relative signal strength etc. Total cost is about $100 for the card plus a laptop computer of your choice.

Also by using a laptop you will have real world results that a user is likely to experience.

Reply to
K Bloch

In message , Robert Lacoste writes Snipped...

AFAIK the usual scheme is to use a balanced mixer with a LO of (say)

2GHz. Then a 400KHz bandpass with an endpoint on upper or lower sideband to give you a signal for analysis/inspection/processing.

Obviously since you are a mixed signal expert - I must be wrong :-)

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Tony Morgan
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Tony Morgan

Mark posted, in part:

How about $24.95 plus shipping? Kensington WiFi Finder detects 802.11 a and g from up to 200'. Order from MACMALL at 800 622-6255 (their part # is 241768). It's a stand-alone unit that has 3 LEDs to indicate signal strength. You could probably modify it with a meter driver very easily.

Don

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Dbowey

mike wrote in news: snipped-for-privacy@juno.com:

yes, you are

not all, it wont show you microwave oven for example

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RusH

In message , RusH writes

You using a microwave oven as an AP? :-)

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Tony Morgan
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Reply to
Tony Morgan

2.4 GHz - 2.0 GHz leaves 400 MHZ, not 400 KHz.
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Michael A. Terrell
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Michael A. Terrell

In message , Michael A. Terrell writes

Thank you soooo.... much.

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Tony Morgan
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Tony Morgan

It's what I call a downconverter...

Friendly, Robert

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Robert Lacoste

WLANexpert mode Air Scan:

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Will show you microwave oven :)

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IVAN KORSHUN
2 snipped-for-privacy@mail.ru (IVAN KORSHUN) wrote in news:d84590a1.0309100251.48b327d0 @posting.google.com:

yez, this is the only free tool that will do that , but only on a Prism 2 card, not on all cards like Mike said - that was my point.

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Reply to
RusH

I don't know who makes them, but there is a simple 2.4 Ghz detctor sold. I think Fellowes, or other computer accessory vendore has it.

Reply to
Gary Tait

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These folks make an 802.11 card and have a utility that makes it behave like an ism band spectrum analyzer.

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At the bottom of this page you will find a PCMCIA card designed specificly for the purpose. $250 new and plenty on eBay for $150 and under. I bought one of these but havn't received it yet. Supposedly runs in DOS only and I've seen a few "package" deals including a 486 based laptop. Could be pretty sweet with an old tablet PC like a Fujitsu Stylistic.

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This is another product designed specificly for the ISM band. It's a bit on the large size and fairly pricey if I remember correctly.

Google for "ISM Band Spectrum Analyzer" and you will find quite a few more traditional spectrum analyzers for the job in the $2k-$5k price range.

Kensington and a few others make small handheld devices for detecting wifi networks. They filter out things like cordless phones, cameras, and microwave ovens. This filtering can be a problem if your looking for a source of intereferance. Also they have just a few LEDs to show signal strength. Cost is something like $30 depending on where you buy.

I'm sure something clever can be done on the cheap but it doesn't appear to be public yet.

Reply to
Slick Willy

see

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extends poor mans spectrum analyser to almost 5 gigahertz. cheaply using cavities made from plumbing pipe and hobby shop brass, plus some low cost microwave vcos.

Steve Roberts

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Steve Roberts

In article , snipped-for-privacy@chiark.greenend.org.uk says... | 802.11b wireless networks run on 14 channels from 2.412GHz to | 2.484GHz. I want to be able to test the signal strength in different | areas of a building cheaply, but the professional meters are rather | expensive. It would be nice to be able to test the channel strengths | individually, but a measure of signal strength over that whole range | of channels would be better than nothing. | |

My Belkin wireless card came with software, which gives an indication of the signal strength it is receiving from the access point it logs into. Set up an access point on the channel you wish to try, then wonder around the building with a laptop fitted with a wireless card, preferably with a plug in antenna.

If more detailed information is required, such as the signal strength from individual channels, then I find NetStumbler can do this. You just pick any one the channels upon which it sees a signal, and a signal strength will be shown for it. With this software you can check if there are any other networks within the vicinity on any of the channels, thus allowing you to avoid using them.

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         Harry (M1BYT)...
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Harry Bloomfield

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