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How to measure Wifi power

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Jane11

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Where can I get any useful info about measuring a level ( power) of a Wifi signal? What equipment to use? Network analyzer or spectrum analyzer?
What kind of antenna? etc.
Thanks
 

dknguyen

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Most Helpful Member
Have you just tried finding an app for your smartphone? That's probably the easiest and cheapest way by far if all you need is a casual measurement.
 

ronsimpson

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Have you just tried finding an app for your smartphone?
My phone does that (more or less) but the real question is what and why do you want to measure.
If you want a strong verses week reading the phone works well. I am using "Wifi Analyzer". It is good to compare two signals.
If you want to know 1.05 watts then you need to spend some money.
upload_2017-12-14_16-9-49.png
 

dknguyen

Well-Known Member
Most Helpful Member
My phone does that (more or less) but the real question is what and why do you want to measure.
If you want a strong verses week reading the phone works well. I am using "Wifi Analyzer". It is good to compare two signals.
If you want to know 1.05 watts then you need to spend some money.
View attachment 109671
I see that you too are a man of culture. I use Wifi Analyzer too to nudge my signal right in between every other signal. I have to go to greater effort to squeeze it in than you did.
 

ronsimpson

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Most Helpful Member
I see that you too are a man of culture.
Fooled you.
That picture is from Wifi web sight.

I do not live in town. There are no neighbored to cause trouble.
I have 1="Simpson-North" that connects the house to the barn and green houses. Includes channel 40 a 5190mhz.
6="Simpson" Which is for the house.
11="Simpson-South" that connects over to the Kid's house. And channel 161 at 5815mhz.
Then on 5ghz (#149). I have a repeater that jumps 5 miles to the grain elevator (tall building). From there it connects to fiber.

I want to add some wifi cameras in the middle of all this. I probably need to clear up one channel just for cameras.
 

Jane11

Member
Thanks for the replies.
I would like to measure a power( level) of an incoming Wi-Fi signal alias wifi AP( access point) and strength/power ( or waveforms) of backscatter signals from that WiFi AP.
Do you have any idea what method/measurement device to use?
 

Mosaic

Well-Known Member
A spectrum analyzer with a 1/4 length aerial should do the trick.

Alternatively an RF wattmeter like a Boonton 4210 if you don't expect much ambient RF noise. You can still do this with a bandpass filter to tune the aerial reception if there is other RF frequency noise in the area.
 

Jane11

Member
Can you please explain a little more that "a 1/4 length aerial" ?What should be the aerial length ?How can I calculate?
Thank you for help
 

RadioRon

Well-Known Member
Measurement of signals off-air (ie. using an antenna to pick up the signal so you can measure the power) is actually very difficult to do precisely. So, you have to set your goal to only getting an approximate measurement, I think. Something within +/- 10 dB is easier and may give you what you want. And for that, a WiFi sniffer app on a PC or phone is good enough, or perhaps a calibrated WiFi locator gadget. If you must have a precise measurement, hire an engineer who specializes in this sort of thing, and be prepared for answers that are somewhat unsatisfying. Even using good test equipment, the answer will be statistical in nature. By this, I mean that the answer is a graph or table showing that your signal has a probability X of having a signal strength Y. This is because the signal strength will probably not be static in nature, or in other words, the power will be jumping around depending on what is moving nearby. But realistically, not many people need that kind of precision.

To give it your best shot, in hopes of something closer to +/- 5dB, use a spectrum analyzer set up to measure channel power in either a 20MHz or 40MHz channel (depending on your WiFi channel width selections), and set the top line reference to 0 dBm. In this case you will probably be able to measure power down to -60 dBm which is still pretty strong. For lower powers, you can adjust the spectrum analyzer vertical range downwards, but you won't be able to get much lower than about -80 dBm without a more complicated setup (like with an external preamp for example). Set your frequency to the channel frequency used by your WiFi setup. Attach an antenna suitable for the frequency range, either 2.4 GHz or 5GHz band. Here's one you can make for 2.4GHz: https://mbharris.co.uk/articles/antenna/omni/quarter/
A simple antenna like this must be held underneath only, and preferably with a stick or something taped to the cable to keep the antenna up and away from your body at least two feet or so. Alternatively, you can tape the antenna to a camera tripod. For approximate measurements, you can assume that the reading on your spectrum analyzer is pretty close to the signal received at the antenna (in other words, I am removing the complication of knowing the antenna gain and cable loss in this case to keep things simple). Just be sure to use a good coax cable (ie. not RG59 or RG59 or RG6) and keep the cable shorter than 6 feet long.

Using this kind of setup will give you a feeling for how much and how fast the signal jumps around, which you can then do some mental averaging to get a final number. Your spectrum analyzer update speed has some bearing on how well you can do this so you may have to fiddle around to find the best settings. Beware that the radio signals are sensitive to their environment, as in, they reflect off anything conductive. Doing measurements indoors means that you may get frustrated trying to measure levels and measurements may not be repeatable.

With a simple antenna like the one above, you won't be able to isolate the direction that the signal is coming from. You mentioned backscatter, which implies that you want to know about direction of signals. For that, it would be necessary to use a directional antenna. There are many plans out there. I found a few using the keywords "DIY directional wifi antenna". These antennas tend to be bigger, which makes them less useful for finding strength in a small space, like a room in a house.
 

cowboybob

Well-Known Member
Most Helpful Member
Can you please explain a little more that "a 1/4 length aerial" ?What should be the aerial length ?How can I calculate?
Thank you for help
First, determine the frequency of the signal you wish to monitor (for example, 5GHz).

Then, using this calculator (or similar), determine the wavelength:
1. enter the frequency (for example, 5) and select the multiplier (GHz)
2. Select the Wavelength (1/4) and finally,
3. the measurement system you prefer. For this example, you should see this:
upload_2017-12-15_17-33-26.png

A single strand of solid copper, of the indicated length (in this case, round to 1.5cm), will suffice as a 1/4ω aerial for most tests. Accurate power tests will take some finessing of aerial's orientation with the source.

How you connect the aerial to your RF power monitoring device will be for you to determine.
 

Mosaic

Well-Known Member
Ok, I was planning to do this as a project with an ESP07s for external antenna attachement. But you can do it with most any ESp8266 chip.
https://www.hackster.io/rayburne/esp8266-mini-sniff-f6b93a

Cost is minimal, but u do need to know about hooking up the ESP series chip. Perhaps one of the prototyping ready made boards for a few bucks from sparkfun or adafruit would be easiest to use.
 
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