# How to build a cheap device to determine where Wi-Fi interference is coming from?

In this post, the ServerFault guys use a fancy tool to map out the WiFi spectrum under normal conditions, with a microwave running, and with a baby monitor on. The baby monitor and microwave cause a surprising amount of interference.

Having had to deal with Wifi interference in the past, a tool like that could be very useful. However, its price - $200 to$1000+ - is way outside my price-range.

I'd like to build something to serve a similar purpose, but my experience with building analog devices is, shall we say, non-existent.

1. What's the cheapest way to determine where Wi-Fi interference is coming from? My idea was rig up an old radio to somehow receive around the 2.4Ghz frequency, but I have no idea how to do that.
2. How can I tell which channel has the least interference?

Please let me know if this is off-topic. Thanks!

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It will be pretty difficult to build your own device. It can be done, but 2.4GHz can be pretty tricky to deal with if you have never messed with RF electronics before. You might have better luck with a wifi card plugged into a computer with a very directional antenna and moving it around to see where it is coming from. –  Kellenjb Feb 27 '12 at 17:19
@Kellenjb: That's disappointing, I was hoping it was a matter of just replacing an inductor or something. Unfortunately, normal WiFi cards do not give direct, raw access to reading analog signals, so the only way to do this with software is to write down the "signal strength" reported in various places, which is a notoriously inconsistent/unreliable measurement :\ –  BlueRaja - Danny Pflughoeft Feb 27 '12 at 17:30
@BlueRaja-DannyPflughoeft I am not sure what card it is, but I had a former lab mate who had a fairly cheap wifi card that was designed for sniffing wifi packets. It also gave the ability to show the power across the whole wifi spectrum. It was pretty easy to tell when the microwave was turned on by just looking at its results. Superuser might be able to help you more for finding out what product he was using. It might have also been that he had a 3rd party program to show him these results. –  Kellenjb Feb 27 '12 at 17:35

The WiSpy is based on a Chipcon (now TI) CC2500 chip, which is a general-purpose 2.4GHz digital radio. By scanning through the channels, it can be turned into a spectrum analyzer. http://tim.cexx.org/?p=646

So you need a board with a CC2500 radio. Search ebay for "CC2500" and you can find them for http://www.ebay.com/itm//260900652131

Here is a similar project with a Chipcon chip at a different frequency: http://ossmann.blogspot.com/2010/03/16-pocket-spectrum-analyzer.html

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Hey markrages, thanks for this. I'm afraid I'm not confident enough in my electronics skills to figure out what to do with this thing once I get it. Could you elaborate a bit? –  BlueRaja - Danny Pflughoeft Feb 27 '12 at 21:18
The ebay CC2500 board has the RF part of the radio taken care of. What you need to do is the digital interface to the CC2500 as described in its data sheet and the example code in the DIYspy link. Or just buy the WiSpy and plug it in. –  markrages Feb 27 '12 at 21:25

Before you go any further with this idea, I would suggest that you try out the free software first. inSSIDer will tell you what channels are being used (by wifi routers only). This will give you a good starting point as to how much wireless router interference you are dealing with. If you truly want to do spectrum analysis I would suggest going with this tool first, at a more reasonable price of \$70 (with free software). Building a analyser will take a long time especially with designing the software as well.

Good luck with this.

EDIT:

shortly after writing this I stumbled upon this article. I hope it helps.

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I dont think a spectrum analyzer is the goal, unless you have a suggestion for a directional antenna. –  Kortuk Apr 2 '12 at 22:18