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SDR -- Software defined radio for those that don't know the acronym.

Some of you know I fly model aircraft. For the longest time, 72MHz FM radios were the standard and if someone turned on a radio on the same channel as you, goodby airplane. Then came 2.4GHz and spread spectrum radios that (theoretically) put and end to that. We have had some incidents at the field where I fly that have caused the lost of aircraft, even on 2.4GHz. Now, there are all sorts of issues with 2.4GHz that don't arise on 72Mhz -- see antenna fade for an example. Anyway, we started talking about doing a site survey to see just what really was whizzing through the ether where we are trying to fly.

Enter software defined radios. Essentially they do a direct analog to digital conversion of the IF signal or even of the RF signal for lower frequencies. High bandwidth, high performance radios are still pretty expensive. However, for $20 US or so, you can get a USB tuner that does a bang up job, within its limitations. Many DVB-T dongles for TV reception have the ability to do raw captures as well. I'm using on based on the Rafael Micro R820T chip. I get about 2.4Mhz of bandwidth in a range from 24-1776MHz. I have a 1998MHz downconverter in front of it, putting the 2.4GHz band right in the 400-500MHz range for the receiver. All for under $100US.

Below is a sprectrogram/waterfall plot of 2.4-2.5Ghz at my house.



In the upper left, you can see my wireless access point, on channel 1. I turn it off about 13:40. All the little blips you see from 13:40 to 13:55 are the various WiFi adapters throughout the house looking for access points, as well as bluetooth adapters. By 13:55 I have almost everything turned off. It is amazing how many little radios I have floating around. Just before 14:24, I took a couple of laptops out of sleep to power them down. The splash of energy at 14:28 is my wife warming a pita bread in the microwave. The blob that starts at 14:52 is my putting a 24oz cup of water in the microwave on high for four minutes. By about 14:40, I had turned off everything I could in the house and yet you will notice there were still blips into the -10db range now and again. I believe those are from the smart meter outside the house. The lower power blips here and there are likely from the neighbors houses.

This is the same data plotted as power vs frequency.


There are 100 scans in the above plot. It gives you a good idea just how noisy 2.4GHz can be.

Pretty cool stuff really. For the radio sauvy among you, you will notice the power scale goes from about -50db to around -2db. The ADC is only 8 bits so I get about 55db of dynamic range. The sensitivity is quite good. Something in -138dBm area, but only hanving 55dB of dynamic range is somewhat limiting when dealing with nearby wifi access points. Add to that the fact that my downconverter has a 37dB preamp in it and low power signals get lost in the noise. That is one of the reasons I was trying to get everything turned off -- to see what was down in the noise.

--SS

Neat, thanks for sharing.

Very cool!

Interesting that the microwave comes through so strong. I thought those were supposed to be shielded?

The Egg wrote:

Interesting that the microwave comes through so strong. I thought those were supposed to be shielded?

"Strong" is relative; Wi-Fi is very low power. The standards for shielding of microwaves were developed decades ago, and were intended to prevent potential health hazards to users, not to avoid interference with wireless devices that were still years in the future!

Neat. Was the point just to see the 'background' radiation at 2.4 GHz, or is there some greater lesson to be learned here?

FireGryphon wrote:

or is there some greater lesson to be learned here?

My guess is that a similar experiment on the other unlicensed bands would show similar results (excluding microwave ovens).

The 5 GHz band would be an interesting comparison. I've been waiting for 5 GHz radios to drop in price so I can make the swap, but they still carry a premium. Every time I see something like this (or my home wifi goes insane for no good reason) I ponder switching again.

What kinda ranges from your various devices were you? It's kinda shocking the microwave just as strong as the wifi, unless they were at very different distances from each other.

SuperSpy wrote:

What kinda ranges from your various devices were you? It's kinda shocking the microwave just as strong as the wifi, unless they were at very different distances from each other.

Well, the wi-fi router is pushing all of 84 milliwatts (assuming no use of 3rd-party firmware) while most microwave ovens are cranking out at least a kilowatt. Remember, all that that antenna was seeing was the leakage from that kilowatt. Certainly makes me want to step away from the microwave after pushing Start.

just brew it! wrote:

The Egg wrote:

Interesting that the microwave comes through so strong. I thought those were supposed to be shielded?

"Strong" is relative; Wi-Fi is very low power. The standards for shielding of microwaves were developed decades ago, and were intended to prevent potential health hazards to users, not to avoid interference with wireless devices that were still years in the future!

***ninja edit*** What the Cap'n said ...

--SS

FireGryphon wrote:

Neat. Was the point just to see the 'background' radiation at 2.4 GHz, or is there some greater lesson to be learned here?

There were several reasons for the exercise. One was to get any idea of what the low level background looked like. Another was to test the usefullness of the system. Given the parameters of the scan and the bandwidth of the radio, it takes a couple of minutes to scan the entire 100MHz. I wasn't sure how well very short transmissions would show up. I certinaly can't get a good idea of what a very bursty signal actually looks like from a spectrum distribution level, but I can at least tell it's there, assuming it is reasonably repetative. Third goal was to see what the other wifi signals in the area looked like. Depending on where I am in the house, I can pick up half a dozen other access points. I've been chasing wifi problems with my Asus UX31A, and while I was pretty certain it was the laptop, this helped prove it. I can see other access points in the waterfall plot, but nothing comes close in strength to mine when I have it turned on. Finally, there was geeky curiosity.

Unfortunately, I can't scan 5.8GHz. My currnet setup stops at 3.774GHz. I am running my AP in dual radio mode. The stuff that supports it is using 5.8GHz though that has it's plusses and minuses. It may be less congested, but it is also more sensative to stuff between you and the AP.

--SS

just brew it! wrote:

The Egg wrote:

Interesting that the microwave comes through so strong. I thought those were supposed to be shielded?

"Strong" is relative; Wi-Fi is very low power. The standards for shielding of microwaves were developed decades ago, and were intended to prevent potential health hazards to users, not to avoid interference with wireless devices that were still years in the future!

The fact that he's picking up a strong signal just makes me question exactly how much leakage is occuring. Especially in these days of cheap, mostly plastic chinese-made units. I assume there are some which are much worse than others. I don't really care about the interference; I'm more concerned with the small child standing 6 inches from the door and staring intently at their food cooking.

Well... yeah, standing and staring right into the microwave is probably not the best idea regardless.

But in terms of total exposure it looks like the Wi-Fi AP may be worse (since under normal circumstances it would always be on), depending on how close to the AP you are. You're also getting a decent slug of EM energy every time you use a cell phone; and in that case the device is intentionally radiating a signal that needs to make it all the way to the nearest cell tower, and doing so while pressed against the side of your head!

Hold a neon mains tester up to the microwave, if it lights up then you are cooking outside the oven as well.

But yes, in this case it's just the sensitivity.

Considering the microwave leakage looks about the same as the wifi access point, I'd consider that not really a health issue but just an interference issue.

ChronoReverse wrote:

Considering the microwave leakage looks about the same as the wifi access point, I'd consider that not really a health issue but just an interference issue.

Yeah, but how far away from the antenna was the microwave compared to the AP. Plug that difference into the inverse square law and recalculate.

ChronoReverse wrote:

Considering the microwave leakage looks about the same as the wifi access point, I'd consider that not really a health issue but just an interference issue.

Couple of points that make this somewhat of an apples-to-oranges comparison:

1. Microwaves generally aren't left running all the time; Wi-Fi access points are.

2. The Egg's concern was with people standing right in front of the microwave and staring at the food while it cooks. I don't know anyone who stares at their Wi-Fi access point, unless they are watching the status lights in an attempt to diagnose a malfunction.

just brew it! wrote:

I don't know anyone who stares at their Wi-Fi access point, unless they are watching the status lights in an attempt to diagnose a malfunction.

Well, when I'm sitting at my desk at home, my AP is within arm's reach and it's not running at stock power.

Captain Ned wrote:

Well, the wi-fi router is pushing all of 84 milliwatts (assuming no use of 3rd-party firmware) while most microwave ovens are cranking out at least a kilowatt. Remember, all that that antenna was seeing was the leakage from that kilowatt. Certainly makes me want to step away from the microwave after pushing Start.

Don't sweat microwaves too much. The radiation it's throwing off is the non-ionizing variety (it's basically just heat). It's the ionizing radiation that starts in the UV bands and nuclear radation that will really cause problems.

Granted, you still won't see me sticking my head in a microwave oven...it's just not the kind of radiation people are worried about when they think of growing additional limbs or turning into the Incredible Hulk.

[/EEDegree]

Those plots are very cool, SS. Really makes me want to see what I can do to make it light up, which I'm sure wasn't your intent

I don't want to throw this off-subject too much, but this brings to mind brain MRIs. I've had a few of them now, and I know that those pump out radio waves, likely in very high dosages. Probably supersedes anything I would accrue elsewhere.

Captain Ned wrote:

ChronoReverse wrote:

Considering the microwave leakage looks about the same as the wifi access point, I'd consider that not really a health issue but just an interference issue.

Yeah, but how far away from the antenna was the microwave compared to the AP. Plug that difference into the inverse square law and recalculate.

In this particular case, they were approximately the same distance away, and probably about the same amount of material between the antenna and source. So, the radiated power (leakage in the case of the microwave) would be about the same.

As superjawes notes, this is non-ionizing radiation. The danger is boiling the water in your tissues, not necessarily flipping bits in your DNA. MRIs don't use ionizing radiation. What they do use is incredibly strong magnetic fields. The radiation in an MRI actually comes from the hydrogen atoms in your tissue being yanked around by the magnetic field. I spent some time at a facility that built MRI systems. Amazing and scary beasts.

--SS

superjawes wrote:

Those plots are very cool, SS. Really makes me want to see what I can do to make it light up, which I'm sure wasn't your intent

That's actually why I put the water cup in the microwave. I hadn't even though about it till I happened to see the blip from my wife heating stuff. It's also why I spent about an hour tracking down and turning stuff off in the house. Took me a while to remember the smart meter outside the house. Next time the wife is gone and I can power everything down, I'm going to turn on one of my 2.4GHz aircraft radios to get an idea of what they look like. I have two of them from different manufacturers so I can see how they interact.

--SS