See Actual Microwaves — No More Faking It

Last week we saw a lot of interest in faux visualization of wireless signals. It used a tablet as an interface device to show you what the wireless signals around you looked like and was kind of impressive if you squinted your eyes and didn’t think too much about it. But for me it was disappointing because I know it is actually possible to see what radio waves look like. In this post I will show you how to actually do it by modifying a coffee can radar which you can build at home.

The late great Prof. David Staelin from MIT once told me once that, ‘if you make a new instrument and point it at nature you will learn something new.’ Of all the things I’ve pointed Coffee Can Radars at, one of the most interesting thus far is the direct measurement and visualization of 2.4 GHz radiation which is in use in our WiFi, cordless phones (if you still have one) and many other consumer goods. There is no need to fool yourself with fake visualizations when you can do it for real.

Modify the MIT Coffee Can Radar

The MIT Coffee Can Radar has introduced the study of radar, electromagnetics, RF/analog design, and signal processing to hundreds of curious students, hackers, and veteran engineers. It is used in both private, government labs, turned into full semester courses at other universities, and has even been featured on Mongolian National Television.

We will use this radar to directly image a 2.4 GHz microwave field emitted from its own transmitter.

The radar will be configured to show the time-varying 2.4 GHz field as if time is standing still. To do this we configure the radar to work in Doppler mode. This radar is unique in that you can DC couple the output of its front-end frequency mixer, thereby creating what is sometimes called a ‘micro Doppler’ measurement device. With this the output of the frequency mixer is a voltage proportional to phase and amplitude of the scattered signal from whatever radar target the radar is pointed at. If you stand in front of the radar in this mode and walk towards it and away from it you will see a slowly time varying waveform directly proportional to where you are standing.

To visualize wireless radiation, I’ve modified the op-amp circuit on the output of this mixer so that it can feed a pair of LEDs, one red and one green. These are wired opposite polarity. I then un-mounted the receiver cantenna, attached an extension microwave cable, and taped the LEDs (with extension wires) to the top of the can. With this new antenna assembly I can freely walk the receiver antenna around my lab with the LEDs attached to it providing me an optical indication of phase and amplitude of the transmitted carrier.

Block diagram.
Block diagram.

When you move the receive cantenna around in front of the transmitter cantenna you will see the LEDs vary from red to green to red, back and forth, tracking the wavefront being radiated out of the transmit cantenna.

To capture the image of the 2.4 GHz wireless wave front, the aperture of a DSLR is left open while the receive cantenna is moved around near the front of the transmitter antenna. All of this must be done in a darkened room.

What 2.4 GHz wireless signals actually look like

If the above description is confusing then watch this video:

This is what 2.4 GHz wireless radiation looks like. From these measurements you can see the wave front curvature as it exits the transmitting cantenna. As we approach the distance 2d^2/lambda (where d = diameter of the transmitting cantenna) we can see the wave front turning into a plane wave. Also, you can see that the brightness of the LEDs drops as the inverse distance from the transmitter, just as one might expect.

Yes, there is granularity to this image limited by how much patience I had for moving the receiver assembly around in front of the transmitter. For the above image I spent about 1-2 minutes. For more fine detail spend more time moving the receiver assembly around while the DSLR shutter is open.

For each wireless router in your home or office this is what the radiated fields look like.

IEEE Microwave Theory and Techniques Association (MTT) Video Competition

This work helped to spur an innovative competition sponsored by IEEE MTT where some of these methods were used for a variety of scenarios and the results are amazing (scroll to about 2 min into video below):

Your turn

Its well within your ability to measure and observe the nature of wireless propagation. See for yourself what wireless signals look like propagating through your lab and your home. If you’re still not convinced that this is for you, take a look at [David Schneider’s] Coffee Can Radar presentation, and my own video demonstrations of Doppler shift using the hardware.

Author Bio

Gregory L. Charvat likes to image wireless radiation, is the author of Small and Short-Range Radar Systems, a visiting research Scientist at MIT Media Lab, co-founder of Hyperfine Research Inc. and Butterfly Network Inc. Greg is editor of the G. L. Charvat Series on Practical Approaches to Electrical Engineering. Greg is a former staff member at MIT Lincoln Laboratory, where he created the MIT Through Wall Radar the MIT Build a Radar course. In addition to this, he has done many more things and is involved in lots of interesting stuff.

51 thoughts on “See Actual Microwaves — No More Faking It

      1. Those guys are actually from eastern Ukraine. In some of their videos you can hear Ukrainian shells exploding near-by. I don’t know how they are keep-up their enthusiasm to make videos when there is an enemy army trying to kill you everyday.

        1. They are used to it by now. Imagine if Detroit or the dodgy areas of Washington DC was a well-equipped army base.

          Drunk soldiers driving tanks or popping off 105’s for fun or “the enemy” – it’s the same thing in Ukraine, nothing new exactly. Been that way since 1990’s at least – when someone I know, who did business there, left because things got a little too hairy.

          1. You should look who is behind most Baltic and Ukraine investment and revenue. If you are an outsider with capital to make things happen your’re more likely to be convicted of a major crime or deported than be shot by military. The more money you have as an outsider the more dangerous it is for you.

            The biggest grocery store franchise in the Baltic region, for example, looks like it has region independent owners/investors, for example, but it’s really 100% Russian.

      1. I wonder if a mesh-wire pair of glasses would help protect the eyes, even when they aren’t grounded.

        I also wonder if he pointed it at the sky if it would send spy satellites in alert thinking it was some radar-guided missile or new radar site.

          1. Of course military broad sweep RADAR is very powerful, but the nose-cone RADAR of a missile less so, and normal RADAR sweeps horizontally so to pick it up with satellite means the satellite needs to look for small signals that bounce or leak. And I know they also have their own signature and frequencies, but I’m sure the military looks for experimental stuff too, you never know what north-korea or some such might cook up.
            So I continue to wonder how it would register.

      2. Why?

        The magnetron outputs about 1 kW and over a distance of ~1 meters it has already spread thin enough that the heating effect per area would be equal to standing outside in the sun. Of course you don’t want to stick it in your face, but other than that there’s nothing special about microwaves.

        And they don’t cook you from inside out. The absorption depth in liquid water is something like 1.7 millimeters.

        1. Are you assuming that it follows inverse-square power density? MW is very directional, which is why it’s used for radar in the first place. I work with one of the guys who developed the safety standards for microwave devices and his reaction to that video isn’t printable. Just don’t do it.

          1. Directional or not microwaves follow the inverse-square law. Focusing 1 kw in a beam of 1/8 steradian just gives you the equivalent of an 8 kw radiator in that direction, but still inverse square. Unless you have some kind of very fancy collimation scheme, which is very hard to do with microwaves, power falls off quickly.

          2. Reply to Skoold, same recursion depth problem: We’re not talking about a waveguide here though. Once microwaves are in free air they act like all other electromagnetic radiation. If you can focus all the energy into a very narrow beam you can create the equivalent of a huge effective radiator in the direction the antenna is pointed, but the dropoff will still be inverse-square, and that coffee can antenna the Ukranians are using isn’t going to get you that kind of collimation. You need something like a radio telescope dish for that.

        2. Why??? Because they are not in a controlled environment, they cannot control the stray microwave reflections from surrounding objects. These reflections can interfere with each other and create “hot” superposition zones where the energy is enough to burn your retina even if it only lasts for a couple of seconds.

      1. Except for the fact that non-ionizing radiation does not cause cancer.
        That’s not to say that these guys aren’t morons, because they definitely are. That microwave magnatron will cook up the flesh in their bodies just as easily as it would have cooked up a sandwich.

        1. Except that, unlike ionizing radiation, you get plenty of warning when you’re being thermally cooked. I think they’re really in more danger from the magnetron HV supply than the actual microwaves. They are sensible enough to keep the transmitter at the end of a long pole, and that’s probably good enough to ward off any unexpected health effects.

    1. “Just don’t do this at home, or anywhere else for that matter” – sorry sir, but this sentence is very insulting to any adult. I’m stunned how it can be so ubiquitous warning in US.
      It’s almost like there were only two kind of ppl living in America. Tinkerers and morons.
      So, as always, my response is:
      -Volenti non fit iniuria-

  1. As has someone has previously mentioned on HackADay…
    Cantenna was a (trademarked?) name of a dummy load antenna (for ham radio) sold by Heathkit. It consisted of a re-purposed metal paint can filled with mineral oil and a high power 50 ohm resistor submerged within connected to an RF connector on the can’s lid.

    Seeing the radar antenna labeled as cantenna (although appropos) is a nit I have to pick.

    1. Yes to this!

      For decades a “Cantenna” has been a dummy load made by putting a non-inductive resistor inside a coffee can filled with oil. They are still constructed in the thousands by Radio Amateurs world wide.

      It’s sad that modern day hackers ignore the thousands of articles written by previous generations of Hams and are forced to constantly reinvent everything.

      This article itself is a good example. Hams (and prof engineers) have for decades visualised RF fields by means of incandescent globes, fluro’ tubes, graphs, etc. It is exactly what Hertz did with his original radio experiments, except he used a spark as a detector..

      1. Now, that you mention it: Before doing anything “RF”, one *could* save oneself a lot of work & worry merely by buying a couple of the ARRL’s handbooks and learn something.

        There is a lot of useful information (and projects that work) in these books, especially the “ARRL Antenna Book” and “Wireless Receiver Design for Digital Communications” – even though they are bloody expensive – I see that the books are now on Amazon too, which is really good (before one had to buy only from ARRL in the US so one gets roughed up by customs also).

      2. Not entirely true, but the first resource is the internet. The internet is an extention of my brain. If google don’t know, I don’t know. Plenty of really old articles get read and used. I learned a lot of what I know about playing with Op Amps by reading some Texas Instruments technical that were decades old. I did that because TI made all of those documents available for free online and are relatively easy to find. If I had to so much as reach into my pocket and pull out a credit card to get it, I would have moved on, because I can’t go shelling out cash at every dead end in hops that it will be useful information.

        If the information is out there in our collective brain, we will find it and use it. If the information is in some other form or behind a paywall, then its not part of our collective brain and it will likely not be used.

      3. I’ve always called them a Dummy Load, and not “cantenna” because they aren’t supposed to be an antenna, but a dummy load. I.E. it’s supposed to NOT radiate, but rather be a 50 ohm load and turn RF energy into heat… “Canned Heat” would have been a better name.

        Cantenna has been used for ANTENNAS made out of various cans for like ever now. Nothing new

        And hackers certainly didn’t come up with it to piss off old crotchty ham guys, which is way many people don’t have an interest in ham anymore – because hams tend to be “I was first!” complex having DICKS and no one wants to be around that. Along with the fact a large part of the wireless spectrum is usable now without having a ham license, such as WiFi and ISM

        Hams are their own worst enemies when they take attitudes like they do. And believe me, I work with a lot of the “old guys” and they are assholes on a whole. Not people I want to spend any amount of time around doing a hobby….

        1. Amazing how you malign amateur radio with comments like, “and they are assholes on a whole”.
          While not all amateurs are just operators but are actual technical enthusiasts. Amateur radio constitutes a large group of folks interested in applying physics to radio frequency systems.
          Maybe you just can’t get a license to practice RF physics.

    1. “We choose to go to the moon and do the other things.” Perhaps not the only editor who went to lunch?

      The bio for the author’s posts are normally more detailed (lengthy). I interpret this shortened bio as humour.

  2. This reminds me of a project I’ve been wanting to do for a while. Set up a near-field probe on a spectrum analyzer and mount that to a Cartesian robot. Then have that scan the space around an active device and map the measurements. Here’s an example of someone doing something similar (maybe it’s a simulation, not sure)

    http://www.interferencetechnology.com/wp-content/uploads/2012/04/Slide8.jpg (also is a good illustration of the counter-intuitive fact that RF return currents will not flow in straight lines)

    1. Just thinking… now that Wifi routers have multiple MIMO antennas, could you do a steerable beam, and actually produce a full scan of a room using one? Using custom firmware admittedly.

  3. Just wanted to say this is awesome, please consider doing more articles like this that actually focus on building stuff (rather then ‘john doe made X’ or ‘jane doe kickstarted Y’)

  4. I wonder how long until someone figures out a way to create a grid of sensors to handle certain RF bands and build an RF imaging system that is more than a single pixel at a time. I’d love to see what RF energy is bouncing around in an urban, suburban, and wilderness environment.

    1. They already have and do. Its called “SAR” or synthetic aperture radar.

      While not – quite – what you’re looking for probably, as it tends to be its own radiator as well as receiver, the gist of it is the same. Except the point of it is to image objects using radio waves. But that’s what you would see using something higher resolution, as the radio waves will reflect much like light does off of surfaces, just differently as the wavelengths in question are different. And its not directly observing anything, as there is some processing going on to turn RF into an image.

      There are a lot of variables too, wavelength being the key, as the longer the wavelength, the larger your sensor would end up being. Huge actually, much below UHF.
      Microwave frequencies are ideal for this as they are such small wavelengths it’s easy to make very directional antennas. And as you can see above, you can see the radiated pattern coming out of the antenna as a parabola shape and turning into a plane wave as it gets further out. For lower frequencies the antenna would be very large and not as directional. In fact, the image would probably just be a wash of noise.

  5. The russian consumer protection authorities advise people to stand 60 centimeters away at least form microwaves during operation if I recall correctly, to prevent eye damage, And that’s a normal one with a shielded door.

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