Listen To The RF Around You

These days, we are spoiled for choice with regard to SDRs for RF analysis, but sometimes we’re more interested in the source of RF than the contents of the transmission. For this role, [Drew] created the RFListener, a wideband directional RF receiver that converts electromagnetic signal to audio.

The RF Listener is built around a AD8318 demodulator breakout board, which receives signals using a directional broadband (900 Mhz – 12 Ghz) PCB antenna, and outputs an analog signal. This signal is fed through a series of amplifiers and filters to create audio that can be fed to the onboard speaker. Everything is housed in a vaguely handgun shaped enclosure, with some switches on the back and a LED amplitude indicator. [Drew] demonstrates the RFListener around his house, pointing it at various devices like his router, baby monitor and microwave. In some cases, like with a toy drone, the modulation is too high frequency to generate audio, so the RF listener can also be switched to “tone mode”, which outputs audio tone proportional to the signal amplitude.

The circuit is completely analog, and the design was first done in Falstad Circuit Simulator, followed by some breadboard prototyping, and a custom PCB for the final version. As is, it’s already an interesting exploration device, but it would be even more so if it was possible to adjust the receiver bandwidth and frequency to turn it into a wideband foxhunting tool.

21 thoughts on “Listen To The RF Around You

  1. You don’t realize how much RF is around yuo, not only at home, but anywhere you go.
    All your local AM, FM, and TV stations, the public service bands, police, fire,
    medical, and the CB’ers and ham operators. You also have NOAA weather radio,
    the time signals on the shortwave bands. All broadcasting 24 hours a day, 7 days a week.
    The only way to get completely away from any RF exposure is to put yourself in a
    sealed faraday cage. It’s amazing if you think about it, the trillions of watts
    used each day to generate RF, and that’s just radio/tv. Then you add in all the
    other things that generate RF, hospital equipment, computers, routers…I could go on
    and on. I remember an article that said the earth is now surrounded by an RF shield.
    Want to invade the earth? Hit it with an EMP and fry every bit of electronics on the
    planet. That would send modern man back to the proverbial stone age.
    Imagine that, no more radio, tv, or internet. Newspapers would make a comeback once
    the motors that run the presses are fixed. I’ve wondered at times what I would hear
    if I went back in time with a modern radio receiver to a time before radio was
    invented, say the 1700’s. I would probably hear the cosmos. Now that would be
    interesting. Yes we are very much awash in RF energy, enough to last a lifetime.

    1. Let’s not forget about the greatest and strongest source of radio emissions around: the sun. Except it’s not modulated, so not much fun to listen to.

      But it does the EMP hit once in a while too — the last time it did the electricity wasn’t so widespread, but it’s bound to happen again.

    2. The worst RF emitter I have run across are my two new Chamberlain garage door openers. They have cheap switching supplies that use the wires going to the sensors at the bottom of the doors, and to the switch at the people door as antenna. An AM radio anywhere near the garage / shop is useless. No more listening to a ball game while working on a project. I can hear them all the way up past 7MHZ. It has taken extreme treatments with toroids and bypass caps to dampen the RFI.

      1. And our GDO (Chamberlain, I think) would not close when I installed LED bulbs in it. I guess the bulbs were jamming the signal from the car remote. Buying a pair of “special” GDO LED bulbs did fix that problem.

  2. Err, your first sentence appears to contradict what you appear to want to say.

    “These days, we are spoiled for choice with regard to SDRs for RF analysis, but sometimes we’re more interested in the source of RF than the contents of the transmission.”

    You then talk about listening to the content.

    1. Nothing. Or, nothing different than random walls inside his house, most likely.

      The Sun’s loud because it’s hot, but it’s not that hot. It’s like, 10-20 times hotter than the walls of your house – it’s crazy to think it’s that *little* but it is. And the Sun’s only ~degree-scale: so to have the Sun fill the antenna’s “eye” it would have to be a decent sized dish (that’s ~50 dB of gain!), and even then it would only be maybe 30 dB than the noise from pointing it at a random bit of your wall.

      The reason you can observe the Sun easily is because the sky is *very* cold and very low noise receivers are cheap at GHz scale. Plus, well, dishes *are* cheap. But compared to just the average noise around you, it’s not that hot. Looking at the Sun with 30 dB of gain gets you less noise power than pointing an antenna at your wall.

      (This of course assumes there’s not a flare active, of course.)

    1. Vivaldi antennas are only around 10 bucks from wa5vjb. Ham radio guys do not pay hundreds for that kind of stuff. Etching your own is easy. The mask can be sign vinyl or self-adhesive shelf paper. Net cost maybe 40 cents. If you tried it you’d also find that sheet metal or copper wire or a bent coat hanger works just as well if it has roughly that shape. Is it important that it’s 50 ohms for an uncalibrated receive antenna instead of 35 or 300? No. “Vivaldi” just sounds cool.

  3. This is a very good starting point listening to RF on a single plane. You could possibly expand the idea and collect more RF data to create a 3-D plot then transfer the image to eye wear. You would be able to see 3-D RF energy in real time, determine where it’s coming from and field strength.

  4. interesting re-invention of the wheel, (pun intended) there are already bug and microphone detectors used by security teams that do essentially the same thing, they use them to sweep a room or an entire building of microphones or wireless cameras & bugs (anti-surveillance) so governments and corporations can protect themselves from espionage and spys

  5. I like the range and directionality on this one. Those (plus the level display) make it something I’d like to experiment with, as big upgrades to the Elektrosluch I’ve been using for this kind of thing.

    (To be fair, the Elektrosluch is intended as an audio toy, not any kind of diagnostic tool. Hackaday covered an older version of it back in 2014; the design has been released as open source in the intervening years.)

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