Radios without power sources

[Goodhart] is sharing his process for building a couple different AM radios. It’s surprising how few components he’s using; the first build is just a germanium diode, some wire, and a piezo earpiece. But it strikes us that both of the radios he gives build instructions for have no power source. We’re also amused by the process of selecting the station. His example uses 770 AM, and requires you to take the wire and place it up in a tree with the two ends about 1216 feet apart. We think there’s something a bit off with the math, but with that much conductor to start with there might be enough induced current for you to actually hear something come out the piezo. We don’t think we’ll be trying this anytime soon, but we’d like to hear comments from those of you who do (or already have).

37 thoughts on “Radios without power sources

  1. I heard that if you take an old AM radio and go right next to the station, you can hear it without turning the volume on. But now with on/off and digital what-not, it’s not possible.

  2. Well, AM signals can be decoded by simply using a rectifier and a low pass filter. Turns out, he’s using the diode to rectify the signal and the looooong cable works as an inductance (which coincidentally is also used as the antenna).

    In antenna theory, it’s known that the size of the antenna can be reduced by adding other types of impedances. Such as the capacitor he’s talking about on the last page. Also the capacitor and the inductance will work together as a resonant filter which will perhaps help him to reduce the interference from other stations.

    Now, FM demodulation would require to actually decode the changes in frequency (very likely to build a PLL), which doesn’t seem quite easy using passive components.

    It’s a very interesting example. Perhaps he can actually use a Yagi antenna for gain and directivity, which can also improve a little bit on portability.

  3. If you take an old AM radio (tube type) near a transmitter, you will not get anything. If you take an old AM radio (passive, crystal) near a you will hear something. The key to this whole magic is the germanium diode (detector) and the piezo earpiece (high impedance, low voltage).

    To answer spiritplumber’s question, most likely not. First off, using a standard SINGLE germanium diode will only give you HALF of the possible power of that signal if you are trying to power something. If you are trying to do wireless power extraction, use germanium (lower forward voltage drop than silicon), in a full wave bridge configuration. This will allow you to extract power from both halves of the alternating current waveform generated during transmission. Even at that point, if you don’t live extremely close to a station and you don’t have a massive antenna, you will more than likely not get more than a few microwatts of power using this method.

    To go back to the roots of these radios, look up “fox hole radios”. These generally used the iron oxide on a razor blade as a semiconductor. Ghetto, but it worked.

    If you wanted to get really fancy with one, you can use either a half wavelength or a 1/4th wavelength antenna, and use a transistor and battery to amplify the incoming signal.

  4. For a strong signal, even a silicon power diode will work. The key with germanium is that it has a much lower voltage drop when forward biased, and thus a much weaker station will be able to bias it, and will be received.

    I led a bunch of grade-school kids in a radio-building afternoon a few years ago. We used 24 awg wire wound on 6″ sonotube sections (about 30 turns), with a 5mm gap between each turn, and the radio was tuned by selecting different turns with an alligator clip (instead of a variable capacitor). Everyone, even the little grade 1 kid left with a working radio, and a huge smile on his/her face!

  5. Sort of thing my grandfather taught me about fortysomething years ago before moving on to single valve trf receivers etc. First one I ever built under his tutelage used a cats whisker as the ‘detector’ as it was known then and indeed used either a long wire antenna strung up the garden or the dipole strung the length of both gardens by way of the roof apex ! (grandmother had the patience of a saint).

    I have heard a story from several sources that way back a few folks living near the 200KHz, high powered MSF transmitter in Droitwich and the 60KHz vlf transmitter in Rugby UK were ‘leeching’ sufficient
    power from them to actually run small appliances. This may have been urban myth but in theory it’s possible so maybe not.

  6. Someone with more knowledge in antenna theory can correct me, but the reason he suggests a distance of 1216ft for his simple wire antenna is c/770KhZ is 1277ft. That is the wavelength for a 770Hz centered signal is REALLY LONG. That is why many AM radios don’t try to detect the Electric field, they try and receive the magnetic field with an inductor

  7. You guys never built a crystal radio? Not even 20years ago, every transistor bread board project kit came with the parts needed. I never got to run wires up a tree, but tying them to the cold water pipe was enough to listen to the local AM station.

  8. You can listen to FM with a crystal set as well, using a process called slope-detection. You tune an AM receiver (such as a simple crystal set) to the edge of the FM channel. As the modulated FM carrier moves toward and away from the center of the receiver frequency (tracking the FM-encoded audio), the detected AM signal strength varies due to the Gaussian bell curve bandwidth envelope of the receiver (set by the choice of tuner inductor and capacitor).

    Here is an FM radio crystal set (and other cool projects):

  9. @Sean — not an urban myth… One of my old circuit manuals (may have even been a Forrest Mims book — can’t remember) has a circuit for running a small DC motor off of a coil, a diode, and a long-wire. It wouldn’t take much to scale something like that up.

  10. Yeah its kinda a neat project and it really impresses some people. Crystal radios used to be huge during WWI for the soldiers in the trenches….

    I made one with about 1000ft. of wire once and it actually worked quite nicely….

  11. I had a cristal-radio kit as a small child.
    it was only a coil, a diode, a adjustable condensator and an piezo earplug
    i could hear about 2-4 radio stations just with a 30cm antenna (2. floor) and a ground wire on the heatpipe

  12. “We think there‚Äôs something a bit off with the math…”

    Really? You think there’s something off with the math? Did you ever take a basic physics class?

    Wow, HAD editor fail. You should delete that sentence, it makes you guys look like a bunch of uninformed idiots. How long have you been claiming to be “hackers”?


  13. Ouch! They got me! The link is an April fool’s joke. Just read the author’s names backwards.

    However, I read something awhile back about creating a very long virtual receive antenna by also transmitting into the antenna, which was especially effective of ELF communications. It seemed logical and reasonable at the time when I read it several years ago. Google is not helping me find that again…

  14. I did actually play with a diode/speaker receiver back in the day. To be fair, I think it was indeed a germanium one (hey, back then even transistors were mostly germanium :P), and the speaker was an electromagnetic, metal-membrane one (lots of turns in the coil, I guess it really was higher-impedance). You’d use a water pipe as a ground, a few feet of wire as the antenna, and the local MW AM came in just fine. Everybody in the class was awestruck when we tested it at school during recess… :D

  15. This is nothing new. The Operator’s for the Military build entire comm array’s in this fashion. Most of the time they can build the thing in plain sight, and you won’t even realize what it is until they point it out to you. How do you thing they operate in Afghanistan? The only time they need power is to transmit, and they have solar panel’s for that.

  16. @asheets

    Oh I know it’s technically feasible, I’ve been a radio amateur for longer than I care to remember but just because it’s possible doesn’t mean anyone was doing it hence my urban myth caveat. The story even appears online such as here – – but I’ve never actually seen hard evidence such as the claimed court case details.


  17. Schottky diodes, these days.
    I’ve got some galena potted in Wood’s metal in brass cups around here somewhere.
    You don’t need a full-wave antenna, though.
    Quarter wave will do just fine.

  18. I used the finger stop on the old rotary phones as the antenna for the crystal radios I made as a kid.
    Worked ok but I don’t know why. The metal finger stop was not connected to anything inside the phone. Maybe it had enough area to form a capacitance with the line wires?

  19. the antenna you spoke of that received longer transmissions was actually a “capacitive” antenna. reference the “b3cks” beer can antenna on google which is a simple make and works better with lower frequencies. the principles date back to N. Tesla and his original patents on wireless electricity. Tesla used a metal orb at the top of a vacuum sealed tube with a insulated wire enclosed. this allowed for no interference from the “medium”. He also insisted on a good ground, according to him the radio waves propagated more so through the ground then the air however he also showed how “hertzian” waves are not the best information carriers and one should use longitudinal waves for their supurb carier ability and distance. legend has it he transmitted around the world and the signal bounced off itself and came back to him. his colorado spring notes which cover high frequency radio currents research is a good read.

    if tesla was still here he would laugh at the battery powered radios for theres just no need.

    P.S. Anyone who thinks marconi is the father of wireless is sadly mistaken.

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