RF transmission in the 9 KHz band

[W1VLF] is on a quest to communicate over long distances with a 9 kilohertz transmitter. He built this giant coil with that in mind. A round concrete form was used as a base and wound with magnet wire by hand. We’d recommend building an automated winding device, but this method seems to have worked. Operating at 50 Watts on the air-core coil at 8.97 KHz he was able to pick up the signal at a distance of 5 kilometers. It’s not breaking any overall distance or portability records, but on a project like this the quest is where the fun is at.

[Thanks Drone]


  1. Alan says:

    Nice. I’m not so much interested in radio, but thanks for taking all the photos and posting them.

  2. FaultyWarrior says:

    I love how this project is only about 5 miles from my house! Maybe I should go up to their store?

  3. Hackaaaaaaaaaaaa says:


    Read that for some background on why the author chose 8.9khz, why he can use it, and why you don’t get a heck of a lot out of it.

    1 to 300 bits/s isn’t bad ;)

  4. 1000100 1000001 1010110 1000101 says:


    Just build a slightly larger coil and contact him at 8.97 kHz :P

  5. Pete says:

    Do you really recommend building an “automated coil winder” for something this large or are you just trying to get links into the post?

  6. Gregg says:

    Make a numbers station!

  7. mrgoogfan says:

    Turn it into a tesla coil.

  8. mrgoogfan says:

    Wait? Didn’t tesla say that the resonont frequency of the earth was 8.97Hz? Well then, scale it up and build a new wardenclyffe tower

  9. mrgoogfan says:

    *resonant* sorry.

  10. jdog says:

    33km band….interesting. Reminds me of the ULF stuff that the US Navy used to run, real slow transmissions of very long wavelength…. could cover the globe from just a few locations.

    Not that I Have a whole lot of personal interest in firing up an antenna like this….

  11. FaultyWarrior says:

    haha, that I should!

  12. Hirudinea says:

    HOLY CRAP, Thats a big coil!

  13. bothersaidpooh says:

    i wonder if a better way to build this antenna would have been to get several hundred feet of 80 core IDE cable and connect the alternate ends?

  14. M4CGYV3R says:

    The resonant frequency of ‘the earth’ varies depending on the material at the site you are trying to resonate as well as the material next to/below that primary material. The size of a certain vein of stone would also determine its resonant frequencies.

    In general, “The Earth” as a planet is made up of too many dissimilar materials mashed together to have a single resonant frequency. You may be able to do it, but you’d likely be talking a resonator larger than any building presently in existence.

    But yes, a tesla coil would be an immense use of this coil. I don’t personally see any benefit to 8.9kHz transmission.

  15. scott says:


    You should totally put that on top of a well-head !

    Low frequencies love a good ground.

  16. Vonskippy says:

    So will they be making a Laptop version?

  17. hydrojerry says:

    Couldn’t find something more interesting to feature here than this!

  18. kabukicho2001 says:

    the dust over the coil may be a kinda high voltage induced by some appliance near by or earth magnetic field?

  19. Benj says:

    If the wavelength is 33 kilometers… I thought the order of magnitude of the antenna length had to be around the same as the wavelength. 33 kilometers of copper? :Q

  20. Jess says:

    I was really hoping it was a gigantic coil gun :(

  21. Schilcote says:

    It SHOULD be a factor of the wavelength (dosn’t have to be, if you pump power through wires EM will come out of it no matter what).

  22. John Smith says:


    The antenna would work much better if it were that length, but it doesn’t have to be. If it did, car AM radio antennas would need to be about 160m long.

  23. pookey says:

    @M4CGYV3R said: “I don’t personally see any benefit to 8.9kHz transmission.”

    LOL… that line of thought is exactly why the government initially gave the shortwave bands to hams. “Let ‘em have those ‘useless’ frequencies above 300 meters.”

    Nobody is going to build a cell phone with a 8.9 khz carrier, true. But who knows what potential application might arise for that band of frequencies?

    In any event, I applaud experimentation.

  24. Leigh says:

    The length of the wire in the coil is unrelated to the wavelength. When W1VLF says “resonant” he means that at the specified frequency, the reactance (imaginary component of the impedance) is zero. If your antenna is too short for your specified frequency, the reactance will be negative (capacitive reactance) and so you can add an inductor of equal and opposite reactance to reach 0j ohms. It’s not the case that the wire length in that inductor is related to the wavelength, though. You just solve the equation 1/(w*C) = (w*L) where C and L are the inductance and capacitance, and w=2*pi*frequency. That’s what he’s doing when he measures the coil as being 168 milliHenries. The real component of the impedance (measured in regular ohms, not imaginary axis ohms) has two parts: the loss from the wire measured as a resistor at DC (where he said 60 ohms for his first coil), and the “radiation resistance” which is also expressed in ohms, and shows what gets “lost” from the antenna by being radiated into space. Assuming reactance X=0, the efficiency is calculated just like a resistor divider, with some power being lost in the coil as heat, and some being radiated. Efficiency = Rr/(Rl+Rr). The Rl=60 ohms is quite large, since the radiation resistance of such a drastically shortened antenna is probably in the milli-ohm range, so his efficiency is probably on the order of 1e-3/(60+1e-3) =~ 16e-6 or about -110dB of loss. So (assuming the milli-ohm radiation resistance guesstimate is OK) his 50W in actually radiates around a milliwatt.

    See http://en.wikipedia.org/wiki/Antenna_%28radio%29#Resonant_frequency and


  25. Anonymouse says:

    It’s very pretty, but why is it so compact? It looks more like it’s been optimized for resonance than for radiation. If I were to attempt something like this, I’d drive three 4 inch PVC sewer pipes into the ground as far apart as my land would support, drive some screws in, and wrap with as many turns of cheap lamp cord as would fit.

  26. Paul says:

    >>It looks more like it’s been optimized for resonance than for radiation.

    It has. Check the article. This is just the loading coil to match to the electrically short antenna supported by his 120 foot tower (turns out that a 33km antenna just isn’t practical).

  27. mrgoogfan says:


    what if you stretch a wire across two mountaintops? Just hanging over the valley. I wonder, if the antenna was the actual wavelength, rather than 1/4 wave or 1/8 wave, would it make much more of a difference?

  28. Brad Hein says:

    Wow… This is AWESOME!

    My 706 only goes down to 30Khz… Wish I had something that receives at 9khz.

  29. Brad Hein says:

    I wonder if it picks up solar flares =)

  30. asheets says:

    Low-and-Slow is the way to go!

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