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]

31 thoughts on “RF Transmission In The 9 KHz Band

  1. 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….

  2. @mrgoogfan
    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.

  3. @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.

  4. 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

  5. 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.

  6. Anonymouse:
    >>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).

  7. @paul

    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?

Leave a Reply

Please be kind and respectful to help make the comments section excellent. (Comment Policy)

This site uses Akismet to reduce spam. Learn how your comment data is processed.