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I built a vlf receiver in the nineties but good grief, that's a long time ago and have looked very little at receivers since.

I'm interested in frequencies from 12 - 18 Hz.

Looking at the following https://www.omnicalculator.com/physics/dipole

The suggested antenna length is around 9 million metres.

Can anyone give some pointers on how to construct an elf receiver with a passband in the range 12 - 18 hz with a sharp 3db cut-off? I'm okay on the amplifier, it's the antenna that has me in a tizzy.

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    $\begingroup$ The answers to that question do go into he realities of doing anything at ELF, though. At these wavelengths it's more or less the same challenges. $\endgroup$ May 13 at 11:30
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    $\begingroup$ @JohnVE3WNA it's really not. Transmitting and receiving are very different challenges. And that other question is... lousy and not really good for anything so it shouldn't be a dupe target. $\endgroup$ May 14 at 0:58
  • $\begingroup$ @hobbs I glanced at the answers and they seem to cover more or less what is covered in the two answers here. I'll undelete my answer if the community wants to salvage this one. $\endgroup$ May 14 at 1:11
  • $\begingroup$ May I ask, What is there to listen to between 12 and 18 Hz? $\endgroup$
    – Mike Waters
    May 15 at 23:08
  • $\begingroup$ Sorry, ask me again in a month, all going well I'll be able to answer then. $\endgroup$
    – SeanJ
    May 15 at 23:51

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There are ELF antenna's designed for this purpose, such as https://www.stormwise.com . They are not half-wave dipoles, so the dimensions can be a bit smaller than the size of the planet. Usually they are magnetic field detectors, similar to inductive pickups and MF or AM radio ferrite loopstick antenna's, except with a larger volume of ferrite material, and lots more turns or windings of fine wire (perhaps Litz). The permiability of the core and number of turns increases coupling to the magnetic field of the EM radiation. Thus changes in magnetic field strength can be detected at frequencies well below 18 Hz.

The sharp cut-off is usually done via DSP, as the Q factor due to the resistance of the length of a vast numbers of windings stuffed into a realistic volume is not high. The limitation is often the volume of high permeability material you can afford to buy (usually ounces, not metric tons).

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Transmitting and receiving aren't the same problem. You don't need a resonant antenna, or a high-efficiency antenna, to receive perfectly well. You just need enough signal from the antenna to get above a preamp's noise floor, and that's not nearly as big of a challenge. I'm able to receive WWVB on 60kHz from 2500km away using an LZ1AQ amplifier with a pair of loops with an effective 1m diameter[*]. Granted, that's only LF, not ELF, but the idea is extensible.

There are three basic designs to start with: a dipole (or a monopole worked against ground) paired with a high-impedance "voltage" amplifier, a single-turn loop paired with a low-impedance "current" amplifier, and a multi-turn resonant loop or loopstick.

For a monopole, aluminum tubing is popular (you can easily buy them in 2m-3m lengths, and in diameters that will nest into each other, and secure them together using hose clamps), and for a loop, wire on a wooden or PVC frame is common. In either case the antenna isn't tuned for a particular frequency, it's just made big enough to work. The bigger you make it, the more signal you will get at lower frequencies — but the higher frequencies will also go get stronger, for the most part, and you will probably need to filter them out to avoid overloading the amp. A lowpass filter with a transition somewhere below 500kHz would be wise, to cut out MW broadcast stations that can have tens of thousands of watts of power.

The last option uses a loop with a larger number of turns — I've seen big loops with tens of turns on an open frame from 2 feet to 8 feet square, or hundreds of turns on a ferrite rod. Unlike the wideband active loop where you keep the inductance as low as possible, for the multi-turn loop you parallel the (relatively large) inductance of the loop with a tuning capacitor to get resonance at a given frequency. This naturally produces a narrower bandwidth. You use this with a high-Z preamp, or perhaps match it directly to feedline with a secondary loop. I haven't built one of these myself but there are materials on the internet; they're basically scaled-up versions of antennas used for AM reception. Scaling up a loopstick might require an unusually large and expensive ferrite.

[*]: Construction details for mine: each loop is a square with a diagonal of 52 inches, making for an enclosed area of 0.87 m^2, the same as a circle with a diameter of 1.05m. Each loop is made of four 12awg wires in parallel, with centers 1/2" apart, which makes for an inductance (and thus loop current) similar to that of 1/2" copper tubing, but easier and cheaper.

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    $\begingroup$ Similar approach for 60kHz WWVB: a 1 m diameter air-coil form wound with #26 magnet wire with no attempt to limit inter-winding capacitance. Keep winding until inter-winding capacitance resonates with the large inductance at the frequency-of-interest. Helps to use a high-impedance FET preamp right at the antenna coil. Resonance is not sharp. For lower frequencies, I'd suggest smaller-diameter magnet wire. $\endgroup$
    – glen_geek
    May 15 at 20:40
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For true ELF the earth is often a significant part of the antenna. This usually means really good and deep grounding systems, and very long leads. The usual experimental approach would be ground dipoles or large ground loops.

The military experiments into transmitting ELF tried some electrical lengthening techniques, but the best results were literally using power lines as radiators.

If you have a large body of water where you live, and the legal right to do so, you could spool out kilometres of wire under water.

No matter what, I think you are well into the realm of experimentation. If you are actually going to try and cut antennas to length remember that ELF waves propagate slower than the speed of light (in a vacuum) so you'll have to adjust the value you use for $c$ in your calculations.

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  • $\begingroup$ Ideally antenna should be 50cm or less and indoor multi storey building environment. Only interested in receive side and not transmission but unfortunately direct contact is not possible. $\endgroup$
    – SeanJ
    May 14 at 13:41
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    $\begingroup$ @SeanJ by all means give anything a try, but physics is physics, and there are only so many electrical lengthening techniques that can be used at these frequencies. 50 cm and no access to a good ground is... going to be a challenge. Even receiving at these wavelengths is tricky when you have the space. $\endgroup$ May 14 at 16:18
  • $\begingroup$ Thanks @JohnVE3WNA the whole thing is becoming more complicated than expected. $\endgroup$
    – SeanJ
    May 15 at 19:06

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