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I need to transmit frequencies as low as 1Hz and I know the wavelength is the speed of light but wondered if there has been any breakthroughs in this field? I need a handheld antenna or at least one that can fit in a backpack.

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    $\begingroup$ For 1 Hz, you could wave a large magnet back and forth manually, or maybe attached the magnet to a spinning turntable of some sort. Detect with a nearby compass needle. For 10's of kHz, something like a lot of turns of Litz wire around a bundle of long ferrite rods might generate/receive some EM. $\endgroup$
    – hotpaw2
    Jul 11 at 16:37
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    $\begingroup$ @hotpaw2 Do you have any references to research about these types of antennas? $\endgroup$ Jul 11 at 21:00
  • $\begingroup$ What distance do you need to cover with your transmission? $\endgroup$
    – webmarc
    Jul 16 at 12:20
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    $\begingroup$ @webmarc Not very far at all maybe 10-20 feet at the most. $\endgroup$ Jul 16 at 16:07
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The wavelength at 1 Hz is several times greater than the circumference of the Earth. At these frequencies, one arguably can't even have an "antenna" for terrestrial communication, since any two such antennas would be so close together they would be more like two plates of a capacitor or two windings of an inductor than an antenna.

1 Hz isn't even ELF, which is defined by the ITU as 3 to 30 Hz. And before you say "but 1 is almost the same as 3!" consider difference between the 7 meter band at the 15 meter band is the same magnitude.

ELF transmitters are (exclusively, to my knowledge) the domain of militaries and natural sources, and they involve multiple large facilities, and operate at frequencies significantly higher than you are proposing. Even with their huge size, the antennas are incredibly inefficient, and so the transmit power is well beyond the capabilities of a private individual, and certainly not portable.

While I wouldn't rule out some clever amateurs constructing an ELF station, it's such a technical challenge given the limited space, power, and money available that I imagine it would take hours, if not days, to transmit a single bit. I'm afraid a portable station that can communicate over any large distance is not feasible: the portable size would make the inefficiencies orders of magnitude worse, and there simply isn't any known technology that could make portable enough energy to overcome that.

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For your required distance of 10-20 ft, you might consider first testing a dummy load.

An antenna approaching any kind of efficiency will be larger than the space you're looking to cover.

And any antenna that is portable will closely approximate a dummy load... So try a few wire wound resistors and see what happens.

Will be interesting to see what your measured distance per transmitted watt will be, please share!

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An electrically resonant circuit or object would be very difficult or extremely large at those low frequencies (possibly county, country, or planet sized).

But mechanically resonant generation of a 1 Hz alternating EM field might be easy.

You can accelerate either a charged object, or a magnet in some cyclic or periodic fashion. Note that according to Maxwell's equations, the electric field and magnetic field are coupled duals, so changing (time varying) either will change the other, and if done in a periodic fashion, will generate EM waves. So you could either wiggle a charged object, or move or spin a magnet, permanent or electromagnetic, at 1 Hz. Although possibly very difficult to detect away from the near field, Maxwell's equations seem to include no lower bound on frequency of EM waves.

I'm halfway tempted to connect a sensitive galvanometer up to one of my magnetic loop antenna's and see how far away I can make the galvanometer needle wiggle by swinging a large permanent magnet on a rope around in a circle (or maybe on an old 78 RPM turntable with a variable speed control) nearby to the loop. Instead of a galvanometer, maybe try a sensitive ADC plus a data logger, feed the data to some SDR code, and demodulate away (e.g. how fast and how large a circle the magnet might be getting swung around).

Range will be limited. Maybe a few steps in distance.

Added:

For semi-portable ELF highly wound ferrite rod antenna's, here are some vendors for your frequency range:

https://www.stormwise.com

https://aaronia-shop.com/products/elf-ulf-antenna

Used for geomagnetic observations, etc. The active ones are Rx only, but the passive ones might take a few milliwatts of transmit power. Again, likely very short range. Maybe more similar to a colossal NFC antenna. But likely more range at ELF than a typical amateur radio mag loop or extremely short dipole.

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    $\begingroup$ I'm no physicist, but I believe that you're wrong. First, I don't think that swinging a permanent magnet in a circle will generate electricity because permanent magnets don't have mobile charged particles, and acceleration of charged particles is what creates electromagnetic radiation. (A magneto generates AC with a moving magnet because the electrons in the wire are in a moving magnetic field, but that's different.) $\endgroup$
    – rclocher3
    Jul 13 at 19:14
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    $\begingroup$ Second, and more importantly, any hand-held apparatus powered by muscles or batteries would generate a miniscule RF field because the efficiency of the antenna would be terrible, since a handheld antenna would be a such tiny fraction of the enormous wavelength. Such a generated RF field would surely be much smaller than thermal noise. I don't need to be a physicist to know this, because in the absence of breakthroughs in Physics, practical VLF antennas are enormous and require power levels measured in megawatts. $\endgroup$
    – rclocher3
    Jul 13 at 19:22
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    $\begingroup$ Another way to view the problem: spinning a magnet might generate a changing magnetic field, but radiation also requires a changing electric field. That said, the answer concedes "Range will be limited. Maybe a few steps in distance." and so the "transmitter" and "receiver" would be coupled through near-field, so no radiation required. Whether that counts as an "antenna" or not depends on definition; I'd say not, but on the other hand NFC "antennas" are a thing. $\endgroup$ Jul 13 at 22:35
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    $\begingroup$ @PhilFrost-W8II isn't a magloop effectively a (very weak) electromagnet whose polarity is reversed rapidly? $\endgroup$
    – webmarc
    Jul 15 at 16:31
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    $\begingroup$ @webmarc Yes, that's the (rhetorical) point. $\endgroup$ Jul 15 at 17:13

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