Most interest seems to focus on the short wavelength end of the spectrum-the microwaves and shorter wavelengths. But what about frequencies down around ).0005 Hz and below? Is anyone actually studying these radio waves?
What’s the wavelength for a frequency like that? I’ve forgotten how to figure it…
There really aren’t any radio frequencies that low.
Our friends at Wiki have a good handle on ELF, which is the closest thing to what you’re inquiring about.
At 3hz, the wavelength is 100,000 km! Since us hams are usually the only folks concerned with antennas vs. wavelength, it’s OK not to know or care, but an antenna to send or receive must be a substantial portion of the wavelength. It’s hard to put a 1,000 mile antenna wire in the yard. Any yard.
The wavelength would be (speed of light)/(5 x 10[sup]-3[/sup] Hz) = 6 x 10[sup]10[/sup] meters, or significantly larger than size of the Moon’s orbit. Given that detection or emission of an electromagnetic wave requires an antenna whose length is a significant fraction of the wavelength, I don’t think that such waves can be effectively studied.
Lincoln Labs (associated with MIT) investigated ultra-long wavelength waves back in the sixties, I think. They have a piece about it on their timeline. I’m not sure they ever had a practical application, for the antenna-length reasons cited above.
How do submarines (the target of ELF radio transmissions) receive signals? Surely they’re not dragging a 100,000-km antenna through the water behind them.
What is it you think we could learn by studying them?
It’s really long though- a kilometer maybe. It’s tuned to a harmonic of the true frequency.
I did some work on this maybe a dozen years ago. Just for a month or so. We were looking into what could be found by looking at frequencies in the mHz to Hz range. Not “radio waves” really, just sensing the magnetic field.
Some things I recall: there was a model of the behavior of the Earth’s magnetic field, and how it varies over the day (probably over the year as well). I believe there was Fortran source code available online to generate the model’s predicted magnetic field. You could hope to detect things like the San Francisco Bay Area’s BART (electric subway) trains stopping and starting from some distanec away, because it was a large DC current. You could make your own sensor pretty cheaply. Just three large loops or coils of wire, in three orthogonal planes, to sense changes in the magnetic field (I don’t think we ever built one).
We kind of just touched the surface. To go further, I think we would have had to invest in a magnetohydrodynamics code to better model changes in he Earth’s magnetic field due to things like BART.
So they’re dragging a kilometer-long cable through the water behind them whenever they’re cruising the open ocean?
They already do for their towed array sonars. See, e.g., the TB-16, TB-29, etc… The link for the TB-29 mentions the cable being 1-2 km long. They’re moving to the TB-34 array, which I imagine will be about as long.
Makes me wonder how often they get the line hung up on things.
Not ‘whenever’, they reel it out to do ELF work then haul it back in.
The E-4B National Command Authority aircraft apparently tows a five mile long (!) ELF antenna. The wiki article mentions comms capabilities down to 14kHz.
I thought the whole point of the ELF system was to be able to send a simple signal to subs so that they would surface to receive more detailed communication. Since the sub has no idea when topside folks are going to send that signal, why wouldn’t they have that antenna deployed at all times?
While electromagnetic signals in that range aren’t studied much, they are of interest for gravitational-wave astronomy. The now-canceled LISA mission would have studied gravitational waves up to an upper limit of about 1 Hz, while pulsar-timing experiments can probe all the way down into the nanohertz range. Even the LIGO detectors, which look for the highest-frequency gravitational waves which we expect to exist in the Universe, top out at a few kHz.
An antenna does not have to be a significant fraction of a wavelength long! Remember the little bar on the back of an AM radio, wound with waxy pink wire? That’s a ferrite rod antenna that can be four or five orders of magnitude shorter than a wavelength. Navigation systems that rely on radio beacons with maybe a couple hundred kilohertz use antennas that are not only on that size scale, but are also directional.
If you can make an antenna something like a wavelength or a half or a quarter in length, there are designs that work well, but it isn’t the only option.
Yeah, and you probably own a device known to be good to a few atto-Hertz. A compass.
An effective antenna does not need to couple to the electrical field, and coupling to the magnetic field alone can be perfectly useful. Hence ferrite rod antennas.