Russians still use extremely low frequencies for communication with submarines (an unidirectional communication system called "Zevs"). Transmitter is located on the Kola Peninsula, frequency is 82 Hz, bit rate is a few bits per minute, power consumption is in the multi-megawatt range. Waveguide is formed by the surface of the earth and the ionosphere.

I'd like some intuition --- is that very low frequency? Is that very high power? Can I have some context for how I should think about 82 hz and multi megawatt? (For example, I know that few bits a minute = well, that's a couple ASCII characters a minute)

If you don't already know just how low 82 Hz is - then the lowest frequency band you're probably familiar with is AM radio. Which uses around 1,000,000 Hz.

And as others have mentioned, yes, megawatts is a lot. That's the same order of magnitude as what most power plants produce.

And your power points in your house use 50 to 60Hz, depending on where you're from - and almost nobody even considers that "radio", even though technically it is.

Typically antennas for radios are 1/4 wavelength of the transmission frequency long (for various reasons but mostly related to transmission efficiency).

So as comparison, the 2.4GHz WiFi/Bluetooth radios in your phone have a 125mm wavelength so probably have 32mm long antennas (most likely folded in interesting ways to make then physically shorter). Old school 27MHz CB radios have an 11m wavelength, so ~2.7m antennas (in practice, at least on car mounted antennas, they'd coil some of that length up at the base out in the middle of they antenna, to keep the size down to 5 or 6 feet.) An 87Hz radio has a 3,500km long wavelength, so would want (for peak efficiency) an antenna almost 1000km long.

On top of that, power requirements to get a certain range go up with the square of the distance. Your phone WiFi probably maxes out at 200mW at it's 125mm wavelength and gets maybe 200m or less of max range. It's about 20,000km to get halfway around the planet, so ignoring a bunch of other important things, that's 100,000 times as far you'd need to transmit with 10 billion times as much power to get the same signal strength at that distance. That'd need ~2GW. Your phone battery wouldn't;t last long trying to do that...

(There's another complication abiyut the power requirement changing with wavelength, but I'm not quite enough of a radio geek to know that and the numbers I quickly googled up doin;'t make much sense, so...)

Here are some names we give to radio bands. These names are somewhat dated, as even “high” frequency is pretty low.

Low Frequency (LF) (30 kHz to 300 kHz)

Medium Frequency (MF) (300 kHz to 3 MHz)

High Frequency (HF) (3 MHz to 30 MHz)

Very High Frequency (VHF) (30 to 300 MHz)

Ultra High Frequency (UHF) (300 MHz to 3 GHz)

HF is used by amateur radio operators to communicate around the world, and has sufficient bandwidth to carry voice or low speed data (300 bits per second). Commercial airliners use HF over the oceans too. Probably some military stuff too.

Most other radios on the planet is operating on a higher frequency, in VHF/UHF or higher.

As for power, 100 watts is enough to send an HF signal around the globe. Commercial AM radio stations might operate up to 50,000 watts, so a megawatt is a lot more.

82Hz is an E2, the 19th white key from the lowest on a piano. [1]

So it's fairly low but completely audible.

For megawatts, imagine a million bookshelf speakers playing the lowest note they can play, or 10,000 subwoofers playing the highest note they can play. Most people listen to music at less than a Watt power.

I certainly wouldn't want to live next to that transmitter.

[1] https://en.wikipedia.org/wiki/Piano_key_frequencies

You're fine with hojillions of neutrinos zipping through you. It's not just about power, it's about interaction. How's living near that 82Hz transmitter compare to constantly living around electrical wires acting as 60Hz transmitters? Or a substation, for that matter?

It's much worse. That's audible. 60Hz wires don't shake the whole house.

82 Hz implies a wavelength of ~4000km in vacuum. It’s extremely low frequency by comparison with the kind of radio spectrum we usually use.

The Wikipedia page gives a good overview: https://en.wikipedia.org/wiki/Radio_spectrum

Isn’t that audible? And with megawatts of power… think of the whales!

Its still electromagnetic waves not pressure waves so they're not audible.

Still, I'd recommend not being very close to the transmitter if its putting out megawatts of power.

You know that hum you get when mains powered audio get gets a certain kind of interference? That's 60Hz (or 50Hz depending on where you are) of moving air. It's not the 50Hz electromagnetism you h4ear though, it's that getting picked up by the amplifier and played out through the speakers which move the air.

87Hz is right about the second lowest F key on a piano. (the 21 key up from the lowest note on an 88key piano), or the low E string on a guitar at the first F fret.

Nah, it’s electromagnetic & your ears are too small to intercept the transmission to any kind of significant extent.

A few MW sounds a lot, but the inverse square law applies - once you get to the ocean the power density is going to be very low. Plus penetration into the ocean itself isn’t going to be great - there’s an impedence mismatch between the water and the air.

I'm reasonably sure VLF and ELF are very much in use but it seems as though ELF was abandoned by the US Navy. The Navy does continue to use VLF though.

https://www.thedrive.com/the-war-zone/25728/chinas-new-york-...

http://www.nukeresister.org/static/nr135/135elfcloses.html

Lighting strikes also feed the very cool https://en.wikipedia.org/wiki/Schumann_resonances

At the frequencies and distances that these cave "radio" systems are operated they act more like two coils of wire with a mutual inductance in the near field (<1/4 wavelength away). This isn't propagating electromagnetic waves (radio), at least not much or with any fraction of efficiency.

> surprisingly not used that often.

VLF (and the lower frequency parts of the radio spectrum) aren't used much anymore because power-efficient antennas are too large to be practical. If RF energy enters an antenna that doesn't resonate at the correct frequency, much of it will be reflected back and forth within the antenna until it all dissipates as heat due to internal resistance.

A simple kind of antenna is a vertical antenna. These need to be 1/4 the wavelength of the RF signal. You can get by with more or less than 1/4 wavelength, but you'll waste increasingly more energy.

Wavelength is inversely proportional to frwquency. If you've got a microwave signal with a wavelength of 30cm, that's practical. VLF frequencies go up to 30 kilohertz at the high end, which is about 1 kilometer (!).

So if you want to transmit at decent power, you need a big antenna and a big power source, because you're still not going to have a perfect match and some will be wasted. This is what a VLF antenna array looks like:

https://en.wikipedia.org/wiki/VLF_Transmitter_Cutler

Notice it's absolutely enormous.

Back when we didn't have high frequency electronics, commerical communication was done in the hundred kilohertz range. You can see here what those coast station antennas looked like:

https://www.radiomarine.org/historic-coast-stations

I'm a noob regarding radio: do I understand it correctly that low frequency radio signals can more easily be used with more solid carriers while higher frequencies allow more/faster bandwidth? (I'm totally oversimplifying here)

You need bandwidth to transfer more data (there is a theoretical limit of how much data you can transfer at a given bandwidth with a given signal to noise ratio).

The lower you go with frequencies, the larger antennas you need (quarter wavelength is usually the minimum... for FM radio (around 100MHz) it's 75cm which is very realistic length for a car or a house radio... at 14mhz (ham radio freq), it's 5.3 meters, which is still doable if you live in a house, and at 1.8MHz (another ham band) is ~40 meters, which is hard.. going lower is even harder, and special "tricks" are needed.

The second problem is the bandwith itself... some wifi equipment supports bands up to 100MHz of width (to simplify, you use all the frequences from eg. 5.1GHz to 5.2GHz), and if you want to stay at low frequencies, it's impossible to create such a wide band, because the antenna sizes are so different at 1MHz and 101MHz, the propagation is different, etc., so usually you're stuck with narrower bandwiths at lower frequencies (and of course, many of the low frequencies are already in use.. eg for fm radio).

But generally yes, higher the frequency, more directly (in a straight line) it goes, and you get more losses in stuff between the transmitter and receiver.

Can the antennas be coiled to achieve a long length while not being strung , for example, 40 meters? I recall car antennas that used to have coils long ago.

There are coil antennas, but they work magnetically. The 40m straight wire works electrically. The key difference is that the electric antenna interacts with more of the wave, so it's stronger in the far-field, while the magnetic antenna interacts more localized. The latter is why/how NFC works at 13 MHz (23 m wavelength) with an antenna that fits on a credit card.

Old school 27MHz CB radios would have 1/4 wave antennas 2.75m tall. In practice, for vehicle mounted use, they'd almost always have bottom or centre coils to allow overall lengths down in the 1.5-2m sort of range, with compromises accepted there for practicalities and convenience. Fixed base stations would often use a "proper" 2.75m antenna though.

Not really, atleast not the classic designs and if you want to transmit. You can bent them a bit, but the total lenght has to be around the quarter wavelength

Thanks!

Generally speaking, yes. With RF there are a handful of encoding methods, but in very simple terms binary 1's and 0's correspond to the high/low peaks of the frequency. Higher frequencies can therefore transmit data faster because you have more "peaks", and opportunities to encode data per interval of time.

Higher frequencies have less power in terms of ability to travel a great distance or through materials. It is very much like audible sounds, you can hear bass notes from someone playing music far away, but you'll lose the highs and then the mids after a relatively short distance.

One of the many tradeoffs in RF networks is trading range for throughput.

> but in very simple terms binary 1's and 0's correspond to the high/low peaks of the frequency

This is AM, "Amplitude Modulation"

Wiki has some good articles on other modulation techniques https://en.wikipedia.org/wiki/Amplitude_modulation

Oh the comparison with audible sounds is brilliant, thank you.

Bandwidth, literally the width of the band. Don’t think of it as transmitting at one frequency but a range between two, a band.

Information rate depends on the width of the band. For example wifi uses 20, 40, or 80 MHz bands. Easy when your signal is around 2.4 or 5 GHz.

But you can’t do that at 100 Hz or 100 kHz or 100 MHz because… well the numbers aren’t big enough. You could have a funny radio that transmitted from 1 to 80 MHz, but you wouldn’t say your signal was “at” 100kHz.

That make sense?

Yes thank you!

Yes

The US Navy still uses VLF, though ELF comms seem to be retired at this point.

https://en.wikipedia.org/wiki/Communication_with_submarines#...

I never understood why visible light goes through water but almost every other frequency is blocked?

The issue is even more complicated. The absorbtion at the red end of the visual spectrum is two magnitudes larger than at the blue end. This is why water appears often blue, even against a white background.

Wikipedia has some more details: https://en.wikipedia.org/wiki/Electromagnetic_absorption_by_...

I just need an ELI5 of that Wikipedia page and I’m all set. Good find though.

This reminds me of the time I went looking for a satisfactory answer for why the sky is blue.

It turns out a surprising number of people have a surprising number of different opinions and explanations on this. You pretty much need a PhD in physics to fully grok most of them. And every time you think you've read something that at least _sounds_ concrete, someone else comes along and says, "yes and no, the _real_ mechanism is this," ad infinitum until you wind up on the fringes of scientific knowledge.

In the end, hours of research later, as best as I could tell, it came down to: the sky is blue because air is blue. But it's a very faint blue, so you can't actually discern the blue until you look through a _lot_ of air, for example in the sky.

> the sky is blue because air is blue.

Things have color because they scatter/reflect/refract/absorb/emit differently for different colors of light.

Air looks blue because it scatters blue light more than other colors. The blue components of sunlight reach you from all parts of the sky, while the non-blue components reach you mostly from the direction of the sun. At sunset, sunlight from the direction of the sun passes through much more atmosphere than during the day and most of its blue light scatters away, leaving red and yellow. [0] If air didn't scatter light, the sky would be black like it is on the moon.

Air is a gas. Gasses do not reflect light.

Air refracts sunlight and makes the sun appear flattened at the horizon [1].

Air absorbs most non-visible colors (UV and IR) [2]. Animals evolved eyes to see only the colors of available light.

During aurora, the air emits green light.

[0] https://en.wikipedia.org/wiki/Rayleigh_scattering

[1] https://en.wikipedia.org/wiki/Atmosphere_of_Earth#Refractive...

[2] http://funnel.sfsu.edu/courses/metr104/F13/summaries/Absorpt...

If there were a open band somewhere else, that is what we would have evolved to see.

Is just the dynamics of how light interacts with the atom and its bonds.

Because people haven't really explained the specific mechanisms:

You can think of a material as a bunch of simple harmonic oscillators, which are driven by an applied field (electric or magnetic). These oscillators have different natural frequencies, and so couple to different wavelengths. For instance, if you play a loud note next to a piano, you can see the corresponding piano string start to vibrate, but the others may not.

The interaction between these oscillators and the wave are what lead to the dielectric constant (and therefore the slowing of waves), as well as absorption (which can be thought of as the imaginary component of the dielectric constant).

In a real material, these "oscillators" are really any method of energy storage that can couple to the motion of charges (i.e. current). These include, but are not limited to: - rotations (in a gas or liquid) - vibrations (in any state) - electronic transitions - electronic movement (in the case of a metal) - displacement (in any state)

In a single molecule, many of these mechanisms would have discrete natural frequencies. But in a solid or liquid, interactions lead to a continuous band structure (especially for things like vibrations).

For water specifically, the below visible range is quickly absorbed by vibrational and rotational energy modes, while the high end of the UV range is absorbed by electronic transitions. Other materials have similar sweet spots for transmission, but at different frequencies. For instance, materials like indium tin oxide (ITO) are designed to be conductive, but not at the high frequencies of visible light, making them transparent. As another example, metals are reflective below their plasma frequency (related to the speed the 'electron sea' can move at), and transparent above (X-rays operate in this region of transparency).

If you want more information, I can recommend "Optical Properties of Solids" by Mark Fox.

It's the other way around. Our eyes evolved to be sensitive to (some of) the wavelengths to which air and water are transparent.

There's nothing innately special about visible light. Just about every substance is transparent to some wavelength or other.

Why? The properties of water were optimized to make life possible!

I think this reasoning may be right, if expressed backward. "Visible light" is just that which life evolved to care most about, because it was what was available in the (shallowish) water.

in the case of the whales, whom returned to the deep sea 40-50mya and have since rarely had a use for visible light, it's interesting to wonder what they have evolved to care most about if no longer visible light, especially in the sense of their song, and when you compare the visual acuity of their cousins the dolphins in the same timespan, and the incredible hyper-evolution of the human eye since ~1mya

The properties of vision were optimized to make seeing through water possible!

I just meant the physics of it.

There's a huge antenna in Colorado, I think managed by the Denver Air Route Traffic Control Center which is located in Longmont, that's used as a backup for transatlantic flights communications.

Trans-oceanic flights go far beyond the VHF radio horizon to land so routinely use HF radio. It's not a backup, it's the norm.

Example: https://www.iaa.ie/air-traffic-management/north-atlantic-com...

"Huge" is probably not that big vs VLF. Maybe on the order of ~50m width (~3MHz / ~100m wavelength for ~50m half-wave antennas). VLF is more like wires across entire valleys - kilometres.

How huge? Aviation uses HF (shortwaves) and not VLF as a backup for translatlantic flights (see for example https://thepointsguy.com/guide/how-pilots-communicate-with-a...). HF antennas can already look huge ;-)