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Extremely / Very Low frequency radio is fascinating and is surprisingly not used that often. Radio propagation characteristics include being able to pass throug

Extremely / Very Low frequency radio is fascinating and is surprisingly not used that often. Radio propagation characteristics include being able to pass through water and ground, and continually bounce across the ionosphere / circle the globe. The navy experimented with this for some time to talk to submarines but seems to have ended the experiment. Additionally I learned that lightning strikes generate a high burst of low frequency radio and commercial lightning detectors (which I use while hiking) actually is just a radio receiver on a harmonic of the lightning signal.

_def 5y ago
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)
- williamtwild 5y ago
  Yes
- ajsnigrutin 5y ago
  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.
  - TedDoesntTalk 5y ago
    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.
    - namibj 5y 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.
    - ajsnigrutin 5y ago
      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
    - bigiain 5y ago
      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.
  - _def 5y ago
    Thanks!
- brk 5y ago
  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.
  - zild3d 5y ago
    > 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 https://en.wikipedia.org/wiki/Amplitude_modulation
  - _def 5y ago
    Oh the comparison with audible sounds is brilliant, thank you.
- colechristensen 5y ago
  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?
  - _def 5y ago
    Yes thank you!
hoseja 5y ago
Lighting strikes also feed the very cool https://en.wikipedia.org/wiki/Schumann_resonances https://en.wikipedia.org/wiki/Schumann_resonances
xvf22 5y ago
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-city-sized-earthquake-warning-system-sounds-more-like-way-to-talk-to-subs https://www.thedrive.com/the-war-zone/25728/chinas-new-york-... http://www.nukeresister.org/static/nr135/135elfcloses.html http://www.nukeresister.org/static/nr135/135elfcloses.html
cmurf 5y ago
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.
- ale42 5y ago
  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-atc-in-air/ https://thepointsguy.com/guide/how-pilots-communicate-with-a...). HF antennas can already look huge ;-)
- tomfanning 5y ago
  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-communications 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.
superkuh 5y ago
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.
mrfusion 5y ago
I never understood why visible light goes through water but almost every other frequency is blocked?
- i000 5y ago
  Why? The properties of water were optimized to make life possible!
  - ohwellhere 5y ago
    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.
    - 867-5309 5y ago
      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
  - mrfusion 5y ago
    I just meant the physics of it.
  - graderjs 5y ago
    The properties of vision were optimized to make seeing through water possible!
- Archelaos 5y ago
  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_water https://en.wikipedia.org/wiki/Electromagnetic_absorption_by_...
mkup 5y ago
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.
- bollu 5y ago
  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)
  - pja 5y ago
    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 https://en.wikipedia.org/wiki/Radio_spectrum
    - TedDoesntTalk 5y ago
      Isn’t that audible? And with megawatts of power… think of the whales!
      - vel0city 5y ago
        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.
      - pja 5y ago
        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.
JoeDaDude 5y ago
The US Navy still uses VLF, though ELF comms seem to be retired at this point. https://en.wikipedia.org/wiki/Communication_with_submarines#Very_low_frequency https://en.wikipedia.org/wiki/Communication_with_submarines#...
tdeck 5y ago
> 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 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 https://www.radiomarine.org/historic-coast-stations

_def 5y ago

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)

williamtwild 5y ago
Yes
ajsnigrutin 5y ago
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.
- TedDoesntTalk 5y ago
  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.
  - namibj 5y 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.
  - ajsnigrutin 5y ago
    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
  - bigiain 5y ago
    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.
- _def 5y ago
  Thanks!
brk 5y ago
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.
- zild3d 5y ago
  > 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 https://en.wikipedia.org/wiki/Amplitude_modulation
- _def 5y ago
  Oh the comparison with audible sounds is brilliant, thank you.
colechristensen 5y ago
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?
- _def 5y ago
  Yes thank you!

hoseja 5y ago

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

xvf22 5y ago

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-city-sized-earthquake-warning-system-sounds-more-like-way-to-talk-to-subs https://www.thedrive.com/the-war-zone/25728/chinas-new-york-... http://www.nukeresister.org/static/nr135/135elfcloses.html http://www.nukeresister.org/static/nr135/135elfcloses.html

cmurf 5y ago

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.

ale42 5y ago
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-atc-in-air/ https://thepointsguy.com/guide/how-pilots-communicate-with-a...). HF antennas can already look huge ;-)
tomfanning 5y ago
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-communications 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.

superkuh 5y ago

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.

mrfusion 5y ago

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

i000 5y ago
Why? The properties of water were optimized to make life possible!
- ohwellhere 5y ago
  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.
  - 867-5309 5y ago
    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
- mrfusion 5y ago
  I just meant the physics of it.
- graderjs 5y ago
  The properties of vision were optimized to make seeing through water possible!
Archelaos 5y ago
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_water https://en.wikipedia.org/wiki/Electromagnetic_absorption_by_...

mkup 5y ago

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.

bollu 5y ago
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)
- pja 5y ago
  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 https://en.wikipedia.org/wiki/Radio_spectrum
  - TedDoesntTalk 5y ago
    Isn’t that audible? And with megawatts of power… think of the whales!
    - vel0city 5y ago
      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.
    - pja 5y ago
      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.

JoeDaDude 5y ago

The US Navy still uses VLF, though ELF comms seem to be retired at this point. https://en.wikipedia.org/wiki/Communication_with_submarines#Very_low_frequency https://en.wikipedia.org/wiki/Communication_with_submarines#...

tdeck 5y ago

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