Talk:Very low frequency

instead of myriameters use SI units!

17.8 kHz ?

Doesn't NAA send on that frequency too?

"Radio Natura" book

I added Renato Romero's "Radio Natura" book in the "Further reading" section. This is the first edition in Italian. It must be replaced with the 2nd edition in English when it is published, possibly in mid-2008. Sv1xv (talk) 22:13, 9 January 2008 (UTC)

Sources

Is there any way we could get sources for the table? Some of the information seems sketchy, such as the 22.3 kHz area. We don't even know where that is from, and many others have weird information as well. I found copies of this table all over the internet, but where was the original? 71.131.23.130 (talk) 05:39, 15 April 2008 (UTC)

There are many listing of VLF transmitters on the Internet but almost all are seriously out of date. The only one I know of that is accurate and actively maintained is http://sidstation.loudet.org/stations-list-en.xhtml — Preceding unsigned comment added by 194.106.52.83 (talk) 05:38, 23 May 2013 (UTC)

PC / radio reception

Quite a bit of space is dedicated to the discussion of PC reception of VLF signals: wouldn't "PC based detection" be more accurate?, and shouldn't it at least be mentioned that many shortwave radios and communications receivers cover VLF as well? —Preceding unsigned comment added by 217.157.224.13 (talk) 21:32, 23 December 2009 (UTC)

removed fact from ELF article! Please consider

Please consider the facts highlighted within this url... http://www.smh.com.au/news/travel/naval-base-link-to-jet-plunge/2008/11/14/1226318890475.html. I recently removed this interesting fact from the Extremely low frequency article. --CyclePat (talk) 05:39, 7 February 2010 (UTC)

Low bandwidth prevents audio signals

The article states that the low bandwidth prevents the transmission of audio signals, but it doesn't explain why (and neither does the bandwidth article.) Why is it not possible to use AM to transmit voice in this band? -- Malvineous (talk) 06:41, 28 April 2012 (UTC)

Clarified that a bit. HTH.--Harumphy (talk) 07:45, 28 April 2012 (UTC)

Thanks! It answered that question but raised another :-) I think I'll head over to a radio forum for that one though! -- Malvineous (talk) 11:17, 5 May 2012 (UTC)

Just to add a little additional information to Malvineous's contribution: the primary reason is the narrowness of the band, compared with the bandwidth of sound. The VLF band is 27 kHz wide. Sound frequencies go up to about 16 kHz, so one high fidelity audio channel would occupy half the band. Even a telephone-quality voice channel requires 3 kHz (although compression can reduce that considerably). An additional 3 kHz must be allocated for channel separation to prevent interference, so a voice station would require 6 kHz. Since VLF transmitters are typically long range, covering most of a continent, only 4 or 5 voice stations could operate per continent, leaving no room for the other VLF services. Radio spectrum bandwidth is scarce and expensive, and regulators feel that audio is an inefficient use of this band.

However, there are additional technical reasons why VLF transmitters can't transmit audio. Existing VLF transmitters have extremely narrow bandwidths. Not only can they not transmit voice, but even text messages are sent very slowly. The US Navy's VLF stations reportedly have a transmission rate of around 200 baud, around 28 characters per second. Although there may be other reasons for the narrow bandwidth, the reason I am aware of is the construction of the antenna. Due to the length of VLF waves, VLF antennas are extremely electrically short, meaning they are much shorter than a resonant antenna, which is one quarter of a wavelength long. This makes them very inefficient. In order to increase its efficiency, the top of the antenna is connected to a large spiderweb of suspended cables that acts as a capacitor plate to ground. The bottom of the antenna is connected to a gigantic loading coil, several stories tall. This makes the antenna act like a huge LC circuit of extremely high Q. Deviation of the frequency during modulation of more than a few hundred hertz causes high voltage arcs at the ends of the antenna. So information is added to the transmitter signal by minimum frequency shift keying (MFSK), shifting the frequency of the carrier back and forth by only about 100 hertz, to indicate binary ones and zeros. This is only capable of sending text at a very slow rate. --Chetvorno^TALK 13:20, 5 May 2012 (UTC)

The Chu-Harrington limit tells you what the bandwidth could be for an antenna size and frequency. Graeme Bartlett (talk) 07:11, 6 May 2012 (UTC)

These statements are misleading. It must be noted that they refer to analogue transmission of signals. It is quite possible to transmit a HD movie across VLF, in digital transmission, it would just be very slow. Its a bit similar to your modem or router in that respect. One way to speed things up is to create hundreds or even thousands of carriers separated by microhertz or millihertz. This can turn a one hertz bandwidth into the equivalent of a broadband connection. With modern DSP and DSP filters, its certainly possible, but the infrastructure required to achieve this makes it prohibitive to anyone but the military. 84.13.194.135 (talk) 09:28, 25 May 2012 (UTC)

Carriers separated by microhertz cannot transmit data quickly, instead they take weeks to send one bit each. Graeme Bartlett (talk) 13:22, 25 May 2012 (UTC)

Yes, and the normal meaning of audio transmission is real time audio, not waiting half an hour for a few minutes of digital audio to come through. --Chetvorno^TALK 15:45, 25 May 2012 (UTC)

Carriers separated by microhertz can be modulated up to the frequency of the wave. The bandwidth is in the modulation, rather than the range of frequencies used. Of course, this is digital transmission, not analogue. It takes some serious equipment to transmit and receive the signals though. That said, a million channels between 19000Hz and 19001Hz provides some serious throughput at low frequencies. 78.146.89.66 (talk) 10:25, 26 May 2012 (UTC)

Just to give you an idea, you can push about 17.7GB (not Gb) per second with this technique between 19000Hz and 19001Hz. 78.146.89.66 (talk) 10:36, 26 May 2012 (UTC)

Got some sources to support this? --Chetvorno^TALK 12:40, 26 May 2012 (UTC)

I'm working on it at the minute. The idea itself is simple, the photons for 19000.00001Hz are different from 19000.00002Hz. It just takes an appropriate filter to separate them. When separated, in this example, you have two different carriers on the 19KHz band. Then you can modulate them on and off. So, whilst the carrier is only 1 microhertz wide, it is still at a frequency of 19000.00001Hz or 19000.00002Hz. The fastest rate of this modulation equals the frequency of the photon, not the bandwidth. The key is developing a bandpass filter that fits within the area of interest (i.e. within 1 microhertz at a given frequency). Obviously, given the amount of filters required per hertz we are looking at a specialized chip. Also, there is a finite amount energy that can be supplied per microhertz band, so some cooling at the receiver may be required to increase the minimum detectable signal. I'm looking for web references at the minute, but I can't see any. If I find some, I'll add them here for you. 78.146.89.66 (talk) 13:37, 26 May 2012 (UTC)

Before someone mentions the Shannon–Hartley theorem, please be aware that this applies only to analogue signals and digital signals where bandwidth forms part of the modulation scheme. In the digital world, apply "Marx Theorem" where capacity equals frequency. There is also approximately 10^29 carriers available in any 1 Hz bandwidth. To calculate the approximate "total theoretical capacity" of any given Hertz multiply the frequency by 10^29. I say approximate because the channel capacity expands as the frequency rises and 10^29 is an approximate value. 78.146.89.66 (talk) 13:57, 26 May 2012 (UTC)

Here is an example of its use in the field. This is an image from a waterfall of the German VLF station DHO38 earlier today. We can see the 5 minute high bandwidth broadcast in the center of the image. We can't tell if its microhertz comms from the picture, but if we assume at least hertz comms with 80 channels, they are pushing about 224MB (not Mbit) per second. The entire broadcast is about 65GB in 5 mins. http://i.imgur.com/RXwQH.jpg

This is almost correct. There is a good post here that provides the math. http://www.physicsforums.com/showpost.php?p=3947771&postcount=86 — Preceding unsigned comment added by 2.100.85.153 (talk) 19:45, 12 June 2012 (UTC)

Just to set the record straight, there are no reliable sources that information can be transmitted faster than the Shannon-Hartley theorem allows. --Chetvorno^TALK 00:09, 27 December 2013 (UTC)

And if you are invoking photons, then the Heisenberg's uncertainty principal comes into play. In order to be certain of a Minute frequency difference you need lots of time. So there is still a trade-off between time and bandwidth. Graeme Bartlett (talk) 04:52, 17 August 2018 (UTC)

22.3 kHz ?

Why should this guy sit in russia? This source says that a 22.3 kHz transmitter is located in Exmouth and that is was used as a "add-on" for omega. http://www.globezoom.info/wbb2/print.php?threadid=21047&page=1

'6 Marinefunkstelle Harold E. Holt (Exmouth, Australien) 21° 49' S; 114° 10' O 22,3 kHz 989 kW'

Further http://nautilus.org/publications/books/australian-forces-abroad/defence-facilities/naval-communication-station-harold-e-holt-north-west-cape/ includes

"In addition, the recent change of transmitting frequency from 22.3 kHz to 19.8 kHz has precipitated concern among station personnel regarding the safety of working on the towers."

So once they used this frequency, maybe they run the transmitter with it for 2 hours every month just to test if it is still operational. — Preceding unsigned comment added by 77.3.138.184 (talk) 18:50, 25 June 2012 (UTC)

Radiation resistance

Hi User: Chetvorno, I just noticed you added a statement about radiation resitance being extremely low on the VLF antenna system, do you have any supporting evidence for this? I feel that it would actually be higher, considering the amount of wire/cabling that the current is traversing through these structureswhich are HUGE. cheers Read-write-services (talk) 05:49, 9 April 2018 (UTC)

Sure, I'll find some sources, I should have sourced it when I added that. Radiation resistance $R_{R}$ is not the total resistance of the antenna seen at its terminals, but an equivalent "resistance" that represents the energy radiated from the antenna as radio waves. As such, it is a desirable resistance, for an efficient antenna you want it to be the largest resistance seen by the input power. The radiation resistance of practical VLF antennas is low because the antennas are so much shorter than a wavelength $\lambda$. For the electrically short monopole antennas used at VLF, in which the length of the antenna $L$ is much smaller than a quarter wavelength $L<<\lambda /4$, even with a large capacitive topload (which all VLF antennas have) the radiation resistance is

$R_{R}=160\pi ^{2}\left({L \over \lambda }\right)^{2}$

Since VLF wavelengths are $\lambda =$ 10 to 100 km (30,000 to 300,000 feet), and it is not possible to build a radio mast taller than about L = 600 m (2000 feet), the radiation resistance of even the tallest VLF antennas is between about 6 and 0.06 ohms, and in smaller masts the radiation resistance scales down with the square of the length. The other resistances in the antenna and transmission line; the ground system resistance, copper losses, eddy current losses, etc. are in series with this small resistance, forming a voltage divider, and the input power divides proportionally between them.

So the major problem of building antennas at VLF frequencies is to (TRY to) keep the other resistances in the antenna/transmission line small compared to the tiny $R_{R}$; otherwise most of the transmitter power is not radiated but dissipated as heat in these resistances. Even the huge mile long antennas of naval VLF transmitters top out at about 50-80% efficient. They keep the copper resistance down by using many wires in parallel for the vertical radiators and horizontal toploads. Ordinary sized wire antennas used at VLF frequencies have $R_{R}$ far below one ohm, and are less than 1% efficient. --Chetvorno^TALK 10:25, 9 April 2018 (UTC)

Bravo! thanks for that, I asked this for two reasons, for the article as well as general interest in the subject of NDB antennas which are also proportionately smaller than about 1/20 lambda.. Also, I get your point about radiation resistance-RR, I completely forgot that this is not the DC resistance without energy input to the antenna, but rather the impedence to the enrgy trying to leave the antenna and into free space-my mistake. Thanks again for the clarification, formula etc. Read-write-services (talk) 04:35, 10 April 2018 (UTC)

Aircraft VLF

There has been a recent addition to the article regarding aircraft/stategic bombers having VLF? Can this be verified?Read-write-services (talk) 23:19, 16 August 2018 (UTC)

it seems that the information exists-it has not been (as yet) included in the articleRead-write-services (talk) 23:44, 16 August 2018 (UTC)

Protective bubble

Original paper at:

https://link.springer.com/article/10.1007/s11214-017-0357-5

©Geni (talk) 08:27, 11 May 2020 (UTC)

Power

in: "Trideco" ... at a power of 1.8 megawatts, the most powerful radio station in the world.

It should be noted from UHF television broadcasting, "A UHF TV station could be licensed for up to five megawatts of carrier power" — Preceding unsigned comment added by 192.225.121.101 (talk) 01:49, 12 August 2022 (UTC)

Okay, but firstly we're discussing VLF Radio (radio carrying audio modulation) vs UHF Television (radio carrying audio and video modulation), so it's technically not a fair comparison. Either way, these would require research, because I think both, "facts" are dubious. Read-write-services (talk) 06:40, 19 August 2022 (UTC)

For the UHF TV they are talking about effective radiated power rather than total power. UHF can use directional antennas to concentrate the power to the horizon, boosting EIRP. Graeme Bartlett (talk) 08:42, 19 August 2022 (UTC)