Extremely low frequency

from Wikipedia, the free encyclopedia

Extremely Low Frequency (ELFforshort,Englishforextremelylowfrequency) is the part of theelectromagnetic spectrumthatincludeselectromagnetic waveswithfrequenciesof 3–30 Hzand correspondinglywavelengthsof 10,000 to 100,000 km.

Along with:

  • Super Low Frequency (SLF, 30-300 Hz)
  • Ultra Low Frequency (ULF, 300-3000 Hz)

According to the IEEE, it forms a frequency band that is usually referred to in the literature on ionospheric radio wave propagation as low frequency or low frequency. This frequency band is followed by the longest waves ( Very Low Frequency , VLF, 3–30 kHz).

According to other definitions, low frequency includes the above. Bands between 3 Hz and 30 kHz.

This article deals with the areas of ELF and SLF.

application

In particular, ELF waves are used for submarine communication , as these electromagnetic waves have a very large surface wave range due to their low frequency and can still be detected in poorly conducting seawater even after a greater penetration depth .

However, only very low data transmission rates are possible with such low frequencies. This is said to have been around 10 bits per minute in the US Navy's Seafarer system in the 1970s , which is sufficient, however, to transmit numerous commands coded in the form of very short groups of characters. There is evidence of only three ELF transmitters currently in existence: The transmitter systems at Clam Lake , Wisconsin and Escanaba River State Forest , Michigan for the American Sanguine system (transmission frequency: 76 Hz) and the transmitter of the Russian ZEVS system (transmission frequency: 82 Hz) nearby from Murmansk .

The lower the frequency of an electromagnetic wave, the greater the associated wavelength , which is calculated from the frequency and the speed of propagation. While the wavelengths in the radio frequency range range from around one millimeter (radar) to a few hundred meters (medium wave), ELF waves range from several thousand kilometers in length.

Since frequencies below 9 kHz, like the ELF range, do not fall under the guidelines of the ITU , you can operate a transmitter in the ELF range in numerous countries (but not in Germany) without a license, provided that it does not generate harmonics with frequencies above 10 kHz. However, such a transmitter with the antennas that can be realized in practice for amateurs should only have a range of a few kilometers at most.

There are also naturally occurring ELF waves: Frequencies of approx. 7 to 8 Hz arise as a so-called Schumann resonance through natural atmospheric disturbances (spherics).

The frequencies of common AC networks and for rail power supply are also in this frequency range. 16.70 Hz are common here (some railway networks, variation from 16 1/3 to 17.0 Hz), 50 Hz and 60 Hz.

In order to achieve a large range, the length of the transmitting antenna should be a multiple of λ / 4 (a quarter of the wavelength). In the case of ELF waves, this corresponds to a few hundred kilometers of wire length. Such transmitting antennas are extremely difficult to implement in practice (for example in the form of an Alexanderson antenna several hundred kilometers long ). That is why the floor dipole is used for transmission in this frequency range .

Receiving antennas

Air core coil as receiving antenna for VLF

Magnetic (inductive) antennas are preferably used to receive very low frequencies, as these are relatively insensitive to radio interference. In addition, the distance to the ground can be small because it is non-magnetic.

As shown in the picture, these can be air-core coils with hundreds of turns for higher frequencies above 1000 Hz. For very low frequencies below 100 Hz, the coils are provided with a soft iron core to increase the reception voltage (see ferrite rod antenna ). Magnetic antennas have a directional effect .

If you only want to receive one frequency, the sensitivity can be increased considerably by connecting a capacitor of suitable size in parallel. The bandwidth of the resonant circuit formed in this way can be very small (a few percent of the center frequency).

Wire antennas, which react preferentially to electrical fields, are poorly suited for various reasons, but can be effectively adapted to the receiving electronics by means of resonance transformers : They are usually considerably shorter than the optimum 1/4 of the wavelength and accordingly have a high resistance. High input impedance preamplifiers are therefore required. Horizontal dipole antennas, which are necessarily less than the wavelength above the (conductive) ground, have high attenuation.

receiver

In addition to analog receivers, a personal computer or a microcontroller with an integrated sound card or analog-digital converter can be used to receive Extremely Low Frequency . The signals are received with a coil with several tens of thousands of turns or a floor dipole . A software can, for example, an FFT perform analysis and a spectrogram represent.

Sources of interference

The interference source density increases significantly towards low frequencies. On the one hand, the large range means that distant sources of interference can impair reception. On the other hand, these frequencies are close to DC fields, the fluctuations of which produce sidebands in the ELF range.

Interactions with the human organism

The frequency spectrum of human brain waves , made visible in the EEG , is also in the range from 0 to 50 Hz. In principle, interactions between strong electromagnetic fields and EEG patterns have been demonstrated in some experiments carried out at the Justus Liebig University in Giessen . There was attenuation , increased activity, or no effect on the EEG was detectable. The EEG changes were always symptom-free in these experiments.

Comparison of the frequency band ELF to the frequency of the human brain in relation to the state of consciousness
(measured with EEG)
EEG frequency band delta Theta alpha beta gamma
typical brain activity Deep sleep
and coma 		Dream sleep , hypnosis
and trance 		relaxed wakefulness
and meditation 		normal
wakefulness 		motor and
cognitive processes
Frequency range / Hz 0.4 ... 3.5 4… 7 8… 13 12 ... 30 25 ... 100
Electromagnetic wave range Sub-ELF ELEVEN SLF

See also

Individual evidence

  1. ^ K. Davies: Ionospheric Radio . Peregrinus Ltd, London 1990. (English)
  2. ^ K. Rawer: Wave Propagation in the Ionosphere . Kluwer Publ., Dordrecht 1993. (English)
  3. ↑ so called by the military, although actually Super Low Frequency (SLF)
  4. ^ Extremely Low Frequency Transmitter Site Clam Lake, Wisconsin. (PDF; 910 kB) United States Navy fact file. Federation of American Scientists , accessed July 9, 2016 .
  5. ^ Meinke and Friedrich-Wilhelm Gundlach : Pocket book of high frequency technology. Springer, 1992, ISBN 3-540-54715-0 , p. N37.
  6. ^ Anne Schienle, Rudolf Stark, Rainer Kulzer, René Klöpper, Dieter Vaitl: Atmospheric electromagnetism: Individual differences in brain electrical response to simulated sferics . In: International Journal of Psychophysiology . tape 21 , no. 2-3 , 1996, pp. 177-188 , doi : 10.1016 / 0167-8760 (95) 00052-6 .
  7. ^ Anne Schienle, Rudolf Stark, Bertram Walter, Dieter Vaitl, Rainer Kulzer: Effects of Low-Frequency Magnetic Fields on Electrocortical Activity in Humans: A Sferics Simulation Study . In: The International Journal of Neuroscience . tape 90 , no. 1-2 , 1997, pp. 21-36 , doi : 10.3109 / 00207459709000623 .
  8. ↑ a b A Schienle, R Stark, D Vaitl: Electrocortical responses of headache patients to the simulation of 10 kHz sferics . In: The International Journal of Neuroscience . tape 97 , no. 3–4 , 1999, pp. 211-224 .
  9. ^ A Schienle, R Stark, D Vaitl: Sferics provoke changes in EEG power . In: The International Journal of Neuroscience . tape 107 , no. 1-2 , 2001, pp. 87-102 .