Very low frequency
As long waves (English very low frequency, VLF short ) refers to electromagnetic waves in the frequency range of 3 to 30 kHz. - ( Not to be confused with low-frequency (<3 kHz ) (see frequency band) ).
Application
Areas of application:
- This frequency range is used nowadays because of in this area only low potential bandwidth of a few hertz almost only for the transmission of commands to submerged submarines (for example, with the transmitter DHO38 ), because the radio waves in this frequency band about 10 to 30 meters into the sea water can penetrate. At a transmission frequency of 15 kHz, the skin depth in marine water is about 20 meters. At 82 Hz ( still below the long waves, then called ELF) is as high as about 300 meters. The boundaries are fluid, since the depth of penetration by the transmission power, the size of the receiving antenna, the sensitivity of the receiver and the characteristics of seawater (salt content, temperature) depends. See also skin effect.
- Another area of application is the mine radio. Here the antenna wires are routed in the shaft, the distance to the receivers is therefore never very large.
- In addition, they are also used for the radio navigation (alpha ) and transmitting the time signal ( beta).
- A more exotic application can be found in the investigation of the reflection behavior in the upper soil layers. In this way, large structures in the order of the wavelength can be found which if they differ in their electrical conductivity of the surrounding area.
In the early days of radio technology was repeated tries telephony using the amplitude or single-sideband modulation in the range from 20 kHz, but was due to the low transmission bandwidth, the result unsatisfactory.
VLF transmitters require large antenna systems, which consist of several towers of more than 100 meters above sea level, and claim an area of several square kilometers. In contrast to other large technical installations, such as airports, however, nature is only marginally impaired on the grounds of a VLF transmitter. There are also mobile VLF transmitter such as TACAMO.
In the frequency range below 30 kHz are also working some time drawing and navigation radio transmitter. The historical VLF transmitter SAQ in Grimeton ( in Varberg in Sweden) is include on special occasions each for about half an hour in operation and can at these times even in action, such as on Alexanderson Day.
As a rule, long -wave transmitters operating in the frequency range between 10 kHz and 30 kHz. There are also stations that operate in the frequency range below 9 kHz. This frequency range is subject to the part of the international telecommunications organization ITU no regulation and may, in some countries (but not in Germany ) can be used without a license by anyone.
The Russian station ZEVS sent to 82 Hertz and the American channels Sanguine to 76 Hertz On frequencies still below the long waves ( Extremely Low Frequency ). They served during the Cold War both the alerting of submarines at great depth - reportedly up to 300 meters.
For hobbyists VLF reception of the PC is increasingly being used with integrated sound card besides specially designed for this frequency range radio receivers. About the sound card with a coil or wire antenna received signals are processed by a software (eg freeware SPECTRAN ) for FFT analysis (English Fast Fourier Transform) analyzed and presented in the form of spectrograms.
Flashes emit electromagnetic pulses ( atmospherics or spherics ) with a maximum energy density in Längwellenbereich. Worldwide, there are about 100 lightning discharges per second major. The measurement of atmospherics is used for recording the thunderstorm activity (see also Atmospheric disturbances ).
Wave propagation
Longitudinal waves propagate in the waveguide between the earth surface and ionospheric D- layer in the altitude range between 70 and 90 km from ( ionospheric wave guide ). ( Exception: Whistler - propagation ). Since their wavelengths (10 to 100 km) already have the dimensions of the waveguide, its spread can only be due to the ray optics (interference between the ground wave and once or twice at the ionosphere reflected waves ) describe. This is possible only for relatively short distances between transmitter and receiver; e.g. at a frequency of 15 kHz up to about 1000 km. For longer distances, a wave-optical approach is necessary. In the low frequency range area eventually ( frequency <3 kHz), the wave- optical solution is still possible. An extreme case are the Schumann resonances. These are the natural oscillations in the cavity between Earth's surface and the ionosphere with a wavelength the size of the circumference of the earth and a frequency of about 7.5 Hz and harmonics.
List of VLF transmitter
The list of standard PC sound cards (which is a max. Sampling rate of 48 kHz have ) receivable in Germany VLF transmitter. Transmitter with frequencies above 24 kHz are not considered.
The well-known VLF transmitter GBR in Rugby ( transmit frequency: 15.95 kHz) has set its transmit activity on 1 April 2003.