Cloud discharges and CG flashes both radiate energy over a wide spectrum of frequencies, predominately the radio frequency (RF) bands. During the “stepped” process that creates new channels, there are strong emissions in the very high frequency (VHF) range. High current discharges along previously established channels (“return strokes”) generate powerful emissions in the low frequency (LF) and very low frequency (VLF) ranges. Medium frequency (MF) emissions are centred in the AM radio band and are responsible for the static we hear on
AM radio during lightning storms.
Cloud and ground flashes produce significantly different RF emissions over different time scales, which can be used to distinguish between these two classes of lightning. With their high current and predominately vertically oriented return strokes that generate magnetic fields, CG flashes produce strong signals that can easily be associated with a single position near the point they strike the earth’s surface.
When high currents occur in previously ionized channels during cloud-to-ground flashes, the most powerful emissions occur in the VLF range. VLF (very low frequency) refers to radio frequencies in the range of 3 kHz to 30 kHz. An essential advantage of low frequencies in contrast to higher frequencies is the property that these signals are propagated over thousands of kilometres by reflections from the ionosphere and the ground.
Waves with a frequency between 30 and 3 kHz have a length between 10 and 100 km. An applicable antenna for these frequencies is a small loop antenna of size less than 1/10000 of the wavelength in circumference.
The electric field of the radio waves emitted by cloud-to-ground lightning discharges is mostly oriented vertically, and thus the magnetic field is oriented horizontally. To cover all directions (all-around 360 degree) it is advisable to use more than one loop. A suitable solution can be obtained by two orthogonal crossed loops as they are used for direction finding systems.
The size of the antenna can extremely be reduced by using ferrite rods. But a high number of turns are necessary to reach the same voltage compared with a loop antenna.
This implies that the ferrite antenna has a lower resonance frequency than the loop antenna.
The resonance frequency of the ferrite antenna for wide-band VLF reception should not fall below 100 kHz.
Waves with a frequency between 30 and 3 kHz have a length between 10 and 100 km. An applicable antenna for these frequencies is a small loop antenna of size less than 1/10000 of the wavelength in circumference.
The electric field of the radio waves emitted by cloud-to-ground lightning discharges is mostly oriented vertically, and thus the magnetic field is oriented horizontally. To cover all directions (all-around 360 degree) it is advisable to use more than one loop. A suitable solution can be obtained by two orthogonal crossed loops as they are used for direction finding systems.
The size of the antenna can extremely be reduced by using ferrite rods. But a high number of turns are necessary to reach the same voltage compared with a loop antenna.
This implies that the ferrite antenna has a lower resonance frequency than the loop antenna.
The resonance frequency of the ferrite antenna for wide-band VLF reception should not fall below 100 kHz.
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