Ferrite Rod Antennas

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The size of a loop antenna can extremely be reduced by using ferrite rods, but a ferrite rod antenna needs a higher number of turns to get the same voltage compared with the larger loop antenna. That is, a corresponding ferrite rod antenna has a lower resonance frequency than the larger loop antenna. This is one reason why the loop antennas are more adequate than small ferrite rod antennas. The resonance frequency of the ferrite rod antenna for wide-band VLF reception should not fall below 100 kHz. Suitable ferrite rod antennas can be obtained from http://www.sfericsempfang.de.

It is not necessary to align the two antennas to north/south and west/east. The antennas only have to be placed horizontally, because the magnetic field of radio waves emitted by cloud-to-ground lightning discharges is mainly oriented horizontally.

Please do not twist the connection cable of the ferrite rod antennas. This creates a capacity and leads to a lower self resonance. If you are using the ferrite rod antennas of http://www.sfericsempfang.de, then one of the two copper enameled wire ends has a knot. Connect these wires with the positive inputs of your amplifier and the other wire with the negative input of the amplifier.

Figure 20: Egon holds a 35 cm ferrite rod antenna. Two 20 cm ferrite rod antennas mounted on a PVC board connected to Amplifier PCB 5 Version 7

Ferrite antennas with a length of 12 or 20cm can be purchased ready-made, or you can wrap them yourself.
10-12 cm is enough . 20cm is excellent. More than 20cm is unnecessary.
Should be mounted at right angles - cross or L-form
Electrical shielding is rarely needed as connecting to pre-amplifier is differential, and common mode signals are attenuated strongly.
We normally use 2, but BLUE allows for an extra - could be a vertical.

Comment from Egon on vertical antennas: We have implemented three H-field channels to make experiments. Initially, in addition to the two horizontal antennas offset by 90 degrees, we connected a third vertical H-field antenna. We wanted to know if this results into better reception characteristics. This has not been confirmed. The interference signals at the vertical channel are much stronger than the lightning signals. So it makes no sense to position a third vertical antenna in addition to the two horizontal antennas. The detector would work almost permanently in interference mode. If you want to use the third channel, then please also use it as a horizontal channel, for example, with three antennas offset by 60 degrees, or to make experiments.

Figure 21: Shielding a ferrite rod antenna with ribbon cable

It is not easy to wind ferrite rod antennas by yourself. The quality of a ferrite rod antenna depends on the used material for the rods and on the type of the coil. A large range of rods can be found at http://www.amidon.de. High quality rods can be obtained much cheaper by http://www.sfericsempfang.de. Coil placement and the length of windings on the rods, bars, plates and tubes affect the effective permeability of the antenna. Greatest inductance will be obtained when the winding is centered on the rod rather than placed at either end. The best ”Q” will be obtained when the winding covers the entire length of the rod. The spacing between the turns has also a significant effect on the ”Q”. The best values for the ”Q” are obtained when the coil turns are spaced one wire diameter apart, with a winding covering the entire rod. This would not increase the inductance to much. Note that we have to design a ferrite rod antenna with a high ”Q” but a low inductance, such that we do not get low resonance frequencies with small capacities.

Figure 22: A 20 cm ferrite rod antenna with an inductance of 22.5 mH and a ”Q” of 125 and a 30 cm ferrite rod antenna with an inductance of 13.4 mH and a ”Q” of 156

Ferrite rod antennas can also be made waterproof, as the image presentations at Figure 23 show. Note: Instead of using holders made from aluminum as shown in the photographs, you should use some wooden or plastic ones.

Figure 23: A guidance for building an outdoor ferrite rod antenna, continued, © Florian Bamberg

Since Amplifier 12 the difference between shielded and unshielded antennas is not as big as it was before. Instead of investing time in assembling a shielding you should first use an unshielded antenna system to search for a place with low electric interference.

A shielding of the E-field component can improve the signal to noise ratio, especially in metropolitan areas. This can improve the detection rate of lightning discharges. As we only want the magnetic part of the signal (H-field), any Electric field (E-field) is unwanted. The shielding of a loop antenna can be realized for example by wires in a copper tube. It is important that the tube is not closed. It can either be separated at the top or at the bottom. If it is separated at the bottom, then only one end of the copper tube has to be connected with the ground of the amplifier.