Ground dipole
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A Ground dipole [ alt: Ground antenna ] is a special antenna which consists of two electrodes positioned in the ground.
[edit] Description
These antenna are usually used for for ELF wave reception and by radio amateurs also for VLF transmission and reception. These electrodes are fed with the signal to be transmitted. The current which is fed in the earth flues through the earth too the other electrode, whereby a conducting loop is formed, which radiates as every loop fed with AC electromagnetic waves.
The Ground dipole works most effective ( for very long waves), when the loop is as big as possible. This is best achieved, when feeders are built as overhead lines and when the ground between the electrodes is of bad conductivity, which forces the current to long ways through the ground. On the first view a ground dipole seems to be a good antenna as it requires no tall antenna towers and cannot be directly hit by lightning, but these arguments did not count as its effiency is very bad ( < 1%). It is also affected by lightning as voltage gradients induced by lightning can result in dangerous high voltages, so overvoltage protection equipment is required by ground dipole facilities.
The bad effiency is the result of the fact, that high frequency currents run always near the surface of the earth, so no large loop diametres cannot be realized by using small masts for the feeder lines. For low frequencies its effiency increases as low frequent AC penetrates deeper in ground, but effiency remains small. Ground dipoles are also used for reception of ELF-signals especially by hams. By conncting the electrodes to the analog input of a PC-soundcard one can easily receive ELF-signals by using a FFT-software. However one has to keep in mind that by potential gradients of nearby lightning strikes dangerous overvoltages can occur, which can destroy the soundcard of the PC.
