Ground dipole
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A Ground dipole (alt: Ground antenna) is a special type of antenna which consists of two electrodes positioned in the ground.
[edit] Description
Ground dipole antennas are usually used for transmitting and receiving on the ELF and VLF bands. The antenna consists of two electrodes, planted in the ground at some distance from each other. Radio signal currents are sent along wires to the electrodes. The current flows in a loop, around the above-ground wires and through the Earth (where conductive), forming a large conducting loop, which radiates electromagnetic waves.
For very long waves, a ground dipole works most effectively when the loop is as large as possible. This is best achieved by feeders built as overhead lines and where the ground between the electrodes has little conductivity. Although the ground dipole seems to be a good antenna as it requires no tall towers and its loop covers a large area with respect to the wire used, the size (with respect to the frequency being used) is still very small, and due to its large size, total resistance is relatively high, giving very low efficiency (<1%). It is also affected by lightning, as voltage gradients induced by lightning can result in dangerously high voltages on the leads.
Ground dipole antennas are popular among amateur radio enthusiasts. By connecting the electrodes to the analog input of a PC soundcard, one can easily receive ELF signals by using FFT software. One must keep in mind that this setup is very sensitive to nearby lightning strikes which can destroy the soundcard input and should therefore be equipped with adequate protection and isolation circuitry.
