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There are two basic types of ground associated with amateur radio stations:
A safety ground is installed to reduce the hazard from lightning strikes to the antenna or "feedline" system, or from short-circuited AC power. A safety ground in its most basic form simply provides a better electrical path to ground than the person or equipment being protected. NEVER connect radios to ANYTHING inside the house for ground purposes.
Some important principles:
The current from a lightning strike won't turn corners, and basically travels only over the skin of what is conducting it, so large diameter wire is less effective than a conductor such as copper tubing (and the tubing is less expensive).
If your equipment is grounded to the AC mains or other existing house wiring, you risk propagating a lightning strike to your entire house, and maybe your neighbors as well.
RF Grounds - Antenna
An antenna needs to be balanced. RF grounds are used to supply the "missing" side of an antenna so that it can work correctly.
One very popular example is the quarter-wave vertical antenna. The radiating side of the antenna is 1/4 of a wavelength long. Ground radials are used to supply the "missing" 1/4-wave side. This type of RF ground is called an antenna counterpoise or "virtual" ground.
The term "ground radial" is used to describe an antenna counterpoise that is constructed from many wires distributed in a star pattern outward (radial) from the center.
RF Grounds - Using the Earth as an RF Ground Plane
The surface of the earth is also an RF ground plane. The height of an antenna above the earth can affect the antenna's radiation pattern.
RF Grounds - Ground Planes in Electronic Circuits
A ground plane on a printed circuit board is a layer of metal, typically copper, which provides a conductive path back to the source.
RF shielding is one way to use a ground plane.
When the RF wavelength is very short (microwaves- about the length of a resistor), ground planes can be used to guide the signals. Examples of signal guiding techniques are microstrip and stripline.
Electronic circuit boards often combine RF ground plane with power supply grounding techniques.
Related links (all on Wikipedia):
Also see this thread in our forum:
»The Science of Effective Grounding...