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TechTalk:  Underground Power

Why Antenna Grounding is Important, How It Works & Is Installed

Assuring a Strong Signal

Every agency that has invested in an information radio station wants a signal with suitable strength to achieve the FCC maximum 2.0 mV/m at .93 miles (yielding the typical 3-5 mile coverage radius). The amount of AM signal an antenna system can generate is related to many factors and one of the most important is the antenna’s counterpoise (grounding) and how it is designed and installed. The vertical “whip” portion of the antenna and the buried portion together are responsible for the creation of the station's signal. The buried counterpoise portion is required to "complete the circuit,” making the antenna system an efficient radiator. Therefore, the more robust the grounding, the more efficient the antenna and the stronger the signal at a given transmitter wattage. The physical size of the grounding becomes most important at the longer wavelengths − that is, at the bottom end of the AM band.
Vertical Antenna Grounding Horizontal Antenna Grounding
Vertical Profile Antenna System (VP9000)

The most cost effective and least intrusive antenna grounding method is afforded by the Vertical Profile Antenna System. The active element is comprised of a cylindrical copper sheath attached to the buried portion of the aluminum antenna support pole. The conductive pole and sheath, together create an effective vertical grounding method. Installation costs are reduced, compared to horizontal grounding methods, because the pole and the grounding are one piece, eliminating the need for horizontal groundplane installation and the resulting cost, time and site disruption.

Unirod

For installations where vertical grounding is required but the antenna must be mounted on an existing pole or structure, a Unirod may be employed. The Unirod can be from 10 to 40 feet in length (longer Unirods for long wavelength frequencies) and is augured into the earth beneath the antenna pole, installed with a highly conductive soil backfill. Unirods are also recommended when the most efficient possible grounding method is required.
PowerPlane Factory-Assembled Groundplane

The PowerPlane Groundplane is composed of 62 copper ground radials (12-gauge) bonded to an inner ring of (4-gauge) copper wire. Each radial is actually a loop, and extends outward from the inner ring 10 feet in every direction, creating a circular fan pattern.

At the upper end of the AM band, the PowerPlane should be installed as described. At the lower end of the AM band (below 1000 kHz), efficiency can be increased if the loops are cut to create 62 straight radials, each 20 feet in length.

Install the groundplane in good earth beneath the antenna/pole at about a 6” depth. It may also be installed on the surface of the ground, covered or uncovered. A version of the PowerPlane (FlexPlane) is available for roof surface installation in special circumstances or for portable applications.

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