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Antenna Capacity Tops

February 20th Fact-of-the-Day

A grounded (or low-impedance base-fed) vertical antenna can be fitted with a capacity top (also called a capacity hat) to lower its resonant frequency. The capacity top can be a sphere, cone, inverted cone, cylinder, disk, spider, or ring with little electrical difference if dimensions are adjusted to provide the same top capacitance. Capacity tops without rotational symmetry, such as T or inverted-L shapes, also can be used with little difference in performance, so long as the top elements are relatively small compared to the height of the vertical radiator. In all these cases, the current distribution in the vertical radiator at resonance is nearly identical to the current distribution that would exist in the same portion of a longer vertical radiator at resonance if a capacity top was not used. ©2005 Martek International All rights reserved.


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