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Folded Unipole Antenna Impedance

February 26th Fact-of-the-Day

A folded unipole antenna is one-half of a folded dipole mounted vertically on a ground plane or buried ground radial system. Just as folding a dipole once raises its radiation resistance by a factor of four, from approximately 72 ohms to approximately 288 ohms, folding a quarter-wave vertical mounted on perfect ground once raises its radiation resistance from approximately 36 ohms to approximately 144 ohms. Just as the diameters of the two conductors of a folded dipole can be made different from each other to change the impedance step-up ratio to something other than four, the diameters of two folded unipole conductors can be made different from each other for the same purpose. The impedance step-up property of folded unipole radiators is especially useful in close-spaced vertical arrays which generally have low radiation resistances that are difficult to match efficiently if unfolded radiators are used. ©2005 Martek International All rights reserved.


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