Folded unipole antennas

Folded unipole antennas less than a quarter wavelength long are used to transform the input resistance of short vertical antennas to a more acceptable value by effecting a transformation within the antenna itself. An equivalent circuit of the folded unipole is derived. The transformation of input impedance is shown to be due to two factors: (1) the division of radiation mode currents between the driven and undriven portions of the antenna, and (2) the flow of currents in a nonradiating transmission-line mode. The effect on the input impedance of reactive elements inserted between the unfed portion and ground is investigated. The theory is verified with an experimental antenna whose input impedance is measured as a function of tuning reactance. The experimental curve is found to be in good agreement with the theory. Two methods of operating the folded unipole are proposed. In the first, only the division of radiation mode currents is utilized to effect the input impedance transformation. This method is applicable to antennas close to a quarter wave. The second method transforms the input impedance by utilizing the transmission-line mode only. It is applicable to short antennas.