A feeding method for small helical antennas

A feeding method is proposed for small helical antennas with diameters of about 1/80 to 1/20 wavelength. The present feeding method consists of excitation of a helix with a total length of about 1/4 wavelength and with its lower edge grounded by an open-ended center conductor oriented coaxially to the helix. In this paper, it is shown that matching with a 50-Ω feed system can be accomplished even when the helical diameter is varied in the same feeding method. The gain and the bandwidth characteristics of the ground plane versus diameter are discussed. As an example, a gain of −7.2 dB_d (with respect to the dipole antenna) on the ground plane is obtained when the helical diameter is 1/46 wavelength and the height is 1/56 wavelength. Like the monopole antenna, the main radiation pattern is omnidirectional vertical polarization in the horizontal plane and figure-eight in the vertical plane. When this antenna is installed on a transmitter case, the gain is about −4 dB_d. When installed on the transmitter case, the antenna radiates the orthogonal polarization component. The null of the main radiation pattern in the zenith direction can be compensated by this polarization. The radiation characteristics of the antenna are numerically calculated by the method of moments and are verified experimentally. © 2006 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 90(3): 33–41, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecja.20324