Gain Characteristic Maintained, Miniaturized LPDA Antenna Using Partially Applied Folded Planar Helix Dipoles

The miniaturization of a compact planar log-periodic dipole array (LPDA) antenna usually comes with a side effect of the gain deterioration phenomenon. To address this issue, a folded planar helix (FPH) dipole is designed as a miniaturized radiating element to replace parts of a conventional LPDA antenna composed of <inline-formula> <tex-math notation="LaTeX">$\lambda /2$ </tex-math></inline-formula> dipoles in this paper. The FPH dipole has a radiation resistance and efficiency close to those of a <inline-formula> <tex-math notation="LaTeX">$\lambda /2$ </tex-math></inline-formula> dipole with 39% reduced size and shows a higher radiation quality factor. The LPDA antenna with the longest <inline-formula> <tex-math notation="LaTeX">$\lambda /2$ </tex-math></inline-formula> dipole replaced by the FPH element and the LPDA antenna with the longest and third-long <inline-formula> <tex-math notation="LaTeX">$\lambda /2$ </tex-math></inline-formula> dipoles replaced by the FPH elements are examined. A conventional LPDA antenna can be miniaturized while avoiding gain degradation in the operation band of 400–800 MHz. The lowest impedance matched frequency and the frequency where the directional beam starts to appear are about the same for the proposed miniaturized LPDA antenna. Moreover, the front–back ratio can be improved by the proposed method. The prototype of the two-FPH-element applied LPDA antenna is built, and the design concept is verified experimentally. A size reduction of 39% and an average gain deterioration of −0.2 dB are achieved by the prototype in comparison with a conventional LPDA antenna. The proposed antenna can be utilized in the mobile radio wave direction-finding system.