| Summary: | 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 λ/2 dipoles in this paper. The FPH dipole has a radiation resistance and efficiency close to those of a λ/2 dipole with 39% reduced size and shows a higher radiation quality factor. The LPDA antenna with the longest λ/2 dipole replaced by the FPH element and the LPDA antenna with the longest and third-long λ/2 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.
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