Design of Non-parallel Feed Line VHF Log-periodic Antenna Based on Branch Loading

In order to ensure that the length of the antenna is reduced while the gain of the log-periodic antenna remains essentially unchanged, a non-parallel feed line log-periodic antenna based on branch loading is proposed. Firstly, based on the traditional log-periodic antenna, the antenna length is reduced by 20% by adjusting the scale factor and spacing factor of the antenna, and the length of the antenna′s aggregate line is reduced from 7.0 m to 5.6 m; Secondly, in order to compensate for the gain loss caused by the antenna length reduction, the antenna′s feed line is changed from a parallel structure to a non-parallel structure, which makes the equivalent aperture of the antenna increase and enhances the antenna′s gain; furthermore, in order to solve the impedance mismatch problem brought by a non-parallel feed line structure, the antenna impedance characteristics are adjusted using the branch loading method, and the reasons for the antenna impedance change are analyzed from the perspective of capacitive loading. The ultra-short wave log-periodic antenna designed based on the above method has a voltage standing wave ratio of less than 1.6 and a gain of 8.02 dBi to 8.61 dBi in 30 MHz to 70 MHz. Finally, a log-periodic antenna working in higher frequency has been designed and developed based on the same method, and experimental tests of the antenna have been carried out to verify the correctness of the design method.