Resonance phenomena of log-periodic antennas: characteristic-mode analysis

The method of characteristic modes is implemented in the form of a general-purpose computer program based on an established moment-method program using piecewise sinusoids. Application of the method to the log-periodic dipole antenna (LPDA) yields characteristic modes that are insensitive to changes in moment-method segmentation and are revealing with regard to parasitic resonance phenomena. A study of the modes on the LPDAs shows that the asymmetry resonance is dominated by a mode that is not restricted to one cell, although the single-resonant-cell postulate remains a good first-order explanation. On the other hand, the symmetric termination resonance is shown to involve more than one mode. A numerical study of various antenna deformations shows that only length extensions cause strong asymmetry resonances. The E-plane array is analyzed, and the results are shown to compare favorably with experiment