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Anomalous behavior in "optimum" log-periodic dipole antennas

It is well known that compressed Log-Periodic Dipole Antennas (LPDAs) can exhibit multiple active regions and anomalous frequency-dependent behavior. However, it is shown here that so-called “optimum” LPDAs can also exhibit anomalous behavior even when the scale factor is of a moderate value. Numerical simulation and experimental data are presented which elucidate this behavior. It is confirmed that the anomalous behavior is due to incomplete power transfer from the feed transmission line to the active region(s). However, the behavior seen here differs from that of previously-studied, highly-compressed LPDAs in that in the frequency dependence of the anomalous behavior does not correspond to energy trapped between the principal active region and a reactive feed line termination. Instead, it appears to correspond to a much longer electrical length, possibly that of the entire antenna. Moreover, the anomalous behavior cannot be rectified by the placement of a resistive termination at the far end of the feed transmission line. Finally, the anomalous behavior cannot be completely eliminated by other traditional techniques such as increasing the feed transmission line characteristic impedance.