Asymmetry anomalies and resonances in hybrid LPDA-broadband dipole antennas

Combinations of log-periodic dipole antennas (LPDAs) and broadband dipole elements have become ubiquitous as broadband measurement antennas for both emissions and immunity testing. It is well known that these combinations of LPDAs and broadband dipole elements can exhibit anomalous, frequency-dependent behavior especially when the broadband dipole elements are heavily end or top loaded and the antenna is operated in vertical polarization over a conducting ground plane according to S.J. Porter and A.C Marvin (1994). While there is more than one possible source for anomalous behavior in such antennas (as well as in LPDAs), we show here that in situations when the antenna is vertically polarized and situated over a ground plane the principal source of anomalies is related to the so-called asymmetry resonance found in E-plane arrays of LPDAs and LPDAs with inherent structural asymmetry according to Z. Chen (1999) and C.C. Bantin and K.G. Balmain (1970). The asymmetry resonance in LPDAs is associated with the existence of common mode (CM) current on the transmission line feed. We show, for the first time, that, to a limited extent, this anomalous behavior can be mitigated through the proper use of common mode chokes/baluns. Numerical simulations and experimental data are presented to verify both the presence of the externally-induced symmetry and the efficacy of several different balun/choke arrangements.