Investigation of the stop region of a compressed log-periodic dipole antenna

The region immediately behind the active region of a log-periodic dipole antenna (LPDA), the stop region, is investigated. In compressed LPDAs, in which one or both of the LPDA parameters are reduced, the attenuation coefficient of the stop region is strongly dependent on the unloaded characteristic impedance of the feed transmission line, and the stop region can fail to confine the incident power to the principal active region. We quantify this behavior using an analytical expression for the shunt loading of the elements in the stop region. It is shown that the stop region transitions to a fast wave region as the shunt inductive susceptance of the elements is canceled by the intrinsic shunt capacitive susceptance of the feed transmission line, leaving a net shunt capacitive susceptance and hence supporting transmission of power.