Log-periodic antennas with modulated impedance feeders

The proper phasing of the active region of a log-periodic array can be obtained by means of a feeder without the conventional transposition of the wires. This is of practical importance for arrays of monopoles and slots where the transposition is not possible. However, the input impedance of such structures, when fed from a line with constant impedance, varies greatly with frequency. The impedance variation has been traced to reflections from a stop region between the antenna feedpoint and the active region. By introducing a step modulation of the characteristic impedance of the feedline, the reflections can be reduced and a stable impedance obtained.