Characteristics of modified spiral and helical antennas

Two types of modified antennas radiating circularly polarised wave are analysed from the viewpoint of improvement in the axial ratio. The outermost arms of a round spiral antenna are transformed to small zigzag sections. It is found that the zigzag sections contribute to the suppression of the standing wave near the arm ends with subsequent improvement in the axial ratio, especially at lower frequencies of operation. In a modified helical antenna which has tapered open end sections, it is revealed that the axial ratio is improved as the tapering cone angle is increased. An axial ratio of less than 0.5 dB is realised over a range of frequencies of 1 to 1.3.