An efficient moderate-size vertical-incidence ionosonde antenna for 2-20-MHz polarization studies

A wide cone-angle conical spiral antenna which has been developed for use as a circularly polarized vertical-incidence ionosonde antenna is described. It exhibits an 80-degree beamwidth, 5-dB gain, axial ratio of 1.2:1 and SWR less than 2:1 typically over the band 2-20 MHz. The entire structure is only 38 meters high and 52 meters in diameter. Extensive measurements on models and the full-size antenna are presented which show that the polarization and radiation patterns are quite uniform over the entire operating band. In particular, the radiation pattern shows no evidence of sidelobes at low elevation angles which could cause unwanted oblique returns. The discovery of a beam-reversing phenomenon in spiral antennas is described along with its elimination by geometry control. An example of ionosphere mode selection by means of polarization control with the full-size antenna is presented.