RESEARCH ON CIRCULARLY-POLARIZED CONICAL-BEAM ANTENNAS

This antenna is intended for communication between a stationary satell ite and mobile stations, and is designed to possess circularly polariz ed conical beams. A small and light-weight antenna was developed. It g enerates a circularly polarized conical beam in the desired direction, by means of changing the inclination angle of the linear-antenna elem ents, and the spacing between the antenna elements, without phase shif ters. The antenna inclination angle (alpha), the antenna length (l), t he distance between antenna elements (d), and the radius of the circul ar reflector (R), are adopted as parameters, and optimum values for th e directivity of the research objective are obtained by calculation. F our elements of the antenna are fed by in-phase signals, the distances between elements are set to appropriate lengths, and by providing spa tial phase differences of 90 degrees, circularly polarized waves are o btained. The antenna elements are fundamentally monopole radiators, ob viating the addition of phase shifters and the like in the feed circui t. Consequently, the antenna's construction is simple, and it can be o ffered at low price. Besides this, because it is relatively small and does not require a tracking device, it can be said to be suited for mo unting on mobile objects. Ultimately, the antenna parameters were dete rmined as follows: l = 0.64 lambda (120 mm), R = 0.43 lambda (80 mm), alpha = 45 degrees, and d = 0.48 lambda (90 mm). In the calculation, t he frequency was set to 1.6 GHz. It was seen that the beam radiation i ntensity took the maximum Value in the angular range of theta = 30 deg rees to 60 degrees, and that the theta and phi components of the elect ric field were approximately equal: In other words, the radiated waves were circularly polarized. The experiment for measuring the directivi ty of the circularly polarized conical-beam antenna was carried out by using four monopole antennas, and the effectiveness of the theoretica l analysis was confirmed. Next, experiments were carried out for the i mpedance and directivity, with representative dimensions. The gain in the maximum direction was 4.5 dBi, with an axial ratio of less than 2 dB, and a standing-wave ratio of less than 1.2. As a result, it was fo und that the calculated and the measured values agreed well, and satis factory axial-ratio characteristics were obtained.