Quadrifilar helical antennas with a variable conical beam

A quadrifilar helical antenna (QHA) can radiate a circularity polarized conical beam. In mobile satellite communication system, circularity polarized omnidirectional antennas are preferable for vehicle antennas because no satellite tracking system in the azimuth direction is necessary. The conical radiation pattern characteristics of the QHA with a constant arm length are analyzed numerically and parameters used to change the beam direction are investigated. When the antenna height changes with a constant arm length, the number of helical turns needs to be varied slightly in order to maintain the shape of a helical cylinder. In addition, by adjusting the antenna height, the elevation angle of the beam direction can be controlled with an almost constant gain. Moreover, for the purpose of reducing a backlobe, we proposed that a spiral antenna with one turn is connected to a QHA, which we call a quadrifilar helical-spiral antenna (QHSA), and its radiation pattern characteristics are investigated. Some experimental results are also presented