Design of variable turn bifilar helical antenna for mobile communication

In recent years, a variety of antennas for mobile satellite communications have been proposed. Attention has been given to bifilar and quadrifilar helical antennas as suitable for base stations because of their simple structure of bar shape requiring only limited installation space. These types of antenna are widely used because of their simple structure and superior axis ratio. With bifilar and quadrifilar antennas, it is possible to attain omnidirectivity in the horizontal plane and make a variable vertical directional beam, i.e., conical beam, and thereby obtain the desired gain by changing the number of turns. These antennas can therefore be fixed and also be mounted on mobile stations (ships, automobiles, and aircraft). This paper discusses type of antenna structure: one is a bifilar antenna of a fixed length and diameter to permit shifting the beam in the desired direction by changing the winding factor manually or automatically. We calculated their excitation currents and radiation directivity by using the moment method