A 7-turn multi-step quadrifilar helix antenna providing high phase center stability and low angle multipath rejection for GPS applications

A 7-turn, multi-step quadrifilar helix antenna is presented that is designed to provide wide angle, hemispheric coverage, while at the same time exhibiting very sharp pattern roll-off near the horizon to enhance multipath signal rejection. Providing near uniform pattern coverage over the entire upper hemisphere, the antenna exhibits a very stable phase center location as a function of observation angle. The quadrifilar helix antenna design presented was developed to a preliminary prototype stage in 1996 as part of an early proposal effort to support the development of the GPS wide area augmentation system (WAAS). Both simulated and measured results are presented. Additionally, some of the practical considerations associated with developing this type of antenna are discussed.