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An analytic model for computing the radiation properties of (he quadrifilar helix volute antenna is discussed and various design considerations for Global Positioning System (GPS) applications are presented. The effects which modifying the antenna length and diameter has upon the antenna amplitude and phase performance are presented and the use of the antenna for dual-frequency operation is discussed. The effects of phase imbalances are presented and compared with measured pattern anomalies. I:'lTRODUCTION T HE QUADRIFILAR helix antenna [1] is one of the most commonly used designs for Global Positioning System (GPS) applications owing to its possible small size (for fractional turn volutes) and circular polarization properties. Various manufacturers of GPS equipment use volutes of diff erent size and construction to cover either of the two GPS frequencies of 1.57542 GHz (Ll) or 1.2276 GHz (L2). Although similar in typc, thcsc antcnnas may show widely differing radiation characteristics depending upon the particu lar design chosen as well as upon the quality of construction. Information which is available in the literature l2J is suitable for selecting or designing quadrifilar helical antennas for many applications. Data from Kilgus [2] and Adams [3] are particularly useful in ubtaining desired amplitude pattern responses, however, owing to the particular absencc of phase data for these antennas. these references are not directly applicable in evaluating the various antenna design choices or in identifying the particular design features which are most critical in determining antcnna performance for GPS applica tions. A trial-and-error approach to this problem is both costly and unreliable. The purpose of this work is to use a method of moments technique to predict antenna radiation properties for a wide range of design parameters and to present the effects which these design changes have upon antenna performance for GPS. GPS A"ITENNA PERFORMA"ICE OBJECTIVES GPS operation requires an antenna that can provide uni form response over approximately the entire upper hemi-

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