Antenna phase center offset (PCO) location and phase center variation (PCV) play an important role in high-precision GNSS ranging applications that use carrier-phase measurements. For ranging systems that use CDMA code-phase measurements, other statistics play a similar role -antenna group delay center offset (GDCO) and group delay variation (GDV). This paper describes the methods of how to measure and compute these statistics based on amplitude and phase pattern measured for a given antenna. A susceptibility analysis of these statistics versus multipath and antenna design parameters is also made. Practical examples on how to deal with undesired movement of PCO or larger variation of PCV is presented as well.

A multi-arm spiral antenna, of N arms, can have N modes of operation. The integer represents the operating mode which is determined by the signal phase increment between adjacent arms as fed from the antenna terminals. For the antenna to radiate effectively, the active region must be enclosed by a circumference of mode number (n) times the operating wavelength. Each mode generates a different radiation pattern. Modes, with between 2 and , are capable of forming radiation pattern nulls. This paper shows a way to excite noninteger modes along with integer modes of radiation, which combined, can create a null which can be steered away from the antenna vertical bore sight. Furthermore, variable reactive loading of spiral arm ends offers additional steering control and positioning of nulls in a 2D angular hemispherical space.Index Terms-Active region, Archimedean spiral, multi-arm, nulls in circularly polarized patterns, reactive loading, spiral arm, spiral array.

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