Modeling radar power and phase antenna patterns and the evaluation of dual-polarization antenna performance

The quality of dual polarization radar measurements depends on the error characteristics of the antenna. All antennas possess polarization errors and it is of interest to evaluate to these errors. Recently it has been shown that the cross coupling of horizontal and vertical polarization signals due to antenna polarization errors can bias dual-polarization measurements, especially for radars that employ the simultaneous transmission of the H and V polarized wave. One figure of merit for performance of an antenna is ICPR (Integrated Cross-Polarization Ratio) defined as the LDR (Linear Depolarization Ratio) when the radar beam is filled with small spherical scatterers. Typically only antenna power patterns are available and not the phase patterns are unknown, especially for crosspolar antenna patterns. Thus only the upper bound of ICPR can be evaluated. It is now possible to very accurately model the complex antenna patterns if the physical antenna can be measured and the antenna pattern of the feedhorn is known. In this paper the performance of the S-Pol (NCAR's S-band polarimetric radar) is evaluated using a GRASP (General Reflector Antenna Software Package) software. The shape and dimensions of the S-Pol antenna and support struts are measured using photogrammetry. A new high performance feedhorn was recently manufactured for S-Pol and its antenna pattern was evaluated using GRASP like software. From this information, the antenna pattern for S-Pol's antenna can be accurately calculated, both magnitude and phase, and thus ICPR can be accurately be predicted. This paper describes this modeling effort and analyses the resulting antenna patterns.