Design and performance characteristics of the new 9 m dual-offset Gregorian antenna for the CSU-CHILL radar

The CSU-CHILL national radar facility has been operated by Colorado State University since 1990. The radar system has been under continuous improvement since then, incorporating state-of-the-art hardware and software capabilities. The radar operates at S-band and is a fully polarimetric Doppler system with the polarization basis being the horizontal (H)-vertical (V) states. Full polarimetric capability is achieved by using a two transmitter and two receiver system. Under normal operations the transmitters are ‘fired' alternately with PRT of 1 ms; the pulse width is 1 micro-sec. By fully polarimetric we mean that the signal processor in real-time computes the three power terms (HH-power, HV-power and VV-power) and the three complex correlations (HH/VV, HH/VH and VV/HV) for each resolution volume. From the latter set of measurements, the common variables such differential reflectivity, linear depolarization ratio, differential propagation phase, copolar correlation coefficient and co-cx correlation coefficient are derived in addition to the conventional radar reflectivity at H-polarization, mean Doppler velocity and spectral width. To various degrees, the accuracy of these meteorological variables depends on antenna performance in terms of main beam symmetry, ‘close-in' sidelobe levels in all planes (and not just in the principal planes) and the cross-polar levels (on-axis as well as off-axis). By ‘close in' we mean approximately 10 beam widths from boresight. In particular, in severe storms the gradients of reflectivity across the main lobe and close-in sidelobes can be very large, of the order of 40 dB/km, which makes high antenna performance vital to ensuring accurate measurements of most polarimetric variables and to avoid large artifacts in the data.

In order to obtain the most accurate measurement of the dual-polarized radar variables, Colorado State University contracted with General Dynamics SATCOM (previously VertexRSI) to build a custom 9 m dual-offset Gregorian antenna to stringent specifications especially in terms of beam symmetry, sidelobe envelopes and the on- and off-axis cross-polarization levels. In addition, the mechanical design was specified for modest acceleration/deceleration specifications which occur during sector PPI/RHI scanning, and the antenna itself was to be designed for transport and precise re-assembly. The manufacturer had proven the dual-offset geometry for their SatCom antennas for many years, but had not yet built a 9 m class of such antennas. In addition, our stringent specifications for weather radar applications far exceeded the usual ITU-R specifications, for example, beam symmetry, sidelobe envelope and off-axis cross-polarization levels. The dual-offset design, among other features, gives the ability to simultaneously control both the sidelobe envelopes as well as the peak off-axis cross polarization levels (which cannot be done with the single offset design). We describe how the stringent electrical specifications were met by careful design, feed/OMT measurements in an anechoic chamber, far-field predictions of electrical performance using the latest software, precise manufacturing methods and final verification of performance at the calibrated test range in Kilgore, TX. The antenna was installed on the CSU-CHILL radar in February 2008 and we have nearly a year's worth of high quality data to report on. In particular, we demonstrate the low sidelobe levels and high polarization purity via data from several storm types. The major improvements have been in a significant reduction in sidelobe-induced artifacts, very high accuracy in differential reflectivity and copolar (HH/VV) correlation coefficient variables, and a system linear depolarization ratio (LDR) limit approaching -40 dB.