First Look Data Collected with the Dual Offset feed Antenna



The offset-feed reflector antenna, recently installed at CHILL promises improved sidelobe rejection as well as improved cross-polar response. In addition, the care taken in matching the sidelobe powers should result in a reduction in artifacts present in . Recent observations, obtained during the first week of June, 2008 are presented here to demonstrate the capabilities of this antenna.

Ground clutter returns from the Rocky Mountains

A series of reference scans were done with the center fed antenna before it was removed from service in July, 2007. The data in the following PPI images was collected with the center fed antenna; the radar was operating in horizontal-only polarization mode with clutter filtering disabled. The widespread, azimuthally-smeared, zero velocity areas located approximately 60 km west of the radar are ground returns due to the mountains. There is a significant amount of clutter present at both the 2.5 and 3.5 degree elevation angles.

Below are the corresponding PPI sweeps taken with the offset feed antenna on 10 June 2008. The reduced sidelobe levels of the new antenna has decreased the extent of the ground clutter as the main beam scans just above the mountainous terrain.

Sidelobe artifacts above thunderstorms

The center fed antenna often produced sidelobe artifacts above the true echo top when RHI scans were done through thunderstorm cores. On 4 June 2008 RHI scans were done through an area of thunderstorms that were moving off to the east of the radar. The reflectivity and radial velocity fields shown below were plotted from the "raw" (unedited / thresholded) data fields. The top of the convective echo is essentially artifact-free in this scan. This is most clearly seen in the unperturbed radial velocity noise pattern immediately above the storm summit.

Note: The uniform signal "ring" at the 120 km range mark is a test pulse.

Measurements of low depolarization levels

The following RHI scan was taken through a stratiform rain area around mid-day local time on 5 June 2008. LDR values below -35 dB are present in much of the very light rain (quasi-spherical drop shapes) located below the bright band within ~9 km range of the radar. LDR is shown in the second (LH) panel.

A histogram of the LDR values within the near range region below the bright band is shown below. It appears that radar's LDR measurement range now reaches about -40 dB. With the center fed antenna, observations of LDR levels of -35 dB or less were quite rare [1].


  1. Bringi and Chandrasekar, 2001: Polarimetric Doppler Weather Radar: Principles and Applications, Figure 7.13