A compact, dual-frequency, dual-polarization, wide-angle-scanning antenna system has been developed as part of an airborne instrument for measuring rainfall. This system is an upgraded version of a prior single-frequency airborne rain radar antenna system and was designed to satisfy stringent requirements. One particularly stringent combination of requirements is to generate two dual-polarization (horizontal and vertical polarizations) beams at both frequencies (13.405 and 35.605 GHz) in such a way that the beams radiated from the antenna point in the same direction, have 3-dB angular widths that match within 25 percent, and have low side-lobe levels over a wide scan angle at each polarization- and-frequency combination. In addition, the system is required to exhibit low voltage standing-wave ratios at both frequencies.
The system (see figure) includes a flat elliptical scanning reflector and a stationary offset paraboloidal reflector illuminated by a common-aperture feed system that comprises a corrugated horn with four input ports - one port for each of the four frequency-and-polarization combinations. The feed horn is designed to simultaneously (1) under-illuminate the reflectors 35.605 GHz and (2) illuminate the reflectors with a 15-dB edge taper at 13.405 GHz. The scanning mirror is rotated in azimuth to scan the antenna beam over an angular range of ±20° in the cross-track direction for wide swath coverage, and in elevation to compensate for the motion of the aircraft.
The design of common-aperture feed horn makes it possible to obtain the required absolute gain and low side-lobe levels in wide-angle beam scanning. The combination of the common-aperture feed horn with the small (0.3) focal-length-to-diameter ratio of the paraboloidal reflector makes it possible for the overall system to be compact enough that it can be mounted on a DC-8 airplane. The input ports are oriented orthogonally and carefully positioned and the depths of the corrugations in the feed horn were chosen carefully, all in an effort to minimize the overall level of cross-polarization and side-lobe level in the system.
For optimum performance, the feed phase center would ordinarily be kept at the focal point of the offset paraboloidal reflector. It would be possible to do so in single-frequency operation, but it is not possible to have a single feed phase center for both of the widely separated frequencies of this system. Instead, the feed horn is designed so that the combination of locations is optimal in the sense that it yields an optimal combination of gains, matched 3-dB widths, low cross-polarization, and low side-lobe levels. The elliptical shape of the scanning mirror was chosen, from among a number of superquadric shapes, as the one that results in the lowest overall side-lobe levels.
This work was done by Ziad A. Hussein of Caltech and Ken Green of Microwave Engineering Corp. for NASA's Jet Propulsion Laboratory. NPO-30506
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Dual-Frequency Airborne Scanning Rain Radar Antenna System
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