Transmit Beamspoiling for Airborne Phased Array Radar Antenna

The University Corporation for Atmospheric Research (UCAR) is presently studying architectures and technologies for Airborne Phased Array Radar (APAR) instrumentation to replace the retired ELDORA X-band radar platform. The proposed dual-polarized C-band radar will utilize large active electronically steered arrays (AESA’s) installed on the top, sides and bottom of a C-130 airplane. A preliminary Technical Requirements Document was recently circulated for the purpose of receiving comments from industry. A maximum transmit beamwidth of 2.2 degrees was specified, with a stated desire to widen (or “spoil”) the transmit beam to 20 degrees in elevation for certain rapid measurements. In this mode, the broadened transmit fan beam would be paired with 12 simultaneously formed and paritially overlapping receive beams, each approximately 2.2 degrees wide, to cover a large volume at once. This presentation examines the challenges involved with producing high quality spoiled transmit beams. It presents an unconventional design approach that produces a power-efficient fan beam with excellent ripple control.

The conventional, and widely-used, approach to elevation beam spoiling is to apply quadrature phase weighting vertically across the AESA elements. While this does, indeed, spread the beam, the resulting radiation pattern has excessive amplitude variations (more than 3 dB) and very large sidelobes. To improve this performance, we adapt a method presented over 20 years ago by Kinsey to calculate phase weights that steer power from each radiating element towards a desired direction. By using a set of desired directions that define a uniform range of angles, a broadened beam is produced. Since the method steers power element by element, it has the side benefit of correctly accounting for variations in element power due to amplitude weights or taper. We therefore choose to add a small amplitude taper to reduce amplitude variations across the broadened beam, and to suppress sidelobes outside of the main beam.

The addition of a modest 22 dB Taylor taper, spread by non-quadratic phase weights that are calculated with Kinsey’s method, produces a 20 degree main beam with just ±0.5 dB maximum amplitude ripple. The first one-way sidelobe is nearly 25 dB down, with additional sidelobe suppression provided by the receive beams. The presentation will include details of the design approach applied to the squared-off elliptical AESA found in the notional APAR Requirements document, with plots of the resulting beam performance.