Smaller Antenna Aperture Draws Interest from Military, Radar Technology Contractor

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  • By Brent Holmes
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ROME, New York --  The Air Force Research Laboratory identified and implemented an effective method of reducing the size of the antenna aperture for Very High Frequency (VHF) Synthetic Aperture Radar (SAR) systems. Under a Small Business Innovation Research (SBIR) project, two antenna elements were used to form a beam while maintaining a high level of signal fidelity in the desired direction. The two antennas are combined using a novel phase steering mechanism that directs energy from horizon to nadir while simultaneously placing a null at the direction of the conjugate beam. The planar wideband phased-array is capable of pointing a beam at a desired depression angle on both sides of the aircraft over the entire bandwidth. A 10 decibel (dB) goal was initially set front to back ratio to make it effective for imaging.

During Phase II of this Air Force program, a flight-worthy prototype of this system was constructed. The electrical and mechanical characteristics of this system also drew interest from another military service and prime contractor for a radar technology program. The success of this SBIR project was further enhanced by the inter-directorate cooperation between the Air Force Research Laboratory's Information and Sensors Directorates, as well as the active sponsorship of the Aeronautical Systems Center's Advanced Imagery Technology Branch.

VHF is a useful operating band for SAR systems for two main reasons: the radar hardware at these frequencies is relatively inexpensive, and the wavelengths allow foliage and ground penetration. However, antenna design is a perpetual problem with VHF SAR systems. In order to produce high fidelity images, SAR systems require high isolation between the antenna main beam and the conjugate beam. The conjugate beam is energy radiated at the opposing depression angle. In order to achieve this high front-to-back (F/B) ratio, conventional arrays need a large number of elements and or area for a reflector. Operating at VHF, this array would be very large and its installation aboard any platform is permanent because of the inherent difficulty in installing an array of its size. The goal of this program was to develop an electrically small array that could meet the requirements for SAR (high F/B ratio) and could be quickly installed and removed.