Ultrawideband and wide-angle scan antenna element and small active phased arrays

Abstract

Wideband, wide-scan active phased arrays (APA) are widely used for multi-function radars, communication and electronic warfare. Although considerable research is available on the design and analysis of such antenna elements and arrays with large aperture, multi-octave wide scan phased antenna arrays with an electronically small aperture finds major application in electronic warfare (EW), is not well-developed. Based on a detailed literature survey on ultrawideband antenna for the wideband APA this research began with investigating different designs of tapered slot antenna (TSA) elements. A study of the performance parameters like bandwidth, beamwidth, cross-polarization and beam-squint have been conducted for candidate antenna elements such as microstrip fed TSA, balanced antipodal TSA and stripline fed TSA (STSA). Based on these studies, STSA was selected as the most suitable candidate for meeting the requirement for antenna element. An STSA element with a wide beamwidth of 120° in H-plane with reflection coefficient less than -10 dB is designed. Two different antenna arrays are proposed in this research to demonstrate the advantages of STAS as small aperture active array element for wideband phased arrays. Challenges in the design, analysis and characterization of moderately sized phased arrays using STSA element have been analyzed. Although seof interest in literature, of these a linear active phased array antenna that operates across 6 to 18 GHz frequency range, an array with wide-scan coverage of ±60° in H-plane is not developed. The element pattern in phased arrays causes a squint in the main beam away from broadside. For small phased arrays this can be reduced by on-line calibration by taking a sample of signal at each element of the array. Therefore STSA is integrated with a -20 dB asymmetrical coupler at the feed for calibration and performance monitoring. In addition, the proposed STSA with integrated coupler is designed to have a wideband transition which allows a direct 50 Ω stripline feed. The isolated port of the coupler is internally terminated with a resistor using Ohmegaply resistive layer to improve reliability in airborne platform. A linear array of 16 elements is designed and experimentally demonstrated. The main figure of merit of the array is realised gain of the array over the scan angle. A minimum element gain of 0 dB in array environment is obtained over the scan angle of ±60°, without any grating lobes at high frequency. It also demonstrated a cross-polarization of better than -25 dB at boresight and beam squint less than 1°. A staggered feeding arrangement is incorporated to accommodate the physical dimensions of connectors for minimum inter element spacing. Conventionally the centre element pattern in array environment is used for obtaining the scanning characteristics of a large phased array. But, for small phased array, since the edge effects may be significant an average scan element pattern is proposed for array characterization. This gives an accurate result for the scan characteristics of array and can be used to correct array synthesis before full scale fabrication. This approach is validated by measuring the array pattern of the fabricated array with transmit/receive module. Second, an 8 element E-plane array operating over 1 to 6 GHz frequency range is proposed with scanning of 45° from broadside. This antenna covers D, E, F and G bands of EW system. A novel STSA has been investigated across this bandwidth. Here the radiating part of tapered slot antenna is ended with an elliptical stub instead of circular stub which gives the same performance that of circular stub with reduced overlap with the radial stripline stub. A compact broadband Wilkinson power divider feed network is designed along with array to characterise this. These edge effects are more severe in this array, especially at the low frequency of the operation band and methods to overcome this are investigated. Approaches of additional dummy edge elements or modifying edge elements have been investigated by simulations to reduce edge effects. An approach of adding 3D caps at the edges has been found to be more effective in this regard. To summarize, methods for designing high performance multi-octave stripline fed tapered slot antenna elements and their arrays in the E- and H-plane have been developed using simulations and validated by experiments. These antennas are particularly suited as small aperture arrays in EW applications.