Ultrawideband Antenna Arrays for Frequency Invariant Beamforming

Abstract

Beamforming is a technique based on the concept of a spatial filter that concentrates electromagnetic energy in a specific direction by controlling the amplitude and phase of array antenna elements. Ow- ing to these characteristics, beamforming has become a key technology in 5G/6G communications, satellite communications, and electronic warfare (EW) systems. In particular, in EW systems, it is es- sential to operate over a wide frequency range to detect signals radiated at unknown frequencies, and thus beamforming must also maintain wideband operation. However, when array antennas are operated over a wide bandwidth, the beam pattern varies with frequency, which necessitates frequency-dependent control schemes. Typically, an array antenna is designed with half-wavelength spacing at the highest operating frequency to suppress sidelobes. As the frequency increases, the electrical aperture size de- creases, resulting in a narrower beam pattern at high frequency. While a narrow beam provides high gain by concentrating radiated energy, it also requires a larger number of beams to cover a given angular sector during beam scanning. To address this limitation, frequency-invariant beamforming algorithms is applied. Furthermore, to enable reliable wideband direction finding, wideband array antennas and a wide hardware architecture capable of supporting such operation are designed. First, we investigate array-optimization methods for designing wideband array antennas. The frequency- invariant beam algorithm requires a larger number of antenna elements as the operating frequency in- creases, which leads to significant cost and spatial complexity. In particular, for the ultrawideband fre- quency range of 2–18 GHz (9:1), commonly employed in EW systems, generating frequency-invariant beams requires an impractically large number of antenna elements. To overcome this problem, it is necessary to identify the optimal array configuration that achieves uniform beam performance with a minimal number of elements. For this purpose, empirical formulas were derived by analyzing the rela- tionship among beam characteristics, target frequency range, and required number of antenna elements. These formulas were then applied to a nested array method, which overlays multiple arrays to reduce the total number of elements. When applied to a linear array, the proposed approach reduced the required number of antenna elements by approximately 40%. Furthermore, when the number of available antenna elements is fixed, an optimization study is con- ducted to determine the array geometry that achieves the best possible frequency-invariant beam perfor- mance. The performance variation according to element spacing and array configuration in planar arrays is analyzed, and the optimal array geometry satisfying the target beam performance is derived. Second, to increase the probability of detection, a wideband array antenna with polarization diversity is required. To achieve polarization diversity, antennas capable of receiving both vertically and horizon- tally polarized signals, such as 45◦ slant-polarized or circularly polarized antennas, are preferred. To realize wideband circular polarization, an artificial dielectric layer (ADL) is designed and to implement on an aircraft, it is designed as 1-D linear array. By exploiting different boundary conditions, orthogonal phase delays are introduced along two directions. Moreover, the cascaded slabs are designed with a gradient of effective permittivities to provide a gradual impedance transition from the guided mode to the radiating mode, enabling wideband impedance matching and circular-polarization performance. The antenna shows 3-dB axial ratio in 12-17.5 GHz and impedance bandwidth 10.1-20 GHz. Furthermore, to realize a slant-polarized antenna in a 1-D connected linear array, an extended-ground configuration is employed. For a low-profile wideband array implementation, a two-slab structure is adopted, and a wideband array antenna operating over 6–18 GHz is designed. Third, in direction-finding applications using beam scanning, the relationship between the beamwidth, required number of beams, and angular accuracy is analyzed. To detect high-speed targets with a limited number of beams across a wide bandwidth, a frequency-invariant beam approach is adopted. Simulation results confirmed that applying the frequency-invariant beam significantly improves direction-finding accuracy compared to conventional methods, especially at higher frequencies. Meanwhile, for tracking electromagnetic signals radiated from high-speed targets such as missiles, a monopulse algorithm is required to estimate the direction more rapidly than beam scanning. However, the monopulse algorithm exhibits a frequency-dependent unambiguous range that defines the reliable direction-finding region. This range becomes significantly narrower at higher frequencies. To overcome this limitation, the frequency-invariant beam is applied to the monopulse algorithm. As a result, a wide and uniform unambiguous range was achieved across the entire frequency band. Fourth, A new monopulse architecture based on frequency-invariant beams is proposed. The array antenna is divided into multiple subarrays, and the relative signal comparison between subarrays enables wide-range direction estimation without beam scanning. The proposed system is implemented as an analog circuit using microstrip transmission lines, eliminating the need for additional RF components, thereby reducing cost and achieving high-speed angle estimation through simplified computation. This approach enables direction estimation over a wide angle, wideband, demonstrating significant potential for next-generation wideband electronic warfare and sensing applications.