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This study investigates non-uniform array configurations to maximize the beamforming performance of passive coherent location (PCL) systems. The proposed array consists of eight dipole elements that are divided into two groups with different distances from the array center. This double-layered non-uniform configuration is designed considering the array antenna characteristics (peak-to-side lobe ratio, first null bandwidth, null width, and null depth). The resulting antenna array can provide more appropriate patterns for PCL systems than a conventional uniform circular array. The target detection performance of the proposed array in PCL systems is tested in a certain scenario. The results demonstrate that the proposed antenna in PCL systems can detect the target with Doppler and range errors of 1 Hz and 1.2 km, respectively, in a given situation.

This article proposes the design of a coupled-fed printed dipole antenna for direction finding (DF) systems. The antenna consists of a radiating 50° angled expanded-arm printed dipole as well as a fan-shaped feeder that is located on the opposite side of the radiating dipole. The 50° angled expanded-arm structure results in a wide beam-width and broadband-matching characteristics. The more the arm angle is bent, the wider the beam-width pattern. Note that the expanded structure to the side of the dipole arm allows the wide beam-width of the radiation pattern. The printed dipole is then electromagnetically coupled with the fan-shaped feeder to obtain broadband-matching characteristics. To obtain additional broadband-matching characteristics, the circular cavity and the four-step tapered structures of the radiating printed dipole are carefully tuned by considering the coupled fields induced by the indirect feeding structure. The reflection coefficients are −14.6 dB and −11.9 dB at 1.575 GHz and 2.4 GHz, and the boresight gains are 4.8 dBi and 1 dBi, respectively. To confirm the DF capability of the proposed antenna, the pattern of the 4 × 4 arrays is observed by varying the steering angle.

This paper presents the design of a negative group delay circuit (NGDC) using the filter synthesis approach. The proposed design method is based on a frequency transformation from a low-pass filter (LPF) to a bandstop filter (BSF). The predefined negative group delay (NGD) can be obtained by inserting resistors into resonators. To implement a circuit with a distributed transmission line, a circuit conversion technique is employed. Both theoretical and experimental results are provided for validating of the proposed approach. For NGD bandwidth and magnitude flatness enhancements, two second-order NGDCs with slightly different center frequencies are cascaded. In the experiment, group delay of 5.9±0.5 ns and insertion loss of 39.95±0.5 dB are obtained in the frequency range of 1.935–2.001 GHz.

This paper presents a design of an impedance transformer (IT) with high frequency selectivity characteristics. The frequency selectivity can be controlled by even- and odd-mode impedance of a shunt coupled transmission line (TL). For experimental validation, a 50- to 20- Ω IT was implemented at a center frequency (f0) of 2.6 GHz for the long-term evolution signal. The measured results were in good agreement with the simulations, showing a return loss higher than 19 dB over a passband bandwidth of 0.63 GHz (2.28–2.91 GHz) and good sharp frequency selectivity characteristic near to the passband. The series coupled TL provides a transmission zero at 5.75 GHz, whereas the shunt coupled TL provides three transmission zeros located at 2 GHz, 3.1 GHz, and 7.14 GHz.