In this paper, a novel ultra-wide-band antenna backed by a suspended ground for an enhancement of bandwidth and a frequency selective surface (FSS) reflector for the enhancement of gain is proposed in order to meet the specific coverage requirements for broadband applications. The impedance bandwidth of the proposed antenna is 13.4 GHz (1.8–15.2 GHz). To enhance the gain of the antenna, a FSS reflector is employed below the suggested ultra wide-band antenna. A novel FSS, which consists of 5 × 5 array of elements, located at a distance of 1.01 λ (36.14 mm) below the proposed antenna. There is 4–5 dBi improvement in antenna gain after application of the FSS. In order to observe the signal correlation of the proposed antenna, the time domain analysis using similar antennas in face-to-face and side-to-side scenarios has been performed using the EM simulation tool CST-STUDIO. The simulated results of the proposed antenna exhibit good agreement with the experimental results of the prototype model antenna.

Tunable microwave devices will play an important role in future wireless systems, in which high-frequency bands, e.g. millimeter waves, will become promising, due to its huge spectrum availability. At such frequency bands, liquid crystals (LCs) exhibit low microwave loss, which is excellent compared with the other tuning elements. In this paper, LC-based microwave components are fabricated by using low temperature co-fired ceramic technology, allowing the integration of the LC into microwave structures. The first component, the amplitude tuner, controls the signal's amplitude by using the interference concept, which exhibits a tunable attenuation range from 11 dB to 30 dB at 30 GHz. The second component is a 3-pole tunable bandpass filter, which is realized by using a substrate integrated waveguide topology, enabling a device with comparatively high-quality factors (Q-factors). The measurement results show Q-factors in the range of 68 to 100 for a frequency tuning of 29.4–30.1 GHz, i.e. a tuning range of 700 MHz, accompanied by an insertion loss 2 dB to 4 dB.

In this paper, a slow-wave transmission line implemented in coplanar waveguide technology, based on simultaneous inductive and capacitive loading, is presented for the first time. The shunt capacitors are achieved by periodically etching transverse strips in the back substrate side, connected to the central strip through metallic vias. The series inductors are implemented by etching rectangular slots in the ground plane. The effect of these reactive elements is an enhancement of the effective shunt capacitance and series inductance of the line, leading to a significant reduction of the phase velocity (slow-wave effect). Consequently, the guided wavelength is also reduced, and these lines can be applied to the miniaturization of microwave components. Moreover, due to periodicity, these artificial lines exhibit stop bands (Bragg effect) useful for spurious or harmonic suppression. A compact harmonic suppressed power splitter, based on a slow wave 35.35 Ω impedance inverter, has been designed and fabricated in order to demonstrate the potential of the proposed approach. The length of the inverter is 48% the length of the conventional counterpart, and measured power splitting at the first (3f0) and second (5f0) harmonic frequencies is rejected more than 49 and 23 dB, respectively.

High-resolution imaging with an airborne synthetic aperture radar (SAR) calls for precise trajectory measurements that can hardly be achieved with common navigation systems. In this paper, an efficient method called the local-quadratic map-drift autofocus is developed for the estimation of residual (uncompensated) motion errors directly from the received radar data. The map-drift autofocus is applied locally on short time intervals to estimate the cross-track components of the aircraft acceleration. The estimated acceleration is then integrated to evaluate the residual trajectory errors on the whole data frame interval. The method has been successfully tested with an X-band airborne SAR system.

In this paper, a novel approach of high-accuracy calibration (HAC) method is employed to improve the resolution of tumor detection within a fibro-glandular breast model, and also an improved 3D back-projection approach to scan each focal point inside of the breast is presented. For these purposes, a simulated hemispherical setup of a multi-static array with a modified UWB bowtie antenna is applied around the breast. The superiority of the proposed HAC method is that all-time delays of multi-static channel paths are taken into account at preprocessing of reflected signals, and therefore the time location of tumor response can be estimated accurately in the late stage of recorded signals. As a result, stronger signals are obtained to detect the location of the tumor with higher spatial accuracy. By using the improved 3D back-projection method, a better approximation of transmission channel paths based on Fermat's principle is achieved. A realistic breast model is proposed with two cases of single and twin spherical tumors. Then, to validate the efficiency of the proposed HAC method to detect the time-dependent tumor location, several scenarios are studied in the mentioned model. Quantitative metrics of successfully reconstructed tumor (with a small radius of 7 mm) images prove the ability of the proposed imaging method for early-stage breast cancer detection.

The present work describes a unique planar low-profile wideband circularly polarized Multiple-Input and Multiple-Output (MIMO) antenna operating in the X-band, with pattern and polarization diversity over the entire axial-ratio bandwidth (ARBW). The design is unique in the sense that a simple grounded stub introduced between two linearly polarized monopole antennas has been used to realize wideband circular polarization, pattern diversity, and high isolation between antennas. The ARBW of the MIMO antenna is 2.45 GHz (8.11–10.56) 3 dB and its impedance matching bandwidth is 3.52 GHz (8.07–11.59). The isolation is better than 20 dB. The antenna can be easily adapted to operate other frequency bands by simple frequency scaling. It has been fabricated on an FR-4 substrate and its performance has been compared against several existing available antennas.

The next generation mobile communication systems impose challenging performance, size, and cost requirements on the power amplifiers (PAs). This paper presents novel DC-biasing circuits, which are compact and yet can control the harmonics almost arbitrarily. The proposed circuit consists of a composite right-/left-handed (CRLH) transmission line (TL) stub, of which the size and harmonics-control function can be tuned by modifying the dispersion diagram of the stub line. As a proof of concept, a compact 2-GHz 7-W GaN HEMT class-F PA using the versatile CRLH-TL stubs was fabricated, demonstrating 85.8% drain efficiency and 77.3% power-added efficiency.

The aim of this study is to increase the capacity of the 1090 MHz secondary surveillance radar (SSR) channel by novel transmission and decoding techniques. In particular, multilateration stations have omni-directional antennae; hence considering aircraft traffic and including the extension of surveillance to cooperating vehicles, the reception of superimposed signals is more and more probable. We propose mitigating this problem by combining frequency agility and source separation techniques; we analyze the pertaining channel capacity improvement, and then evaluate the detection and timing estimation for frequency-shifted and separated signals.

The problem of the plane H-polarized electromagnetic wave scattering by flat pre-fractal impedance strips' gratings is examined. For this purpose, a mathematical model in the form of the first kind singular integral equation system is modified for correct usage. Considerable attention is focused on the asymptotic model of the H-polarized electromagnetic wave scattering by sparsely filled grating, which has an explicit solution. The scattered electromagnetic field in the far-zone is considered in details. Dependences of scattering integral characteristics on values of the strip impedance and angles of the plane H-polarized electromagnetic wave incidence upon the grating are studied.

In this work, a new compact, low profile, frequency, and end-fire pattern reconfigurable antenna is presented. The proposed antenna consists of four parasitic elements and an electric-inductive-capacitive (ELC) resonator enclosed with a closed ring resonator (CRR). The reconfigurability in the proposed antenna is achieved with the help of five PIN diodes (D1–D5) embedded on the top surface of the substrate. The diode (D1) is implanted between ELC and CRR resonators for frequency reconfigurability. The other four diodes (D2–D5) are implanted between the ground plane and four parasitic elements to control the electrical length of the ground plane to achieve pattern diversity. The ground plane and parasitic elements steer the primary omni-directional beam to bi-directional and uni-directional end-fire radiation at multiple frequencies. The proposed antenna exhibits multiband operation and end-fire pattern diversity depending upon the different states of PIN diodes. The overall size of the proposed antenna is 0.20λ0× 0.17λ0× 0.009λ0, where λ0 is calculated at the lowest resonance frequency. The impedance bandwidth of the antenna ranges from 1.45 to 26.22%, while peak gain varies from 0.86 to 3.86 dBi depending upon the state of operation. The measured results are in agreement with the simulated results, which confirm the frequency and pattern diversity performance of the antenna. The proposed antenna can be used in back-to-back repeater systems.

Amplifiers for the next generation of T/R modules in future active array antennas are realized as monolithically integrated circuits (MMIC) on the basis of novel AlGaN/GaN (is a chemical material description) high electron mobility transistor (HEMT) structures. Both low-noise and power amplifiers are designed for X-band frequencies. The MMICs are designed, simulated, and fabricated using a novel via-hole microstrip technology. Output power levels of 6.8 W (38 dBm) for the driver amplifier (DA) and 20 W (43 dBm) for the high-power amplifier (HPA) are measured. The measured noise figure of the low-noise amplifier (LNA) is in the range of 1.5 dB. A T/R-module front-end with mounted GaN MMICs is designed based on a multi-layer low-temperature cofired ceramic technology (LTCC).

A single-layer coaxial-fed compact rectangular microstrip antenna with very low voltage standing wave ratio (VSWR) is presented in this paper. The simulated VSWR of the proposed antenna 1.00374 is obtained near the center frequency of the operating band (3.5 GHz). Simulation and measurement results indicate that the bandwidth (simulated: 3.36–3.715 GHz, and measured: 3.295–3.645 GHz) of the antenna exceeds 10% below VSWR 2, when the size reduction of the antenna is about 81.6%. The realized peak gain is obtained about 2.15 dBi at 3.5 GHz. For the verification of the computational results, two designs were fabricated and measured. Good agreements between simulated and measured results were found.

This work describes the design of a rectenna array exploiting orthogonal, closely-spaced UHF monopoles for orientation-independent RF energy harvesting to energize a passive tag, designed for UWB localization, with wake-up radio (WUR) capabilities. To reach this goal, different RF networks are studied to simultaneously realize RF decoupling of the antenna elements and matching of the radiating elements to the non-linear network of rectifiers. The design is performed for a wide power range of the RF incoming signals that need to be exploited for both energizing the passive tag and for providing energy autonomy to a WUR sub-system, used to minimize the long-term power consumption during tag standby operations. Two meandered cross-polarized monopoles, located in close proximity, and thus highly coupled, are adopted for orientation-insensitive operations. The combining RF network is reactive and includes an unbalanced power divider to draw a fraction of the harvested energy to a secondary way for WUR operations. The performance of the harvester is first optimized by EM/non-linear co-design of the whole system over an interval of low RF power levels. The system has been realized and experimentally validated: the superior results obtained, in terms of both dc voltage and power, with respect to a standard single-monopole rectenna, justify the deployment of the presented tag for the energy autonomy of future generation radio-frequency identification tags for indoor localization.

An innovative and simple method for attaining broadband frequency reconfigurable antenna structure is presented using low-cost materials and a compact design. The frequency reconfigurability is attained by the “OFF” and “ON” mechanisms of three PIN diodes. Many performance observations are carried out such as reflectance coefficient, bandwidth (BW), resonance frequency, electric field, feed position variation, and gain among different configurations. The suitability of the presented work for the different applications lies in the X frequency band. The resonating frequency for all switch OFF modes is achieved at 13.2 GHz and one switch ON mode at 10.7 GHz. The proposed design yields the frequency tunability behavior over the broadband of 2.5 GHz. The design yields the directivity of 5.58 dB, the minimum reflectance coefficient of −17.27 dB, and a total gain of 3.87 dB. This design offers an electric field of 46 558 v/m, a BW of 340 MHz, and a normalized directivity of 87° using low-cost substrates. The results of the presented design were also fabricated and related to simulated results. Performance observation of the proposed work with previously published research work is also included. The presented design provides the solution of the low-cost, compact, reconfigurable antennas, which can be used for next-generation communication systems.

In this paper, a novel microwave oscillator incorporating miniaturized nested split-ring resonators is proposed. The high-quality (Q) factor and wide spurious-free band of the NSRR contribute to low-phase noise and high-harmonic suppression of the proposed oscillator circuits. In addition, the NSRR is featured by compact size of 0.12λg × 0.12λg, where λg is the guided wavelength of resonance frequency. The fabricated 2.4 GHz oscillator has an output power of 11.7 dBm with 5 V DC supply and 10 mA current consumption. The second harmonic suppression is −45.49 dBc, the phase noise is −110 dBc/Hz @100 kHz, and the DC–RF conversion efficiency is measured as 30%.

A compact co-planar waveguide (CPW) fed planar elliptical antenna has been designed and presented for super wideband (SWB) characteristics. The designed antenna has an overall size of 30 × 30 × 1.57 mm3, and it consists of an elliptical patch radiator fed using a modified 50 Ω CPW-fed tapered microstrip feed line. By using a semi-ring shaped structure with a tapered feed line, an impedance bandwidth of 180.66% has been observed from 1.27 to 25 GHz with a ratio bandwidth of 19.68:1. To validate simulation results, the designed antenna has been fabricated and measured, and a reasonable agreement has been observed between simulated and measured results. It has also been observed that the designed antenna offers good radiation properties over the entire operating bandwidth. The simulated average gain and radiation efficiency of the proposed SWB antenna is noted to be 4.3 dBi and 95.77%, respectively; while the measured average gain and radiation efficiency is 3.8 dBi and 94.69%, respectively.

A fast imaging algorithm for real-time use in short-range (ultra-wideband) radar with synthetic or real-array aperture is proposed. The reflected field is presented here as a convolution of the target reflectivity and point spread function (PSF) of the imaging system. To obtain a focused 3D image, the proposed algorithm deconvolves the PSF out from the acquired data volume with high speed due to fast Fourier transform and implementation in frequency-wavenumber domain. Then the result is tested against two numerical criteria for efficiency, namely error and instability, whose optimal values can be obtained iteratively. Since the PSF differs with distance, the algorithm suits mainly applications with relatively small objects such as concealed weapon detection. Using several PSFs allows us to image a certain range of interest by their successive deconvolution from the same data. Performance of the algorithm has been evaluated experimentally and compared with that of Kirchhoff migration. Measurements were carried out by a 5–25 GHz synthetic aperture radar in the lab, and scenarios included a gun and a ceramic knife in free space, on a large metal plate, and a gun concealed on a dummy under a thick raincoat. The results demonstrate sufficient image quality obtained in a fraction of time.

A compact printed quadrant shaped monopole antenna is introduced in this paper as a good prospect for ultra wideband- multiple-input multiple-output (UWB-MIMO) system. The proposed MIMO antenna comprises two perpendicularly oriented monopoles to employ polarization diversity. An open circuit folded stub is extended from the ground plane of each radiating element to enhance the impedance bandwidth satisfying the UWB criteria. Two ‘L’ shaped slots are further etched on the radiator to provide good isolation performance between two radiators. The desirable radiator performances and diversity performances are ensured by simulation and/or measurement of the reflection coefficient, radiation pattern, realized peak gain, envelope correlation coefficient (ECC), diversity gain, mean effective gain (MEG) ratio and channel capacity loss (CCL). Results indicate that the proposed antenna exhibits 2.9–11 GHz 10 dB return loss bandwidth, mutual coupling <−20 dB, ECC < 0.003, MEG ratio ≈ 1, and CCL < 0.038 Bpsec/Hz, making it a good candidate for UWB and MIMO diversity application.

In the framework of the Top Amplifier Research Groups in a European Team (TARGET) project, we developed a new electrical method for the temperature measurement of HEMTs and performed several unique studies on the self-heating effects in AlGaN/GaN HEMTs. This method, in combination with transient interferometric mapping (TIM), provides a fundamental understanding of the heat propagation in a transient state of HEMTs. The AlGaN/GaN/Si HEMT thermal resistance was determined to be ~70 K/W after 400 ns from the start of a pulse, and the heating time constant was ~200 ns. Our experimental methods were further applied on multifinger high-power AlGaN/GaN/sapphire HEMTs. The TIM method indicates that the airbridge structure serves as a cooler, removing approximately 10% of the heat energy. In the next study we used TIM and the micro-Raman technique to quantify thermal boundary resistance (TBR) between different wafer materials and GaN epi-structure. We found TBR to be ~7 × 10−8 m2K/W for GaN/Si and ~1.2 × 10−7 m2K/W for GaN/SiC interfaces. The role of TBR at the GaN/sapphire interface was found to be less important.

Multipath propagation occurs in many situations of radar measurements in complex environments. The unwanted effects range from interference over the radar channels, which causes amplitude fading and a corrupted direction of arrival (DOA) estimation, to the detection of ghost targets in an angle of arrival of the multipath direction. Due to the different number of reflections, polarimetric radars are capable to separate certain multipaths from direct paths if the target is known in advance. Furthermore, it is possible to separate objects with different polarimetric features in DOA that are located in the same radial distance to the radar. In this paper, a new approach to DOA detection based on the coherent Pauli decomposition is presented. With this approach, important multipath and DOA effects are analyzed and measurement results at 77 GHz on canonical objects in an anechoic chamber are presented. The results prove the feasibility of the approach and demonstrate the occurring effects.