Design and development of 2 kW, 3 dB tandem hybrid coupler for the frequency range of 155–225 MHz has been presented in this paper. The developed 3 dB coupler is to be used in a prototype of Ion Cyclotron Resonance Frequency (ICRF) system of Tokamak, which has been developed to test the resilience of ICRF network during continuously variable RF load excursions. The 3 dB coupler divides the RF power between two antennae of the prototype and protects the RF source by coupling of reflected power to the isolated port. The developed coupler shows excellent coupling flatness of −3 ± 0.3 dB over 38% of fractional bandwidth and also provides voltage standing wave ratio (VSWR) <1.3, isolation better than 32 dB and return loss better than 25 dB in full band. The presented work establishes a technique which can be useful for the development of high-power hybrid coupler in the range of high frequency (HF), very high frequency (VHF) and ultra high frequency (UHF).

A novel online antenna array calibration method is presented in this paper for estimating direction-of-arrival (DOA) in the case of uncorrelated and coherent signals with unknown gain-phase errors. Conventional calibration methods mainly consider incoherent signals for uniform linear arrays with gain-phase errors. The proposed method has better performance not only for uncorrelated signals but also for coherent signals. First, an on-grid sparse technique based on the covariance fitting criteria is reformulated aiming at gain-phase errors to obtain DOA and the corresponding source power, which is robust to coherent sources. Second, the gain-phase errors are estimated in the case of uncorrelated and coherent signals via introducing an exchange matrix as the pre-processing of a covariance matrix and then decomposing the eigenvalues of the covariance matrix. Those parameters estimate values converge to the real values by an alternate iteration process. The proposed method does not require the presence of calibration sources and previous calibration information unlike offline ways. Simulation results verify the effectiveness of the proposed method which outperforms the traditional approaches.

A new time reversal (TR) method for target imaging is proposed in this paper. Through single measurement by the antenna array, the received signals are utilized to form the space–frequency–frequency multistatic data matrix (MDM). Singular value decomposition is applied to the matrix to obtain the left singular vectors which span the signal subspace. The obtained vectors are divided into multiple subvectors by two different schemes and used to provide target signatures in the form of coarse frequency dependence and relative phase shifts that can be exploited to construct the imaging function. The performance of the proposed method is investigated through numerical simulations for both single and multiple targets, and the results are compared with the traditional TR method using the frequency–frequency MDM. It turned out that the proposed method is able to achieve high resolution with limited array aperture and shows satisfactory robustness in noise environment. Furthermore, experimental results are provided to show the availability of the method in practical applications.

In this paper, a new path-loss model for electromagnetic wave in an indoor multipath environment is proposed based on matching coefficient, polarization matching factor, and normalized field intensity direction function. This model is called the Friis-extension (Friis-EXT) model, because it operates as the Friis model under certain conditions. In addition, in the modeling process of the path-loss in an indoor environment, the reflective surfaces in the environment and form of the antenna are considered. Afterwards, the path-loss data in an indoor corridor environment are measured, and the maximum error between the theoretical value and the measured data is <7.5 dB. Finally, the Friis-EXT model is compared with some other traditional models, and the results show that the Friis-EXT model is the best one that matches the measurement data.

Modern telecommunications are moving towards (massive) multi-input multi-output (MIMO) systems in 5th generation (5G) technology, increasing the dimensionality of the systems dramatically. In this paper, the impairments of radio frequency (RF) power amplifiers (PAs) in a 3 × 3 MIMO system are compensated in both the time and the frequency domains. A three-dimensional (3D) time-domain memory polynomial-type model is proposed as an extension of conventional 2D models. Furthermore, a 3D frequency-domain technique is formulated based on the proposed time-domain model to reduce the dimensionality of the model, while preserving the performance in terms of model errors. In the 3D frequency-domain technique, the bandwidth of the system is split into several narrow sub-bands, and the parameters of the model are estimated for each sub-band. This approach requires less computational complexity, and also the procedure of the parameters estimation for each sub-band can be implemented independently. The device-under-test consists of three RF PAs including input and output cross-talk channels. The proposed techniques are evaluated in both behavioral modeling and digital pre-distortion (DPD) perspectives. The experimental results show that the proposed DPD technique can compensate the errors of non-linearity and memory effects in the both time and frequency domains.

Microwave holography technique reconstructs a target image using recorded amplitudes and phases of the signals reflected from the target with Fast Fourier Transform (FFT)-based algorithms. The reconstruction algorithms have two or more steps of two- and three-dimensional Fourier transforms, which have a high computational load. In this paper, by neglecting the impact of target depth on image reconstruction, an efficient Fresnel-based algorithm is proposed, involving only one-step FFT for both single- and multi-frequency microwave imaging. Numerous tests have been performed to show the effectiveness of the proposed algorithm including planar and non-planar targets, using the raw data gathered by means of a scanner operating in X-band. Finally, a low-cost and high-speed hardware architecture based on fixed-point arithmetic is introduced which reconstructs the planar targets. This pipeline architecture was tested on field programmable gate arrays operating at 200 MHz clock frequency, which illustrates more than 30 times improvement in computation time compared with a computer.

This paper describes the development of a power amplifier operating over a 2.4–2.5 GHz frequency range with the output power level more than 15 W and 60% PAE. The transistor applied was the 10 W (13 W Psat) power GaN HEMT (CGH40010F from Wolfspeed) recommended up to 6 GHz. A harmonic tuning method was used to achieve even 30% more output power than the CGH40010 transistor was specified to deliver while maintaining high gain and high efficiency. Furthermore, an accuracy analysis of amplifier design was also conducted. It included validation and correction of the available transistor models as well as validation of the models of microstrip circuits implemented in ADS. Finally, it was concluded that both the mentioned sources of errors contributed at a similar level.

Guaranteeing charge conservation of empirically extracted Gallium Nitride (GaN) High-Electron-Mobility Transistor (HEMT) models is necessary to avoid simulation issues and artifacts in the prediction. However, dispersive effects, such as thermal and charge-trapping phenomena, may compromise the model extraction flow resulting in poor model accuracy. Although GaN HEMT models should be extracted, in principle, from an isodynamic dataset, this work deals with the systematic identification of an approximate, yet most suitable, charge-conservative empirical model from standard multi-bias S-parameters, i.e., from non-isodynamic data. Results show that the obtained model maintains a reasonable accuracy in predicting both small- and large-signal behavior, while providing the benefits of charge conservation.

The scattering of the H-polarized plane electromagnetic wave by a finite multilayer graphene strip grating is considered. The properties of the whole structure are obtained from the set of integral equations, which are written in the operator form. The scattering operators of a single layer are used and supposed to be known. Scattering and absorption characteristics as well as diffraction patterns are presented.

Though compact polarimetric approaches have been developed and applied in space and geo researching systems they have not been taken into consideration in automotive applications, yet. A sensor system has been designed to conduct polarimetric measurements in the 77 GHz frequency band, which is permitted for automotive usage. This system is able to transceive linearly as well as circularly polarized electromagnetic continuous waves. Depending on the case of application, the frequency output can be set statically or modulated over time within adjustable parameters. Hence, a variety of compact polarimetric modes can be performed and compared with full polarimetric approaches. Two compact polarimetric modes, dual-circular polarimetric mode, and circular-transmit-linear-receive, will be introduced and applied in this contribution. Their operability in this frequency range will be investigated after the microstrip antennas as well as the beam focusing dielectrical lense are characterized. Finally, results of a realistical measurement set-up will confirm the practicability of compact polarimetric approaches for double bounce recognition.

In this paper, the design theory of an 8 GHz oscillator with a new structure of active dielectric resonator (DR) is presented. The new structure emphasizes on phase noise reduction by using only one active device. The proposed structure uses additional feedback from transistor to resonator in order to increase the quality factor. Measurement results report that phase noise is reduced to −145.19 dBc/Hz at 100 kHz offset frequency which represents 12 dB improvement compared with oscillators with passive DR. Also, in comparison with conventional active resonator oscillators, noise source of the second amplifier which makes spurious oscillation is removed. The size and power consumption are reduced due to the use of a single transistor. This structure has the lowest phase noise in comparison with other DR oscillators. In order to implement the proposed oscillator, a circuit including amplifier, resonator, coupler, and phase shifter is designed and realized.

This paper presents a hybrid design of 1 × 2 and 1 × 4 arrays operating in 0.277–0.292 THz on 65 nm Complementary metal–oxide–semiconductor (CMOS) technology. Each of the chips has an X-band input with 3 ×3 multiplier stages and connected at the output to an on-chip ring antenna. A wideband microstrip Wilkinson four-way and two-way power dividers have been developed on a multilayer printed circuit board to feed the array elements with proper radio frequency and direct current inputs. Demonstrating improvements in effective isotropically radiated power and in total radiated power compared to a single CMOS element, the hybrid integration approach proves effective in implementing coherent THz transmitter arrays. Theoretical and practical factors that reduce the radiated power compared with ideal arrays are also discussed.

A series of novel wideband filtering power dividers (WFPDs) with wide stopband rejection performances is proposed in this paper. The proposed WFPD structure consists of a parallel-coupled line, two transmission line (TL) sections, four loading terminations, and an isolation resistor. The coupled line is applied at the input port to provide wideband impedance transformation, while different types of loading terminations and TL sections are adopted to realize various out-of-band rejection performances. To verify the proposed concepts, three WFPDs operating at 3.0 GHz are designed and fabricated with 3 dB bandwidth of 79.7, 79.0, and 74.4%. In WFPD1, the measured out-of-band rejection of better than 13.4 dB extends to 2.57f0. Moreover, the measured out-of-band rejection of better than 17 dB extends to 2.47f0 (4.75f0) in WFPD2 (WFPD3), respectively. Good agreements between the simulated and measured results validate the presented ideas.

A flexible and adaptive energy-efficient high-speed wireless hub is developed in polymer foil as a Hybrid System-in-Foil (HySiF) using Chip-Film Patch (CFP) technology. In this matter, the SiGe BiCMOS silicon chips (2.39 × 1.65 mm2) are thinned down to 45 μm and are embedded face-up inside a two-polymer CFP carrier. The active pads of the embedded silicon chips inside foil are extended to the surface of the foil to interconnect to the antenna on the foil. The integrated hybrid system has a signal transmission at 5–6 GHz frequency band. The overall thickness of the system is below 100 μm and its bendability is down to 4 mm radius of curvature. The designed and fabricated PA silicon chips operate at 50 mA with a 1.5 V supply voltage. Therefore, in addition to the high lateral thermal resistance of the thinned chip, self-heating loop inside polymer due to the low thermal conductivity of the embedding polymer raises the system temperature. Consequently, the thermal behavior and RF performance of the PA chip under different conditions are investigated. Moreover, the antenna with the required carrier frequency is simulated, fabricated, and measured on top of the polymer foil as a stand-alone system in the flexible CFP.

The measurement accuracy of low-frequency narrow-band antenna is heavily influenced by its environment, which is also difficult to remove the clutter with a time gating. This paper proposes a method to improve the measurement accuracy of low-frequency narrow-band antenna using signal processing technique. The method is to predict the unknown value out of received original signal with an auto-regressive model (AR model) based on modern spectral estimation theory, and the parameters in AR model are calculated by maximum entropy spectral estimation algorithm. Thus, a wideband signal compared with the original band is obtained, and then the time-domain resolution is enhanced. The time gating is more exactly to separate the antenna radiation signal from multipath signals. The simulation and experimental results show that about 50% extended data for each ends of original band can be obtained after spectral extrapolation, and the time-domain resolution after extrapolation is twice than the original narrow-band signal, and the influence of measurement environment can be eliminated effectively. The method can be used to improve accuracy in actual antenna measurement.

A compact high-isolation power divider with bandpass response and high-frequency selectivity is presented in this letter. Two dual-mode resonators are used to realize filtering response. The circuit size of the proposed power divider can be reduced by using dual-mode capacitance loaded square meander loop resonators. Due to capacitive load, the resonator can exhibit slow-wave characteristics, which can be utilized to suppress harmonics and reduce size. The simulated and measured results show reasonable agreement.

Accurately and efficiently modeling the drain-lag effects is crucial in nonlinear large-signal modeling for Gallium Nitride high electron mobility transistors. In this paper, a simplified yet accurate drain-lag model based on an industry standard large-signal model, i.e., the Chalmers (Angelov) model, extracted by means of pulsed S-parameter measurements, is presented. Instead of a complex nonlinear drain-lag description, only four constant parameters of the proposed drain-lag model need to be determined to accurately describe the large impacts of the drain-lag effects, e.g., drain-source current slump, typical kink observed in pulsed IV curves, and degradation of the output power. The extraction procedure of the parameters is based on pulsed S-parameter measurements, which allow to freeze traps and isolate the trapping effects from self-heating. It is also shown that the model can very accurately predict the load pull performance over a wide range of drain bias voltages. Finally, the large-signal network analyzer measurements at low frequency are used to further verify the proposed drain-lag model in the prediction of the output current in time domain under large-signal condition.