This paper presents a continuous frequency tuning Fabry–Perot Cavity (FPC) antenna of which the operating frequency can be varied from 4.87 to 5.84 GHz (18.1%). The cavity of the proposed antenna is excited by a double-layer antenna structure, which is made up of a main square patch and a parasitic one. The superstrate is a frequency-selective surface, consisting of 6 × 6 square-patch-type unit cells. The frequency tuning property is realized by employing 48 phase-changing elements placed around the main patch, which forms a reconfigurable high impedance surface (HIS). By controlling the biasing voltages of the varactors inserted in the HIS element, its capacitances and reflection phases can be varied continuously, which leads to a variation of the operating frequency. An antenna prototype has been fabricated and measured for validation. The measured results are in good agreement with the simulated ones. In the frequency tuning range, the measured realized gains have a smaller variation from 10.2 to 14.1 dBi compared with other reported frequency tuning FPC antennas.

A frequency reconfigurable microstrip patch antenna with a combination of an asymmetric armed U-slot and a reversed L-slot etched on a rectangular base patch of 6 GHz resonant frequency designed on an FR4 substrate (ɛr = 4.4) is presented in this paper. Three RF PIN diodes are positioned at inimitable sites of these slots to achieve frequency reconfiguration. A DC bias circuitry, which includes DC blocking capacitors and RF blocking inductors, is integrated with the antenna structure for switching (ON/OFF) the PIN diodes. Six reconfigurable modes with resonant frequencies at 4.33, 4.63, 5.24, 5.87, 5.96, and 6.29 GHz is obtained with different ON-OFF combinations of these PIN diodes. These reconfigurable resonant frequencies cover two continuous bands from 4.21 to 5.43 GHz and 5.69 to 6.6 GHz and is considered to be useful for the applications like aeronautical radio navigation (4.3 GHz), sub-6-GHz 5G (4.4–5 GHz), WLAN (5.2 and 5.8 GHz) and Wi-Fi 6E (5.925–6.425 GHz). Measured gain of the antenna for all the modes varies between 5.86 dBi and 8.72 dBi.

Short-channel Gallium Nitride (GaN) high-electron-mobility transistors (HEMTs) often utilize T-shape gates due to their large gate-line cross-sectional area and subsequent fMAX increase. In this paper, we report the linearity trade-offs associated with varying the T-gate geometries of AlGaN/GaN HEMTs on Si, specifically the gate extensions which serve as field plates and their impact on the large-signal performance. Small-signal characterization and modeling, in addition to TCAD, provide initial guidelines for the optimal dimensions for the gate field plates using the ratio of fT and the product of the gate resistance and the gate-to-drain capacitance. We utilize various characterization methods, including 6 GHz non-linear vector network analyzer characterization in addition to load-pull, to quantify the amplitude and phase distortion and their subsequent impact on the large-signal metrics of the devices under differing matching conditions and bias points. We deduce that the influence of the gate field plates on the amplitude and phase distortion is non-negligible, particularly under matched conditions.

This paper presents a miniaturized, tunable, high-power, eight-port hybrid coupler, based on a lumped element hybrid coupler topology. The 90° hybrid couplers are ubiquitous elements used for feeding RF coils in quadrature in magnetic resonance imaging (MRI) systems. Due to the low Larmor frequency (64 MHz) of 1.5 T MRI, distributed elements are too large for practical circuits to drive multi-port RF coils. Thus, miniaturization with MRI-compatible, non-magnetic, and high-power components is necessary. First, a miniaturized hybrid coupler is proposed for MRI systems with non-magnetic variable capacitors. Afterwards, the miniaturization methodology is applied to develop an eight-port coupler, capable of supporting both a transmit and a receive quadrature RF coil system. The high-power (up to 1 kilowatt) extended coupler measures 10 cm × 6 cm. Test results show that each port has a return loss of more than 16 dB, each input-isolated port is isolated by more than 24 dB, and each output has an insertion loss of less than 2.5 dB and output phases of 0.0°, 90.8°, 182.1°, and 278.7°.

This paper introduces a symmetric bar chart-shape (SBCS) microwave sensor for measuring permittivity of fluidic samples. For designing purposes, the introduced sensor was used based on the field changes between the SBCS and rectangular loop microstrip (RLM) structure. When a sample is placed on the sensing location, interaction between SBCS and RLM varies the field intensity. The vinegar samples were combined with water and then they are placed on the sensor. The change in field intensity changes the resonance frequency. However, there is a relationship between the permittivity of samples and the resonance frequencies. The proposed sensor is implemented on the substrate of RO4003C. The relative permittivity of samples changed from 59 to 77 and the resonance frequencies changed from 2.3 to 1.4 GHz. The quality factor is 3544 and the sensitivity is 2.2%.

Due to increasingly complex and automated manufacturing processes, the demands on the control parameters of these processes are also increasing. In many applications, such a parameter is the fill quantity, whose precise determination is of ever growing importance. This paper shows with which accuracy and precision an M-sequence ultra-wideband radar can determine levels in small metallic and non-metallic containers with contact-based and contactless measurements. First, the principle of level measurement using guided wave radar is explained and the measurement setup is described. Afterward, the measurement results are shown and discussed. The measurements show that the level can be measured with an accuracy of better than 0.5 mm. In addition, level fluctuations can be detected with a precision of 3 ${\rm \mu }$m. Based on the results of the guided wave radar, the possibilities of volumetric contactless measurement using an electrically small patch antenna are discussed. A particular challenge in contactless level measurement is the high number of multipath components, which strongly influence the accuracy. In addition, there are near-field effects when measuring close to the antenna. Exploiting these near-field effects, an additional method to accurately determine the full state of the container is investigated.

Two wideband bandstop filters (BSFs) for single and dual-band are proposed and then extended to reflectionless BSFs based on the analysis from input impedance/admittance perspective. Also, topologies of higher-number-stopband input-reflectionless BSF are provided to broaden the design scope. Open/shorted coupled lines are adopted to obtain multi transmission zeros and desired stopband bandwidth by adjusting the even-/odd-mode impedance of coupled lines. Resistor-loaded coupled lines are connected with Port 1 to absorb unwanted signals and obtain input-reflectionless behavior. For validation of the proposed theory analysis, BSFs with corresponding absorptive prototypes are constructed and measured.

This article presents a flexible wearable KIT monopole antenna for biomedical application. A metamaterial unit cell is proposed to improve the antenna performance. The proposed antenna and the metamaterial are fabricated on 1 mm-thick polydimethylsiloxane substrate to operate in the ISM frequency band of 2.45 GHz. Integration of the metamaterial improves the gain and reduces the specific absorption rate (SAR) of the antenna. The overall dimension of the antenna with the metamaterial is 49 × 49 × 19 mm3. The designed antenna is investigated for the loading effect of the body by placing on the hand phantom model. Bending tolerances are also analyzed for x and y direction with various bend radii. Gain and SAR of the proposed antenna are 4.61 dBi and 0.868 W/kg. The results of the fabricated prototype show that the proposed wearable antenna is safe for biomedical applications.

Electromagnetic simulation software has become an important tool for antenna design. However, high-fidelity simulation of wideband or ultra-wideband antennas is very expensive. Therefore, antenna optimization design by using an electromagnetic solver may be limited due to its high computational cost. This problem can be alleviated by the utilization of fast and accurate surrogate models. Unfortunately, conventional surrogate models for antenna design are usually prohibitive because training data acquisition is time-consuming. In order to solve the problem, a modeling method named progressive Gaussian process (PGP) is proposed in this study. Specially, when a Gaussian process (GP) is trained, test sample with the largest predictive variance is inputted into an electromagnetic solver to simulate its results. After that, the test sample is added to the training set to train the GP progressively. The process can incrementally increase some important trusted training data and improve the model generalization performance. Based on the proposed PGP, two monopole antennas are optimized. The optimization results show effectiveness and efficiency of the method.

We propose the techniques for automatic processing of measurement results in the context of golden (typical) device selection and noise figure measurement. These techniques are for golden (typical) device selection and noise figure measurement processing. Automation of measurement result processing and microwave element modeling speeds up a modeling routine and decreases the risk of possible errors. The techniques are validated through modeling of 0.15 μm GaAs pHEMTs with 4 × 40 μm and 4 × 75 μm total gate widths. Two test amplifiers were designed using the developed models. The amplifier modeling results agree well with measurements which confirms the validity of the proposed techniques. The proposed algorithm is potentially applicable to other circuit types (switches, digital, power amplifiers, mixers, oscillators, etc.) but may require different settings in those cases. However, in the presented work, we validated the algorithm for the linear and low-noise amplifiers only.

In this paper, a simple and compact coplanar waveguide (CPW)-fed four coexisting inverted-L-shaped radiating elements based wideband (ILW) multi-input and multi-output (MIMO) antenna has been presented for wearable wireless applications. The 50 Ω CPW feed structure has been used to excite the four inverted-L-shaped radiators in a wide square slot of the proposed four-port wearable MIMO antenna. The four diagonal strips are extended from the corners of the wide square slot to form a square split ring structure in the center, which suppresses the coupling of electromagnetic waves among coexisting compact quad inverted-L-shaped radiators. The proposed ILW wearable MIMO antenna achieves a wide −10 dB simulated impedance bandwidth of 58.18% (3.9–7.1 GHz) and more than 30 dB isolation along with much reduced envelope correlation coefficient (<0.0002), high diversity gain (>9.9 dB), and specific absorption rate lower than 1 W/kg in the entire operational band. The simulated results are in good agreement with the measured ones. This ensures its utility for several wearable wireless applications in the C-band that comprises WLAN (5 and 5.2 GHz), Wi-MAX (5.5 GHz), and ISM band (5.8 GHz) applications.

This paper describes a multiple-input multiple-output (MIMO) antenna for low millimeter (mm)-wave applications based on dielectric resonators. This is the first time that a filtering response is used in conjunction with an MIMO antenna operating at a low mm-wave frequency. The antenna is simulated using an asymmetrical U-shaped aperture and a microstrip line feed. The suggested filtenna has two distinguishing characteristics: (i) the diversity parameters of the proposed MIMO are increased by including pattern and spatial diversity, and (ii) the proposed feed mechanism of a dielectric resonator provides the filtering response. Between the two ports, a metallic plate tilts the radiation pattern by 45°. The anti-parallel locations of the ports increase the isolation value by >30 dB. To validate the performance of the suggested antenna, the proposed filtenna was built and confirmed. The proposed antenna operates between the frequencies 27.9 and 28.5 GHz. Within the operating frequency range, the observed gain is ~4.5 dBi. On the contrary, the gain suppression level beyond the operational frequency range is ~15 dB. The stable radiation properties and high diversity parameter values of the suggested filtenna make it an effective solution for 5G Internet of Things sensing applications.

This note proposes a log-periodic microstrip antenna (LPMA), particularly after developing possible feed structures. The process develops an appropriate feed such that the LPMA can exhibit almost end-fire characteristics such as a log-periodic dipole antenna. The study simulated two LPMAs, one with five elements and the other with seven elements, followed by measurements on a five-element prototype. The five-element prototype LPMA with such a feed exhibits a bandwidth of 2.2–3 GHz. Over this band, radiation patterns remain consistent with a gain of 9.8 dBi. Simulated results follow the measured ones closely.

This contribution presents miniaturization techniques for folded circular substrate integrated waveguide (FCSIW) cavity filters. The cavities support dual-mode operation. Two of these cavities are combined to form a fourth-order filter. Two filter designs are implemented. The first one serves as a reference and exhibits good manufacturability. For the second one, the cavity is modified, which leads to $50\%$ more compactness. The designs are experimentally verified with 10 GHz bandpass filters. Even though the achieved level of compactness requires higher technological effort, the measurements validate the miniaturization concept. The out-of-band rejection exceeds 40 dB between 11.2 and 30 GHz for the reference filter and between 11.3 and 33 GHz for the miniaturized one. The implemented components are compared with state-of-the-art filters.

A gain enhanced dual-band filtenna operating at frequency bands of 3.5 and 5.3 GHz for 5G sub-6 GHz applications is presented in this article. A dual-band antenna is integrated with a filter to improve selectivity. The antenna uses printed monopoles as the radiating element, and the filtering response is achieved using a modified feedline. The designed filtenna is 50 × 38 × 1.63 mm3 in size with a maximum gain of 2.5 dBi. The novel filtenna design is equipped with an artificial magnetic conductor (AMC) for the first time to enhance the performance. A unit cell for the AMC is designed that offers zero phase shift in the desired frequencies. A four by four AMC array is designed using this unit cell, which is used as a reflective surface to improve the radiation characteristics of the filtenna. The gain of the filtenna is improved three times up to 7.5 dBi in both bands. The proposed design's overall planar dimensions are 80 × 80 mm2. Using AMC, the gain has improved with a very minute or no change in the other characteristics. The measured results are congruent with the simulated ones, illustrating that the filtenna has good impedance matching and excellent gain in both bands.

In this communication, a compact flower shaped printed antenna for ultra-wideband communication is proposed. The “flower-shaped” structure is capable of transmitting UWB-band signals. The designed antenna exhibits a return loss ranging from 2.3 to 11 GHz with an 8.7 GHz central frequency and has 130% relative bandwidth. The suggested design includes a patch in the shape of a flower with several slots fed by a microstrip line. Multiple slots have been designed for better resonances at lower modes. The antenna is constructed with an FR4 substrate, and a 50 Ω A-type connector feeds it. The optimum dimensions of the designed antenna are 12 × 16 × 1.6 cubic-millimeters and 0.092λ × .12λ × 0.012λ in lambda. The proposed structure also demonstrates stable radiation patterns across the operating bandwidth. The proposed radiator has a high gain of 2.67 dBi, and an efficiency of 85%. It is compact, lightweight, and easy to make. Therefore, it can be used for UWB applications.

This article shows the design of two different low phase noise (LPN) planar X-band frequency oscillators using two various microstrip filters (MFs). These two MFs act as a frequency stabilization part in the loop of the microwave oscillator. The first one, the modified Jerusalem MF (MJ-MF), is based on the Jerusalem scheme. The second one, the complementary modified Jerusalem MF (CMJ-MF), is complementary of the MJ-MF. Finally, by employing the branchline coupler, the LPN MF oscillator is achieved. The MJ-MF (narrowband filter) LPN X-band oscillator operates at 8.17 GHz and denotes a phase noise (PN) of −161 dBc/Hz at 1-MHz frequency offset. The CMJ-MF (wideband filter) LPN X-band oscillator operates at 8.14 GHz and mentions a PN of −157 dBc/Hz at 1-MHz frequency offset.

This paper presents a planar microwave sensor for the noninvasive detection of glucose concentration in diabetic patients. The designed sensor operates from the 3.8 to 6.2 GHz frequency band, which covers the 5.8 GHz Industrial Scientific and Medical (ISM) band. The designed sensor shows a percentage bandwidth of 23.8% with a reflection coefficient (S11) of −50 dB at the resonance frequency of 5.7 GHz. The detection was carried out by varying the relative permittivity of the blood in accordance with the glucose concentration based on the Cole–Cole model. The measured result is calculated in terms of varying resonance frequency with variation in the reflection coefficient |S11| of the designed sensor. The observed frequency shift and corresponding sensitivity of the sensor are found at 1.7 GHz and 0.089 MHz/mg dL−1, respectively. An experimental validation has also been performed, and the frequency shift is analyzed by interacting the human thumb with the sensor. The simulated and experimental results of the designed sensor suggest that it can be useful for detecting glucose concentration noninvasively for diabetic patients.

A two-element low-profile closely coupled dual-band MIMO antenna is demonstrated for WiMAX applications. Based on the principle of metasurface (MTS) decoupling, a double-layer MTS consisting of pairs of elliptic patches with two different sizes is proposed. The MTS is loaded above a coupled dual-band MIMO antenna, and the mutual coupling in the lower and upper band is reduced by the larger and smaller elliptic patches, respectively. The edge-to-edge distance of antenna elements is only 0.01λ0 (λ0 is the free-space wavelength at 2.6 GHz). The measured results show that the working bandwidths of the MIMO antenna are 2.5–2.69 and 3.4–3.69 GHz. The −10 dB impedance bandwidths in two bands are 8.83% (2.49–2.72 GHz) and 8.50% (3.38–3.68 GHz), and the isolation between antenna elements is enhanced by 13.5 and 18.4 dB in two bands, respectively. Moreover, broadside radiation performances in two bands are obtained.

In order to merge the advantages of the traditional compressed sensing (CS) methodology and the data-driven deep network scheme, this paper proposes a physical model-driven deep network, termed CS-Net, for solving target image reconstruction problems in through-the-wall radar imaging. The proposed method consists of two consequent steps. First, a learned convolutional neural network prior is introduced to replace the regularization term in the traditional iterative CS-based method to capture the redundancy of the radar echo signal. Moreover, the physical model of the radar signal is used in the data consistency layer to encourage consistency with the measurements. Second, the iterative CS optimization is unrolled to yield a deep learning network, where the weight, regularization parameter, and the other parameters are learnable. A quantity of training data enables the network to extract high-dimensional characteristics of the radar echo signal to reconstruct the spatial target image. Simulation results demonstrated that the proposed method can achieve accurate target image reconstruction and was superior to the traditional CS method, in terms of mean squared error and the target texture details.