This work introduces a method for plasma state supervision, based on a frequency-modulated continuous wave radar sensor and a suitable signal evaluation enabling a continuous supervision method for the plasma state. Highly precise phase evaluation of the signal allows us to detect and visualize smallest changes in the plasma state. Assuming the plasma to act like a frequency-dependent dielectric material, the propagation of the electromagnetic wave depends on the plasma state and hence, also the measured phase. Broadband measurements are carried out at center frequencies of 80 and 140 GHz in a low-pressure plasma. The radar-based setup can be used for a very flexible application, capable for spatially resolved measurements in the plasma bulk. At the same time, the high measurement rate allows for quasi real-time monitoring, so that transient processes in the plasma are recorded. Due to the simple setup, this approach is most suitable for industrial applications to improve process control. The chosen different frequencies will show a change in the influence of the plasma on the electromagnetic wave demonstrating the advantages of multi-frequency approaches in future applications.

Power amplifier (PA) nonlinearities and the digital predistortion (DPD) required for linearization are modeled at baseband using the memoryless polynomial, memory polynomial, and pruned Volterra series. The sampling requirements for the nonlinear basis waveforms created from a band-limited signal are determined as a function of the polynomial order. A Gaussian approximation of the channel filtering is used to make the mathematical analysis tractable. For the memory polynomial and pruned Volterra series cases, the spacing between memory taps, referred to as the delay offset, is selected to trade off model accuracy and the condition number of the coefficient estimation. Examples are provided showing how to select the appropriate delay offset for a memory-based amplifier model and its effect on the DPD performance. A pruned Volterra series is proposed, which demonstrates improved DPD performance compared with the memory polynomial.

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.

A coplanar waveguide-fed planar hexagonal monopole ultra-wideband antenna with dual-band rejection characteristics is proposed in this paper. The desired notch frequencies at 3.5 and 5.5 GHz are realized by incorporating mushroom structures. The input impedance and surface current distributions are used for analysis and explanation of the effects of mushroom cells. The prototype and proposed antennas are fabricated and tested. From the measured results, the proposed antenna provides an operating band of 2.81–14.32 GHz for 2 ≤ voltage standing wave ratio (VSWR), while the dual-band stop function is in the frequency bands of 3.3–3.7 GHz and 5.10–5.88 GHz. Moreover, the antenna model also exhibits constant group delay and linear phase in the pass band. The proposed antenna has appreciable gain and efficiency over the whole operating band except the notch bands.

A waveguide shunt slot-fed microstrip patch antenna (WGMPA) element is proposed and analyzed with method of moments (MOM) using entire-domain basis functions. The developed analysis has been utilized to obtain parametric observation of power-coupling versus transverse offset of feeding slot from the waveguide axis. Expressions for the radiation pattern as a summation of contributions of individual basis functions are reported. The proposed element is amenable to building-up series-fed linear arrays by a simple cascading of elements at the through-end of the feeding waveguide. The authors propose that arbitrary amplitude excitations may be applied to such linear arrays for desired tailored array pattern characteristics. The required transverse offsets for each array element may be computed using the reported parametric result. As a demonstration of concept, two distributions are designed – uniform amplitudes and Dolph–Chebyshev for reduced side lobes. Computed element patterns from MOM are used with an array factor formulation for arbitrary element positions. Both modules show radiation characteristics closely matching the expected directivity and sidelobe envelopes. Analysis validation is achieved using a proven finite element method (FEM)-based solver; the comparison is close and is reported. Efficacy of the waveguide shunt-slot fed patch element for building linear array modules with prescribed amplitude distributions is thus established.

In this paper, we propose a new design of metamaterial and electromagnetic bandgap (EBG) structure based on double layers of metallic grids. Using finite integration technique, the electromagnetic properties of the grid structure have been investigated and this structure has been employed to enhance the performance of a patch antenna working at 11 GHz. The surface waves in the substrate of antenna have been suppressed by EBG behavior of the grid when used as a substrate, and it improves its gain to 9.21 dB from 5.64 dB. When this structure is also used as superstrate of the antenna, the gain up to 13 dB has been achieved due to its metamaterial behavior. Congregation effect of metamaterial provides a huge improvement in the directionality of the antenna and its half power beam width (HPBW) has been improved to 32.7° and 31.5° from 108.3° and 93.5° in E-and H-planes, respectively.

In this paper, a compact, dual-band patch antenna is proposed over Minkowski fractal defected ground structure (DGS) for bandwidth enhancement of global positioning system (GPS) applications. The proposed design combines the truncated dual L-shaped slits cut on diagonal corners of radiating patch and fractal defect on the metallic ground plane. This concept shifts the frequencies to lower bands with improvement in antenna radiation properties. By deploying symmetrical and asymmetrical boundaries to the structure for the fractal DGS on metallic ground plane, improvement in bandwidth and gain are obtained. Compact antenna size is achieved for dual-band GPS frequencies of L1 (1.575 GHz) and L2 (1.227 GHz). The measured results for antenna prototype are (1.2–1.245 GHz): L2 band and (1.51–1.59 GHz): L1 band for 10 dB return loss bandwidth with better pattern radiation. Gain value with and without DGS is observed for compact antenna overall volume of 0.32λ0 × 0.32λ0 × 0.024λ0.

Wireless-connected wearable electronics are finding extensive usage for diagnostic and therapeutic purposes after the globally spread pandemic disease of COVID-19. Although they are undoubtedly helpful for keeping physical distance, their health effects are still under investigation from different aspects and are still a concern for the end-users. In this study, a custom M-shaped wearable antenna covering the wireless body area network and wireless local area network frequencies is designed, built, and measured. A beret cap made from a 2 mm thick textile is used as a substrate. The specific absorption rate (SAR) in a realistic human-head model due to electromagnetic energy produced by the antenna is evaluated using the finite-difference time-domain method. The SAR distributions for 1-g and 10-g tissues are calculated at 2.4 and 5.8 GHz. It is shown that the obtained maximum SAR values for 1-g and 10-g tissues at each frequency of interest were less than the limits determined by IEEE RF exposure guidelines and standards.

The paper is devoted to coherent radiometric imaging systems. The investigated systems may be considered as a version of bistatic noise waveform passive radar with synthetic aperture having external reference. Reference signal in this imaging system is received from the investigated emitting object using auxiliary receiver. Although a number of theoretical studies on aperture synthesis imaging algorithms exist, relatively few of them deal with experimental investigation. In particular, no information concerning range-azimuth coherent radiometric imaging has been found. In this paper, an experimental investigation of possibility to generate coherent radiometric images using two antennas with beam synthesizing is carried out. Indoor radiometric imaging has been validated using absorbers as the emitting objects. These experiments enabled to estimate range resolution of the radiometric system achieved by focusing antennas with beam synthesizing for range span limited by means about bistatic baseline.

This paper studies the corona discharge power thresholds in microstrip bandpass filters (BPFs) and, in particular, is focused on a solution based on λ/2 cover-ended resonators to enhance their peak power handling capability (PPHC). First, a parametric analysis is carried out to evaluate the variation of the maximum electric field and the unloaded quality factor (Qu) as a function of the cover's geometrical dimensions (i.e. height, length, and width). Next, several microstrip BPFs centered at 1.6 GHz are designed, and their behaviors under moderate-to-high applied RF power signals are simulated to corroborate the previous study. A suitable number and size of covers are selected to enhance PPHC without barely degrading the filters’ electrical performance and, consequently, without hardly increasing the insertion losses. Finally, two third-order filters with covers and without covers (benchmark prototype) are manufactured, by way of illustration, and they are tested in the European High-Power RF Space Laboratory to validate the good performance of the proposed solution, where a PPHC enhancement of 3.1 dB at high pressures is achieved as compared to the benchmark prototype.

In this paper, a new empirical path loss model based on frequency, distance, and volumetric occupancy rate is generated at the 3.5 and 4.2 GHz in the scope of 5G frequency bands. This study aims to determine the effect of the volumetric occupancy rate on path loss depending on the foliage density of the trees in the pine forest area. Using 4.2 GHz and the effect of the volumetric occupancy rate contributes to the literature in terms of novelty. Both the reference measurements to generate a model and verification measurements to verify the proposed models are conducted in three different regions of the forest area with double ridged horn antennas. These regions of the artificial forest area consist of regularly sorted and identical pine trees. Root mean square error (RMSE) and R-squared values are calculated to evaluate the performance of the proposed model. For 3.5 and 4.2 GHz, while the RMSEs are 3.983 and 3.883, the values of R-squared are 0.967 and 0.963, respectively. Additionally, the results are compared with four path loss models which are commonly used in the forest area. The proposed one has the best performance among the other models with values 3.98 and 3.88 dB for 3.5 and 4.2 GHz.

This paper proposes a fully integrated 160-GHz transmitter and receiver in package for millimeter-wave applications. The monolithic integrated circuits were designed with a harmonic approach and were fabricated using a SiGe:C HBT production technology with an fT and fmax of 170 and 250 GHz, respectively. The manufactured 2006 × 1865 µm2 bare dies were integrated in 6 × 6 mm2 embedded wafer level ball grid array packages, where they were interconnected with highly directional antennas built on the redistribution layer of the packages. With a total frequency multiplication factor of 36 and an active balun at the first stage, the transmitter allows the use of a 4.5-GHz input signal driven from a single-ended signal source [1] and distributed on a standard low-cost printed circuit board. The receiver comprises a Gilbert-cell-based subharmonic mixer with a simulated 1-dB input compression point of −4 dBm, and a minimum double-sideband noise figure of 16.5 dB. The functionality of the proposed system was successfully demonstrated in a quasi-monostatic FMCW radar measurement with a 1-ms up-chirp frequency sweep from 157 to 160 GHz and in a forward-scatter imaging experiment with an 8-GHz frequency ramp from 157 to 165 GHz.

This paper proposes the design of tunable dual-band resonators for multi-band multi-standard systems. The main objective is to provide frequency tunability in the second resonance while maintaining the first resonance fixed. To this end, two tunable resonators are proposed: the capacitive-loaded stepped-impedance resonator and the capacitive-loaded hole resonator. The work is divided into two main parts. In the first part, an in-depth analysis of the capacitive-loaded stepped-impedance resonator (SIR) structure is done; it provides analytical closed-form design equations that ease the resonator design in contrast to the several approaches available in the literature to date. The analysis is also particularized to the case of the capacitive-loaded constant-section resonator and extended to the capacitive-loaded hole resonator. In addition, a study of the quality factor in capacitively tuned SIRs is also provided. In the second part, resonators are integrated in three dual-band tunable filters, one based on the capacitive-loaded constant-section resonator, another one on the capacitive-loaded SIR, and finally on the capacitive-loaded hole resonator. Two of these filters demonstrate operation in wireless local-area network frequency bands, with a fixed first band at 2.45 GHz and a second band which can be tuned between 5.75 and 5.25 GHz.

Radio Frequency Micro-Electro-Mechanical System (RF-MEMS) represents a feasible solution to obtain very low power dissipation and insertion loss, very high isolation and linearity switch with respect to “solid state” technologies. In this paper, we demonstrate the full integration of RF-MEMS switches in the GaN-HEMT (Gallium Nitride/High Electron Mobility Transistor) fabrication line to develop RF-MEMS devices and LNA-MMIC (Low Noise Amplifier/Monolithic Microwave Integrated Circuit) prototype simultaneously in the same GaN wafer. In particular, two different coplanar wave (CPW) LNAs and a series of discrete RF-MEMS in ohmic-series and capacitive-shunt configuration have been fabricated. RF-MEMS performances reveal an insertion loss and isolation better than 1 and 15 dB, respectively, in the frequency range 20–50 GHz in the case of pure capacitive shunt switches and in the frequency range 5–35 GHz for the ohmic-series switches. Moreover, the GaN HEMT device shows an Fmax of about 38 GHz and a power density of 6.5 W/mm, while for the best LNA-MMIC we have obtained gain better than 12 dB at 6–10 GHz with a noise figure of circa 4 dB, demonstrating the integration achievability.

The design, realization, and characterization of highly efficient powerbars and monolithic microwave integrated circuit (MMIC) high-power amplifiers (HPAs) with AlGaN/GaN high electronic mobility transistors (HEMTs) are presented for the frequency range between 1 and 30 GHz. Packaged powerbars for the frequency range between 1 and 6 GHz have been developed based on a process called GaN50 with a gate length of 0.5 μm. Based on a GaN25 process with a gate length of 0.25 μm, high-power MMIC amplifiers are presented starting from 6 GHz up to advanced X-band amplifiers and robust LNAs in microstrip transmission line technology.

Novel designs of circular waveguide components for lowpass, highpass, bandpass, and bandstop applications are presented. The filters are formed by cascaded TM11-mode resonator sections which share a common axis, thus preserving the polarizations of input signals due to rotational symmetry. Moreover, the filters are smaller than standard TE11-mode bandpass filters. Basic design considerations are discussed with respect to the connection of TM11-mode resonators through circular irises. Design principles for all four filter types are presented and their performances computed by the coupled-integral equation technique (CIET). All filter performances are verified by the commercially available software package μWave Wizard, and structural dimensions are provided.

This paper presents a radio frequency micro-electro-mechanical-systems (RF MEMS) fabrication process based on a stacked structural layer and Au–Au thermocompression bonding, and reports on the performance of a sample RF MEMS switch design implemented with this process. The structural layer consists of 0.1 µm SiO2/0.2 µm SixNy/1 µm Cr–Au layers with a tensile stress less than 50 MPa deposited on a silicon handle wafer. The stacked layer is bonded to a base wafer where the transmission lines and the isolation dielectric of the capacitive switch are patterned. The process flow does not include a sacrificial layer; a recess etched in the base wafer provides the air gap instead. The switches are released by thinning and complete etching of the silicon handle wafer by deep reactive ion etching (DRIE) and tetramethylammonium hydroxide (TMAH) solution, respectively. Millimeter-wave measurements of the fabricated RF MEMS switches demonstrate satisfactory up-state performance with the worst-case return and insertion losses of 13.7 and 0.38 dB, respectively; but the limited isolation at the down-state indicates a systematic problem with these first-generation devices. Optical profile inspections and retrospective electromechanical analyses not only confirm those measurement results; but also identify the problem as the curling of the MEMS bridges along their width, which can be alleviated in the later fabrication runs through proper mechanical design.

The paper presents millimeter-wave (mm-wave) signal sources using a hetero-integrated InP-on-BiCMOS semiconductor technology. Mm-wave signal sources feature fundamental frequency voltage-controlled oscillators (VCOs) in BiCMOS, which drive frequency multiplier–amplifier chains in transferred-substrate (TS) InP-DHBT technology, heterogeneously integrated on top of the BiCMOS wafer in a wafer-level bonding process. Both circuits are biased through a single set of bias pads and compact low-loss transitions from BiCMOS to InP circuits and vice versa have been developed, which allows seamless signal routing through both technologies exhibiting 0.5 dB insertion loss up to 200 GHz. One VCO operates at 82 GHz with a tuning range of 600 MHz and an output power of approximately 8 dBm. A frequency doubler combined with this VCO circuit delivers 0 dBm at 164 GHz and a frequency tripler with a similar VCO delivers −10 dBm at 246 GHz. Another hetero-integrated W-band doubler–amplifier circuit demonstrates 12.9 dBm saturated output power with 5.9 dB conversion gain at 96 GHz. A direct comparison of the TS InP-DHBT MMIC with either silicon or traditional AlN carrier substrates shows the favorable properties of the hetero-integrated process discussed here. The results demonstrate the feasibility of hetero-integrated circuits operating well above 100 GHz.

This paper presents the development of two dual-band radio-frequency (RF) harvesters optimized to convert far-field RF energy to DC voltage at very low received power. The first one is based on a patch antenna and the second on a dipole antenna. They are both implemented on a standard FR4 substrate with commercially off-the-shelf devices. The two RF harvesters provide a rectified voltage of 1 V for a combined power, respectively, of −19.5 dBm at 915 MHz, −25 dBm at 2.44 GHz, of −20 dBm at 915 MHz, and −15 dBm at 2.44 GHz. The remote powering of a clock consuming 1 V/5 µA is demonstrated, and the rectenna yields a power efficiency of 12%.

In this paper, a triple-band bandpass filter using a multi-mode half-mode substrate integrated waveguide (HMSIW) cavity is presented. In this design, back-to-back connected half-mode defected ground structure (DGS) is loaded into the HMSIW cavity, which generates the first passband. The first four modes of HMSIW cavity are TM110, TM120, TM310, and TM320. They have been used to generate second and third passbands. The slot line perturbation is used to shift the modes close to each other. Here, TM110 and TM120 constitute second passband, and TM310 and TM320 give third passband. Moreover, the notch band is designed using embedded open-circuited stub to improve the attenuation between the second and third passbands. The proposed filter is designed and fabricated on RT/Duroid 5880 substrate with a thickness of 0.508 mm. The measured results are in line with simulated ones for each of the passbands, and the center frequencies are 6.28, 13, and 19.12 GHz.