A double step attenuation measurement technique using a non-isolating gauge block attenuator (GBA) has been proposed for accurate measurements of radio frequency and microwave high attenuation. For fixed attenuator as a device under test (DUT), a medium value (≤ 60 dB) attenuator is used as the GBA which connected directly between the test ports, then high attenuation of the DUT is measured in two setups as follows. 1) Thru and GBA with normal power level and 2) GBA and DUT with higher power level. This approach removes the need to isolate the GBA, therefore, accurate measurements of high attenuation can be obtained simply over a broad frequency range. For variable or step attenuator as a DUT, one of the attenuation sections of the DUT is applied as the GBA. Detailed analyses and those verification measurements are carried out both for fixed attenuator, as well as for variable attenuator and show good agreement.

We propose microwave heating via electromagnetic coupling using zeroth-order resonators (ZORs) to extend the uniform heating area. ZORs can generate resonant modes with a wavenumber of 0, which corresponds to an infinite guide wavelength. Under this condition, uniform heating is expected because the resulting standing waves would not have nodes or antinodes. In the design proposed in this paper, two ZORs fabricated on dielectric substrates are arranged to face each other for electromagnetic coupling, and a sample placed between the resonators is heated. A single ZOR was investigated using a 3D electromagnetic simulator, and the resonant frequency and electric field distribution of the simulated ZOR were confirmed to be in good agreement with those of the fabricated ZOR. Simulations of two ZORs facing each other were then conducted to evaluate the performance of the proposed system as a heating apparatus. It was found that a resonator spacing of 25 mm was suitable for uniform heating. Heating simulations of SiC and Al2O3 sheets were performed with the obtained structure. The heating uniformity was evaluated by the width L50% over which the power loss distribution exceeds half the maximum value. This evaluation index was equal to 0.397λ0 for SiC and 0.409λ0 for Al2O3, both of which exceed λ0/4, the distance between a neighboring node and antinode of a standing wave, where λ0 is the free-space wavelength. Therefore, the proposed heating apparatus is effective for uniform microwave heating. Because of the different electrical parameters of the heated materials, SiC can be easily heated, whereas Al2O3 heats little. Finally, heating experiments were performed on each of these materials. Good uniformity in temperature was obtained for both SiC and Al2O3 sheets.

In Japan, research on spatial transmission Wireless Power Transfer/Transmission (WPT) for long-distance power transmission has been conducted ahead of the rest of the world; however, until 2022, there has been no category under the Radio Law, and it has been treated as an experimental station. The authors are working on Japanese institutionalization (revision of ministerial ordinances) and global standardization of this spatial transmission WPT for social implementation. This paper describes the Japanese and international institutionalization and standardization trends. In addition, as the latest trend in R&D trends, as the next step of institutionalization, the author introduces two national projects that are being worked on by industry, academia, and government for Step 2, which can be used for a wider range of applications by relaxing the scope of use and restrictions from Step 1, which has various restrictions. The first is about the Cross-ministerial Strategic Innovation Promotion Program (SIP) Phase 2. In SIP Phase 2, we conducted R&D on “WPT system for sensor networks and mobile devices”. This R&D is research on detecting and avoiding people so that radio exposure does not exceed protection guidelines and detecting incumbent radios and avoiding harmful interference so that more power can be transmitted under coexistence conditions. The other is “Research and Development for Expansion of Radio Resources” to be conducted by the Ministry of Internal Affairs and Communications (MIC), which is scheduled for four years from FY2022. This is also a more concrete research and development project for Step 2 institutionalization, along with the results of the SIP mentioned above.

GaN solid state power amplifiers (SSPA) for wireless power transfer and microwave heating have been reviewed. For wireless power transfer, 9 W output power with 79% power added efficiency at 5.8 GHz has been achieved. For microwave heating, 450 W output power with 70% drain efficiency at 2.45 GHz has been achieved. Microwave power concentration and uniform microwave heating by phase control of multiple SSPAs are demonstrated.

Japan encounters an urgent issue of “Carbon Neutrality” as a member of the international world and is required to make the action plans to accomplish this issue, i.e., the zero emission of CO2 by 2050. Our world must change the industries to adapt to the electrification based on the renewable powers. Microwave chemistry is a candidate of electrification of industries for the carbon neutrality on the conditions of usage of renewable energy power generation. This invited paper shows an example of “Microwave Pidgeon process” for smelting magnesium in which heating with burning fossil coals can be replaced with microwave energy for discussing how microwave technology should be developed for that purpose from both the academic and industrial sides.

With the emphasis on personal information privacy protection in wireless communications, the new dimension low-interception covert transmission technology represented by the vortex wave with Orbital Angular Momentum (OAM) has received attention from both academia and industry. However, the current OAM low-interception transmission techniques all assume that the eavesdropper can only receive plane wave signals, which is a very ideal situation. Once the eavesdropper is configured with an OAM sensor, the so-called mode covert channel will be completely exposed. To solve this problem, this paper proposes a vortex microwave photon low-interception transmission method. The proposed method utilizes the differential operation between plane and vortex microwave photons signals to construct the covert differential channel, which can hide the user data in the mode domain. Compared with the traditional spread spectrum transmission, our proposed covert differential channel schemes need less transmitted power to achieve reliable transmission, which means less possibility of being intercepted by the eavesdropper.

Previously a method was reported to determine the mathematical representation of the microwave oscillator admittance by using numerical calculation. When analyzing the load characteristics and synchronization phenomena by using this formula, the analysis results meet with the experimental results. This paper describes a method to determine the mathematical representation manually.

This study investigates the improvement of the channel capacity of 5-GHz-band multiple-input multiple-output (MIMO) communication using microwave-guided modes propagating along a polyvinyl chloride (PVC) pipe wall for a buried pipe inspection robot. We design a planar Yagi-Uda antenna to reduce transmission losses in communication with PVC pipe walls as propagation paths. Coupling efficiency between the antenna and a PVC pipe is improved by attaching a PVC adapter with the same curvature as the PVC pipe's inner wall to the Yagi-Uda antenna to eliminate any gap between the antenna and the inner wall of the PVC pipe. The use of a planar Yagi-Uda antenna with a PVC adaptor decreases the transmission loss of a 5-GHz-band microwave signal propagating along a 1-m-lomg straight PVC pipe wall by 7dB compared to a dipole antenna. The channel capacity of a 2×2 MIMO system using planar Yagi-Uda antennas is more than twice that of the system using dipole antennas.

Transcatheter renal denervation (RDN) is a novel treatment to reduce blood pressure in patients with resistant hypertension using an energy-based catheter, mostly radio frequency (RF) current, by eliminating renal sympathetic nerve. However, several inconsistent RDN treatments were reported, mainly due to RF current narrow heating area, and the inability to confirm a successful nerve ablation in a deep area. We proposed microwave energy as an alternative for creating a wider ablation area. However, confirming a successful ablation is still a problem. In this paper, we designed a prediction method for deep renal nerve ablation sites using hybrid numerical calculation-driven machine learning (ML) in combination with a microwave catheter. This work is a first-step investigation to check the hybrid ML prediction capability in a real-world situation. A catheter with a single-slot coaxial antenna at 2.45 GHz with a balloon catheter, combined with a thin thermometer probe on the balloon surface, is proposed. Lumen temperature measured by the probe is used as an ML input to predict the temperature rise at the ablation site. Heating experiments using 6 and 8 mm hole phantom with a 41.3 W excited power, and 8 mm with 36.4 W excited power, were done eight times each to check the feasibility and accuracy of the ML algorithm. In addition, the temperature on the ablation site is measured for reference. Prediction by ML algorithm agrees well with the reference, with a maximum difference of 6°C and 3°C in 6 and 8 mm (both power), respectively. Overall, the proposed ML algorithm is capable of predicting the ablation site temperature rise with high accuracy.

Recently, “Both-Side Retrodirective System” was proposed, as a beam convergence technique, for microwave high power transmission. To demonstrate the effectiveness of the both-side retrodirective system by experiment, the authors propose a 2-dimensional measurement equipment. Propagation in the parallel plate waveguide was analogized based on free-space propagation, and the theory and characteristics were clarified by simulation. The electric field distribution in the waveguide was measured by electric probe with the proposed equipment. Two types of measurement equipment were developed. One is a 4-element experiment system, which is a small-scale device for principle verification. The other is a 16-element measurement equipment, which is intended to evaluate beam convergence of a both-side retrodirective system in the next step. The measured results were compared with simulation results. As a result, it was confirmed that the beam formed in the waveguide was successfully measured. Thus, the effectiveness of 2-dimensional measurement equipment for evaluation of beam convergence was shown.

The generation and reduction reaction of magnesium plasma were studied using a cylindrical transverse magnetic-mode applicator in magnetic and electric field modes. By heating Mg powder using the magnetic field mode, plasma was generated with the evaporation of Mg and stably sustained. When the Mg plasma sample was introduced into the reaction zone and exposed to microwave and lamp heating, a reduction reaction of scandium oxide also occurred. The results of this study provide prospects for the development of a larger microwave refining system.

A high-efficiency uniform/selective heating microwave oven was developed. Because the power amplifier requires high-efficiency characteristics to function as a microwave source, a free-standing Gallium Nitride (GaN) substrate was applied in this study. By applying a harmonic tuning circuit, an output power of 71 W and PAE of 73% were achieved in pulsed operation, and an output power of 63 W and PAE of 69% were achieved in CW operation. Moreover, we fabricated a prototype PA module that consists of an oscillator, a driver amplifier, PA, and other RF circuits. The output power was controlled by pulse width modulation to maintain high efficiency regardless of output power. We evaluated the arrangement of antenna polarizations to isolate each antenna. By suppressing the interference of output from adjacent antennas, it is possible to irradiate the object on the top surface of the antenna, thereby demonstrating heating characteristics with small temperature unevenness. The prototype microwave oven successfully demonstrated uniform/selective heating.

A novel displacement sensor was proposed based on a frequency delta-sigma modulator (FDSM) employing a microwave oscillator. To demonstrate basic operation, we fabricated a stylus surface profiler using a cylindrical cavity resonator, where one end of the cavity is replaced by a thin metal diaphragm with a stylus probe tip. Good surface profile was successfully obtained with this device. A 10 nm depth trench was clearly observed together with a 10 µm trench in a single scan without gain control. This result clearly demonstrates an extremely wide dynamic range of the FDSM displacement sensors.

In recent years, microwave wireless power transfer (WPT) has attracted considerable attention due to the increasing demand for various sensors and Internet of Things (IoT) applications. Microwave WPT requires technology that can detect and avoid human bodies in the transmission path. Using a phantom is essential for developing such technology in terms of standardization and human body protection from electromagnetic radiation. In this study, a simple and lightweight phantom was developed focusing on its radar cross-section (RCS) to evaluate human body avoidance technology for use in microwave WPT systems. The developed phantom's RCS is comparable to that of the human body.

We achieved detailed characterization of resonant tunneling chaos generator circuits in microwave frequency range. The circuit is analogous to Duffing oscillator, where the third-order nonlinear potential term is emulated by the nonlinear current-voltage curve of the resonant tunneling diode. The circuit includes a periodic reset mechanism to output identical chaos signal, which is essential to observe chaos signal on a sampling oscilloscope. Though this was shown to be effective in our previous papers, the length of the waveforms to observe is limited to rather short period, and it was unclear if this technique can be used for detailed characterization of such high-frequency chaos. In this paper, we improved the circuit design to observe longer waveforms, and demonstrated that the detailed characterization is possible using this periodic resetting technique with a sampling oscilloscope. The hybrid integration scheme is also used in this paper, which allows the easiest and shortest way to mimic a circuit as per circuit design, and precise estimation of circuit parameters aiming to eliminate circuit-related abnormalities. We provide deep insight into the dynamics associated with our circuit, starting from the single period, double period, chaos, and triple period regimes, by extracting power spectra, return maps, phase portraits, and bifurcation diagrams from acquired time series using sampling oscilloscope. Our method to study microwave chaotic signals can be applied to much higher frequency ranges, such as THz frequency range.

Testing the radio frequency compatibility between Cn-band Satellite Navigation and Microwave Landing System (MLS) has included establishing a specific interference model and reporting the effect of such interference. This paper considers two interference scenarios according to the interfered system. By calculating the Power Flux Density (PFD) values, the interference for Cn-band satellite navigation downlink signal from several visible space stations on MLS service is evaluated. Simulation analysis of the interference for MLS DPSK-data word signal and scanning signal on Cn-band satellite navigation signal is based on the Spectral Separation Coefficient (SSC) and equivalent Carrier-to-Noise Ratio methodologies. Ground tests at a particular military airfield equipped with MLS ground stations were successfully carried out, and some measured data verified the theoretical and numerical results. This study will certainly benefit the design of Cn-band satellite navigation signals and guide the interoperability and compatibility research of Cn-band satellite navigation and MLS.

In this study, we propose an accurate range-Doppler analysis algorithm for moving multiple objects in a short range using microwave (including millimeter wave) radars. As a promising Doppler analysis for the above model, we previously proposed a weighted kernel density (WKD) estimator algorithm, which overcomes several disadvantages in coherent integration based methods, such as a trade-off between temporal and frequency resolutions. However, in handling multiple objects like human body, it is difficult to maintain the accuracy of the Doppler velocity estimation, because there are multiple responses from multiple parts of object, like human body, incurring inaccuracies in range or Doppler velocity estimation. To address this issue, we propose an iterative algorithm by exploiting an output of the WKD algorithm. Three-dimensional numerical analysis, assuming a human body model in motion, and experimental tests demonstrate that the proposed algorithm provides more accurate, high-resolution range-Doppler velocity profiles than the original WKD algorithm, without increasing computational complexity. Particularly, the simulation results show that the cumulative probabilities of range errors within 10mm, and Doppler velocity error within 0.1m/s are enhanced from 34% (by the former method) to 63% (by the proposed method).

This paper describes the efficiency-limiting factors resulting from transistor current source in the case of class-F and inverse class-F (F-1) operations under saturated region. We investigated the influence of knee voltage and gate-voltage clipping behaviors on drain efficiency as limiting factors for the current source. Numerical analysis using a simplified transistor model was carried out. As a result, we have demonstrated that the limiting factor for class-F-1 operation is the gate-diode conduction rather than knee voltage. On the other hand, class-F PA is restricted by the knee voltage effects. Furthermore, nonlinear measurements carried out on a GaN HEMT validate our analytical results.

The expansion of the communication area is expected for Beyond-5G/6G networks using the High Altitude Platform Station (HAPS), Internet of Things (IoT), and sensor devices. Beyond-5G/6G networks constitute the vast amounts of devices that require the latest power utilization system. We expect Microwave Power Transfer (MPT) plays a role in the wireless power supply to HAPS, IoT, and sensors in this network. This work discusses the link design and techniques of MPT for the newest power utilization system required on Beyond-5G/6G networks.

A detailed investigation of DC and RF performance of AlGaN/GaN HEMT on 3C-SiC/low resistive silicon (LR-Si) substrate by introducing a thick GaN layer is reported in this paper. The hetero-epitaxial growth is achieved by metal organic chemical vapor deposition (MOCVD) on a commercially prepared 6-inch LR-Si substrate via a 3C-SiC intermediate layer. The reported HEMT exhibited very low RF loss and thermally stable amplifier characteristics with the introduction of a thick GaN layer. The temperature-dependent small-signal and large-signal characteristics verified the effectiveness of the thick GaN layer on LR-Si, especially in reduction of RF loss even at high temperatures. In summary, a high potential of the reported device is confirmed for microwave applications.