Masaharu Takahashi

In this paper, we propose a 3D undersea positioning system using VLF electromagnetic waves as a system to support divers’ activities in seawater and show results of a basic study for the proposed system in electromagnetic field simulations. Assuming that a transmitting antenna is attached to a diver, the antenna’s position in seawater is estimated using the Received Signal Strength (RSS [dB]) of multiple receiving antennas placed near the sea surface. To assist in the installation and retrieval of the proposed system, we propose a receiving antenna array that integrates twelve dipole antennas into a single receiver. Furthermore, we also propose a position estimation algorithm optimized for the proposed receiver. In the proposed algorithm, Support Vector Regression (SVR) was utilized to estimate the distance between the transmitting antenna and the receiver from the RSS. We confirmed that position estimation using the receiver was possible within a circle with an approximately 12 m radius centered on the receiver, demonstrating the viability of our proposed system for implementation.

Mobile communication devices have become widely used in various situations in our daily lives. Not only are our opportunities for exposure to radio waves increasing, but the exposure environment is also changing owing to developments in telecommunications technology. Conventional studies on radio wave exposure evaluation have focused on only a single frequency, and only a few studies have considered radio wave exposure at multiple frequencies, including the frequencies used in fifth-generation (5G) mobile communications. In this study, specific absorption rate (SAR) evaluations were conducted assuming the exposure of the human body to radio waves with multiple frequency components. To simulate the actual use of a smartphone, analytical models were created using a smartphone model as the electromagnetic wave source and a numerical human body model as the exposure target. In the case of multiple frequencies, SAR was calculated for each frequency component, and SARs were summed. In addition, by changing the power ratio of each frequency component to the total radiated power, the distribution of SAR and the shift in the peak spatial SAR$ _{\text{10g}}$ (psSAR$ _{\text{10g}}$) were investigated. As a result, it was confirmed that the SAR distribution changes with the power ratio of each frequency component in the radio wave. In the case of radio waves with multiple frequency components, no specific power ratios were found to cause a significant increase in psSAR$ _{\text{10g}}$.

Meander Line Antenna (MLA) is widely employed in compact electronic devices, such as cellular phones and WLAN terminals, owing to its electrically small size. To facilitate practical antenna design, essential equations encompassing self-resonant structure, input resistance, antenna efficiency, and Q factor have been systematically developed. However, the prior self-resonant equations included only inductive reactance (XC), neglecting the capacitive reactance (XD) equation. This manuscript addresses this gap by introducing new design equations, presenting a newly derived $\rm X_{D}$ equation and an improved Q factor expression. The inadequacies of the existing Q factor equation, reliant on the radius of a sphere encompassing the antenna, are addressed by proposing a more fitting expression that incorporates antenna structural parameters using the ratio of reactance to resistance. The overview of existing design equations sets the stage for the introduction of these newly developed equations. To assess the accuracy of electromagnetic (EM) simulation results, a comparative analysis is conducted between simulated and theoretically calculated input resistance values. The derivation of new reactance equations involves the development of $\rm X_{C}$ equations based on electromagnetic theory. The $\rm X_{D}$ equation is established by deriving the stored charge equation from electrical near-field distributions obtained through EM simulations. By applying the relationship between charge and capacitance, a new $\rm X_{D}$ equation is obtained. Subsequently, a new self-resonant equation is derived, and the validity of the newly derived equations is confirmed through EM simulation results, ensuring their accuracy. Two MLA prototypes, with lengths of 0.05 and 0.1 wavelengths at 405 MHz, are fabricated and experimentally validated. Smith chart measurements confirm the self-resonant condition and input resistance. By correlating with the VSWR characteristics, the obtained Q factor of approximately 100 aligns successfully with the results from the reactance equation. The antenna gain is verified at −7.7 dBi and −3.5 dBi for antennas with lengths of 0.05 and 0.1 wavelengths, respectively. These findings establish the practical applicability of the proposed equations for antenna design and elucidate the performance of practical antennas.

In recent years, the use of millimeter-wave bands has been considered for radio communication systems between train and trackside. Millimeter-wave band radio waves are known to have strong linearity and high attenuation on Non-Line-of-Sight (NLoS). On the other hand, it has been reported that they propagate to NLoS by the effects of rails and side walls in sections with side walls and in tunnel sections. Therefore, we simulate to evaluate the influence of side walls in curved sections for the 40 GHz band, which is being considered for use in future mobile communication systems and train radio systems. The simulation results shows that the influence of the side wall tends to be larger, and the path loss tends to be smaller for smaller curve radiuses.

Obstacles on railway tracks are a factor hindering safe and stable operation. For this reason, obstacle detection systems have been introduced at level crossings, slopes and under platforms. However, they have been introduced only in limited sections for limited applications. Therefore, the authors considered to introduce an obstacle monitoring system that combines 90 GHz band millimeter‐wave and Radio over Fiber (RoF) technology, as an obstacle monitoring system over tracks to monitor the entire line. Since the millimeter‐wave band has a wide bandwidth, high precision detection is possible when they are used in radar. In addition, since radar control units can be grouped in one place by introducing the RoF technology, we can build a system at a low cost. In order to study the feasibility of introducing a new obstacles monitoring system combining 90 GHz band millimeter‐wave and RoF technology in railway environment, a field test using prototype equipment was conducted on a railway track. As a result, it was confirmed that the prototype radar system could detect humans and non‐metallic objects entering a track at a distance of about 200 m from the location of the prototype system. These results shows that the proposed system has the potential to be introduced as a new railway obstacle monitoring system using 90 GHz band millimeter‐waves and RoF technology. © 2023 The Authors. IEEJ Transactions on Electrical and Electronic Engineering published by Institute of Electrical Engineer of Japan and Wiley Periodicals LLC.

Currently, wireless power transmission technology is being developed for capsule endoscopes. By removing the battery, the capsule endoscope is miniaturized, the number of images that can be taken increases, and the risk of harmful substances leaking from the battery when it is damaged inside the body is avoided. Furthermore, diagnostic accuracy is improved by adjusting the directivity of radio waves according to the position of the capsule endoscope to improve efficiency and adjusting the number of images to be taken according to position by real-time position estimation. In this study, we report the result of position estimation in a high-definition numerical human body model and in an experiment on an electromagnetic phantom.

Various small focus spot applicators are being investigated for hyperthermia therapy, which requires microwave concentration to heat tumors in the human body. Dielectric lens antenna is frequency independent and has strong focusing capability to achieve a very small focusing spot. In this paper, lenses with diameters of 30, 50 and 70 cm were designed to evaluate the size of the focal spot in the human body model. The electromagnetic simulator, FEKO was used to generate rays and near-field focusing data of dielectric lenses at a frequency of 2.45 GHz. The simulated focal spot sizes agreed well with the theoretical values. An analytical investigation into the power at the focal spot was conducted using the proposed power relations of the focused lens novel equation. The theoretical propagation loss is used to represent the power density degradation at the focal spot caused by microwave absorption by the human body. The simulation results of the focused lens in the human body indicated that the 30 cm lens achieved a larger focal spot with a greater focusing power, 0.714 mW compared to the 70 cm lens, which achieved a smaller focal spot but a lower focusing power, which was 0.393 mW.

When divers rescue people in accidents at sea, they are exposed to dangers such as injuries by obstacles, and so on. If the divers can confirm their position, their rescue activities will become safer. In the previous study, assuming that we specify the positions of the divers performing rescue operations to support their work, we developed a 3D undersea position estimation algorithm communicating between the undersea and the sea surface. However, we did not yet consider the effects and countermeasures of sea waves. In this paper, we indicate the effects of sea waves on the algorithm and investigate wave countermeasures.

The impedance expansion method (IEM) is a circuit modeling technique for electrically very small devices based on the method of moments. In the authors' previous work, a circuit model of a wireless power transfer system was developed using the modal IEM, which utilizes the IEM and eigenmode analysis, wherein the currents on the coils were expanded into thin-wire basis functions. However, surface basis functions are required to apply the modal IEM to arbitrarily shaped conductors. In this work, the problems arising when the surface basis functions are applied to the modal IEM are pointed out, and the cause of these problems is clarified. Then, the resolvability of these problems is demonstrated using the loop-star basis functions.