Ichirou Ida

The SAR evaluation method by a combination of thermographic experiments and biological tissue-equivalent phantoms

Many different phantoms have been proposed as human head models for experimental estimation of the electromagnetic energy field induced in the human head caused by the use of an RF device. Here, a biotic tissue-equivalent solid phantom is introduced as the human head model realizing the same relative dielectric constant and conductivity as brain tissue and skull layer. This brain-equivalent solid phantom and skull-equivalent phantom make it possible to accomplish highly reliable and precise estimation of specific absorption rate (SAR) in the human head. The phantom models of cube, sphere, and realistic human heads are fabricated. Measurements are performed to estimate the SAR in the human head models exposed to microwave sources by using the thermographic method.

Dependence of the efficiency-bandwidth product on electrical volume of small dielectric loaded antennas

The efficiency fractional efficiency bandwidth product (EB) that is expressed as a ratio of the radiation resistance to the absolute value of an input reactance of an antenna is used as a performance measure for small dielectric-loaded monopole antennas (DLMAs). The dependence of the efficiency bandwidth (EB) on the permittivity of the dielectric loading (i.e., the electrical volume) is experimentally investigated for the first time after we succeeded in measuring the small radiation resistance of the DLMAs precisely on the basis of the Wheeler cap method. As a result, it was found that the EBs of some DLMAs were enhanced over a bare monopole antenna and an EB characteristic curve has a maximum point. This result suggests the presence of the optimum electrical volume for the dielectric loading in order to obtain the best EB performance. A calculation by the finite-difference time-domain (FDTD) method was also conducted to confirm the presence of the maximum EB value under a certain condition. A general reason for the existence of the peak value is also explained using a mathematical deduction.

Accurate measurement of small input resistances using a conventional network analyzer

Precise and accurate measurement of input resistance is essential to characterize small antennas whose input resistance is very small. However, this is very difficult because uncertainty, which includes imprecision and inaccuracy, sometimes exceeds the resistance being measured. A method for precise measurement of small input resistance using a conventional network analyzer is presented. Inaccuracy, which includes an actual conductor loss of the antenna under test (AUT), manufacturing error, and the residual systematic errors is then estimated. The former two sources of inaccuracy are obtained by comparing a radiation efficiency measurement of a small loop antenna using the Wheeler cap method with a calculated radiation efficiency. Using these values in the calculation of the input resistance, a true input resistance is acquired. Finally, the actual value of the residual systematic errors for each instrument is estimated by comparison between the true input resistance and the measurement.

An Adaptive Beamforming Method for Phased Array Antenna with MEMS Phase Shifters

Adaptive array antennas, which control their own patterns by means of feed-back or feed-forward control, are effective tools for gain enhancement and interference suppression. However, when applying them to mobile terminals, the problems of hardware complexity and power consumption need to be taken into consideration. One solution is the use of analog device-based adaptive array antennas, such as Reactively Steered Adaptive Array (RESAA) antennas and phased array antennas, which have the attractive characteristics of low cost and power consumption. In this paper, we propose an adaptive beamforming method based on a one-dimension search algorithm for phased array antennas with Micro Electro Mechanical Systems (MEMS) phase shifters, taking into consideration their slow operating speed due to mechanical structure of the devices. Furthermore, a smoothing processing is introduced to prevent the effect of noise and a multi-resolution alogrithm is proposed to help the system form beams more quickly and stably. Numerical results based on the IEEE 802.11 a Wireless Local Area Network (WLAN) standard show that the proposed method has good interference suppression and gain enhancement capabilities in multipath fading channels.

A simple circularly polarized beam-switching patch array antenna for satellite communication

The performance of a simple beam-switching four-patch array antenna, radiating circular polarization at 2.6 GHz, is presented. This antenna generates a directive beam allowing the tracking of the signal emitted by a satellite. Two methods of feeding the antenna, to realize the beam-tracking, are analyzed. First, when the feeding source is switched off. Second, when one element is fed with a different phase than the others. This four-patch array antenna has good performances in terms of minimum gain, axial ratio and voltage standing wave ratio but also small size and low cost.

Precise measurement of input impedance and radiation efficiency of small loop antennas

The precise measurement of the impedance characteristics of a small loop antenna has been proved to be possible by the repetition of experiments. Three different types of network analyzers, HP 8719A, 8753B, and 8753C were used in the experiments. It was confirmed that the uncertainty of each instrument does not significantly affect the measurement of the radiation efficiency of the small loop antenna. Good agreement has been obtained between calculated and measured values, considering the surface loss of the antenna.<<ETX>>

Reduction and characterization of a drift error in measurement of small antennas using a network analyzer

The radiation resistances of a monopole antenna and a dielectric loaded monopole antenna have precisely been measured on the basis of the Wheeler cap (1947) method. The precision of the measurement has been proved to improve with the use of the stub tuner. To give a theoretical background for the improvement in the measured radiation resistance, the drift error of the network analyzer has mathematically been characterized using the error vector model. Using the model, we have theoretically estimated the enhancement of measurement precision when the antenna is matched to the transmission line. The use of such impedance converters as the stub tuner can generally be applied to measure the radiation resistance of small antennas having a small resistance and a large reactance, where the drift error of the instrument gives a serious degradation to the measurement precision.

Accurate measurement and FDTD simulation of the efficiency-bandwidth product for dielectric loaded antennas

There are several criteria for antenna evaluation. The authors have studied on the efficiency-bandwidth product (EB) for small dielectric loaded antennas (DLAs). The EBs of DLAs were accurately measured and compared with calculated ones. The calculation simulated the procedure of the Wheeler (1947) cap method. Enhancement of the EB was observed even with the loading of lower permittivity. There seemed to be an optimum value for permittivity of the loading. It was also confirmed that some DLAs with a smaller electrical volume gave the better EB than the larger ones.

Evaluation for dielectric loaded small dipole antennas using efficiency-bandwidth product

The efficiency-fractional bandwidth (EB) product is used to evaluate the performance of small dielectric antennas (DLAs). When applied to electrically small antennas, the EB product is equivalent to the radiation power factor (PF) proposed by Wheeler (1947) and easy to confirm by calculation and experiment. Several shapes of DLAs with almost the same volume are compared with a bare dipole using the EB product. As a result, it was found that a rectangular dielectric coating of lower permittivity (/spl epsiv//sub r/ less than about four) gives an enhancement of the EB product over the bare dipole.