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Toshimi OKADA Satoshi YAGITANI Isamu NAGANO Tomohiko IMACHI Misaki MUKAINO Yasumasa KASABA Hiroshi MATSUMOTO
A feasibility study has been made of the detection possibility of radio wave noises, i.e., Martian atmospherics, emitted from discharges in the Martian atmosphere during large dust storms. The spacecraft NOZOMI, which was launched in 1998, is to be placed on an elliptic orbit around Mars with perigee of 150-200 km. An onboard-equipment LFA (Low Frequency Plasma wave Analyzer) has capability to measure the low frequency plasma waves in the frequency range from 10 Hz to 32 kHz. In order to know if the LFA can detect the atmospheric radio noises, the propagation characteristics of electromagnetic waves through the Martian ionosphere are studied theoretically by using a full-wave method. The ionosphere is modeled as a magneto-ionic medium based on the recent observations of magnetic anomaly by Mars Global Surveyor spacecraft, and the atmospheric constituent and electron density by Viking observations. Our calculation shows that the waves at frequencies less than a hundred hertz can propagate with low attenuation and reach to altitudes above 200 km in the whistler-mode in the regions of magnetic anomalies in the dayside ionosphere. It is shown that the radio noises emitted from electric discharge in an intense dust storm, with the intensity over -30 dBV/m/Hz at the ionospheric entry point, can be sensed by the LFA. The observational identification of Martian atmospherics will contribute to the physical study of charge/discharge process in the Martian atmosphere.
Toshihide TOSAKA Isamu NAGANO Satoshi YAGITANI
We have developed an estimation system of the electric parameters for liquid materials without a sensor connecting to the testing materials. A PC can be used for the calculation, and the calculation time is about 2 minutes. The accuracy is such that the estimated values are different from the nominal values by less than 2%.
Mitsunori OZAKI Satoshi YAGITANI Kazuhisa MIYAZAKI Isamu NAGANO
Using a single-site lightning location technique, a new portable lightning location system is developed. We incorporate an attitude detection technique using inertial sensors to detect an accurate electromagnetic field vector of sferics by palm-sized electromagnetic sensors which can have arbitrary attitude. The present paper describes the concept and the performance of the developed prototype of the portable system.
Tu NGUYEN VAN Satoshi YAGITANI Kensuke SHIMIZU Shinjiro NISHI Mitsunori OZAKI Tomohiko IMACHI
A metasurface absorber capable of monitoring two-dimensional (2-d) electric field distributions has been developed, where a matrix of lumped resistors between surface patches formed on a mushroom-type structure works as a 2-d array of short dipole sensors. In this paper absorption and reflection of a spherical wave incident on the metasurface absorber are analyzed by numerical computation by the plane-wave spectrum (PWS) technique using 2-d Fourier analysis. The electromagnetic field of the spherical wave incident on the absorber surface is expanded into a large number of plane waves, for each of which the TE and TM reflection and absorption coefficients are applied. Then by synthesizing all the plane wave fields we obtain the spatial distributions of reflected and absorbed fields. The detailed formulation of the computation is described, and the computed field distributions are compared with those obtained by simulation and actual measurement when the spherical wave from a dipole is illuminated onto a metasurface absorber. It is demonstrated that the PWS technique is effective and efficient in obtaining the accurate field distributions of the spherical wave on and around the absorber. This is useful for evaluating the performance of the metasurface absorber to absorb and measure the spherical wave field distributions around an EM source.
Hiroyuki UNO Yutaka SAITO Yoshio KOYANAGI Satoshi YAGITANI Isamu NAGANO
This paper presents the switched-beam slot antenna over the electromagnetic band-gap (EBG) reflector. This antenna is composed of two slot elements fed with a phase difference and the EBG reflector, which is used in order to realize a low profile structure. The radiation characteristics of this antenna are calculated using the FDTD method. Calculations show that the height of the antenna using the EBG reflector is 60 % lower than that of the antenna using a perfect electric conductor (PEC) reflector. The radiation characteristics at the center of the operating frequency band in the EBG reflector are equivalent to that in the PEC reflector. It is shown that the tilt angle of the main beam in the elevation plane varies with the operating frequency, and the variation in the case of the EBG reflector is caused by its frequency-dependent reflection phase. Moreover, the radiation pattern of the fabricated antenna is measured. The results demonstrate that the low profile design can be achieved by using the EBG reflector, and reveal the influence of the EBG reflector on the antenna efficiency.
Satoshi YAGITANI Mitsunori OZAKI Hirotsugu KOJIMA
A sensor network consisting of a number of palm-sized nodes with small electric and magnetic sensors has been proposed to monitor local electromagnetic activities in space plasmas. In the present study, a compact loop antenna system is designed and fabricated for use in sensor nodes that can capture magnetic vector fields from ELF to MF frequencies. The performance of the developed system is shown to be sufficient to allow measurement of the magnetic field activity around artificial structures in addition to intense natural plasma waves in geospace.
Isamu NAGANO Paul A. ROSEN Satoshi YAGITANI Minoru HATA Kazutoshi MIYAMURA Iwane KIMURA
The Akebono satellite observed the Australian Omega signals when it passed about 1000km over the Omega station. In this paper, we compare the observed Omega signal intensities with the values obtained using a full wave calculation and we discuss a mechanism of modulation of the signals. The relative spatial variations of the calculated Omega intensities are quite consistent with those observed, but the absolute calculated intensities themselves are several dB larger than the observed intensities. This difference in intensity may be due to the horizontal inhomogeneity of the D region, which is not modeled in the full wave calculation, or to an incorrect assumption about radiation characteristics of the Omega antenna. It is found that modulation of the observed signals is caused by the interference between the waves with different k vectors.
Satoshi YAGITANI Keigo KATSUDA Masayuki NOJIMA Yoshiyuki YOSHIMURA Hirokazu SUGIURA
A thin electromagnetic band-gap (EBG) absorber is employed to capture the 2-d image of radio-frequency (RF) power distribution. The EBG absorber consists of an array of mushroom unit cells formed on a thin dielectric substrate with a metal backplane, and lumped resistors interconnecting the surface patches of the mushrooms. Around the resonance frequency at which the EBG structure acts as a high-impedance surface, the RF power incident on the surface is absorbed by the lumped resistors which are matched with the incident wave impedance. By detecting directly the amounts of power consumed by the individual resistors, an “RF power imager” can be constructed which captures the 2-d distribution of the RF power illuminating the EBG surface, where polarization discrimination is possible. The resonance (i.e., absorption) frequency is made tunable by adding varactor diodes in parallel with the lumped resistors. The EBG absorber tunable in the frequency range of 700 MHz–2.7 GHz is designed and fabricated, and its performance is evaluated by an equivalent-circuit analysis, simulation and measurement. It is shown that the small resistance of the varactors have a considerable effect on the absorption performance. RF power distributions radiated from a dipole antenna are successfully measured by a matrix of sensitive power detectors installed on the backside of the absorber. Using such an RF power imager, the power distributions of even impulsive RF signals and/or noises can be captured and visualized in situ and in real-time, while the electromagnetic environment is almost undisturbed by the EBG absorber.