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Dive into the research topics where Masaharu Fujita is active.

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Featured researches published by Masaharu Fujita.


Journal of Atmospheric and Solar-Terrestrial Physics | 1980

Severe disturbances of VHF and GHz waves from geostationary satellites during a magnetic storm

Tadahiko Ogawa; K. Sinno; Masaharu Fujita; Jun Awaka

Abstract Severe night-time scintillations of VHF and GHz waves (0.136, 1.7, 4 and 11.5 GHz) emitted from the three Japanese geostationary satellites were observed during a geomagnetic storm on February 15, 1978. The GHz scintillations were strongly enhanced when irregular fluctuation of total electron content (TEC) was superposed on the background TEC which was varying sharply with time. This indicates a close relationship between the scintillations and irregularities. Detailed analysis shows that the one-dimensional wavenumber power spectrum of irregularities obeys a form of k−2 and that the scintillation spectra vary as ƒ s −3 . The scintillation index, S4, seems to have a frequency dependence of ƒ −0.5 for 0.136 ⪅ ƒ ⪅ 1.7 GHz, f−1 for 1.7 ⪅ ƒ ⪅ 4 GHz and ƒ −2 for 4 ⪅ ƒ ⪅ 11.5 GHz. It is found that this scintillation event was accompanied with large decrease in the plasmaspheric electron content, which continued for one and a half days after the storm onset, and with appearance of localized electron density trough in the F-region ionosphere.


International Journal of Infrared and Millimeter Waves | 1994

Design and analysis of a folded Fresnel Zone Plate antenna

Yu Ji; Masaharu Fujita

Based on the Kirchhoff-Huygens diffraction theory, a simple analytical method of a planar folded Fresnel zone-plate (FZP), that is the case when a planar reflector is placed behind the zone plates, has been developed. According to the numerical calculation results, the design procedure of the FZP antenna has been presented, and its focusing characteristics and gain-optimized conditions have been discussed. The variations of the focal field distribution with the antenna parameters such as zone numbers, focal length and antenna diameter and the radiation power patterns of the FZP have been simulated numerically. To take a good balance of both receiving and transmitting antennas, at 60GHz operating frequency, the focal length should be designed as a half of the antenna diameter and the zone number should be from 10 to 15. The results in this work show that the folded FZP has good focal characteristics and off-axis performance, and its antenna gain can be optimized by the suitable antenna parameter design. The possibility of applying the folded FZP as a low cost and high gain antenna without strict manufacturing requirement for millimeter-wave communications has been shown.


Journal of Atmospheric and Solar-Terrestrial Physics | 1978

1.7 GHz scintillation measurements at midlatitude using a geostationary satellite beacon

Masaharu Fujita; Tadahiko Ogawa; Kunimasa Koike

Abstract Preliminary results of 1.7 GHz scintillation measurements made in Japan using a geostationary satellite during May–August in 1977 are presented. Since the propagation path below about 100km altitude is nearly parallel to the geomagnetic field, it was possible to observe irregularities along the field direction. The scintillation activity was enhanced at night in June and the maximum peak-to-peak variation of scintillations observed was about 2.3 dB. Simultaneous measurements of the total electron conterrt show that irregular electron density structures play an important role in 1.7 GHz scintillations.


IEEE Transactions on Aerospace and Electronic Systems | 1980

ETS-II Experiments Part IV: Characteristics of Millimeter and Centimeter Wavelength Propagation

Masaharu Fujita; Takashi Shinozuka; Toshio Ihara; Yoji Furuhama; Hisao Inuki

Preliminary results of the ETS-II millimeter and centimeter wavelength propagation experiments are presented based on data collected over one year. Rain attenuation is discussed primarily from the statistical point of view. At 34.5 GHz attenuation in excess of 5, 10, and 19.5 dB were observed 1, 0.3, and 0.1 percent of the test time (5500 hr), respectively. At 11.5 GHz attenuation in excess of 2.5, 4.3, 6.3, 10, and 15.5 dB were observed 0.1, 0.03, 0.01, 0.003, and 0.001 percent of the test time, respectively. Duration time of attenuation is also examined for the benefit of a link design. The longest duration of attenuation of the 34.5-GHz wave exceeding 5 dB was about 150 min. At 11.5 GHz, the longest duration of attenuation in excess of 3 dB, was 60 min and that in excess of 6 dB was 10 min. Rain attenuation is discussed also with relation to the radar and the rain gauge data. Depolarization data of 34.5-GHz and 11.5-GHz waves are orocessed statistically, and it is shown that cross-polarization discriminations in excess of 21.5 dB and 29 dB are observed 0.1 percent of the test time at 34.5 GHz and 11.5 GHz, respectively.


Annales Des Télécommunications | 1981

Propagation characteristics of millimetre and centimetre waves of ETS-II classified by rainfall types

Yoji Furuhama; Toshio Ihara; Masaharu Fujita; Takashi Shinozuka; Kenji Nakamura; Jun Awaka

A propagation experiment at the frequencies of 1.7, 11.5 and 34.5 GHz using Engineering Test Satellite Type IIEts-II was conducted at the Kashima earth station of the Radio Research Laboratories (Rrl).Several meteorological parameters were also measured simultaneously at the station using rain gauge networks, a C-band rain radar, a 35 GHz radiometer and meteorological instruments.The whole data for one year in this experiment were classified into three rain types such as stratus, cumulusand others by using radar data along the path mainly to determine the size of precipitation area and whether the radar bright band existed or not.Rainfall classified as stratus occurred for 45 % of the total rainfall time, whilst rainfall classified as cumulus occurred for 10 %, and others for 45 % of the total rainfall time. For the co-polar attenuation, a prediction method based upon an assumption of spatial homogeneity is found to be applicable to the Earth-space path for the stratus type, though the method is not applicable for the cumulus type. On the contrary, in the degradation ofXpd,a prediction method based upon an assumption of deformed raindrops is not applicable to the path for the stratustype, while the method is applicable for the cumulus type.AnalyseUne expérience de propagation à des fréquences de 1.7, 11.5 et 34.5 GHz utilisant le satelliteEts-II a été réalisée à la station de Kashima du Radio Research Laboratories. Quelques paramètres météorologiques ont simultanément été mesurés à la station, à l’aide de pluviomètres, d’un radar météorologique dans la bande C, d’un radiomètre à 35 GHz et de divers instruments météorologiques. Tous les résultats de l’expérience pour une année ont été classés suivant trois types de précipitations: stratus, cumuluset autres cas;ceci, grâce aux enregistrements du radar qui ont permis de déterminer l’aire couverte par la précipitation et l’existence ou non-existence de la bande brillante du radar. L’occurrence de précipitations du type stratus est observée durant 45% de la durée totale de précipitation, le type cumulusdurant 10% et le type autres cas45%. On trouve pour l’atténuation copolaire une méthode de prévision basée sur l’hypothèse d’homogénéité spatiale qui est utilisable sur un trajet Terre-espace pour une précipitation du type stratus mais ne l’est pas pour le cas cumulus.Par contre, pour la dégradation du découplage de polarisation croisée, une méthode de prévision supposant les gouttes déformées n’est pas applicable au type stratusmais l’est au type cumulus.


Journal of Atmospheric and Solar-Terrestrial Physics | 1982

Frequency dependence of ionospheric scintillations and its application to spectral estimation of electron density irregularities

Masaharu Fujita; K. Sinno; Tadahiko Ogawa

Abstract Analytical results of simultaneous observations of 136 MHz and 1.7 GHz ionospheric scintillations at mid-latitude during summer are presented. The causal relation between scintillation occurrence and ionospheric condition is discussed to examine the influences of spread- F and sporadic- E on the scintillation. The diurnal variations of scintillation occurrences show that the time of maximum occurrence for the nighttime scintillation depends on the scintillation amplitude. The frequency dependence of the scintillation depth is, on the average, f t −1.38 during nighttime and f t −1.52 during daytime, where f t , is the transmitted wave-frequency, which may reflect the difference in the spectral form of electron density irregularities. Finally, the temporal variations of the frequency dependence index are investigated for the nighttime period. The result indicates a clear variation of the spectral form of irregularities with time.


Electronics and Communications in Japan Part I-communications | 2000

A slot‐array antenna on a coaxial cylinder with a circularly polarized conical beam

Kyouichi Iigusa; Tasuku Teshirogi; Masaharu Fujita; Shin‐Ichi Yamamoto; Tetsushi Ikegami

In mobile communication, omnidirectional directivity and circular polarization are convenient properties because when they are used, acquisition and track operations for direction and polarization are not needed. As an antenna which has such properties, we developed a slot-array antenna on an oversize coaxial cylinder. This array antenna has less loss than microstrip arrays because energy is transported to each slot in the oversize coaxial waveguide. Also we can shape the beam in the vertical plane by controlling the size and position of slots. We developed cosecant and conical beam antennas. They are scale model antennas for respectively indoor communication by millimeter waves and mobile satellite communications. We show the performance of these antennas.


Journal of Atmospheric and Solar-Terrestrial Physics | 1977

Measurement of total electron content by means of differential phase shift method using coherent SHF signals from a geostationary satellite

Jun Awaka; Masaharu Fujita

Abstract The total electron content along the radio wave path (TEC) is directly measured by observation of the differential phase shift between two SHF signals, which are transmitted with phase coherence from the Japanese geostationary satellite ETS-II. It is shown that the differential phase shift method has advantages over the Faraday rotation method, viz., independence of geomagnetic field and greater sensitivity.


Radio Science | 2003

Fundamental experiment of a polarization‐rotating reflector of Van Atta array design for polarimetric calibration

Masaharu Fujita; Hisashi Okubo; Yoshiyuki Fujino; Masato Tanaka

[1]xa0Polarization-rotating reflectors are important as a reference target for polarimetric calibration of a synthetic aperture radar. A polarization-rotating retrodirective reflector of Van Atta array design has already been proposed by one of the authors. This paper describes the result of an experimental verification of the proposed polarization-rotating retrodirective reflector. The experimental system consists of two subarrays each composed of 2 × 2 dual-polarized elements for horizontal and vertical polarizations working at 2.4-GHz frequency band. In the experiment, two subarrays are arranged side by side, and the respective orthogonal polarization ports of the subarrays are connected with lines of the same electrical length to form a one-dimensional polarization-rotating Van Atta array. To increase the radar cross section, amplifiers are inserted into the lines. The reflection pattern of the reflector is measured in a radio anechoic chamber by rotating the reflector around the axis perpendicular to the array normal direction. From the experiment, a wider angular response of the proposed reflector in the horizontal direction, which is provided by the retrodirective characteristics of the Van Atta array, than the conventional array reflector of the same element number as the proposed reflector is verified in addition to its polarization-rotating capability introduced by the dual-polarized antenna elements.


IEEE Transactions on Aerospace and Electronic Systems | 1980

ETS-II Experiments Part V: Effect of Ionosphere

Jun Awaka; Masaharu Fujita; Tadahiko Ogawa; K. Sinno; M. Kan

Propagation characteristics of gigahertz and very high frequency waves through the mid-latitude ionosphere have been studied applying the coherent beacon signals of 1.7, 11.5, and 34.5 GHz and the 136-MHz telemetry signal, which are emitted from the geostationary Engineering Test Satellite Type II (ETS-II). Total electron content (TEC) was obtained by measuring the differential phase shift between 1.7 and 11.5 GHz waves and also by the Faraday rotation of the 136-MHz carrier, the former method being more sensitive than the latter one for detecting fluctuating TEC. During the observations, amplitude scintillations at 136 MHz and 1.7 GHz, usually accompanied with an irregular variation of TEC, were recorded frequently in the nighttime and sometimes in the daytime. Specially noted is the 11.5-GHz scintillations less than 0.8 dBp-p that were observed during the severe magnetic storm on February 15, 1978, simultaneously with 136-MHz and 1.7-GHz scintillations. This was the first observation showing that the disturbed ionosphere affects waves above 10 GHz. Presented first are the techniques for the ETS-II ionospheric wave propagation experiment, including a description of the coherent receiver system in the gigahertz frequency range at Kashima and a description of the 136-MHz Faraday rotation receiver at Kokubunji. The capabilities of the facilities for monitoring the mid-latitude ionosphere by giving some observational results are then presented.

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Yoshiyuki Fujino

National Institute of Information and Communications Technology

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Toshio Ihara

Ministry of Posts and Telecommunications

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Kenji Nakamura

Ministry of Posts and Telecommunications

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Kyouichi Iigusa

Ministry of Posts and Telecommunications

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Tasuku Teshirogi

Ministry of Posts and Telecommunications

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Masahiko Onda

National Institute of Advanced Industrial Science and Technology

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Yoji Furuhama

Ministry of Posts and Telecommunications

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