Katsufumi Ehata

Fabrication of low surface resistance YBCO films and its application to microwave devices

Abstract We have examined a fabrication process of low surface resistance (Rs) YBa 2 Cu 3 O 7− δ (YBCO) films on CeO 2 /Al 2 O 3 substrates by a laser ablation technique. The surface morphology of CeO 2 films depended on the film thickness of CeO 2 . A critical value of the CeO 2 film thickness at ∼90 nm was found above which the surface roughness increased abruptly. The Rs of YBCO films also depended on the surface morphology of CeO 2 films. The Rs value of YBCO films on thinner CeO 2 films ( 2 films (>90 nm). We have also examined the high-temperature superconducting (HTS) antenna as one of the HTS passive devices, and measured the power handling capability. The compact cooling system for HTS patch antenna was constructed.

Effect of surface impedance on the antenna properties in superconducting microstrip antenna

Abstract The complex surface impedance ( Z s ) of a superconductor is comprised of two unequal components, surface resistance ( R s ) and surface reactance ( X s ). We have investigated the effects of R s and X s on the YBa 2 Cu 3 O 7− x (YBCO) superconducting microstrip antenna properties (resonant frequency, input impedance, and gain) by theoretical analysis and experimentation. The antennas were fabricated from YBCO thin films on (100) MgO single crystal substrate at 11.7 GHz. The investigation of the antenna properties reveals that the X s of the superconducting thin film mainly influences the antenna resonant frequency ( f r ) and has small influence on the antenna input impedance ( R in ). On the other hand, R s mainly affects the input impedance of the antenna. The antenna gain, and hence the efficiency, is found to be influenced by the film surface resistance. The antennas were characterized in the temperature range of 10–90 K. Possible influences of the substrate dielectric properties (permittivity, e r and loss tangent, tan δ ) on the antenna properties were also considered in the investigation.

Miniaturized cooling systems for HTS antennas

Two types of miniaturized cooling systems for High-Temperature Superconducting (HTS) antennas were designed and built. In this paper, details of these systems, measured and simulated properties of patch antennas installed in these systems, and results of a seal-off trial for 1 month are reported. Both the systems had portable size and weight and could cool the HTS devices. When a patch antenna with resonant frequency of 4.9 GHz was installed in the 1st cooling system, the radiation pattern of the antenna was strongly focused in the forward direction. This is because the vacuum chamber in which the antenna was packaged influenced the microwaves radiated from the antenna. Due to this effect, the directive gain of the antenna was enhanced by 11.2 dB. In the 2nd system, a seal-off trial was carried out to investigate the airtight property that is necessary for thermal isolation. The temperature of the sample stage could be kept under the critical temperature of YBCO for 1 month without continuous vacuum pumping. The cooling systems developed in this work are expected to promote the practical use of HTS antennas.

Fabrication of low Rs YBCO films and its application to patch antennas

We have examined that how to reduce the surface resistance, Rs of YBCO films, and found that control of the in-plane alignment of the YBCO films was very important to realize that. The in-plane alignment of the YBCO films on MgO substrates could be controlled by the BaSnO3(BSO) buffer layers, and self-template method. We also examined YBCO patch antenna and its compact cooling system. The compact cooling system was easy to keep YBCO patch antennas in the superconducting state.

Examination on power handling capability of superconducting patch antenna

Measured results of power handling capability characteristics of superconducting patch antenna and its improvement are reported. A direct-fed patch antenna is fabricated by YBa/sub 2/Cu/sub 3/O/sub y/ thin film with 50 nm thickness. The gain of the antenna does not change with the applied power up to 26.3 dBm at 12.3 K, but drop of the gain is observed more than 26.5 dBm. It is found by electromagnetic analysis that this gain drop occurs due to huge current concentration in the direct-fed patch. However, this current concentration can be reduced using the gap-coupled feed, and breakdown power is expected to be three times larger than that of direct feed.

Fabrication and characterization of HTS antennas for satellite communication and security system

We have examined the HTS receiving patch and patch array antennas for satellite broadcast and microwave security system. The HTS antennas were made by YBCO films prepared by plasma coupled sputtering system. The gain of the HTS patch antenna was approximately 3 dB higher than that of the copper patch antenna with the same geometry. We have constructed the mini-cryocooler cooling system for the HTS antennas. The directivity was not affected by the cooling system.

Characterization of Superconducting Patch Antenna Fabricated by Both-Side Superconducting Thin Film

We have fabricated a patch antenna with a both-side superconducting thin film. The fabricated antenna shows 1dB and 4dB higher gain than that of a single-side superconducting antenna and a normal conducting antenna, respectively. We observed a change of the input impedance characteristics near the T c in the both-side superconducting antenna. This is caused by the temperature dependence of the surface impedance of the superconducting thin film. The gain of this antenna does not change by applying power less than 25dBm below 80K.

Development of a miniaturized cooling system for HTS antennas and its seal-off property

Abstract Measured results and discussion on vacuum seal-off property of a cooling system for high-temperature superconducting antennas are reported. The cooling system was designed and fabricated and its cryogenic packaging was investigated. Vacuum seal-off tests were performed and high vacuum for thermal isolation could be kept for 25 days. On the cold stage of the system, some kinds of gas, including H 2 O and CO 2 , were adsorbed on a cold stage.

Development of a Miniaturized Cooling System for HTS Antennas

A miniaturized cooling system for HTS antennas was developed using a Stirling cryocooler. Its structure and the property of a patch antenna installed in this system are reported. The system showed sufficient cooling ability to cool HTS microwave devices in spite of its portable size and weight. When a patch antenna was installed in this system, radiation from the antenna was focused to the front direction and its gain was enhanced by 11.2 dB. This fact was confirmed not only experimentally but also theoretically.

Superconducting Planar Array Antenna at 11.7 GHz for Satellite Communication

The designed and experimental results of microstrip patch and patch array antenna at 11.7 GHz have been reported. Two different methods- “Transmission Line Model” and a microwave circuit simulator, em based on “FFT”- have been used in the design. The antennas were fabricated by wet etching process and subjected to resonance frequency, input impedance and gain measurements.The superconducting array antenna has shown more than 3 dB higher gain over its metal counterpart with same planar geometry. Therefore, extreme low profile and high gain present superconducting planar arrays as a promising candidate for satellite-based communication systems.