In any mobile communications system, there are poorly covered areas where radio waves are weak or cut-off because of transmission obstacles. These obstacles can be effectively eliminated by using economical relay systems, called “boosters”, between the base and mobile stations. To incorporate a booster in a W-CDMA system, high sensitivity (low noise) and high selectivity performance are required to expand the coverage and resolve or reduce problems such as interference caused by other systems working at adjacent frequencies or a reduction in user capacity that could be caused by booster transmission noise. To eliminate areas of weak or even no coverage, we found that an open-area relay "booster" system using a cryogenic superconductor front-end is indispensable. This paper presents the feasibility of practical application of a cryogenic front-end for an open-area booster. In our experiment, we made a cryogenic front-end with a pinched-off vacuum vessel and 2-W cryocooler. We used a 17-pole high-temperature superconducting filter (HTSF) and, -achieved 20-dB attenuation only 200 kHz from the band edge, for up-link and down-link input filters.
The study of nonbolometric responses caused by the vortex motion and other factors of YBa2Cu3O7-δ (YBCO) microbridges has received much attention because of possible applications for sensitive and fast response detectors. In this study, we fabricated a slot antenna-coupled YBCO microbridge and investigated IF properties at 94 GHz to evaluate the response speed of the device in the resistive state. The response mechanism of the device biased in the resistive state consists of the bolometric response and the nonbolometric response caused by the flux creep and flux flow. From the measurements of voltage-current (I-V) characteristics and video detection properties, the response mechanism was investigated and operating conditions that dominated the bolometric and nonbolometric responses were obtained. The IF bandwidth of the device, having its response dominated by the bolometric process, showed a constant value of about 60 MHz and did not depend on the bias current. On the other hand, the bandwidth dominated by the flux flow response increased as the bias current and temperature increased, up to 1.2 GHz, and depended on the vortex velocity.
The characteristic features of film boiling in both sub-cooled and saturated He II were experimentally investigated. Special emphasis was placed on the film boiling state around the lambda pressure (pλ= 5.04 kPa). It was found that there are two film-boiling modes clearly divided by a line just above pλ. One is a typical sub-cooled boiling mode that is normally found at higher pressures than pλ. The other mode resembles noisy film boiling in saturated He II. The boundary between the two boiling modes lies at the pressure just above pλ, the pressure of which slightly increases as the bath temperature decreases. It was found that noisy film boiling does not occur and the regions of the sub-cooled and silent film boiling modes connect directly in the vicinity of the lambda point. At the same time, the pressure and temperature variation were also measured and the cause of the occurrence of those variations was clarified. The features of each boiling mode were characterized.