In mobile communication system, it is a serious problem that radio wave signal from base stations is blocked by the high and dense buildings in a downtown, high-building district, especially in narrow streets, which will result in a weak signal level and poor communication qualities, and this kind of areas are typically called blind spots. Many efforts were made in order to eliminate blind spots which dramatically degrade the efficiency of data transmission. The general way is to add RF boosters on the top of high-building but with high installing expenses (activity components and RF generator are required) and bulkiness power supply system. Moreover, installation of the heavy boosters is also a critical issue because quake-resistance regulations should be taken into account in areas with frequent earthquakes, especially in Japan. Therefore, from the economic and technological points of view, a passive device is generally desired.