We report on the spurious suppression effect in low-microwave power transmitters by high temperature superconducting (HTS) bandpass filters (BPFs) which are promising for devices requiring BPFs with high-frequency selectivity. Some of the major issues on the power BPFs with HTS planar circuits for wireless communication applications are reviewed. As a case study for the HTS filter and its spurious suppression effect, this paper describes an example of the measured power spectrum density (PSD) on the suppression effect by one of our developed power BPFs with YBCO films for the 5 GHz band. It was designed with equivalent cascade resonators of 16 poles. We demonstrated the effect by HTS power filter in a power amplifier for the 5 GHz band.

We have developed a high-temperature superconducting (HTS) filter with narrow bandwidth characteristic for receiver of weather radar in order to reduce interference between adjacent radar channels. To realize a filter with which a narrow bandwidth and low insertion loss are compatible, resonators with high unloaded Q (Qu) value are required. Hairpin microstrip resonators with 1.5 times wavelength were adopted to suppress the radiation loss and achieve a high Qu value. The developed HTS filter has 8-pole quasi-elliptic function response for sharp skirt characteristic. The measured frequency response of the developed filter shows center frequency of 5370 MHz, insertion loss of 2.04 dB and maximum return loss of 15 dB, which agrees with the designed responses.

An ultra low-noise and highly selective, experimental 2-GHz band cryogenic receiver front end (CRFE) has been newly developed for cellular base stations. It utilizes a high-Q superconducting filter, a very low noise cryogenic amplifier, and a highly reliable cooler that is very compact. Fundamental design of the CRFE is investigated. First, the equivalent noise temperature of the CRFE and the effect of improving CRFE sensitivity on base station reception are discussed. Next, essential technologies and fundamental characteristics of each component are described. Finally, influence of antenna noise, such as ground noise and man-made noise, is estimated through field tests both in urban and suburban areas.

High-temperature superconducting (HTS) films have low surface resistance in the microwave region. This indicates that HTS passive microwave devices, for example, filters and antennas, are promising for commercial use. For superconducting filters, low insertion loss, sharp skirt property and high attenuation value out of band are important, and for superconducting antennas, electrically small antennas and super-directive high-gain antennas are useful. It is known that the elliptic filter has a sharp skirt property compared with that of a Chebyshev filter. We examined a cross-coupled filter which is one type of elliptic filter. The insertion loss of the filter was approximately 0.3 dB. The skirt property of the four-resonator cross-coupled filter was almost equal to that of an eight-resonator hairpin filter. We also examined a cryocooler cooling system for superconducting antenna and measured the relative gain, directivity and power capability of an HTS antenna. The gain of the HTS patch antenna was approximately 10 dB larger than that of a normal conducting antenna. The large gain of the HTS antenna was caused by the low surface impedance and enhancement effect of the dielectric window of the cooling system.

We propose a novel planar filter design for narrow-band applications. The filter consists of half-wavelength ring resonators with open gaps. This design has three advantages over conventional planar designs: a smaller size despite narrow bandwidth, a sharper skirt response at the passband edge without notch, an excellent out-band attenuation. We demonstrated these advantages by fabricating an 8-poles filter centered at 1.95 GHz with a 5 MHz bandwidth using YBCO films on a 2 inch diameter MgO substrate.