The HTS microwave effort at Saitama University is developing cavity filters with conventional screw tuning mechanisms. The HTS elements are cylindrical cavity walls; internal to each cavity is a dielectric puck. While both conventional wireless and satellite communication links currently use similar non- superconducting filters, and these cavity devices have the potential of handling higher powers than the compact planar structures, this particular approach is currently limited to unloaded Q values of 40,000 at 77 K, a performance no better than that achievable with cold copper cavities. The bulk HTS material used has a surface resistance Rs (10 GHz, 77 K) of 20 m?, which must be improved substantially in order to create new capabilities. For comparison, it is noted that the thick-film HTS resonators reported by Illinois Superconductor Corp. have Rs (10 GHz, 77 K) of 2.7 m?, and hence unloaded Qs approaching 300,000.
A wireless-related application area for insertion of HTS microwave technology is satellites for "bent pipe" transponder relays. The same performance advantages and issues apply here as for the terrestrial wireless, except that the longer distance link and power/weight constraints of the platform accentuate the dual benefits of increased sensitivity and selectivity, but they also simultaneously place higher demands on power handling for transmit. Furthermore, the remote platform demands long-life ultra- reliable cryocoolers. The AMTEL team has conducted a paper study of a variant of satellite communication in which a shipborne receiver would exploit HTS for improved filtering in order to reject an adjacent band interference created by a transmit source much closer than the satellite providing the link to the ship. Although this application assessment did consider IMD arising in the receiver preamplifier, it did not consider the effect of HTS nonlinearities on filter distortion.
There is also a delay-line memory activity at NEC in which relatively short (2.26 ns) coplanar HTS lines have been characterized as buffer memories for asynchronous transfer mode (ATM) data switches. At a temperature of 65 K and below, these lines provide below 1 dB loss for signals of up to 10 GHz. The notion is to pack a number of parallel lines with semiconductor switches into a crossbar configuration and provide thereby a 1.6 Tb/s 16 x 16 switch. The application seems less than compelling, as the semiconductor switches will likely limit the switching times to those achievable by standard semiconductor architectures.
Finally, there are oscillator/mixer/antenna combinations in LTS and HTS being explored at both ISTEC and the Kansai Applied Research Center (KARC) of the Ministry of Posts and Telecommunication. These efforts are parallel to university activities in the United States: they stress the submillimeter wavelength regime and appear motivated more by science than application issues.