HTS materials support the flow of dc current without resistive loss. At radio or microwave frequencies, f, resistive losses increase at approximately f2, but are still much smaller than those found in even cold normal conditions. The basic HTS device concept is to exploit this advantage of low loss for microwave signals to form high-performance and complex filters in more compact, more easily integrated structures than can be accomplished with normal technology. The rf spectrum is in global short supply for the various communications services offered and proposed. Recent license auctions in the United States underscore the commercial value of the radio spectrum. Obviously, the more customers connected and the broader variety of services that a provider can support within a licensed band, the greater the return on investment. This places a premium on bandwidth efficiency, and thus sharp frequency skirts are important in order to mitigate interference from adjacent licensed bands. The sharper the skirt, the less "guard band" is wasted within the licensed spectrum. The notion is captured for both satellite and ground links in Figure 4.1, provided by AMTEL.
In addition to the improved utilization of spectrum through improved selectivity, it is highly desirable to reduce the size of the base station. This is particularly relevant as the "smarter," less visually obtrusive, and more power-efficient base stations are deployed, because these will site additional transmitter and receiver assets on the antenna mast as a means to improve transmit efficiency and receive sensitivity. Optimally, these improvements can be obtained by installing the amplifiers and filters right behind the antenna(s). The base stations targeted for such enhancements would be those supporting large numbers of customers, as these sites could more readily accommodate the cryoelectronics and amortize the increased cost.