Why are superconductors useful for wireless filters?
Microwave filters made from superconductors have the advantage that the tradeoffs between filter selectivity and filter efficiency that exist with conventional materials do not apply to superconductors. Ordinary filters are increasingly inefficient (the signal is wasted in the form of heat) as they are made more selective (more filter stages or poles). Superconducting filters have extremely low insertion losses even when they are made with a large number of poles. Thus better filters become practical when they are made from superconductors. A 19-pole superconducting cellular filter. Because of the increasing losses in copper, conventional filters are limited to no more than 9 poles. Conductus has made cellular filters with as many as 19 poles. In addition, thin-film superconducting filters are much more compact than the resonant cavity filters currently used in base stations. Those filters can be up to three feet long; the 19-pole filter shown above is 3-inches on a side.
