“…Then the required level of the coupling factor is strongly dependent on the Q factor of the resonators. In fact, consulting my own vintage reference on the subject [3], I note that for coupled resonators, the critical coupling coefficient is given by subject, and industry, in its own right, and, in all honesty, largely a complete mystery to me. My simple observation as a potential user would be that escalating losses become problematic as the gigahertz frequency range is approached, up to 1 GHz.…”