Some Fundamental but Rarely Discussed Properties of Passive, non-Foster, and Time-varying Matching of Small Antennas

There are dozens of successful implementations of non-Foster and time-varying matching of small antennas in the literature, but a clear comparison with passive cases in terms of bandwidth and efficiency is often lacking. This is due to the fundamentally different physical backgrounds of passive and active matching, which are not yet well understood. Here, we discuss the most significant differences and their importance in practical implementations.