Toward Self-Oscillating Non-Foster Unit Cell for Future Active Metasurfaces

The concept of a self-oscillating non-Foster unit cell, intended for use in future active metasurfaces, is introduced. It is based on two orthogonally polarized small antennas connected via a negative impedance converter (NIC). An NIC converts the admittance of the first antenna into a negative admittance, canceling the admittance of the second antenna within a theoretically infinite bandwidth. This system behaves as a special kind of a nearly perfectly matched small active antenna that supports oscillations at every frequency within NIC's operating bandwidth. Frequency tuning is achieved either by inclusion of a resonant circuit or by injection locking from an external source. The influence of NIC dispersion, non-linearity, and antenna type on the system performances is discussed, and appropriate design guidelines are given. Several scaled experimental demonstrators of non-Foster self-oscillating unit cell, operating in lower RF range, were designed, manufactured, and tested. The experimental results proved the correctness of the basic idea and showed stable self-oscillations with a tuning range from 1:2 to 1:3.