Stable multiple non-Foster circuits loaded waveguide for broadband non-dispersive fast-wave propagation

Periodically loading non-Foster circuits (NFCs) in a conventional waveguide has been proved to be possible for achieving broadband non-dispersive fast-wave (FW) propagation. However, the unconditional stability of such structures has been argued due to the multiple NFC loadings. Despite the potential instability, it is demonstrated that the NFC-loaded waveguide can still be stable for specific terminations, in particular, with the 50 Ω load that is commonly used in radio frequency/microwave applications. For purpose of demonstration, a NFC-loaded waveguide that contains substantially more unit cells than in prior work has been designed and measured, showing a stable and causal FW propagation with a 1.2 c phase velocity over the bandwidth of 80-120 MHz.