Equivalent Circuit of OCS Negative Capacitor based on Linvill’s Floating NIC

An open-circuit stable (OCS) non-Foster negative capacitor is frequently used for broadband matching of a short dipole. The most common implementation is based on Linvill’s floating negative impedance converter (NIC). Unfortunately, assurance of stable operation of such a system is a rather challenging engineering problem. It is usually solved by many cut-and-try development-fabrication cycles, often without clear understanding of the cause of instability. Here, we propose two simple equivalent circuits of an OCS negative capacitor that give clear insight into the influence of particular design parameters on instability and dispersion, enabling a simpler design.