A Dual-Source RF Transmitter System Makes Broadband Matching of a Highly Reactive Antenna Feasible Without Non-Foster Elements

The principle of non-Foster broadband matching of electrically small (high-Q) transmitting antennas has been around for more than a few decades. However, there have been only a handful of experimentally proven examples, with highly limited scope of application. The main obstacle is the inherent instability of non-Foster elements, since those are actually active circuits with positive feedback. This research investigates the idea that a common non-Foster transmitter antenna matching network can be replaced with the arrangement of multiple frequency-coherent sources, rendering positive feedback unnecessary. Two different approaches were analyzed, one with a coherent RF current source, the other one with a coherent voltage source. The viability of this idea has been verified by simulations and experimental results. Unlike non-Foster matching, the proposed principle cannot support instability because it does not contain any positive feedback loops. In addition, it offers multiple-octave operational bandwidth, while the efficiency is limited only by the linear operation of the used circuitry, thus remaining at least equal to that of its non-Foster equivalent. While the proof-of-concept experiments were conducted in the lower HF (up to 20 MHz) frequency band, where common antennas are electrically small and highly reactive, there are no physical obstacles to scaling the same principle of operation to higher frequencies.