Wideband, Non-Foster Impedance Matching of Electrically Small Transmitting Antennas

Electrically small antennas (ESAs) can be passively matched only over very narrow bandwidths and the resulting antennas have low gains. These are the major limiting factors for ESAs used in transmit applications, especially at high-frequency (HF) and lower Very HF frequency bands. This paper discusses the challenges of transmit ESA matching circuit design and the design process of a new non-Foster transmit matching architecture for electrically small monopole antennas that achieves wide bandwidth, high transmission efficiency (transducer power gain), and stability at the same time. The proposed circuit is composed of a current buffer (for high isolation), a transformer (for real-part matching), and a negative impedance converter (for imaginary-part matching). The measured −6 dB (−10 dB)
$|S_{11}|$
fractional bandwidth of the proposed non-Foster transmitting system is 110% (39%), while the maximum bandwidth that can be achieved is 0.076% (0.047%) when matched with the conventional passive matching. The transmission efficiency of the system is improved by as much as 34.4 dB compared to the same antenna without the proposed non-Foster matching circuit, and it retains an enhanced efficiency over the entire frequency band of operation (26–89 MHz). The system remains stable within this frequency band. The measurement results of the compact and broadband transmitting antenna prototype verify the design concept.