non-Foster Matching Network Research Articles

Digital non-Foster-inspired electronics for broadband impedance matching

Narrow bandwidths are a general bottleneck for applications relying on passive, linear, subwavelength resonators. In the past decades, several efforts have been devoted to overcoming this challenge, broadening the bandwidth of small resonators by the means of analog non-Foster matching networks for radiators, antennas and metamaterials. However, most non-Foster approaches present challenges in terms of tunability, stability and power limitations. Here, by tuning a subwavelength acoustic transducer with digital non-Foster-inspired electronics, we demonstrate five-fold bandwidth enhancement compared to conventional analog non-Foster matching. Long-distance transmission over airborne acoustic channels, with approximately three orders of magnitude increase in power level, validates the performance of the proposed approach. We also demonstrate convenient reconfigurability of our non-Foster-inspired electronics. This implementation provides a viable solution to enhance the bandwidth of sub-wavelength resonance-based systems, extendable to the electromagnetic domain, and enables the practical implementation of airborne and underwater acoustic radiators.

Power-Handling Capabilities of Non-Foster Impedance-Matching Networks: Using Gridded Vacuum Tubes and Power Transistors

Non-Foster networks can be used to match the impedances of highly reactive loads to resistive loads over very broad bandwidths. They are used to design ultrawideband, electrically small antennas (ESAs) in which negative impedance converters (NICs) eliminate the very large input reactances of the antennas over wide bandwidths. NICs, however, use active elements that put upper bounds on their power-handling capability. We examine the limitations of the power-handling capabilities of non-Foster impedance-matching networks that use gridded vacuum tubes and compare them with those of the networks using power transistors. Specifically, we examine the requirements for transmitting 5 W of power from a 5.18-m monopole antenna and from an ideal, minimum-quality factor (Q) dipole antenna over the 3n30-MHz band. We describe the situations in which tubes outperform transistors and examine the efficiency levels that can be realistically achieved. The results provide important guidelines for determining the suitability, or lack thereof, of using non-Foster matching networks for ultrawideband transmitting ESAs. Moreover, we offer a method for selecting an active device and the antenna type that can deliver the required power level given the bandwidth and efficiency constraints.

Non-Foster Matched Antennas for High-Power Applications

A non-Foster matching network has been designed for a small cylindrical slot antenna, and its broadband matching capability has been examined at varying signal power levels. It has been observed that the non-Foster circuit (NFC) impedance changes considerably as the signal power increases, thus reducing its matching capability as well as introducing stability problems. To analyze its true advantage in transmit applications, the third-order intermodulation output intercept point (OIP3) and the gain-bandwidth product of the non-Foster matched antenna have been compared with the unmatched antenna, when both are attached to the same power amplifier. Simulations show that the unmatched antenna has a higher OIP3 and gain-bandwidth product, despite being more poorly matched. Although the OIP3 can be slightly improved by increasing the bias current of the NFC, improved linearization techniques should be studied to offset distortion effects in NFCs.

Gain and Noise Analysis of Non-Foster Matched Antennas

Non-Foster matching networks can overcome the gain-bandwidth limitations of passive small antennas, but at the cost of increased noise generated by the active matching circuit. Gain and noise measurements of a non-Foster matched small loop antenna were compared with those of the passive loop antenna to quantify the improvement in received signal-to-noise ratio (SNR) with non-Foster matching. The average gain improvement of 7 dB from 30–135 MHz was accompanied by an average added noise of 8.9 dB, indicating that the non-Foster matched antenna did not provide any improvement in the received SNR compared with the passive antenna. These results led to a study of the various noise sources in a non-Foster receiving system. Noise models were developed for the balanced and unbalanced Linvill negative impedance convertor circuits and verified with simulations. Simulations were then done to analyze the noise figure (NF) of a non-Foster receiving system for various small antennas, different biasing conditions, and receiver noise levels. These were then compared with NF simulations of small antennas attached to an amplifier and a noisy receiver. The results indicate that for receivers with low-noise floor levels, a receiving system consisting of a passive small antenna and an amplifier can provide better SNRs compared with a receiving system with a non-Foster matched small antenna.

Non-Foster Matching of a Resistively Loaded Vee Dipole Antenna Using Operational Amplifiers

A non-Foster matching network using negative impedance converters (NICs) is proposed for a resistively loaded vee dipole antenna (RVD). The RVD suffers from considerable reflection at the feeding transmission line at low frequencies because of its large negative input reactance. To reduce the reflection, a non-Foster matching network, which is composed of NICs based on operational amplifiers (op-amps), is proposed. For the practical design, the effects of input parasitic capacitances of the op-amp and the finite open-loop gain are considered. The stability analysis is performed according to the Barkhausen stability criterion. The non-Foster matching network is implemented with the RVD as a load. The measured impedance demonstrates that the matching network significantly reduces the input reactance of the RVD. In the time-domain analysis, the matched RVD is shown to reflect significantly less incident pulse. A target was imaged through a series of radar echo experiments. The images show that the non-Foster matching network enhances the performance of the RVD as an antenna for the ground-penetrating radar.

Non-Foster matching network design for VLF receive loop antenna

Non-Foster matching network design for VLF receive loop antenna

Sensitivity Analysis for Active Matched Antennas With Non-Foster Elements

During the last years many researchers have been working on the active matching or on non-Foster matching networks for one- and two-port electrically small antennas (ESAs). A new parameter on the sensitivity of the two-port electrically small antenna when loaded with a non-F oster network is presented. This sensitivity analysis will allow us to choose what kind of antennas can be properly matched with non-Foster networks and their position in order to optimi ze the performance of the design. Then, a typical high Q two-port antenna will be harder to match over a broad bandwidth, since |S21| is very small and only agrees with |S11| over very small frequency bands, yielding very large sensitivity values. However, for these two-port antennas, if high levels of coupling can be engineered for a high Q multiple-port antenna, the return and insertion losses can be similar over larger bandwidths and, hence, the sensitivity can be kept low over larger bandwidths, enabling broader impedance matched bandwidths to be achieved, even for highly resonant antennas.

Two-Port Representation of an Antenna With Application to Non-Foster Matching Networks

A new theoretical tool is introduced for treating an antenna as a two-port network. This tool is very useful for allowing the antenna and its matching network to be analyzed and optimized in a circuit simulator. In this paper, we apply the idea to the situation where the matching network of the antenna incorporates non-Foster (active) matching elements, and so overall circuit stability becomes an overriding consideration. Non-Foster matching networks are particularly interesting because they offer the potential to transform the antenna into an active component that is no longer limited by the gain-bandwidth-size constraints of passive antennas, and whose performance can be improved as semiconductor technology advances.