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Electrically-small antennas present high-Q impedances characterized by large reactances and small radiation resistances. For such antennas, the effectiveness of passive matching is severely limited by gain-bandwidth theory, which predicts narrow bandwidths and/or poor gain. With receivers, the inability to resolve this impedance mismatch results in poor signal-to-noise (S/N) ratio, as compared to using a full-size antenna. With transmitters, the consequence is poor power efficiency. However, in many applications full-size antennas are impractical, and a means is required to effectively match their electrically-small counterparts. This paper presents the technique of non-Foster impedance matching, which employs active networks of negative inductors and capacitors to bypass the restrictions of gain-bandwidth theory. We first review the origins and development of non-Foster impedance matching, and then present experimental results for the non-Foster impedance matching of electrically-small dipoles and monopoles. For receivers, our best measurements on the antenna range demonstrate up to 20 dB improvement in S/N over 20-120 MHz; for transmitters, we show a power efficiency improvement which exceeds a factor of two over an 5% bandwidth about 20 MHz with an average signal power of 1 W to the radiation resistance.

A new microwave active allpass network is proposed. In a computer simulation, it is shown that the network, even with markedly non-ideal transistors, can provide a true allpass response over 8 - 12 GHz.

A methodology for microwave active filter design has been presented by this author (see ibid., vol.37, no.9, p.1418-24, 1989). The present work serves to verify aspects of this design approach. A practical implementation of a network discussed in that work is given. First, an ideal preliminary design for a second-order bandpass filter is obtained. Next, the nonideal effects of the active devices and the lossy reactive elements are introduced, and then optimization is used to obtain the final design. Finally, the filter is constructed and tested. The measured response is found to agree quite closely with the design specifications. The filter section is constructed in hybrid form with lumped components, and uses two NEC 900100 GaAs FETs. The prototype accurately realizes a second-degree bandpass response with a center frequency of 6 GHz and a pole q of 2.5. >

A comprehensive history of microwave active filter development to date is given. A design methodology is presented for the realization of precision broadband filters in the face of problems such as the nonideal behavior of microwave GaAs FETs and the low Q and related parasitic effects of monolithic-microwave-integrated-circuit (MMIC) inductors and capacitors. The feasibility of this approach is illustrated by the computer-simulated design of a cascadable, second-order microwave bandpass filter. >

An improved model for the GaAs MESFET has been implemented in the source code of the circuit-simulation program SPICE. New features include 1) an accurate model for the Schottky barrier, which allows simulation of both enhancement- and depletion-mode devices; 2) detailed modeling of the nonlinear gate-source and gate-drain capacitance; and 3) a user-specifiable value for the exponent in the expression for the dependence of the dc drain current upon the gate-source voltage. Also discussed are some important points concerning the charge-voltage equations that must accompany the new model's capacitance-voltage equations within SPICE. The new GaAs FET SPICE model is believed to be the most comprehensive one available to date in the public domain.

The author describes models for the lossy transmission line and the Schottky diode that are incorporated into the source code of the circuit simulation program SPICE (version 2G.6). The model used for the lossy transmission line was derived by A.J. Gruodis (1979). The DC model for the Schottky barrier diode as shown was formulated by D.B. Estreich (1983). Examples used to verify the models are given. >

A model for the gallium arsenide MESFET has been implemented into the source code of the well-known circuit simulation program SPICE. Both large-signal and small-signal simulations of MESFET circuits are now possible within the familiar framework and data format of SPICE. The line-by-line modifications made to SPICE to achieve the model implementation will be made available to interested researchers.