Ryan S. Adams

Bandwidth Optimization of an Integrated Microstrip Circulator and Antenna Assembly: Part 2

The integration of a ferrite circulator with an arbitrary load - such as a wideband antenna - is considered by examining the conditions for perfect circulation over large bandwidths. These conditions relax the requirements of the loading impedance from being purely real (e.g., 50 ohms) to being complex, per a circulation impedance equation. The impedance equation is couched in terms of open-circuit impedance parameters, which are the natural parameters for describing an embedded microstrip circulator. By means of a rigorous field analysis, the impedance parameters are computed by modeling the microstrip ferrite circuit as a closed PEC/PMC cavity. The justification, features, and limitations of this model are discussed in detail, it is shown herein how the Z parameters can be invoked in some search algorithm to optimize the parameters of the ferrite, to achieve either superb circulator bandwidth or isolation performance. An example of a wideband circulator is given for which both simulation and experimental data are provided. The high degree of correlation between these two data sets corroborates the methodology

Performance of digital discrete-time implementations of non-Foster circuit elements

There is renewed interest in the use of non-Foster circuit elements in a variety of important applications such as wideband impedance matching and artificial magnetic conductors. Although non-Foster devices such as negative capacitors and negative inductors can be realized using current conveyors and Linvill circuits, a digital design approach may offer an important alternative in some applications. Therefore, digital discrete-time implementations of non-Foster circuit elements are investigated, and simulation results are presented for the implementation of a discrete-time negative inductor and a discrete-time negative capacitor.

Stability conditions for a digital discrete-time non-Foster circuit element

Digital discrete-time implementations of non-Foster circuit elements offer an alternative to conventional analog circuit approaches. In particular, the design of a discrete-time negative capacitor is investigated, since such non-Foster circuit elements offer significant potential in wideband antenna, metamaterial, and artificial magnetic conductor applications. As with analog non-Foster circuits, stability is an important design consideration for digital non-Foster elements. Therefore, stability conditions and simulation results are presented for a discrete-time negative capacitor, and the onset of instability is shown near the predicted stability boundary.

A novel unit cell and analysis for epsilon negative metamaterial

A novel unit cell and analysis are presented for a metamaterial with effective negative permittivity. The structure consists of two disks connected by a central post, and the analytic approach directly provides the electromagnetic field coupling to the resonant structure. By using Amperes circuital law in the development, the electromagnetic field effects are directly coupled to the resonant behavior of the structure. In this, the theoretical approach does not depend upon plasma resonance. The proposed structure is shown to exhibit resonant behavior with negative permittivity in a frequency band near resonance. As predicted, electromagnetic field simulations show inversion of the electric field as the frequency crosses resonance.

A Robust AlGaN/GaN HEMT Technology for RF Switching Applications

The authors report results suggesting that GaN HEMTs developed for RF switching applications do not suffer from an inverse piezo effect related reliability failure mechanism. A critical voltage was not observed prior to breakdown voltage, suggesting a robust GaN HEMT technology well suited for RF switching applications.

Integration of a Microstrip Circulator With Planar Yagi Antennas of Several Directors

An integration methodology is presented to design antenna systems incorporating a ferrite circulator. Two integrated systems comprised of a microstrip circulator and Yagi planar-type antennas are presented as validation of the proposed methodology. Excellent pass band characteristics are observed in the experimental and numerical data for both systems. Furthermore, one system is magnetically tuned to realize 30 dB isolation over a bandwidth of 850 MHz. Both systems maintain excellent return loss and show improved insertion loss specifications even though the ferrite is biased into partial saturation. Extensive measurement data of the ferrite material and system network parameters are provided to corroborate the claims made herein.

Wideband negative permeability metamaterial with non-Foster compensation of parasitic capacitance

The analysis and simulation of negative effective permeability of a magnetic metamaterial is presented, including parasitic effects. Beyond known issues of non-Foster circuit stability, such parasitics can limit bandwidth improvement. Based on the analysis, ideal non-Foster elements are added to split rings to achieve broadband negative effective permeability while compensating parasitic effects. Results indicate that both a negative capacitance and negative inductance are needed to achieve negative permeability from 0.5 to 4.5 GHz.

Multi-Polarized Microwave Power Imaging Algorithm for Early Breast Cancer Detection

A new image reconstruction algorithm for early breast cancer detection using ultra-wideband microwave signals is proposed. In this algorithm, the backscattered electric and magnetic flelds are measured and combined in a novel way; the direction of power ∞ow with respect to a given focal point is used to localize tumors. Signiflcant improvement in signal-to-mean raito (SMR) and signal- to-clutter ratio (SCR) are achieved when driving signals consist of waves with multiple polarizations. Numerical results demonstrate nearly 5.5dB improvement of SMR and SCR over the traditional Confocal Microwave Imaging method when a single 8mm breast tumor is present.

A Second-Order Accurate Time-Stepping Algorithm for Dominant Mode Propagation in Ferrite Media

This communication presents a fully second-order accurate time-domain simulation scheme that incorporates magnetic materials operating in the dominant mode. The proposed scheme is a modification of the Yee scheme by incorporating the Landau-Lifschitz equation of motion including the dominant and loss terms. Additionally, a rigorous dispersion analysis is presented for the proposed scheme. It is demonstrated that the proposed scheme has no numerical losses (when the phenomenological loss term is set to zero), and the pertinent resonant frequencies experience a second-order shift as a consequence of the numerical discretization.

The Circulator and Antenna as a Single Integrated System

An integration methodology is presented for antenna systems that incorporate a ferrite circulator wherein the geometrical and material parameters of the circulator are adjusted to effect integration; no impedance equalizers are required. To validate the proposed methodology, a system comprised of a microstrip circulator and Yagi planar-type antenna is presented. The overall system exhibits excellent passband characteristics in both the experimental and numerical data. Additionally, the ferrite properties and system network parameters were extensively measured to substantiate the claims made herein.