G.R. Branner

Design of unequally spaced arrays for performance improvement

Classical antenna array synthesis techniques such as Fourier, Dolph-Chebyshev and Taylor synthesis efficiently obtain array current distributions for equally spaced arrays that generate a desired far-field radiation pattern function or keep important parameters like beamwidth and sidelobe level within prescribed performance bounds. However, the concept of optimization of the field pattern (e.g., by decreasing sidelobes or beamwidth) of an given equally spaced array realization by altering its element spacings still represents a challenging problem having considerable practical advantages. These include reduction in size, weight, and number of elements of the array. This paper describes a new approach to synthesis of unequally spaced arrays utilizing a simple inversion algorithm to obtain the element spacings from prescribed far-zone electric field and current distribution, or current distributions from prescribed far-zone electric field and element spacings.

Generalized analytical technique for the synthesis of unequally spaced arrays with linear, planar, cylindrical or spherical geometry

An effective method for optimizing the performance of a fixed current distribution, uniformly spaced antenna array has been to adjust its element positions to provide performance improvement. In comparison with the default uniform structure, this approach yields performance improvements such as smaller sidelobe levels or beamwidth values. Additionally, it provides practical advantages such as reductions in size, weight and number of antenna elements. The objective of this paper is to describe a unified mathematical approach to nonlinear optimization of multidimensional array geometries. The approach utilizes a class of limiting properties of sinusoidal, Bessel or Legendre functions that are dictated by the array geometry addressed. The efficacy of the method is demonstrated by its generalized application to synthesis of rectangular, cylindrical and spherical arrays. The unified mathematical approach presented below is a synthesis technique founded on the mathematical transformation of the desired field pattern, followed by the application of limiting forms of the transformation, and resulting in the development of a closed form expression for the element positions. The method offers the following advantages over previous techniques such as direct nonlinear optimization or genetic algorithms. First, it is not an iterative, searching algorithm, and provides element spacing values directly in a single run of the algorithm, thereby saving valuable CPU time and memory storage. Second, It permits the array designer to place practical constraints on the array geometry, (e.g., the minimum/maximum spacing between adjacent elements)

A Wideband Multiharmonic Empirical Large-Signal Model for High-Power GaN HEMTs With Self-Heating and Charge-Trapping Effects

A complete empirical large-signal model for high-power AlGaN/GaN HEMTs (GaN HEMT) utilizing an improved drain current (Ids) formulation with self-heating and charge- trapping modifications is presented. The new drain current equation accurately models the asymmetric bell-shaped transconductance (gm) for high Ids over a large range of biases. A method of systematically employing dynamic IV behavior using pulsed-gate IV and pulsed-gate-pulsed-drain IV datasets over a wide variety of thermal and charge-trapping conditions is presented. The composite nonlinear model accurately predicts the dynamic IV behavior, S-parameters up to 10 GHz, and large-signal wideband harmonic behavior for a multitude of quiescent gate-source and drain-source biases as well as third-order intermodulation distortion (IM3).

Optimization of active microwave frequency multiplier performance utilizing harmonic terminating impedances

A primary factor affecting optimum performance of microwave multipliers employing nonlinear devices is the proper termination of the fundamental and other harmonic frequency components. The objective of this paper is to present a quantitative analysis leading to the assessment of optimum terminating impedances in the design of active frequency multipliers with special attention given to harmonics other than those desired. The analysis includes computer modeled HEMT data and supporting measured data for corresponding circuit realizations. Circuit designs are presented utilizing HEMT transistors as the active element to verify modeled results. Based on available literature, the results demonstrate, for the first time, the quantitative effects of harmonic termination on active multiplier conversion gain and fundamental and higher harmonic suppression. An experimental design reveals an improvement in multiplier gain of 124% over the conventional approach and data is presented which quantitatively illustrates the advantages of impedance termination considerations under optimal bias conditions.

Design and optimization of large conversion gain active microwave frequency triplers

In this work, novel design and optimization techniques for frequency triplers are presented. Accurate CAD techniques are utilized to develop bias, input power, input network, and output network configurations for optimum third harmonic response. As a result, a microwave frequency tripler is developed which exhibits an unprecedented level of conversion gain.

Novel MIC bipolar frequency doublers having high gain, wide bandwidth and good spectral performance

High-efficiency bipolar microwave frequency multipliers with wideband performance, high conversion gain, and good spectral properties have been developed. Experimental conversion gains of up to 7 dB have been attained for narrowband designs ( approximately=12% BW) and greater than 0 dB for wideband designs (>or=40%) at C-band. Corresponding fundamental and third-harmonic rejections are greater than 45 dBc and 30 dBc, respectively. Extensive modeling and computer-oriented design utilizing harmonic balance have been employed. >

An Empirical Large-Signal Model for SiC MESFETs With Self-Heating Thermal Model

An empirical large-signal model for high-power microwave silicon-carbide MESFETs capable of predicting self-heating thermal behavior is presented. A generalized drain-current equation based on pulsed-gate IV characteristics measuring up to 2 A and 58 V is presented along with its dependence on temperature. A thermal subcircuit with a nonlinear thermal resistance characterized by a dc method is used to model the temperature behavior of the device. The effect of substrate trapping is modeled as a gate-source voltage correction. The complete drain-current model accurately predicts pulsed-gate and pulsed-gate-and-drain IV characteristics for various quiescent biases, as well as static IV characteristics. The complete large-signal model is shown to accurately predict S -parameters, large-signal output, and input reflected power across biases and frequencies, and third-order intermodulation products.

Optimized design of unique miniaturized planar baluns for wireless applications

The high frequency balun network has proven to be an important component in the design of certain RF and microwave system topologies-especially in wireless communications system architectures. This work describes the optimized design of planar balun circuits which operate in the 900 MHz wireless frequency band. The designs are an outgrowth of extensive in-depth computer analysis and fabrication and testing of a multiplicity of circuit realizations. A novel feature of these designs is their compact size which is almost one sixteenth that of conventional quarter wavelength-coupled line designs. Size reduction and excellent coupling are effectively obtained by novel use of discrete capacitors.

Active microwave filters with noise performance considerations

Active microwave filters offer a unique solution for certain filtering problems in modern microwave integrated circuit applications. A number of authors have reported novel active filter topologies in this connection; however, noise considerations for these exotic designs have seldom been addressed, and only Bonetti and Williams (1990, 1992) have provided actual reliable repeatable measured noise data across a band of frequencies. The main objective of this paper is to present a precision computer-oriented technique to accurately predict and quantify the noise performance of various active microwave filter realizations. Experimental examples are presented which validate the computer-oriented simulation methodology. Noise performance results are presented for a representative realization of each class of active microwave filter considered. >

The far-field of a spherical array of point dipoles

In this paper, the far-field of a spherical array of point dipoles is related to the array currents in an elegant manner by invoking the properties of the associated Legendre functions. The inverse problem, viz., determining the array currents from the far-field data is solved in a recursive manner thereby precluding the need for a matrix inversion. >