W. Richards

An improved theory for microstrip antennas and applications

Abstract : An improvement to a recently reported theory for the analysis of the pattern and impedance loci of microstrip antennas is developed. It yields a theory which is simple and inexpensive to apply. The fields in the interior of the antennnas are characterized in terms of a discrete set of modes. The poles corresponding to these modes are complex and depend on the losses in the antenna. The representation of the fields in terms of these modes in rigorous only for a bona fide cavity with no copper loss. The proper shift in the complex poles due to the addition of copper and radiative losses is approximated by lumping the latter two together with the dielectric loss to form an effective loss tangent. By so doing, it is found that the resulting expressions for impedance of the microstrip antenna is in good agreement with measured results for all modes and feed locations. The theory is applied to the evaluation of impedance variation with feed location, multiport analysis, and to design of circularly polarized microstrip antennas. (Author)

An experimental investigation of electrically thick rectangular microstrip antennas

The electromagnetic properties of electrically thick rectangular microstrip antennas were investigated experimentally. Antennas were fabricated with different patch sizes and with electrical thicknesses ranging from 0.03 to 0.23 wavelengths in the dielectric substrate. The resonant frequencies were measured and compared to existing formulas. The bandwidth was calculated as a function of electrical thickness and the antenna radiation patterns were measured.

Dual-band reactively loaded microstrip antenna

A previously derived theory is applied to a microstrip antenna with a reactive load to produce a dual-band radiator. A model consisting of a rectangular patch radiator loaded with a variable length short-circuited coaxial stub was investigated experimentally. Comparisons of theoretical predictions and experimental data are made for the impedance and resonant frequencies as a function of the position of the load, the length of the stub, and the characteristic impedance of the stub.

A theoretical and experimental investigation of annular, annular sector, and circular sector microstrip antennas

An analysis is presented for three related shapes of microstrip antenna elements: the annular; annular sector; and circular sector. The method of analysis involves the full expansion of resonant modes within the cavity formed by the radiating patch and the ground plane. Experimental results for representative radiators are also included for comparison.

THEORETICAL AND EXPERIMENTAL INVESTIGATION OF A MICROSTRIP RADIATOR WITH MULTIPLE LUMPED LINEAR LOADS

ABSTRACT A simple theory is developed for the analysis of microstrip patch elements which are loaded at one or more points with lumped linear load impedances. The analysis is based on a cavity model in which the shape of the field distribution between the patch and ground plane is assumed to be well approximated by that of the resonant modes of a corresponding magnetic- and electric-walled cavity. The resonant mode of the loaded cavity is represented as an appropriate superposition of the modes of the corresponding unloaded cavity. The characteristic equation for the resonant frequencies of the loaded cavity is obtained in terms of the load impedances and the unloaded cavity multiport open-circuit parameters. An analysis of the input impedance of a rectangular microstrip element shorted at an arbitrary point has been implemented and the results show good agreement with experiment. An ancilliary result showing the equivalence between a thin strip and a circular cylinder model of a feed current distribution...

Series expansions for the mutual coupling in microstrip patch arrays

A dominant-mode mutual coupling theory is developed for an array of microstrip patches. One of the key features of the formulation is that only values for isolated self- and mutual impedance are needed in the formulation, making the method suitable for either a cavity model or spectral-domain analysis, or compatible with experimental measurements. The formulation is equivalent with D.M. Pozars (1982) moment-method formulation. A series expansion of the solution is derived, allowing for approximate formulas accurate to any specified order. These formulas do not require matrix inversion. Formulas up to third order require less computation time that the exact solution, for single elements of the impedance, admittance, and scattering matrices. These expansions are computationally efficient for large sparse arrays. >

EXPERIMENTAL AND THEORETICAL INVESTIGATION OF THE INDUCTANCE ASSOCIATED WITH A MICROSTRIP ANTENNA FEED

ABSTRACT It has been demonstrated that a microstrip antenna excited in a non-degenerate resonant mode has a driving point impedance which can be modeled as a resonant parallel R-L-C circuit in series with the an inductor. The latter is an inductance which can be associated with the feeding probe or microstrip line feed of the antenna. This paper investigates the variation with feed location of this inductive component both experimentally and by a full modal expansion of the field using the “cavity model”. The results demonstrate that the simple, feed position-independent formula for this inductive component is inadequate for certain applications involving loaded microstrip elements. The results also show that the cavity model can predict the variation of feed inductance with feed location reasonably well except near the edge of the patch. The physical mechanism for the variation of this inductive component as well as the physical reason why the cavity model overestimates this parameter with feeds near the...

Impedance control of microstrip antennas using reactive loading

It is shown that it is possible to change the input impedance of a microstrip antenna over a wide range without affecting its resonant frequency by moving short circuits along prescribed loci. This theory has been verified by experiment; specifically, it is shown that the resonant frequency of the mode is left relatively unchanged by the appropriate placement of the short circuit loads. The experiment also shows that the agreement between the predicted and observed input impedance is quite reasonable in most cases. There was qualitative agreement between theoretical antenna patterns and measured antenna patterns although the measured cross-polarized components in the double-loaded element were higher than expected from the theory. A single load, varying in position, was seen to provide a reasonable range of impedance variation while retaining a very nearly constant resonant frequency, although the resulting asymmetries in the magnetic current distribution caused a high level of cross-polarized fields. The use of two loads, instead, produced an even greater range of possible impedances, with reduced cross-polarization levels as well. >

Pattern adaptation using loaded, dual-mode microstrip antennas

INTRODUCTION. Presented in this summary are applications of dual-mode microstrip elements, which have two independently excitable modes resonant at he frequency. Square patch elements and circular disk elements are structurally invarient under plane rotations of 900. As a result, these structures support pairs of linearly independent degenerate modes. One mode of each pair can be obtained by rotating the distribution of the other by 900. Thus, the patterns produced by these degenerate modes are also identical except for this 900 rotation. Antennas which support these kinds of degenerate modes have been used to produce circular polarization and polarization-agile 1 antennas.

An exact mutual coupling theory for microstrip patches

An accurate determination of mutual coupling between microstrip antennas is important in the design of finite arrays of patch elements. The classical formula for mutual impedance based on reciprocity is popular for determining such coupling. However, this formula ignores the presence of other open-circuited elements and consequently has serious limitations in accuracy due to resonant currents which may exist on such elements. In this presentation an exact theory for the mutual coupling between eatch elements will be presented, which circumvents this difficulty. For very weak coupling the exact formula reduces to the classical formula.