Microstrip antennas that can be made to perform well over relatively wide frequency ranges but are mechanically and electrically simpler than prior such antennas have been invented. These antennas are designed to support traveling waves, in contradistinction to being designed traditionally to support standing waves. The exploitation of traveling waves to enable wideband operation is not new in itself; the novelty of the present invention lies in the electrical and mechanical antenna configuration for supporting traveling waves.

Simple microstrip antenna elements, considered by themselves, have inherently narrow bandwidths. One of the basic characteristics of a microstrip antenna element that limits its bandwidth is its resonant behavior. The resonance of a traditional antenna, due to the reflection of the electric-current waves at the open-circuited ends of the radiating element(s), causes a standing wave of electric current to form along the antenna structure. The bandwidth of the antenna is narrow because the standing wave can be supported efficiently only when the length of the antenna is a multiple of a half wavelength (in the case of a dipole antenna) or a quarter wavelength (in the case of a monopole antenna).

Heretofore, it has been necessary to stack or interlace microstrip antenna elements having different resonance frequencies and, hence, different sizes, in order to construct microstrip antennas having sufficient bandwidth required in some applications. Antennas designed in this way tend to be larger than desired and have complex feed configurations. In contrast, the present traveling-wave wideband microstrip antennas can have simple feed configurations and can be constructed as units that are intermediate in size and complexity between simple microstrip antenna elements and traditional stacked- and interlaced-element wideband microstrip antennas.

According to the invention, an antenna is loaded with distributed reactance by incorporating capacitive gaps at various locations into the antenna structure. Through suitable choice of the capacitances and the locations of the gaps, the spatial distribution of electric current can be shaped to suppress resonances and, thus, support traveling waves and increase bandwidth.

A Monopole Microstrip Antenna according to the invention includes a coaxially fed central disk with series capacitive coupling between the disk and concentric rings.
The figure depicts a monopole microstrip antenna according to the invention. The antenna includes an electrically conductive central disk on the top surface of a dielectric substrate and an electrical ground plane on the bottom of the substrate. The central disk is connected to a coaxial probe feed. The antenna also includes concentric electrically conductive rings having various diameters. Some rings are on the top surface of the dielectric substrate; other rings are embedded within the substrate. The diameters of the disk and rings are chosen to suit the desired frequency range of the antenna. The number of rings (typically at least five or six) is chosen to obtain the desired bandwidth: in general, the bandwidth increases with the number of rings.

Each ring on the surface of the substrate overlaps the neighboring ring(s), and the innermost ring overlaps the central disk. The depths of the embedded rings and the amounts of overlap are chosen, in consideration of the permittivity of the substrate material and the desired frequency range, to obtain the amounts of capacitance at the gaps needed to produce a radially traveling wave of current. Proceeding radially outward from the central disk, the capacitance at each gap is made smaller than that of the preceding gap so as to obtain a greater impedance by an amount chosen to obtain the desired radial taper. If the capacitances are chosen properly, then at the outer edge of the outer ring, there remains little energy for an electric-current wave to be reflected back toward the coaxial probe feed to produce a standing wave.

This work was done by David A. Tonn of the Naval Research Laboratory. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Physical Sciences category. NRL-0007

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Traveling-Wave Wide-Band Microstrip Antennas

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This article first appeared in the April, 2007 issue of Defense Tech Briefs Magazine.

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