The present invention relates to small microstrip antennas for use in electronic devices. Particularly, the invention relates to efficient and compact microstrip antennas comprising a plurality of patches.
Advances in digital and radio electronics have resulted in the production of a new breed of personal communications equipment posing special problems for antenna designers. As users demand smaller and more portable communications equipment, antenna designers are pressed to provide smaller profile antennas. Additionally, users of such communications equipment desire high data throughput, thus requiring antennas with wide bandwidths and isotropic radiation patterns. Moreover, antennas in such portable equipment are often randomly oriented during use, or used in environments, such as urban areas and inside buildings, that are subject to multipath reflections and rotation of polarization. Thus, an antenna in such devices should be sensitive to both horizontally and vertically polarized waves.
Wire antennas, such as whips and helical antennas are sensitive to only one polarization. As a result, they are not optimal for use in portable communication devices. One solution is to utilize microstrip patch antennas. In general, microstrip antennas are known for their advantages in terms of light weight, flat profiles, and compatibility with integrated circuits. A microstrip patch antenna comprises a dielectric sandwiched between a conductive ground plane and a planar radiating patch. Thus, microstrip patch antennas are useful alternatives for applications requiring a small and particularly thin overall size.
Microstrip patch antennas are commonly produced in half wavelength sizes, in which there are two primary radiating edges parallel to one another. It is known that the size may be further reduced if all of one of the primary radiating edges of a microstrip patch antenna is short circuited, permitting the size of the radiating patch to be reduced to a quarter wavelength. Additionally, it is known that the size may be reduced even further, to approximately one third the size of a half-wavelength antenna, if one of the primary radiating edges is partially shorted circuited. The short circuit is typically created by wrapping a thin sheet of copper foil to electrically connect the ground plane to the radiating patch. To simplify the manufacture of these antennas, shorting posts have been used in lieu of copper foil.
However, microstrip patch antennas are resonant structures with a relatively small bandwidth of operation and, therefore, are not optimal for wide bandwidth applications, such as data communications. It is known to improve the bandwidth of a rectangular patch antenna by placing non-driven, parasitic, patches parallel to the nonradiating edges of the driven patch. For example, U.S. Pat. No. 5,955,994 discloses a rectangular, half-wavelength microstrip patch antenna flanked at both non-radiating edges by identically shaped parasitic patches. However, this antenna is of a relatively large size.
In order to further improve the bandwidth of a rectangular patch antenna with parasitic patches, the shapes of the parasitic patches may be changed from that of the driven patch. For example, Keith Carver and James Mink, Microstrip Antenna Technology, I.E.E.E. AP-29 Trans. on Antennas and Propagation 2, 13-14 (Jan. 1981) discloses a square patch antenna having parasitic patches with smaller widths and longer lengths than the driven patch. The bandwidth may also be improved by spacing each parasitic patch at a different gap width from the driven patch.
However, these parasitic microstrip patch antennas have several drawbacks. For instance, the efficiency of such antennas may differ significantly with frequency within the resonant frequency range, and the antennas often have a reduced overall efficiency. Also, these antennas often have a highly asymmetric radiation pattern.
It would be desirable to provide a microstrip patch antenna with greater bandwidth as well as an efficiency symmetric with frequency. It would also be desirable to provide such a broadband microstrip antenna with a symmetric radiation pattern.
According to the invention, an antenna structure is provided. The antenna structure includes a ground plane, a layer of dielectric material having a first surface overlying said ground plane and an opposing second surface, and an electrically conductive layer overlying said second opposing surface of said dielectric layer. The electrically conductive layer is differentiated into a plurality of antenna elements including a driven antenna element and first and second non-driven, parasitic antenna elements. Each of said elements has a shape of a parallelogram having parallel first edges of length L and parallel second edges of length W, wherein one of said first edges of said first parasitic element is disposed substantially along one of said first edges of said driven element at a gap width WG, and wherein one of said first edges of said second parasitic element is disposed substantially along the opposite one of said first edges of said driven element at said gap width WG. Each of said antenna elements includes means for shorting said electrically conductive layer to said ground plane at a region proximate to one of said second edges of said electrically conductive layer. Also, each of said antenna elements has a resonant frequency, wherein said resonant frequencies are varied from each other using only said means for shorting. The antenna structure further includes means for coupling radio frequency energy to said driven antenna element of said electrically conductive layer.
The antenna structure according to the invention provides improvements in the symmetry of the antenna""s efficiency and radiation pattern because the shape of the antenna elements are substantially the same and the gap widths between the elements are substantially the same. The resonant frequencies of the elements are varied from each other using only the means for shorting.