Microstrip patch antenna

Wednesday, 31 January 2007

An antenna is an element used for radiating or receiving electromagnetic wave. Although antennas may seem to be available in numerous different shapes and sizes, they all operate according to the same basic principles of electromagnetics. As a general principle, a guided wave traveling along a transmission line which opens out will radiate as a free-space wave, also known as an electromagnetic wave. When an antenna is receiving, the antenna transforms free-space propagating waves by inducing a guided electromagnetic wave within the antenna. The guided electromagnetic wave is then fed into an integrated circuit. The integrated circuit then deciphers the signal being transmitted. When an antenna is transmitting, the antenna receives the guided electromagnetic wave for transmission from a feed line and induces an electric field surrounding the antenna to form a free-space propagating electromagnetic wave. The features of antenna can be known by the parameters of operation frequency, radiation patterns, reflected loss, and antenna gain, etc. An antenna may be that component of a personal communication device, a radio, a television, or a radar system that directs incoming and outgoing radio waves between free space and a transmission line. Antennas are usually metal and have a wide variety of configurations, from the whip or mastlike devices employed for radio and television broadcasting to the large parabolic reflectors used to receive satellite signals and the radio waves generated by distant astronomical objects. Many types of portable electronic devices, such as cellular phones, GPS receivers, palm electronic devices, pagers, laptop computers, and telematics units in vehicles, need an effective and efficient antenna for communicating wirelessly with other fixed or mobile communication units. 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. Personal wireless communication devices have created an increased demand for compact antennas. The increase in satellite communication has also increased the demand for antennas that are compact and provide reliable transmission. In addition, the expansion of wireless local area networks at home and work has also necessitated the demand for antennas that are compact and inexpensive. Wire antennas, such as whips and helical antennas are sensitive to only one polarization direction. As a result, they are not optimal for use in portable communication devices which require robust communications even if the device is oriented such that the antenna is not aligned with a dominant polarization mode.

A microstrip patch antenna is a type of antenna that offers a low profile, i.e. thin, and easy manufacturability, which provides a great advantage over traditional antennas. Patch antennas are planar antennas used in wireless links and other microwave applications. The microstrip technique is a planar technique used to produce lines conveying signals and antennas coupling such lines and radiated waves. It uses conductive strips and/or patches formed on the top surface of a thin dielectric substrate separating them from a conductive layer on the bottom surface of the substrate and constituting a ground for the line or antenna. A patch is typically wider than a strip and its shape and dimensions are important features of the antenna. Planar antennas are generally grouped into microstrip-line antennas and slot-line antennas. Microstrip patch antennas are attractive due to their compact structure, light weight due to the absence of heavy metal stamped or machined parts, and low manufacturing cost using printed circuit technology. They also provide low profiles, conformity to surfaces and direct integration with microwave circuitry. Consequently, microstrip patch antennas are used widely in antenna arrays. For example, reflector or dish antennas are commonly used in residential environments for receiving broadcast services, such as the transmission of television channel signals, from geostationary, or equatorial, satellites. Reflector antennas, however, are bulky and relatively expensive for residential use. Furthermore, inherent in reflector antennas are feed spillover and aperture blockage by a feed assembly, which significantly reduces the aperture efficiency of a reflector antenna. An alternative antenna, such as a microstrip antenna, overcomes many of the disadvantages associated with reflector antennas. Microstrip antennas require less space, are simpler and less expensive to manufacture, and are more compatible than reflector antennas with printed-circuit technology. Microstrip array antennas, i.e., microstrip antennas having an array of microstrips, may be used with applications requiring high directivity. A parabolic reflector antenna has a curved surface. A microstrip reflector antenna can be made having a planar surface. Further, a microstrip reflector antenna can achieve the concentration of antenna beam in a particular direction by means of the application of one of several methods. Microstrip antennas are particularly suitable for use as active antennas. Active antenna is an antenna having all of the necessary components, such as an antenna element, a feeding circuits, active devices or active circuits, integrally provided on a monolithic substrate, thus producing a compact, low cost and multi-function antenna equipment. Additionally, the planar structure of a microstrip antenna permits the microstrip antenna to be conformed to a variety of surfaces having different shapes. Microstrip antennas can be designed to produce a wide variety of patterns and polarizations, depending on the mode excited and the particular shape of the radiating element used. This results in the microstrip antenna being applicable to many military and commercial devices, such as use on aircraft or space antennas. There is an increasing demand for the use of microstrip antennas in wireless communication due to their inherently low back radiation, ease of conformity and high gain as compared to wire antennas. The microstrip antenna design allows for a small amount of radiation produced in one direction, the back of the antenna.

Patch antennas comprise one or more conductive rectilinear or ellipsoidal patches supported relative to a ground plane and radiate in a direction substantially perpendicular to the ground plane. As opposed to a conventional wire-based antenna, the microstrip antenna comprises a plurality of generally planar layers including a radiating element, an intermediate dielectric layer, and a ground plane layer. The radiating element is an electrically conductive material imbedded or photoetched on the intermediate layer and is generally exposed to free space. Depending on the characteristics of the transmitted electromagnetic energy desired, the radiating element may be square, rectangular, triangular, or circular and is separated from the ground plane layer. A exemplary patch antenna may include a transmission line feed, multiple dielectrics, and a metallized patch on one of the dielectrics. In a typical microstrip patch antenna, the radiator element is provided by a metallic patch that is fabricated onto a dielectric substrate over a ground plane. The dual-band signal-layer microstrip antenna has been widely used in applications like radar and communication systems, because of its advantages over a conventional antenna, such as lighter weight, lower profile and lower cost. Generally, dual-band single-layer microstrip antennas can be categorized into categories include stub-type microstrip antenna, notch-type microstrip antenna, pin-and-capacitor-type microstrip antenna, and slot-loaded-type microstrip antenna. The patch antenna has a very low profile and can be fabricated using photolithographic techniques. It is easily fabricated into linear or planar arrays and readily integrated with microwave integrated circuits. Microstrip patch antennas are commonly produced in half wavelength sizes, in which there are two primary radiating edges parallel to one another. The performance of an antenna is determined by several parameters, one of which is efficiency. For a microstrip antenna, "efficiency" is defined as the power radiated divided by the power received by the input to the antenna. A one-hundred percent efficient antenna has zero power loss between the received power input and the radiated power output. Microstrip array antennas typically rely on traveling waves and require a complex microstrip feed network which contributes significant feed loss to the overall antenna loss. Furthermore, many microstrip array antennas are limited to transmitting and/or receiving only a linearly polarized beam. The substrate is mounted on a larger ground plane, which serves as the return path for current induced on the patch element. A microstrip antenna operates by resonating at a frequency. The microstrip patch antenna performs optimally when it is sized such that the cavity beneath the patch resonates in its fundamental mode at the frequency of interest.

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