Chapter 2 线极化微带贴片天线.ppt

* * Chapter 2 Linear Polarized MPA 2.1 Introduction The rectangular and circular patches are the basic and most commonly used microstrip antennas. These patches can be used for the simplest and the most demanding applications. For example, characteristics such as dual and circular polarizations, dual-frequency operation, frequency agility, broad bandwidth, feeding flexibility, beam scanning, omnidirectional patterning, and so on are easily obtained in these patch shapes. A large number of microstrip patch antenna have been studied to date. An exhaustive list of these geometries alone with their salient features is available. * The simplest microstip patch configuration is undoubtedly the rectangular MPA. When the patch is excited by a feed, a charge distribution is established on the underside of the patch metallization and the ground plane. 2.2 Rectangular Microstrip Patch Antenna At a particular instant of time, the underside of the patch is positively charged and the ground plane is negatively charged. The attractive forces between these sets of charges tend to hold a large percentage of the charge between the two surfaces. * 2.2.1 Models for Rectangular MPAs Figure 2-1 Configuration of a Rectangular MPA * Rectangular patch antennas can be designed by using a transmission-line model suitable for moderate bandwidth antennas. Patches with bandwidths of less than 1% or greater than 4% require a cavity analysis for accurate results, but the transmission line model covers most designs. The lowest-order mode, TM10, resonates when the effective length across the patch is a half-wavelength. Radiation occurs from the fringing fields. These fields extend the effective open circuit (magnetic wall) beyond the edge. The extension is given by 1 Transmission Line Model Analysis * The microstrip patch antennas are narrow-band resonant antennas. They are termed lossy cavities. Therefore, the cavity model becomes a natural choice to analyze the patch antennas 2