Chapter2线极化微带贴片天线

1、1 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 ba

2、ndwidth, 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. 2The simplest mic

3、rostip 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. At a particular instant of time, the underside of the patch is positively charged and the ground

4、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. 32.2.1 Models for Rectangular MPAsFigure 2-1 Configuration of a Rectangular MPA4Rectangular patch antennas can be designed by using a transmission-

5、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

6、half-wavelength. Radiation occurs from thefringing fields. These fields extend the effective open circuit (magnetic wall) beyond the edge. The extension is given by1 Transmission Line Model Analysis 5The microstrip patch antennas are narrow-band resonant antennas. They are termed lossy cavities. The

7、refore, the cavity model becomes a natural choice to analyze the patch antennas 2 Cavity Model Analysis 62.2.2 Design Consideration for Rectangular Patch AntennasDielectric substrate selectionWidth of patchLength of patchSize of SubstrateRadiation patterns Input admittanceRadiation resistance, Q fac

8、tor and efficiency Directivity and GainBeamwidthBandwidthFeed point location7Figure 2-2 Gain vs Frequency of a Rectangular MPA8Figure 2-3 Bandwidth vs Frequency of a Rectangular MPA9Example1 Design a square microstrip patch antenna at 10 GHz on a 1.588 mm substrate with a dielectric constant of 2.2.

9、 The patch will be approximately a half-wavelength long in the dielectric.2.2.3 Design Examples of Rectangular MPAsExample2 Design a airplane Finder with Rectangular MAP. (1) fr=444MHz (2) VSWR 2.5 BW 0.05fr;(3) Size of substrate 300mm X 180mm;(4) BE 50, BH 40, first sidelobe level -17dB;(5) Horizon

10、tal linear polarization;(6) Match the environmental requirements for aviation.10 In some applications, a circular patch fits in the available space better than a rectangular one. In a triangularly spaced array, they maintain a more uniform element environment. No suitable transmission-line model pre

11、sents itself, and the cavity model must determine the resonant frequency and bandwidth. The patch with its magnetic walls and TM modes is the dual of the waveguide. The resonant frequencies are given by112.3.1 Cavity Model for Circular MPAsFigure 2-4 Configuration of a Circular MPA12Figure 2-5 Field

12、s and Surface Current Patterns for Various Modes at resonance (m=1) 13Substrate selection and disk radius2.3.2 Design Consideration for Circular Disk AntennasRadius of disk antenna as a function of frequency for two different dielectric substrate14Input ImpedanceRadiation patternsRadiation patterns

13、of the microstrip disk antenna for the TM11 mode. (a) E plane; (b) H plane.15Radiation power, lossy power, radiation efficiency16Radiation resistance, quality factorVariation of Q with resonant frequency for various microstrip disk antennas17Impedance bandwidthBandwidth vs FrequencyFeed point locati

14、on18Example Design a circular microstrip patch antenna (TM11 mode) at 3 GHz on a 1.6mm substrate that has a dielectric constant of 2.55 (woven Teflon fiberglass).The physical radius will be slightly less.192 : 1 VSWR bandwidth of circular microstrip patches versus substrate thicknessin free-space wa

15、velengths, including surface-wave radiation.202.3.3 Comparison of the Characteristics of Rectangular and Circular MPAs21Consider a circular ring micrstrip antenna with the coordinate system as shown below. It comprises a ring shaped conductor on the one side of a dielectric substrate with a ground p

16、lane on the other side. The cavity model of the ring is obtained by replacing its peripheries with magnatic walls. Because there is no variation of the fields along the z direction for thin substrates, the modes are designed as TMnm modes.2.4.1 Field and Currents222.4.1 Field and Currents232.4.2 Res

17、onant Frequency242.4.3 Radiation FieldsThe radiation fields of a circular ring antenna can be obtained either from the magnetic current approach or the electric current distribution on the surface of the ring. We shall use the equivalent magnetic current formulation because of its simplicity. Calculations for the radiation fields of the ring are a straight forward extension of