7-16x9 Handbook / Antenna Engineering Handbook / Volakis / 147574-5 / Chapter 7Microstrip AntennasDavid R. JacksonUniversity of HoustonCONTENTS7.1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-27.2 BASIC PRINCIPLES OF OPERATION . . . . . . . . . . . . . . . . . . . . . . . . . . 7-47.3 CAD MODEL FOR THE INPUT IMPEDANCE . . . . . . . . . . . . . . . . . . . . 7-57.4 RADIATION PATTERNS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-87.5 CAD FORMULAS FOR RECTANGULAR PATCH . . . . . . . . . . . . . . . . . 7-117.6 RESULTS FOR RECTANGULAR PATCH . . . . . . . . . . . . . . . . . . . . . . . 7-177.7 CAD FORMULAS FOR CIRCULAR PATCH . . . . . . . . . . . . . . . . . . . . . 7-197.8 CIRCULAR POLARIZATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-227.9 MICROSTRIP ANTENNAS WITH IMPROVED PERFORMANCE . . . . . 7-24Chapter 7ch07.indd 1 5/1/07 6:01:57 PM7-2 CHAPTER SEVEN 6x9 Handbook / Antenna Engineering Handbook / Volakis / 147574-5 / Chapter 77.1 INTRODUCTIONMicrostrip antennas (often called patch antennas) are widely used in the microwave fre-quency region because of their simplicity and compatibility with printed-circuit technology, making them easy to manufacture either as stand-alone elements or as elements of arrays. In its simplest form a microstrip antenna consists of a patch of metal, usually rectangular or circular (though other shapes are sometimes used) on top of a grounded substrate, as shown in Figure 7-1. In this chapter the basic principles of operation are discussed, and CAD formulas are given for the microstrip antenna. The CAD formulas are fairly accurate for thin substrates and illustrate the basic principles. For thin substrates the CAD formulas may even be accurate enough for final design purposes. For thicker substrates these formulas can still be used for initial design work, with full-wave simulation tools used to complete the final design.HistoryThe origin of microstrip antennas apparently dates back to 1953, when Deschamps pro-posed the use of microstrip feed lines to feed an array of printed antenna elements.1,2 The printed antenna elements introduced there were not microstrip patches, but flared planar horns. The microstrip patch antenna was first introduced by Munson in a symposium paper in 1972,3 which was followed by a journal paper in 1974.4 These papers discussed both the wraparound microstrip antenna and the rectangular patch. Shortly after Munson’s sympo-sium paper, Howell also discussed rectangular patch antennas in another symposium paper5 in which he credits Munson with the basic idea by referencing a private communication. In a later journal paper, Howell introduced the circular patch as well as the circularly polar-ized patch antenna.6 Soon after the introduction of the microstrip antenna, papers appeared describing methods of analysis for these antennas, including the transmission-line model,7 the cavity model,8 and the spectral-domain method.9 A good review of the early history of microstrip antennas is provided in the article by Carver and Mink.10 A discussion of microstrip antennas may be found in a variety of books devoted to this type of antenna11–22 as well as in more general antenna books and handbooks.23–27Feed MethodsVarious methods may be used to feed the microstrip antenna, as shown in Figure 7-2 for the rectangular patch. The coaxial probe feed shown in Figure 7-2a is one of the most com-mon feeds for a stand-alone element. The inset feed in Figure 7-2b is common for array applications. The proximity-coupled feed in Figure 7-2c requires multilayer fabrication, (a) (b)FIGURE 7-1 (a) Rectangular microstrip patch antenna and (b) circular microstrip patch antennah xyahW xyLGround PlaneGround Planech07.indd 2 5/1/07 6:01:57 PMMICROSTRIP ANTENNAS 7-36x9 Handbook / Antenna Engineering Handbook / Volakis / 147574-5 / Chapter 7but reduces spurious radiation from the feed line. The aperture-coupled feed shown in Figure 7-2d has the advantage of eliminating feed-line radiation (at the expense of some back radiation from the aperture) and also allows for relatively thick substrates, since probe reactance is not an issue.Advantages and DisadvantagesMicrostrip antennas usually have the important advantage of being low profile, and if the substrate is thin enough, they may also be conformable, meaning that the substrate can be bent to fit a curved surface, making the antenna very unobtrusive. Because the lateral size of a microstrip antenna on a substrate board is typically on the order of a half wavelength in the dielectric, size considerations usually dictate that these antennas are used in the UHF frequency band or higher, up through millimeter-wave frequencies, with microwave fre-quency applications being the most common. The main disadvantages of microstrip anten-nas include potentially lower radiation efficiency compared with other antennas (although this depends significantly on the substrate permittivity and thickness) and small bandwidth. These issues are discussed further next and in Section 7.9.Radiation Efficiency and BandwidthRadiation efficiency depends largely on the substrate permittivity and thickness. A sub-strate that has a higher permittivity or that is thicker will suffer from increased surface-wave excitation, which will lower the efficiency. (Using a foam substrate is a simple way to eliminate surface-wave excitation. Removing the substrate outside of the patch cav-ity will also eliminate surface-wave excitation.) On the other hand, if the substrate is too FIGURE 7-2 Feeding methods for a microstrip antenna: (a) coaxial feed, (b) inset feed, (c) proximity-coupled feed, and (d) aperture-coupled feedxzLh xyWLxzLh2h1Microstrip line xzLh bAperture Microstrip line(a) (b)(c)(d)Ground PlaneGround Planech07.indd 3 5/1/07 6:01:57 PM7-4 CHAPTER SEVEN 6x9 Handbook / Antenna Engineering Handbook / Volakis / 147574-5 / Chapter 7thin, the efficiency will be low due to conductive and dielectric losses. Assuming a typical Teflon substrate (er = 2.2 with a loss tangent of 0.001) and copper for the patch and ground plane with a conductivity of 3.0 × 107 S/m, the radiation efficiency is maximum for a sub-strate thickness of about 0.02l 0, reaching about 90 percent. When using a foam substrate, the efficiency continuously increases with