The Fundamentals of Patch Antenna Design and Performance
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High Frequency Design From March 2009 High Frequency Electronics Copyright © 2009 Summit Technical Media, LLC PATCH ANTENNAS The Fundamentals of Patch Antenna Design and Performance By Gary Breed Editorial Director icrostrip patch Here is a basic tutorial on antennas (also microstrip patch antennas, Mjust called patch intended to familiarize antennas) are among the engineers with the struc- most common antenna ture, feed and radiation types in use today, partic- properties of this common ularly in the popular fre- type of antenna quency range of 1 to 6 GHz. This type of anten- na had its first intense development in the 1970s, as communication systems became common at frequencies where its size and per- formance were very useful. At the same time, its flat profile and reduced weight, compared Figure 1 · Basic configuration of the to parabolic reflectors and other antenna microstrip patch antenna. options, made it attractive for airborne and spacecraft applications. More recently, those same properties, with additional size reduc- the shortening effect of the dielectric constant ε tion using high dielectric constant materials, ( r) of the material between the patch and the have made patch antennas common in hand- conducting surface (or substrate) below. sets, GPS receivers and other mass-produced The two edges of the patch that are con- wireless products. nected to, and opposite from, the feed connec- This tutorial article is intended to provide tion provide the radiation, acting as slot basic information on patch antenna design antennas, where each slot is the gap between and operation, directed to engineers who are the edge of the patch and the ground plane mainly designers of RF/microwave circuits. We beneath the intervening dielectric layer. The hope that this information will assist them as arrows at the left and right edges of the patch they design circuitry connected to these represent the currents between the patch con- antennas, or as they are called on to evaluate ductor and ground plane. At the edges, where and specify a vendor’s antenna product for they are not contained, these currents result their current project. in the desired radiation of electromagnetic waves from the two edge slots. Basic Patch Antenna Design We can easily see that the microstrip feed- Although there are many variations on line excites the center of the slot formed by the patch antenna design, the basic configuration edge of patch that to which it is connected. is shown in Figure 1, where l is then length Between the underside of the patch and the (relative to the feedpoint) and w is the width. substrate ground place, a low impedance λ In the simplest configuration, l = w = eff/2, or transmission line is formed that subsequently an electrical one-half wavelength, including feeds the slot at the opposite side. Since the 48 High Frequency Electronics High Frequency Design PATCH ANTENNAS Figure 2 · Feeding the patch antenna with the feedline Figure 3 · A patch may be fed at two points along on the underside of the substrate laminate. The loca- adjacent edges. This creates two independent radiat- tion of the feed is selected for the proper impedance, ed waves that are orthogonally polarized. When the and to create a current distribution that results in the phase shift between the two modes is set to 90º circu- desired polarization of the radiated wave. larly polarized radiation is achieved. electrical length of this line is λ/2, the routing of the feed, to obtain a desired ous feeds, delivering two radiated impedance at the fed edge is repeated feedpoint impedance, to control the waves that are orthogonal in polariza- at the other, which effectively feeds polarization, or for convenient layout tion. Figure 3 shows how these two the two slots in-phase with nearly (or all of these characteristics simul- feeds are connected, and how the radi- equal antenna currents. Thus, the taneously). Figure 2 shows the most ating currents appear at the edge patch operates as an array of two common of these alternate feed meth- slots. The feed system can be designed slots with a free-space separation ods, which routs the feedline under to provide a phase shift between the somewhat less than λ/2. Maximum the ground plane, isolating it from the two polarizations, with 90 degrees radiation is normal to the plane of radiating side of the structure. The delivering circularly-polarized radia- the patch. Polarization is at right connection to the patch is made tion. Of course, a –90 degree phase angles to the length of the slots, par- through a hole in the substrate using shift will result in circular polariza- allel to the feedline orientation a via or connecting wire. tion of the opposite sense. shown in Fig. 1. Removing the feedline from the The methods of Figs. 2 and 3 can Radiation toward the back of the radiating side of the structure elimi- be combined is various ways. For substrate is greatly reduced by the nates possible interaction between shielding effects of the ground plane the feed system and antenna. It has layer. The amount of reduction the additional advantage of allowing depends on the extent to which the the designer to include more ground plane extends beyond the microstrip circuitry on the backside. patch area. To make a smaller prod- These circuits may be impedance uct, many commercial antennas have matching networks, filters, phase ground planes that are only slightly shift networks and power dividers, larger than the patch. While they for an individual patch or for arrays have less reduction of rearward radi- comprising multiple patch elements. ation than a larger design, they still In a more complex system, active have useful gain and directivity. One elements may be included as well, familiar application of this type of such as a low noise amplifier, trans- patch antenna is the rectangular mit-receive switching diodes, and antenna (usually in a plastic hous- active variable phase shifters. In Figure 4 · Measured pattern of a ing) used widely in 2.4 GHz or 5 GHz wide bandwidth or multiband sys- patch antenna, with a typical WLAN systems. tems, the supporting circuitry may beamwidth of 65º and fairly well also switch or tune impedance match- suppressed rearward radiation. Alternate Feed Methods ing components. Pattern asymmetry is common due A common variation in patch The configuration of Fig. 1 can be to both construction and measure- antenna design is the location and expanded to support two simultane- ment variations. 50 High Frequency Electronics example, it is possible to choose a single through-hole feed Disadvantages location that results in circular polarization. The neces- The most significant disadvantage is that the rela- sary phase shift is the result of different current flow tively large x-y dimensions of a patch create higher-order paths in the region between the patch and ground plane. modes that allow radiation, or reception of interference, at other frequencies. The bandpass filtering effect found Radiation Pattern and Gain in other antenna types must be replaced with additional The simple patch of Fig. 1 has a radiation pattern like circuitry to eliminate any problems. However, it should be that shown in Figure 4. As noted earlier, the two radiat- noted these additional modes also allow the intentional ing slots create a two-element array, which results in a design of multi-band patch antennas. narrower beam width than a simple slot (which is like a dipole). In addition, the backing surface reduces rearward Notes on Design Variations radiation, which increases gain in the forward direction. We conclude this short tutorial by noting that The array gain, dipole-like directivity and uni-directional microstrip patch antennas have as many variations as characteristic are combined for approximately 8 to 9 dBi any component in RF/microwave technology! There are an gain (approx. 6-7 dB relative to a dipole, or dBd). astounding number of different shapes, feed systems, and The beamwidth is similar in both the x and y planes, array configurations. These design options make this type creating roughly a “cone” of radiation outward from the of antenna a fascinating area of study, since many prob- patch. For typical patch construction, the –3 dB lems can be addressed regarding frequency of operation, beamwidth is about 65 degrees, or ±32.5 degrees from pattern shape, gain and multi-frequency coverage. A boresight. search of the literature on this topic will clearly show the The radiation pattern is relatively narrow for a simple wide range of performance that can be obtained. antenna, which makes patch antennas desirable for assembly into arrays. Modern computer-aided design even allows the arrays to follow curves con- tours of aircraft or vehicle bodies. Adaptive tech- nologies that can vary both phase shift and power to each element allow those arrays to be electroni- cally steerable. An example of this type of array is airborne satellite communications antennas used on most commercial passenger aircraft. With this capability, the former shortwave radio communica- tion used on trans-oceanic flights can be replaced with higher capacity and more reliable satellite links. Advantages of Patch Antennas The unique property of the microstrip patch antenna is its two-dimensional structure. As a flat antenna, arrays can have a large aperture, with corresponding high gain, but having low volume and weight. As noted above, curved implementa- tions can be made to conform to aircraft hulls. The dielectric layer can be manipulated to fit different applications. Air dielectric has very low loss, making patch arrays useful for wireless com- munications systems, where its low weight and high gain are also valuable. When used with a high dielectric constant material such as ceramics, the effective wavelength is much smaller and the antenna can be greatly reduced in size.