International Journal of Electronics and Computer Science Engineering 306 Available Online at www.ijecse.org ISSN- 2277-1956 Sunil Singh 1, Neelesh Agarwal 2, Navendu Nitin 3, Prof.A.K.Jaiswal 4 1 2 3 4 Department of Electronics and Communication Engineering 1 Research Scholar, SHIATS, Deemed-To-Be-University, Naini, Allahabad, U.P., India 2 3 4 SHIATS, Deemed-To-Be-University, Naini, Allahabad, U.P., India 1 Email- singh.er.sunil@gmail.com Abstact- The study of microstrip patch antennas has made great progress in these days. This is mostly due to their versatility in terms of possible geometries that makes them applicable for many dierent situations. Compared with conventional antennas, microstrip patch antennas have more advantages and better prospects like low cost, lighter in weight, low profile, smaller in dimension, low volume and conformity and ease of fabrication. Theses microstrip patch antennas can provide dual-frequency operation, dual and circular polarizations, feedline flexibility, frequency agility, broad band-width, beam scanning omnidirectional patterning. In this paper we discuss the microstrip antenna, feeding techniques and application of microstrip patch antenna with their advantage and disadvantages over conventional microwave antennas. Keywords- Microstrip Patch Antenna I. INTRODUCTION Microstrip or patch antennas are becoming increasingly useful because they can be printed directly onto a circuit board. Microstrip antennas are becoming very widespread within the mobile phone market. Patch antennas are low cost, have a low profile and are easily fabricated. Fig1: Basic configuration of the microstrip patch antenna. This introduces some of the basic concepts of patch antenna. Patch antennas (also known as a rectangular microstrip antenna) are among the most common antenna types in use today, particularly in the popular frequency range of 1 to 6 GHz. A patch antenna is a wafer-like directional antenna suitable for covering single-floor small offices, small stores and other indoor locations where access points cannot be placed centrally. Patch antennas produce hemispherical coverage, spreading away from the mount point at a width of 30 to 180 degrees. Patch antennas are also known as panel, flat panel or microstrip antennas. They are formed by overlaying two metallic plates, one larger than the other, with a dielectric sheet in the middle. This type of antenna is usually encased in white or black plastic, not only to protect the antenna, but also to make it easy to mount. Because they are flat, thin and lightweight, patch antennas are often hung on walls or ceilings where they remain visually unobtrusive and blend easily into the background.
307 The main focus will be on explaining the general properties of patch antennas by using the simple rectangular probe fed patch. It will cover topics including: principles of operation, impedance matching, radiation pattern and related aspects, bandwidth, and efficiency. II. MICROSTRIP PATCH ANTENNA In its most basic form, a Microstrip patch antenna consists of a radiating patch on one side of a dielectric substrate which has a ground plane on the other side.the patch is generally made of conducting material such as copper or gold and can take any possible shape. The radiating patch and the feed lines are usually photo etched on the dielectric substrate. A microstrip patch antenna (MPA) consists of a conducting patch of any planar or non-planar geometry on one side of a dielectric substrate with a ground plane on other side. Fig2: Structure of a Microstrip Patch Antenna It is a popular printed resonant antenna for narrow-band microwave wireless links that require semi-hemispherical coverage. The rectangular and circular patches are the basic and most commonly used microstrip antennas. These patches are used for the simplest and the most demanding applications. Rectangular geometries are separable in nature and their analysis is also simple. The circular patch antenna has the advantage of their radiation pattern being symmetric. In order to simplify analysis and performance prediction, the patch is generally square, rectangular, circular, triangular, elliptical or some other common shape.for a rectangular patch, the length L of the patch is usually 0.3333λ < L < 0.5λ, where o λ is the free-space wavelength. The patch is selected to be very thin such that o t << λ (where t is the patch thickness). The height h of the dielectric substrate is usually 0.003 λ h 0.05λ. The dielectric constant of the substrate (r ε) is typically in the range 2.2 12 r ε. Fig3: Common shapes of microstrip patch elements
IJECSE, Volume2, Number 1 Sunil Singh et al. 308 Microstrip patch antennas radiate primarily because of the fringing fields between the patch edge and the ground plane. For good antenna performance, a thick dielectric substrate having a low dielectric constant is desirable since this provides better efficiency, larger bandwidth and better radiation. However, such a configuration leads to a larger antenna size. In order to design a compact Microstrip patch antenna, higher dielectric constants must be used which are less efficient and result in narrower bandwidth. Hence a compromise must be reached between antenna dimensions and antenna performance. A. PRINCIPLE OF PATCH ANTENNA Fig4: Patch Antenna Configuration The basic operating principle of a patch antenna is that the space between the patch and ground plane acts like a section of parallel plate waveguide. Neglecting radiation loss, the edge of the patch is an open circuit, so that energy reflects and remains below the patch. The patch antenna is therefore a resonant cavity with relatively high quality factor. One disadvantage of a high-q system is narrow bandwidth, so patch antennas have limited bandwidth, meaning that the input impedance of the antenna only remains near the desired value for a small range around the designed center frequency. B. PROPERTIES OF A BASIC MICROSTRIP PATCH A microstrip or patch antenna is a low profile antenna that has a number of advantages over other antennas it is lightweight, inexpensive, and easy to integrate with accompanying electronics. While the antenna can be 3D in structure (wrapped around an object, for example), the elements are usually flat, hence their other name, planar antennas. Note that a planar antenna is not always a patch antenna. The following drawing shows a patch antenna in its basic form: a flat plate over a ground plane (usually a PC board). The centre conductor of a coax serves as the feed probe to couple electromagnetic energy in and/or out of the patch. The electric field distribution of a rectangular patch excited in its fundamental mode is also indicated.
309 Fig5: Basic form of patch antenna The electric field is zero at the centre of the patch, maximum (positive) at one side, and minimum (negative) on the opposite side. It should be mentioned that the minimum and maximum continuously change side according to the instantaneous phase of the applied signal. The electric field does not stop abruptly at the patch's periphery as in a cavity; Rather, the fields extend the outer periphery to some degree. These field extensions are known as fringing fields and cause the patch to radiate. Some popular analytic modelling techniques for patch antennas are based on this leaky cavity concept. The field components of interest are: the electric field in the z direction and the magnetic field components in x and y direction using a Cartesian coordinate system, where the x and y axes are parallel with the ground plane and the z- axis is perpendicular. In general, the modes are designated as tmnmz. The z value is mostly omitted since the electric field variation is considered negligible in the z-axis. Hence tmnm remains with n and m the field variations in x and y direction. The field variation in the y direction (impedance width direction) is negligible; Thus m is 0. And the field has one minimum to maximum variation in the x direction (resonance length direction), thus n is 1 in the case of the fundamental. Hence the notation TM10. C. Feed Techniques Typically, a patch antenna is fed by a microstrip transmission line, but other feed lines such as coaxial can be used. Microstrip patch antennas can be fed by a variety of methods. These methods can be classified into two categoriescontacting and non-contacting. In the contacting method, the RF power is fed directly to the radiating patch using a connecting element such as a microstrip line. In the non-contacting scheme, electromagnetic field coupling is done to transfer power between the microstrip line and the radiating patch. A feed line is used to excite to radiate by direct or indirect contact. There are many different techniques of feeding and four most popular techniques are coaxial probe feed, microstrip line, aperture coupling and proximity coupling. i. Microstrip Line Feed In this type of feed technique, a conducting strip is connected directly to the edge of the microstrip patch. The conducting strip is smaller in width as compared to the patch and this kind of feed arrangement has the advantage that the feed can be etched on the same substrate to provide a planar structure.microstrip line feed is one of the easier methods to fabricate as it is a just conducting strip connecting to the patch and therefore can be consider as extension of patch. It is simple to model and easy to match by controlling the inset position.
IJECSE, Volume2, Number 1 Sunil Singh et al. 310 Fig6: Microstrip Line Feed The purpose of the inset cut in the patch is to match the impedance of the feed line to the patch without the need for any additional matching element. This is achieved by properly controlling the inset position. Hence this is an easy feeding scheme, since it provides ease of fabrication and simplicity in modelling as well as impedance matching. However as the thickness of the dielectric substrate being used, increases, surface waves and spurious feed radiation also increases, which hampers the bandwidth of the antenna. The feed radiation also leads to undesired cross polarized radiation. However the disadvantage of this method is that as substrate thickness increases, surface wave and spurious feed radiation increases which limit the bandwidth. ii. Coaxial Feed The Coaxial feed or probe feed is a very common technique used for feeding Microstrippatch antennas. Coaxial feeding is feeding method in which that the inner conductor of the coaxial is attached to the radiation patch of the antenna while the outer conductor is connected to the ground plane. Fig7: Probe fed Rectangular Microstrip Patch Antenna The main advantage of this type of feeding scheme is that the feed can be placed at any desired location inside the patch in order to match with its input impedance. This feed method is easy to fabricate and has low spurious radiation. However, its major disadvantage is that it provides narrow bandwidth and is difficult to model since a hole has to be drilled in the substrate and the connector protrudes outside the ground plane, thus not making it completely planar for thick substrates ( h > 0.02λo ). Also, for thicker substrates, the increased probe length makes the input impedance more inductive, leading to matching problems [9]. It is seen above that for a thick dielectric substrate, which provides broad bandwidth, the microstrip line feed and the coaxial feed suffer from numerous disadvantages. The non-contacting feed techniques which have been discussed below, solve these problems. iii. Aperture Coupling Feed
311 Aperture coupling consist of two different substrate separated by a ground plane. On the bottom side of lower substrate there is a microstrip feed line whose energy is coupled to the patch through a slot on the ground plane separating two substrates. This arrangement allows independent optimization of the feed mechanism and the radiating element. Normally top substrate uses a thick low dielectric constant substrate while for the bottom substrate; it is the high dielectric substrate. The ground plane, which is in the middle, isolates the feed from radiation element and minimizes interference of spurious radiation for pattern formation and polarization purity. Fig8: Aperture-coupled feed Advantages: Allows independent optimization of feed mechanism element. iv. Proximity Coupled Feed This type of feed technique is also called as the electromagnetic coupling scheme. As shown in Figure, two dielectric substrates are used such that the feed line is between the two substrates and the radiating patch is on top of the upper substrate. The main advantage of thisfeed technique is that it eliminates spurious feed radiation and provides very high bandwidth (as high as 13%), due to overall increase in the thickness of the microstrip patch antenna. This scheme also provides choices between two different dielectric media, one for the patch and one for the feed line to optimize the individual performances. Fig9: Proximity-coupled Feed Matching can be achieved by controlling the length of the feed line and the width-to-line ratio of the patch. The major disadvantage of this feed scheme is that it is difficult to fabricate because of the two dielectric layers which need proper alignment. Also, there is an increase in the overall thickness of the antenna. Comparing the different feed techniques: Characteristics Microstrip Line Feed Coaxial Feed Aperture Coupled Feed Proximity Coupled Feed
IJECSE, Volume2, Number 1 Sunil Singh et al. 312 Spurious feed More More Less Minimum radiation Reliability Better Poor due tosoldering Good Good Ease of fabrication Easy Soldering and Alignment required Alignment required drilling needed Impedance Matching Easy Easy Easy Easy Bandwidth (achieved with impedance matching) 2-5% 2-5% 2-5% 13% III.PATCH ANTENNA ARRAY In order to increase main beam gain, reduce side lobe radiation, and increase directivity, the patch antenna design was expanded to a four element array. The design layout is shown in Figure. Four elements are used, separated by λ/2. The patch length and width for each element is the same as the single patch antenna described above. The probe position was optimized in HFSS to ensure a 50 Ohm match including adjacent patch coupling. A rectangular distribution of antenna elements was chosen to obtain identical E-plane and H-plane array factor patterns. Four identical antenna elements were used to allow array factor application to the measured patch radiation pattern for array predictions. The xyz coordinates are defined to the left of the figure. The array substrate is in the xy-plane. The z-direction is perpendicular to the substrate. Radiation Pattern Parameter Patch Width Patch Length Element Spacing E-Plane Separation Distance H-Plane Separation Distance Probe feed location Dimension W = 37.5759 mm L = 28.2084 mm λ/2 = 61.551 mm 33.35 mm 23.97 mm 6.95mm Table: Fundamental Specifications of Patch Antennas The patch's radiation at the fringing fields results in a certain far field radiation pattern. This radiation pattern shows that the antenna radiates more power in a certain direction than another direction. The antenna is said to have certain directivity. This is commonly expressed in db. This case is often described as a perfect front to back ratio, all radiation towards the front and no radiation towards the back. This front to back ratio is highly dependent on ground plane size and shape in practical cases. Another 3 db can be added since there are 2 slots. The slots are typically taken to have a length equal to the impedance width (length according to the y-axis) of the patch and a width equal to the substrate height. Such a slot typically has a gain of about 2 to 3 db (cfr. simple dipole). This results in a total gain of 8 to 9 db. The rectangular patch excited in its fundamental mode has a maximum directivity in the direction perpendicular to the patch (broadside). The directivity decreases when moving away from broadside towards lower elevations. The 3 db beamwidth (or angular width) is twice the angle with respect to the angle of the maximum directivity, where this directivity has rolled off 3 db with respect to the maximum directivity. An example of a radiation pattern can be found below.
313 Fig10: Typical radiation pattern of a simple square patch. So far, the directivity has been defined with respect to an isotropic source and hence has the unit dbi. An isotropic source radiates an equal amount of power in every direction. Quite often, the antenna directivity is specified with respect to the directivity of a dipole. The directivity of a dipole is 2.15 dbi with respect to an isotropic source. The directivity expressed with respect to the directivity of a dipole has dbd as its unit. i. Antenna Gain Antenna gain is defined as antenna directivity times a factor representing the radiation efficiency. This efficiency is defined as the ratio of the radiated power (Pr) to the input power (Pi). The input power is transformed into radiated power and surface wave power while a small portion is dissipated due to conductor and dielectric losses of the materials used. Surface waves are not excited when air dielectric is used.antenna gain can also be specified using the total efficiency instead of the radiation efficiency only. This total efficiency is a combination of the radiation efficiency and efficiency linked to the impedance matching of the antenna. ii. Polarization The plane wherein the electric field varies is also known as the polarization plane. The basic patch covered until now is linearly polarized since the electric field only varies in one direction. This polarization can be either vertical or horizontal depending on the orientation of the patch. A transmit antenna needs a receiving antenna with the same polarization for optimum operation. In a circular polarized antenna, the electric field varies in two orthogonal planes (x and y direction) with the same magnitude and a 90 phase difference. The result is the simultaneous excitation of two modes, i.e. the TM10 mode (mode in the x direction) and the TM01 (mode in the y direction). One of the modes is excited with a 90 phase delay with respect to the other mode. A circular polarized antenna can either be right hand circular polarized (RHCP) or left hand circular polarized (LHCP). The antenna is RHCP when the phases are 0 and 90 for the antenna in the figure below when it radiates towards the reader, and it is LHCP when the phases are 0 and 90. iii. Bandwidth Another important parameter of any antenna is the bandwidth it covers. Only impedance bandwidth is specified most of the time. However, it is important to realize that several definitions of bandwidth exist impedance bandwidth, directivity bandwidth, polarization bandwidth, and efficiency bandwidth. Directivity and efficiency are often combined as gain bandwidth.
IJECSE, Volume2, Number 1 Sunil Singh et al. 314 iv. Impedance bandwidth/return loss bandwidth This is the frequency range wherein the structure has a usable bandwidth compared to certain impedance, usually 50 Ω. The impedance bandwidth depends on a large number of parameters related to the patch antenna element itself (e.g., quality factor) and the type of feed used. The plot below shows the return loss of a patch antenna and indicates the return loss bandwidth at the desired S11/VSWR (S11 wanted/vswr wanted). The bandwidth is typically limited to a few percent. This is the major disadvantage of basic patch antennas. v. Directivity/gain bandwidth This is the frequency range wherein the antenna meets a certain directivity/gain requirement (e.g., 1 db gain flatness). vi. Efficiency bandwidth This is the frequency range wherein the antenna has reasonable (application dependent) radiation/total efficiency. vii. Axial ratio bandwidth This bandwidth is related to the polarization bandwidth and this number expresses the quality of the circular polarization of an antenna. IV. APPLICATIONS The Microstrip patch antennas are well known for their performance and their robust design, fabrication and their extent usage. The advantages of this Microstrip patch antenna are to overcome their de-merits such as easy to design, light weight etc., the applications are in the various fields such as in the medical applications, satellites and of course even in the military systems just like in the rockets, aircrafts missiles etc. the usage of the Microstrip antennas are spreading widely in all the fields and areas and now they are booming in the commercial aspects due to their low cost of the substrate material and the fabrication. It is also expected that due to the increasing usage of the patch antennas in the wide range this could take over the usage of the conventional antennas for the maximum applications. Microstrip patch antenna has several applications. Some of these applications are discussed as below: i. Mobile and Satellite Communication Application Mobile communication requires small, low-cost, low profile antennas. Microstrip patch antenna meets all requirements and various types of microstrip antennas have been designed for use in mobile communication systems. In case of satellite communication circularly polarized radiation patterns are required and can be realized using either square or circular patch with one or two feed points. ii. Global Positioning System Applications Microstrip Patch Antenna for GPS application microstrip patch GPS antenna thinner and cheaper than the thick ceramic patch antenna usually employed for automotive applications was proposed and analysed. This configuration allows to easily integrating a low noise amplifier on the substrate used for the feeding circuitry. Comparing microstrip patch GPS antenna to the thicker high permittivity ceramic patch solution, a gain reduction of about 1.5 D bias observed. The use of the quadrature feeding permits a better purity of the circular polarization and a wider impedance bandwidth allowing reducing the environment dependence with respect to ceramic antennas. On the other hand ceramic patch is less sensitive to the windscreen closeness because of its high permittivity substrate and consequently its smaller dimensions. iii. Radio Frequency Identification (RFID)
315 RFID uses in different areas like mobile communication, logistics, manufacturing, transportation and health care [2]. RFID system generally uses frequencies between 30 Hz and 5.8 GHz depending on its applications. Basically RFID system is a tag or transponder and a transceiver or reader. iv. Worldwide Interoperability for Microwave Access (WiMax) The IEEE 802.16 standard is known as WiMax. It can reach upto 30 mile radius theoretically and data rate 70 Mbps. MPA generates three resonant modes at 2.7, 3.3 and 5.3 GHz and can, therefore, be used in WiMax compliant communication equipment. v. Radar Application Radar can be used for detecting moving targets such as people and vehicles. It demands a low profile, light weight antenna subsystem, the microstrip antennas are an ideal choice. The fabrication technology based on photolithography enables the bulk production of microstrip antenna with repeatable performance at a lower cost in a lesser time frame as compared to the conventional antennas. vi. Rectenna Application Rectenna is a rectifying antenna, a special type of antenna that is used to directly convert microwave energy into DC power. Rectenna is a combination of four subsystems i.e. Antenna, ore rectification filter, rectifier, post rectification filter. in rectenna application, it is necessary to design antennas with very high directive characteristics to meet the demands of long-distance links. Since the aim is to use the rectenna to transfer DC power through wireless links for a long distance, this can only be accomplished by increasing the electrical size of the antenna. vii. Telemedicine Application In telemedicine application antenna is operating at 2.45 GHz. Wearable microstrip antenna is suitable for Wireless Body Area Network (WBAN). The proposed antenna achieved a higher gain and front to back ratio compared to the other antennas, in addition to the semi directional radiation pattern which is preferred over the omni-directional pattern to overcome unnecessary radiation to the user's body and satisfies the requirement for on-body and off-body applications. An antenna having gain of 6.7 db and a F/B ratio of 11.7 db and resonates at 2.45GHz is suitable for telemedicine applications. viii. Medicinal Applications of Patch It is found that in the treatment of malignant tumors the microwave energy is said to be the most effective way of inducing hyperthermia. The design of the particular radiator which is to be used for this purpose should posses light weight, easy in handling and to be rugged. Only the patch radiator fulfils these requirements. There is a simple operation that goes on with the instrument; two coupled Microstrip lines are separated with a flexible separation which is used to measure the temperature inside the human body. A flexible patch applicator can be seen in the figure below which operates at 430 MHz V. ADVANTAGES AND DISADVANTAGES Microstrip patch antennas are increasing in popularity for use in wireless applications due to their low-profile structure. Some of their principal advantages discussed by Kumar and Ray are given below: Light weight and low volume. Low profile planar configuration which can be easily made conformal to host surface. Low fabrication cost, hence can be manufactured in large quantities. Supports both, linear as well as circular polarization. Can be easily integrated with microwave integrated circuits (MICs). Capable of dual and triple frequency operations.
IJECSE, Volume2, Number 1 Sunil Singh et al. 316 Mechanically robust when mounted on rigid surfaces. Microstrip patch antennas suffer from a number of disadvantages as compared to conventional antennas. Some of their major disadvantages discussed by and Garg et al are given below: Narrow bandwidth Low efficiency Low Gain Extraneous radiation from feeds and junctions Poor end fire radiator except tapered slot antennas Low power handling capacity. Surface wave excitation VI. CONCLUSIONS In this paper, we covered the basic principle of patch antennas with their basic properties. We defined a basic set of specifications that allow the user to understand and write a set of requirements for a specific application.we understand the feeding techniques of patch antenna configurationapplication of microstrip patch antenna with their advantage and disadvantages. Besides the ones covered here, many more design options and different implementations of patch antennas are available. VII. REFERENCES [1] Pozar, D.M. and D.H., Schaubert, 1995. Microstrip antennas, the analysis and design of microstrip antennas and arrays, New York: IEEE. [2] S. Pinhas and S. Shtrikman, Comparison between computed and measured bandwidth of quarter-wave microstrip radiators, IEEE Transactions on Antennas and Propagation, 36, pp. 1615-1616, 1988. [3] Sze, J.Y. and K.L., Wong, 2000. Slotted rectangular microstrip antenna for bandwidth enhancement, IEEE Transactions on Antennas and Propagation 48, pp. 1149-1152. [4] James j., and P.S. Hall (Eds), Handbook of microstrip antenna, Peter Peregrinus, London, UK, 1989. [5] Ramesh Garg, Prakash Bartia, InderBahl, ApisakIttipiboon, Microstrip Antenna Design Handbook, 2001, pp 1 68, 253 316 Artech House Inc. Norwood, MA. [6] Noah Snyder Directional Patch Antenna Array Design For Desktop Wireless Internet, Senior Project Electrical engineering department, California Polytechnic State University, San Luis Obispo, 2010 [7]D. Orban and G.J.K. Moernaut The Basics of Patch Antennas [8] The Fundamentals of Patch Antenna Design and Performance From March 2009 High Frequency Electronics. [9] E. Alboni, M. Cerretelli Microstrip Patch Antenna for GPS application.