DESIGN OF 2.4 GHZ RECTANGULAR PATCH ANTENNA USING EMC FEEDING

Transcription

1 DESIGN OF 2.4 GHZ RECTANGULAR PATCH ANTENNA USING EMC FEEDING Dina Angela 1, Agustinus Kristiono 2 1, 2 Departemen Teknik Elektro dan Sistem Komputer, Institut Teknologi Harapan Bangsa Jln. Dipati Ukur no Bandung dina_angela@ithb.ac.id 1, agus86cena@yahoo.com 2 ABSTRACT This report presents the design simulation, fabrication and measured performance of a 2.4 GHz rectangular patch antenna for wireless communication applications. To increase the bandwidth of antennas, there are numerous and well-known methods including increase of the substrate thickness and feeding techniques. The design adopts EMC (electromagnetically coupled) feeding techniques with L-strip feeder. The bandwidth of the antenna is about 3.4% that support wideband. The frequency range is MHz ,50 MHz to support WiFi (Wireless Fidelity) technology. The proposed patch has a compact dimension of 0.509λ λ 0 (where λ 0 is the guided wavelength of the centre operating frequency). The material uses a copper plate with 1 mm of thickness. Numerical, simulated, and measured results of the antenna radiation characteristics, including input SWR, radiation pattern coverage and polarization-diversity, are presented and compared. Keywords: electromagnetically coupled, VSWR, gain, rectangular patch, wideband, ellipse polarization. 1 INTRODUCTION An antenna can also be viewed as a transitional structure (transducer) between freespace and a transmission line (such as a coaxial line), so it makes antenna the most important part in radio communications. An important property of an antenna is the ability to focus and shape the radiated power in space e.g.: it enhances the power in some wanted directions and suppresses the power in other directions[1]. There are many types of antenna according to its application. Microstrip antennas, known as conformal antenna, are increasing in popularity for use in wireless applications due to their low-profile structure. They are extremely compatible for embedded antennas in handheld wireless devices such as cellular phones, pagers etc. The telemetry and communication antennas on missiles need to be thin and conformal and are often microstrip antennas. Some of their principal advantages are light weight and low volume, low profile planar configuration which can be easily made conformal to host surface, mechanically robust when mounted on rigid surfaces, low fabrication cost (hence can be manufactured in large quantities), supports both, and linear as well as circular polarization. Microstrip antennas suffer from a number of disadvantages as compared to conventional antennas. Some of their major disadvantages are narrow bandwidth, low efficiency, and low gain (6 dbi). High speed, broadband and high capacity in or outdoor wireless local area networks (WLAN) are becoming more and more predominant today, its interesting to become familiar with some of the aspects of wireless design that must be faced and overcome. The advantages of microstrip antennas have made them a perfect candidate for use in the wireless local area network (WLAN) applications. Though bound by certain disadvantages, microstrip antennas can be tailored so they can be used in the new high-speed broadband WLAN systems and other applications, e.g. PCS, Bluetooth, RFID, etc. This research is to design a microstrip antenna with rectangular patch shape using EMC feeding method that has no spurious radiation and can give wideband characteristics without any matching circuit. The proposed antenna fulfills the requirements of WLAN applications and increases the coverage area. The antenna works at 2.4 GHz (1.29 GHz of bandwidth). The result of this research is a prototype antenna. 2 MICROSTRIP ANTENNA DESIGN AND IMPLEMENTATION A microstrip antenna is a radiating patch on one side of a dielectric substrate, which has a ground plane on the underside. The electromagnetic waves fringe off the top patch into the substrate, reflecting off the ground plane and radiates out into 47

2 48 The 5 th International Conference on Information & Communication Technology and Systems width c L = 2 L 2 f ε o reff (4) patch ground plane Figure 1. Operations of microstrip antenna substrate the air. Radiation occurs mostly due to the fringing field between the patch and ground (Fig.1). The radiation efficiency of the patch antenna depends largely on the permittivity of the dielectric. Ideally, a thick dielectric and low insertion loss is preferred for broadband purposes and increased efficiency. 2.1 Rectangular Patch Antenna The first mathematical analyze of a wide variety of patch shapes was published in 1977 by [2], which he used the modal-expansion technique to analyze to rectangular, circular, semicircular and triangular patch shapes. Rectangular patch antenna is usually modeled as transmission line that has length (L), width (W), and height of the substrate (h). Microstrip antenna has resonance frequency as shown in eq.1.[9] f = 2 c ( L + 2 L) εreff (1) The fringing fields along the width can be modeled as radiating slots and electrically the patch of the microstrip antenna looks greater than its physical dimensions. The dimensions of the patch along its length have now been extended on each end by a distance L, which is given empirically as[3]: L = h ( ε ) reff W h W ( ε ) reff h (2) The effective length of the patch L eff now becomes: L ef = L + 2 L (3) For a given resonance frequency f 0, the effective length is given as: The expression for ε reff is given, for W/h > 1, as: 1 εr + 1 εr 1 h 2 εreff = (5) 2 2 W In addition, for W/h < 1: εreff ε = r + 1 εr h W W h (6) which ε r is relative permitivity. For efficient radiation, the width W is given as: 1 λo εr = 2 2 W (7a) which λ o is wavelength in free-space. In other term can be written as: c 2 W = (7b) 2 f o εr Electromagnetically Coupled (EMC) Feeding technique affects to the input impedance and polarization of the antenna. Three most common structures that feed planar printed antennas are coaxial probe feeds, microstrip line feeds and aperture coupled feeds. Aperture-coupled feed is adopted in the design. The RF energy from the feed line is coupled to the radiating element through a common aperture in the form of a rectangular slot. EMC feeding technique will not cause spurious radiation and can give wideband characteristics without using any matching circuits. Pozar[4] first proposed this type of feed. The aperture coupled feeding mechanism is shown in Fig. 2. It shows the geometry of the basic aperture coupled patch antenna that will be designed. L-strip feed structure is a serial L-C circuit that is connected to parallel R-L-C resonance elements (represented by the patch). The horizontal part of L-strip structure is as the capacitance elements to reduce the inductance effects at the vertical part. Most of coupling mechanisms are

3 D08 - Design of 2.4 GHz Rectangular Patch Antenna Using EMC Feeding - Dina Angela 49 from capacitive elements. C c is coupling between L-strip feeder and antenna is any other antenna whose gain can be calculated or it is known[3]. This is accomplished P s G s G r P r (a) P s G s G t P t (b) Figure 3. (a) Measurement of received power at standard antenna, P s. (b) Measurement of received power at tested antenna, P t. Figure 2. EMC feeding method at rectangular patch antena the patch. Coupling is controlled by three factors: L-strip feeder inset length (D), patch width (W), and L-strip feeder height (h 1 ). Antenna design simulator will help this work to simulate some variations of those sensitive parameters of antenna characteristics to get an optimum result. 2.3 Antenna Basic Characteristics The bandwidth of the patch is defined as the frequency range over which it is matched with that feed line within specified limits. In other words, the frequency range over which the antenna will perform satisfactorily. This means the channels have larger usable frequency range and thus results in increased transmission. The bandwidth of an antenna is usually defined by the acceptable standing wave ratio (SWR) value over the concerned frequency range. If the center of operating frequency f c, the lower frequency f 1, and the upper frequency f 2, then the bandwidth can be calculated by using (in procentage)[3]: f2 f1 BW = 100% (8) f c Input impedance is measured at antenna s terminal. The input impedance is complex and it includes both a resonant and a nonresonant part which is usually reactive. Antenna input impedance is represented by: Z in = R in + j X in (9) Antenna gain is the ratio of the power gain in a given direction to the power gain of a reference antenna in its referenced direction. The power input must be the same for both antennas. The reference by multiplying the directivity (D) by the efficiency (k) to result in the gain (G), as written below: G = k.d (10) A basic method that can be used to measure the gain is gain-transfer measurement. It must be used in conjuction with standard gain antennas to determine the absolute gain of the antenna under test (AUT)[3]. This process will be described in Fig.3. Antenna acts as the radiation source is fed by a constant power, P in, from a transmitter. Another antenna, whose gain G s has known, acts as the receiver and receives power P s. Then, the receiver antenna is replaced by antenna that will be measured. This antenna will receive power P t. The gain of this antenna can be calculated by: P t G t = Gs (11a) Ps Alternatively, in decibel terms: G t (d) = P t (db) - P s (db) + G s (db) (11b) The radiation patterns (amplitude and phases), polarization, and gain of an antenna, which are used to characterize its radiation capabilities, are measured using standard spherical system[3]. Antenna pattern is usually represented in two orthogonal principal E-plane and H-plane patterns (or vertical and horizontal) as a function of angular direction (θ, Φ) either in polar or in rectangular coordinates plotting. The pattern of most wireless LAN antennas contains a main lobe and several minor lobes, termed side lobes. A side lobe occurring in space in the direction opposite to the main lobe is called back lobe. Radiation pattern is called by field pattern when in E.M. field strength

4 50 The 5 th International Conference on Information & Communication Technology and Systems functions and it is called by power pattern when represents poynting vectors. E y polarization ellipse E x E direction of propagation Figure 4. Illustration of elliptical polarization. Usually the antenna in a particular direction radiates the polarization of the wave. This is usually depending on the feeding technique. In general, all electromagnetic waves are elliptically polarized. In this general case, the total electric field of the wave is comprised of two linear components, which are orthogonal to one another. Each of these components has a different magnitude and phase. At any fixed point along the direction of propagation, the total electric field would trace out an ellipse as a function of time. This concept is illustrated in Figure 4, where, at any instant in time, E x is the component of the electric field in the x- direction and E y is the component of the electric field in the y-direction. The total electric field E, is the vector sum of E x plus E y. 2.4 Microstrip Antenna Design and Implementation Process The aim of the design is to provide an optimum performance at input impedance resonant frequency matching for WLAN requirements. The design steps are as follow: 1. Define the desired antenna specifications: Frequency: ,50 MHz (according to IEEE ). Impedance: 50 Ω, SMA connector. VSWR 1.5. Gain > 6 dbi. 2. Define antenna frequency, f o. 3. Define patch width W (eq.5a or eq.5b), patch length L (eq.6), dielectric height H. 4. Define material specifications: Copper (Cu), air and teflón TM. Relative permittivity of air, ε o = 1. Relative permittivity of material, ε r = 1. Conductor thickness is 1 mm. Teflon (spacer) height is 14 mm, and diameter is 8 mm. Feeder L-strip is made from Cu with width is 8 mm and 1 mm of height. 3 RESULTS AND ANALYSIS Simulation is done repetitively by changing sizes or positions variously of the patch, groundplane, feeder, and air gap height in order to get desired specifications. The parameter results are concluded in Table 1. Antenna parameters values from the simulation process are as follow: VSWR (Voltage Standing Wave Ratio) is 1.35 at 2.4 GHz that give the bandwidth antenna is 1290 MHz (Fig. 5). Antenna gain is 9.07 db (Fig. 6). Radiation pattern is unidirectional (Fig.7). After simulation has accomplished and has got the results, the antenna (or the prototype) is built (see Fig. 12). Then, it will be measured and the results will be compared with the simulation results to get the optimum ones. In measurement process, there are some antenna basic characteristics values, such as VSWR, input impedance, radiation patterm, gain, and polarization. The antenna was measured at Puslit Telkoma PPET-LIPI in Bandung. Table 1. Simulation results of antenna dimensions Parameters Dimension Groundplane (P x L) 120 x 120 mm Patch rectangular (W x L) 62.5 x 44.5mm Air gap height (H) L-strip to groundplane (h 2 ) L-strip to patch 14 mm 1 mm 2 mm L strip length (A1 x A2 x A3) 46 x 9 x 11 mm L-strip length to the edge of patch (D) 6 mm Figure 5. Antenna VSWR Figure 6. Antenna gain from simulation result

5 D08 - Design of 2.4 GHz Rectangular Patch Antenna Using EMC Feeding - Dina Angela 51 gives the best value. If we choose VSWR 1.35, bandwidth percentage (eq.8) is: BW = 100% = 3. 4% 2442 (a) Figure 7. Radiation pattern in (a) Azimuth direction (b) Elevation direction Figure 8. Measurement of VSWR f = 2,4 GHz f = 2,4 GHz Figure 9. Measurement of input impedance at Smith Chart The comparison results of simulations and measurements are (see Table 2): VSWR is in measurement (see Fig.8) and 1.35 in simulation. Simulation result (b) VSWR = 1,074 Z in = Ω It means that the bandwidth is not narrow ( 1%). Input impedance is Ω in measurement (Fig.9) and Ω in simulation. Measurement result gives the best value. The connector has 50 Ω of impedance. This difference may be caused by feed line position that is not precision when prototyping process. Both of measurement and of simulation, radiation pattern is unidirectional (Fig. 10). However, the pattern is quite different each other due to the area of measurement that is not ideal, many interferences from surrounding and the limited precision of measurement equipments. Both of measurement and of simulation, polarization is ellipse (Fig.11). At receiver antenna (AUT), minimum received power at minor axis is dbm or x 10-9 Watt and maximum received power at major axis is dbm or x 10-7 Watt. In gain measurement, helix antenna is used as the reference antenna. Specifications of the helix: Gain is 2.14 dbi toward dipole, power range is -43 to -40 db toward dipole, and frequency is 2.4 GHz. By using eq.11b, the antenna gain in measurement is 8.5 dbi. It is different from the simulation result, 9.07 db. It may be caused by reflected waves around it. 4 CONCLUSION AND DISCUSSION After the calculation theoretically, simulation and implementation, we can conclude all of the results: 1. This microstrip antenna has wide bandwidth, 1.29 GHz at frequency range GHz. 2. The VSWR of this microstrip antenna is under two, means that this antenna can fulfill the main requirement of ideal antenna specification. 3. Radiation pattern is unidirectional. 4. The gain of this microstrip antenna is 9 db (taken from the simulation as the optimum result).

6 52 The 5 th International Conference on Information & Communication Technology and Systems This research can be developed to increase higher gain by using array method, change the patch shape and change the material. Table 2. Comparisons of measurement and simulation results Parameters Simulation Measurement Bandwidth at 2.4 GHz Impedance Ω Ω Polarization Ellipse Ellipse Radiation pattern Unidirectional Unidirectional Gain 9.07 db 8.5 dbi REFERENCES [1] J.D. Krauss (1988) Antennas. Singapore: McGraw-Hills International Editions. [2] Bahl, I.J. and P. Bhartia (1980). Microstrip Antennas Design Handbook. Artech House. [3] C.A. Balanis (1997) Antenna Theory: Analysis and Design. 2 end. Chichester: John Wiley. [4] Pozar, D.M. (1996). A review of aperture coupled microstrip antennas: history, operation, development and applications. University of Massachusetts at Amherst, pp: 1-9. radiation pattern at azimuth direction (simulation) radiation pattern at azimuth direction (measurement) [5] W.L. Stutzman and G.A. Thiele (1981) Antenna Theory and Design. Chichester: John Wiley. [6] E. Hammerstad, F.A. Bekkadal (1975) Microstrip Handbook, ELAB Report, STF 44 A University of Trondheim at Norway. radiation pattern at elevation direction (simulation) radiation pattern at elevation direction (measurement) Figure 10. Comparison of radiation pattern [7] Dase, Sulwan (2003) Antena Susunan Mikrostrip Rectangular Patch Elemen pada Band 6 GHz. MSc Dissertation, Magister Institut Teknologi Bandung. Bandung. [8] Judawisastra, Herman. EL-366 Antena dan Propagasi. Lecture Dictate, Institut Teknologi Bandung. Bandung. [9] S.J., Joo. Technical Feature: Design of Wideband Patch Antennas for PCS and IMT 2000 Service. polarization from simulation result polarization from measurement result Figure 11. Comparison of polarization Figure 12. Rectangular patch antenna with L-strip structure.