Indian Journal of Pure & Applied Physics Vol. 46, August 2008, pp. 593-597 Design, fabrication and characterization of microstrip square patch antenna array for X-band applications Deepak Sood*, Gurpal Singh**, Chander Charu Tripathi, Suresh Chander Sood & Pawan Joshi Electronics & Communication Department, Ambala College of Engineering & Applied Research (Near Mithapur), Ambala Cantt., Haryana **Computer Science Department, Baba Banda Singh Bahadur Engineering College, Fathegarh Sahib, Punjab *E-mail: deepaksood1980@yahoo.com Received 9 August 2007; revised 20 March 2008; accepted 17 April 2008 Microstrip patch antenna array to be used for X-band short-range microwave applications has been introduced. This antenna maintains a maximum gain of 14.6 db for far field region w.r.t paraboloidal rectangular aperture antenna. It is actually a probe (coaxial) fed antenna with quarter wave sections that are fabricated as an arrangement for impedance matching with 50 Ω coaxial cable. This antenna works well in the frequency range 8.45-9.33 GHz with a maximum at 8.53 GHz. It is basically a low cost, lightweight, medium gain and narrowband antenna and suited for short-range microwave applications such as a feeding element for other antennas and in research institutes as a reference antenna. It can also be used at the output of signal conditioning circuit for receiving signals from micro electromechanical sensors. Keywords: Square patch, X-band, Antenna array 1 Introduction Communication using electromagnetic radiation (except for light) began early in the last century. Most of the early systems used very long wavelengths (low frequencies), which travelled great distances. Later it was discovered that higher frequencies could bring other advantages to communications. Microwaves are easier to control (than longer wavelengths) because even small antennas could direct the waves very well. This control leads to confinement of energy into a tight beam (expressed as narrow beam width). This beam can be focused on another antenna dozens of miles away, making it very difficult for someone to intercept the conversation 1. Microwaves are commonly used in various aspects of everyday life. As the frequency increases the wavelength decreases and thus it becomes easier to construct an antenna system that is large in terms of wavelength and which therefore can be made to have greater directivity. At microwave frequencies the physical size of high gain antenna becomes small enough to make practical the use of suitably shaped reflectors to produce the desired directivity 2. Due to short wavelength at microwave frequencies, highly directive antennas are smaller and therefore more practical than they would be at longer wavelengths. Nowadays, for various commercial applications such as short range transmission of signals, medical applications, mobiles and laptops there is requirement of an antenna which consumes very less space and can be mounted easily in the equipment and still have efficient directive radiation pattern. One such antenna design, which is very famous these days, is microstrip patch antenna. Different techniques have been suggested to achieve antenna integration within a single chip 3-5. In this paper a simple design of microstrip patch antenna array for X-band applications is proposed. Microstrip patch uses conductive strips and/or patches formed on the top surface of a thin dielectric substrate separating them from a conductive layer on the bottom surface of the substrate and constituting a ground for the antenna. A patch is typically wider than a strip and its shape and dimensions are important features of the antenna. 2 Design Considerations 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 simplest patch antenna (Fig. 1) uses a half wavelength long patch and a larger ground plane. Larger ground plane gives better performance but of course makes the antenna bigger. Common microstrip
594 INDIAN J PURE & APPL PHYS, VOL 46, AUGUST 2008 Fig. 1 Basic structure of microstrip antenna. antenna radiator shapes are square, rectangular, circular and elliptical. Infact, any continuous shape is possible. A microstrip antenna may be defined 6 as an antenna which consists of a thin metallic conductor bonded to a thin grounded dielectric substrate. The resonant length of the antenna determines the resonant frequency. The patch is, in fact, electrically a bit larger than its physical dimensions due to the fringing fields. The deviation between electrical and physical size is mainly dependent on the PCB thickness and dielectric constant 7. The patch that introduced here has been made of conducting material copper. The design parameters define the operation and performance of the patch antenna. The proposed patch antenna consists of 4-square elements each of size 1.1cm 1.1cm decided according to the design formulas 8 for patch. For good antenna performance, a substrate having a low dielectric constant is desirable since this provides better efficiency, larger bandwidth and better radiation 9,10. The substrate used in this study is PVC sheet. The spacing between adjacent squares is nearly 2.2 cm. All of the four elements are so arranged as to make a bigger square of size 4.32 cm 4.32 cm. The center-to-center spacing between squares is 3.26 cm. Coaxial feed arrangement is done at center of bigger square with the help of quarter wave sections designed end to end (each of length λ 0 /4) for proper impedance matching with 50 Ω coaxial cable. The ground plane size controls the gain of the antenna while distance between ground plane and patch affects the bandwidth. All the parameters are calculated by taking the frequency of operation as 9 GHz. The impedance of the single patch comes out to be 365.2 Ω. Bandwidth and directivity have been calculated to be 1% and 4.5 dbi respectively. To match the single patch to a 50 Ω microstrip line requires a λ 0 /4 section of impedance 135 Ω that has a length nearly 0.166 λ 0. The designed patch antenna array is shown in Figures 2(a) and 2(b) respectively. Fig. 2(a) Front view of 4-element patch Fig.2 (b) Side view of 4-element patch 3 Fabrication Methodology Various techniques are available for fabrication of microstrip patch antenna 10-12. A Simple and low cost method which has been used in this study to fabricate this patch array is discussed. As per design, mask for the fabrication of strip antenna was designed using AutoCAD. In order to precisely transfer the mask image on electro plated copper PCB board photolithography technique was used. The cut size pieces of PCB sheets were cleaned to remove the surface impurities using organic solvents and then dried with hot air gun before coating the positive photo resist (PPR) on it. The PPR coated substrate was pre-baked in an oven at 90 C to remove the solvent and stuffing the film. The baked substrate was exposed on indigenously developed mask aligner with inbuilt exposure system and mask as prepared earlier. Basic fabrication steps are shown in Figs 3(a) and 3(b). The exposed substrate was developed in 20% KOH solution. After transferring the mask images on substrate, it was placed in oven at temperature 130 C for 30 min for hard baking. The exposed unwanted
SOOD et al.: MICROSTRIP SQUARE PATCH ANTENNA ARRAY 595 Fig. 3 (c) Mask layout of 4-elements patch array Fig. 3(a) Fabrication steps Fig. 4(a) Basic test set-up for measurement Fig. 3(b) Patch array layout features were etched out in solution of FeCl 3. The etched Cu pattern was rinsed in DI water and airdried. A number of micro strip antennas were fabricated using above technique to achieve best quality. The dimension details were tested and verified using tool room microscope. The processing parameters were established to achieve best quality reproducible antenna patch strips. The fabricated microstrip antenna was mounted on properly designed and prefabricated aluminum sheet. This aluminum sheet acts as ground plate. N-type female (SMA) connector is soldered at center of PCB from the backside. Fig. 3(c) shows the mask layout of 4-element patch antenna array. 4 Results and Characteristics Various test methods have been suggested earlier 13. In this study the designed antenna is tested with commonly available microwave lab equipments. The fabricated microstrip patch antenna array was used as the transmitting antenna and a pyramidal horn antenna as the receiving antenna. This was mounted on an antenna stand and could be rotated from 0 to 360 both in the E-plane and H-plane. The distance between the two antennas was kept 52.6 cm, which ensured the presence of far field region. The microwave characteristics of the microstrip patch antenna were measured point by point in the frequency range 8.2-12.4 GHz using the wave-guide microwave bench consisting of source, isolator, attenuator and detector. The radiated output was measured in terms of voltage. Basic test set-up is shown in Fig. 4(a) 4.1 Radiation characteristics The radiated field from a microstrip antenna can be determined either from the field distribution between patch and the ground plane or from the surface current distribution on the patch 14. Here patch antenna is tested to determine its radiations with the help of Reflex Klystron (2K25) operating under X-band up to
596 INDIAN J PURE & APPL PHYS, VOL 46, AUGUST 2008 a maximum of 400 volts of dc power supply with a maximum of 25 ma of current. The E-plane and H- plane patterns of 4-element patch antenna array are shown in Fig. 4(b) and 4(c) respectively. These graphs are shown as angle vs output voltage. From the E-plane pattern it is observed that the output is high near 0 on both sides. Also, very low side lobes are observed at equal distances w.r.t 0 after 50 angles on both sides. These side lobes can be minimized by a slight modification in the design. Output level was observed to remain high up to nearly 40 angles on both sides of antenna i.e. in clockwise as well as anticlockwise directions. In the H-plane pattern low output is observed in both directions i.e. clockwise and anticlockwise directions w.r.t 0 as compared to E-plane pattern. Maximum output in H-plane pattern is observed at 0. There is very large variation in output around 0. Downfall in output is observed near 0 in the left direction which shows non-uniform nature of H-plane radiation pattern of patch antenna. This signifies that it is a directive antenna as it responds only to a confined range. 4.2 Frequency response The frequency response of 4-element patch antenna is shown in Fig.4 (d). From this graph it is observed that it responds best at 8.53 GHz and works reasonably well in the range 8.45-9.33 GHz. After 9.4 GHz the output observed to be very low. 4.3 Gain measurement Gain is an important parameter of every antenna. Basically, gain is the ratio of the radiated field intensity by test antenna to the radiated field intensity by reference antenna. In this study the paraboloidal rectangular aperture horn antenna is used as reference antenna. To calculate gain, the patch array is tested as a transmitting antenna w.r.t pyramidal rectangular Fig. 4(b) E-plane pattern of 4-element patch antenna array Fig. 4(d) Frequency response of 4-element patch antenna array. Fig. 4(c) H-plane pattern of 4-element patch antenna array Fig. 4(e) Radiation characteristics of 4-element patch array w.r.t pyramidal rectangular horn.
SOOD et al.: MICROSTRIP SQUARE PATCH ANTENNA ARRAY 597 aperture horn antenna that is used as a receiving antenna as shown in Fig. 4(e). For the calculation of gain, radiation field has been measured at far field of horn antenna. Gain is calculated to be 14.68 db at 52.6 cm of distance from patch array. 5 Conclusions An X-band 4-element patch antenna array has been designed. The designed antenna has been fabricated, tested and analyzed using an X-band microwave source i.e Reflex Klystron. The results have been plotted using MATLAB. Designed antenna has been tested as a transmitter as well as receiver. This 4- element patch antenna provides best radiation properties with a medium gain for far field radiation w.r.t pyramidal rectangular horn. From the tests it has also been observed that it is best for the frequency 8.53 GHz and also shows good performance in the frequency range 8.45 GHz to 9.33 GHz w.r.t a simple single element patch antenna. This type of antenna is best suited for short-range microwave applications, as a feeding element for other antennas. It can also be used in research institutes as a reference antenna. References 1 Thomas D Williams, Introduction to Microwaves Conference on microwave communications, Prague (2002). 2 Microwaves, WWW. ENCYCLOPEDIA. ORG. 3 Chris R Trent & Tom M Weller, IEEE Symposium on microstrip antennas, 1 (2002) 402. 4 Zhang Y P, IEE electronics Letter on Integration of microstrip patch antenna, Nanyang Technological University, Singapore, 38 (2002) 207. 5 Garg R, Bhartia P, Bahl I & Ittipiboon, Microstrip antenna design handbook, Artech house, London, (2001). 6 Christopher J B, editor, IEEE Standard Dictionary of Electrical and Electronics Terms, (IEEE Press), Piscataway, New Jersey, (1995). 7 Orban D & Moernaut G J K, Basics of patch antennas, Orban Microwave Products, WWW.ORBANMICROWAVE.COM. 8 Kraus J D & Marhefka R J, Antennas with all applications, TMH, New York, (2005) 322. 9 Abdel-Aziz M, Ghali H, Ragaie H, Haddara H, Larique E,Guillon B & Pons P, Design, Implementation and Measurement of 26.6 GHz Patch Antenna, Electronics and Communication Engineering Department, Ain Shams University, EGYPT, (2006) 2. 10 Pan B, Yoon Y, Papapolymerou J, Tentzeris M M & Allen M G, A high performance surface-micromachined elevated patch antenna, Georgia Institute of Technology, Atlanta, USA, (2005) 0250. 11 Keith C H, Microstrip antennas: Broadband radiation patterns using photonic crystal substrates, Thesis, Faculty of the Virginia Polytechnic Institute and State University, Blacksburg, (2002). 12 Kim S G, Wideband two-dimensional and multiple beam phased arrays and microwave applications using piezoelectric transducers, Thesis, Graduate Studies of Texas, A&M University, (2005). 13 Ahsan N & Kayani J K, Design of an X-band microstrip monopulse antenna for monopulse tracking radar, International Conference on Applied Sciences and Technology, Bhurban, Pakistan, 2 (2003). 14 Norrman H, Development of a microstrip antenna for a miniaturized transponder, Thesis, Lulea University of Technology, Sweden, (2006) 24.