DESIGN AND DEVELOPMENT OF A COMPACT WIDEBAND CONFORMAL ANTENNA FOR WIRELESS Abstract APPLICATIONS R. Sreekrishna 1, B.R.Karthikeyan 2, Govind R. Kadambi 3 1-M.Sc. [Engg.] Student, 2-Assistant Professor, Department of EEE, 3-Deputy Director (Academics) M.S.Ramaiah School of Advanced Studies, Bangalore 560 058 The modern wireless communication system possess an important constraint on the design of antennas for its conformal structural characteristics. The typical application such as avionics warrants the antenna design to be in flush with the airframe. The antennas for these systems have to be designed with the unique features. The antenna radiator design along with the desired radiation performance in addition to the conformal body shaping is a challenging task. This paper presents, design and simulation study of a wideband conformal antenna that operates in the frequency band ranging from 1.3 GHz to 2.5 GHz. The main objective of the paper is to perform design and simulations to obtain a conformal antenna geometry with omnidirectional radiation pattern in more than one principal plane. Planar antenna geometries such as planar monopole and cross dipole are identified as the suitable candidates. These planar geometries are designed for the desired specification and its planar structure is modified to a conformal shape without significant changes in its performance. This paper details the iterative design and simulation studies of planar antenna geometries and their modified versions. The planar configurations of the simulated antenna designs are developed in to corresponding prototypes. CATIA modelling software is used for the conformal shaping. All the simulations are performed using Empire 3D EM Solver and HFSS Solver. The results of the prototype antenna are in close agreement with the simulated antenna results. Keywords: Conformal Antenna, Return Loss, Radiation Pattern, Monopole, Dipole Symbol Unit Description I As -1 Time changing current L m Length of current element Q C Charge Z 0 Ohms Characteristic impedance f L Hertz Lower cut off frequency f c Hertz Upper cut off frequency υ ms -2 Time change of velocity ε F/m Permittivity ε r - Relative permittivity Г - Reflection coefficient Λ mm Wavelength Abbreviations 3-D 3-Dimension db Decibel GHz Giga Hertz HFSS High Frequency System Simulator KHz Kilo Hertz MHz Mega Hertz RF Radio Frequency VNA Vector Network Analyzer VSWR Voltage Standing Wave Ratio 1. INTRODUCTION Radio frequency bands ranging from 3 MHz to 300 GHz are normally used for long distance wireless communication. The interface between the wired and the wireless medium is of great importance to any wireless communication equipment. From any such equipment, the signal or the RF wave to be transmitted will be carried by a transmission line towards an antenna. An antenna is a structure that matches the impedance of this transmission line to that of free space (free space impedance = 377 Ω). The transmitting antenna acts as the region of transition from a guided wave in a transmission line to a free-space wave and the receiving antenna acts vice-versa. The dimensions and the structure of the antenna mainly depend on the range of frequency it has to transmit or receive. A conformal antenna is an antenna whose structure conforms to a prescribed shape. There are various factors other than electromagnetic considerations, which prescribe the shape and orientation of a conformal antenna. The IEEE standard definition of terms for antennas (IEEE Std 145-1993) gives the following definition for conformal antenna; An antenna that conforms to a surface whose shape is determined by considerations other than electromagnetic; for example, aerodynamic or hydrodynamic Conformal antennas are not a new topic of research for antenna designers. The elevation-plane s (plane of antenna) radiation pattern of the antenna discussed by C.-C. Lin et.al [1] has two deep nulls 180 apart, which turns the antenna blind towards radiation in those directions. The wide band helmet-mounted antennas designs presented by David Herold et.al [2] have poor vertical radiation pattern similar to that of an ordinary monopole antenna. Jarrod Fortinberry et.al [3] has presented a study about the effects on GPS antenna when mounted on helmets. It was seen that the material on which the antennas are flush mounted greatly affects the return loss and the resonant frequency. The antennas discussed in [4] by Dejan S. Filipovic and John SASTECH Journal 36 Volume 10, Issue 1, May 2011
L.Volakis, have near omnidirectional horizontal coverage but a poor vertical coverage. The analysis of conformal patch antennas in [5] reveals no dramatic differences in return loss when compared to the results. 2. ANTENNA PARAMETERS The strength and purity of signal transmitted or received by a system greatly depend on the characteristic properties of the antenna which are defined by the parameters such as Return loss, Bandwidth and Radiation Pattern. Return loss of an antenna is the parameter which specifies how well the impedance of the antenna is matching with that of the transmission line. The return loss is related to the reflection coefficient as: where, (1) antennas are low profile, conformable to planar and non-planar surfaces, simple and inexpensive to manufacture using printed-circuit technology and mechanically robust when mounted on rigid surfaces [7]. Numerous microstrip and planar antenna geometries are explained in literature by designers and researchers, which can give a wide bandwidth. But most of the geometries are unsuitable for the work taken up in this paper since they don t give omnidirectional radiation patterns in more than one principal plane, in contrary to specifications in the problem statement. Based on the literature survey, some of the geometries like planar monopole and cross dipole were identified to be suitable for obtaining a wide bandwidth and to be mounted on conformal surfaces. The identified base geometries, a) linear monopole and b) cross dipole which are modeled and simulated in Empire 3D are shown in Figures 1 and respectively. (2) = Reflection coefficient =Antenna Impedance = Characteristic Impedance of the feed line The impedance bandwidth of the antenna is usually measured in terms of return loss. It is the range of frequencies for which the return loss is less than -10 db. An antenna radiation pattern or antenna pattern is a mathematical function or a graphical representation of the radiation properties of the antenna as a function of space coordinates. It is a 2 or 3 dimensional representation of radiation of an antenna. If the antenna is having an essentially non-directional pattern in a given plane such a pattern is known as omnidirectional radiation pattern. It is essential to have an omnidirectional pattern in cases where the position of receiver (transmitter) is unknown at the transmitter (receiver).the radiation pattern is an important parameter that determines the position of a system or more specifically conformal surface in this case. As a convention the radiation pattern of an antenna is measured in three principal reference planes; Phi 0 (XZ plane), Phi0 90 (YZ plane) and Theta 90 (XY plane). Depending on the orientation of the antenna and the direction in which it is fed, one of the three planes mentioned above will be known as the Azimuth plane and the other two will be known as Elevation planes. 3. 3. PROBLEM DEFINITION The following is the problem statement taken up for this paper; Design and develop a compact wideband conformal antenna for wireless applications to operate in the frequency range 1300 MHz 2500 MHz, giving omnidirectional radiation patterns in at least two reference planes. 4. MODEL CONSTRUCTION Out of several types of antennas that can give wide band characteristics, printed antennas are a suitable choice to be mounted on conformal surfaces. Such Figure 1 Modelled Planar Antenna Geometries Linear Monopole Cross Dipole In order to attain the desired design specifications such as a wide bandwidth from 1.3 GHz to 2.5 GHz and omnidirectional radiation pattern in at least two principal planes, the planar monopole and cross dipole antenna geometries discussed in prior have been modified iteratively. After iterative optimization through simulations, three planar geometries have been finalized since they give satisfactory performance in accordance with the given specification. The 3D models are created in Ganymede graphical editor which comes along with Empire 3D electromagnetic field solver using which simulations are carried out. The final monopole and cross dipole antenna profiles are shown in Figures 2 and respectively. Figure 2 Planar Monopole Cross Dipole SASTECH Journal 37 Volume 10, Issue 1, May 2011
A generic description of the different antenna elements in the designed geometries are given in the succeeding sections. 4.1 Radiator Since the design requirement specifies that the antenna has to be flush mounted on to a conformal surface, the radiating elements and ground plane has to be positioned on to the surface of mounting platform. Hence here the radiator is placed on the top surface of the substrate while simulation. Radiator is the metalized portion of the antenna from where the electromagnetic waves get transmitted to free space. By changing the paths of current flow through the radiator by various techniques such as introducing slots, steps, tapered slots, curved slots, etc, the resonant frequency of the antenna can be varied. The profile of the radiating element can be rectangular, circular or any unsymmetrical shape whose selection is an important design criterion. The thickness of the radiator is usually taken as 0.02 mm, and the material assigned is copper (ε r = 1). 4. 2 Ground Plane Ground plane is also the metallic portion, which serves as the ground or reference for the radiator. The shape of ground plane is usually rectangular, and its length should be at least λ/4 to get the maximum performance where λ is the minimum wavelength of operation of the antenna. Based on the consideration of parameters like resonance frequency, bandwidth and compactness, length can be further increased. Thickness and material properties of the ground plane is same as that of the radiator. Figure 3 Conformal Monopole Antenna 4. 3 Substrate Substrate is an important component of the planar antenna, which acts as the base for the ground plane and the radiator. The material used and the thickness of the substrate is an important design consideration. Substrate serves as the dielectric to the antenna whose dielectric constant ε r (permittivity) and loss tangent defines the amount of radiation and compactness of the antenna. Higher value of the dielectric constant of the substrate generally tends to minimize the antenna dimensions. Here FR-4 epoxy (ε r = 4.3) is used as the substrate material with a thickness of 1.52 mm. 4.4 Feed Port Feed port is used to excite the antenna with the source signal from the transmission line. There are different types of feed points that are commonly used to excite the antenna such as coaxial port, lumped port, microstrip port, stripline port, coplanar wave guide, etc. The input impedance of the feed port defines the desirable impedance that the antenna has to exhibit in its desired frequency band. In most of the practical applications coaxial port is used. They are available with characteristic impedance of either 50 or 75 Ω which matches with that of the antenna. It contains a core conductor which carries the signal, a dielectric and an outer shielding which acts as the reference or ground. The radius of the core, dielectric and outer diameters helps in determining characteristic impedance of the coaxial feed. For the purpose of simulation Lumped port with 50Ω characteristic impedance is used. 4.5 Generic Working Lumped port is connected in between the radiator and the ground plane which are placed at the top surface of the dielectric substrate. The feed port is directed such that the direction of current flow is towards the radiator i.e. the signal that has to be transmitted flows along the paths of the radiator. Due to the design and material properties of the antenna, the currents which belong to the frequency band for which antenna is designed to function gets radiated from the antenna. The path of flow of current decides the impedance of the radiator and it should be matched with the feed port impedance at the desired frequency band of operation. 4.6 Modeling Conformal Antennas The planar antenna models explained previously has to be modified in order to be mounted on conformal surfaces. A sphere of radius 15 cm is considered as an example for a conformal surface on which the antennas have to be mounted. Even though the planar geometries are giving simulation results in accordance with the design specifications, when they are made conformal to another surface their radiation characteristics might vary. In order to study the effect of conforming planar antennas to a spherical surface the antenna models are made to conform to a spherical surface of 15 cm radius, using CATIA modelling software. Then the conformal antenna models are exported to HFSS software for simulation. Figures 3 and 4 show the conformal monopole geometry and conformal dipole geometry respectively. Figure 4 Conformal Dipole Antenna 5. ANTENNA SIMULATIONS The basic planar antenna design dimensions are arrived from theoretical and empirical basis and drawn in Ganymede Graphical editor. The created models are then simulated using Empire 3D electromagnetic field solver. The model is also verified using Ansoft High Frequency System Simulator (Ansoft HFSS) as well. Various techniques have been used to optimize antenna performance. The final planar antenna geometries are made conformal using CATIA modelling software and exported to HFSS for simulation. SASTECH Journal 38 Volume 10, Issue 1, May 2011
5.1 Simulation of Planar Monopole Antenna A Planar monopole antenna with rectangular radiator is designed such that its resonant frequency is approximately equal to the centre frequency of the bandwidth of interest. Design simulations are carried out iteratively on this base model, to obtain the required bandwidth and radiation pattern. A conventional planar monopole antenna gives omnidirectional radiation pattern in the azimuth plane. The antenna radiator is shaped iteratively on the basis of the field distribution patterns on the radiator surface, in order to avoid deep nulls in the radiation pattern in the plane of antenna. The electric field distribution pattern plots obtained on simulating the geometry, gives the strength of electric field on the surface of the antenna, at different operating frequencies. The field distribution plots show peak intensity on the radiator surface, for those frequencies at which the antenna radiates power. The red and blue colours represent the maximum and minimum intensities respectively. The ground plane in a planar monopole antenna does not radiate power. Therefore, the intensity of electric field will be minimal at the surface of the ground plane at all frequencies. The path through which current flows is an important parameter that determines the shape of the radiation pattern in each principal plane. Since field distribution patterns show the actual path of the current through surface of different antenna elements, the antenna structure can be modified to direct the current flow through desired direction. Thus the antenna structure is optimized to get omnidirectional radiation pattern in the plane of the antenna. The antenna design is further optimized by modifying the ground plane and setting the feed port at an appropriate position. The position of the feed point is another important factor that decides the path through which current flows in the radiator. An optimised feed port location helps in obtaining maximum bandwidth. 5.2 Simulation of Modified Planar Cross Dipole Antenna A planar dipole antenna is identified as the base model and it is modified to obtain cross dipole geometry. The ground plane is absent in this geometry. Similar to planar monopole geometry, dipole also has deep nulls in the radiation pattern in the plane of the antenna. The cross dipole geometry is iteratively modified to get the desired bandwidth and to get radiation patterns without deep nulls in the principle planes. The radiators of the cross dipole are kept symmetric to each other for each iteration. Here also the radiators are modified on the basis of the current distribution patterns on antenna surface. Both the symmetric radiators of the designed antenna radiates simultaneously. Radiation can be viewed with the help of the current distribution pattern for which an example is shown in Figure 5. 5.3 Simulation of Conformal Models The final planar monopole and dipole antenna designs are made conformal to a spherical surface to study the effect of conforming planar antenna geometry. As the planar geometries are giving the desired radiation characteristics, when they are mounted on a conformal surface it is desirable that the characteristics don t change drastically. The complicated conformal geometries are modelled in CATIA modelling software. The planar models created in empire 3D are exported to CATIA and made to conform to a spherical surface of radius 15 cm. The conformed models are then exported to HFSS software as Standard Lithography (STL) objects. The antennas are simulated after assigning material properties to the conformal models in HFSS. The source signal is fed to the antennas with the help of a lumped port. For measuring the radiation pattern a far field box is created around the model. Each face of the box is at a distance of λ/4 from the faces of the antenna, where λ is the minimum operating wavelength. 6. RESULTS AND DISCUSSIONS The simulation results of the designed antennas are discussed and are validated in here with the help of test results of the developed prototype antenna. For antenna structures designed in this paper, there are no standard design formulas available. Therefore the designs are optimized through iterative design simulations. Conventional planar monopole and cross dipole geometry are taken as the base model and later modified for optimization through iterative design simulations. 6.1 Development and Testing of Planar Prototype Antennas The prototypes of the final antenna designs discussed in the previous sections are developed for the validation of simulation results. The antenna geometries are fabricated on ordinary copper-clad (general purpose) boards with FR-4 epoxy substrate. The unwanted metal portion on copper-clad board is removed to get the desired antenna profiles. Coaxial cables with a coaxial ports having 50 Ω characteristic impedance is used to feed the antenna unlike the ideal lumped ports used during simulation. The developed planar monopole antenna and dipole antenna prototypes are shown in Figure 6 and respectively. Figure 5 Current Distribution Pattern on the Surface of Modified Planar Cross Dipole Antenna Figure 6 Prototypes Developed Planar Monopole Cross Dipole SASTECH Journal 39 Volume 10, Issue 1, May 2011
The developed prototype antennas are tested for its bandwidth using suitable experimental setup. A Vector Network Analyzer (VNA) that can give the return loss plot of a RF network connected to its port is used for the bandwidth measurement. A well calibrated network analyzer can precisely show how much of the source power is reflected back by the RF network. These plots are further compared with the available simulation results for validation. The testing for radiation pattern for the antenna is usually carried out in an anechoic chamber where the near field radiation patterns are transformed to far field radiation pattern. 6.2 Bandwidth and Return Loss The bandwidth of operation considered here is the impedance bandwidth of antenna. It is calculated by noting the crossover points of the S11 curve at 10dB line. Using the crossover point f L at low frequency and f U at high frequency, the bandwidth is calculated as Bandwidth = (3) The bandwidth percentage is given as Bandwidth Percentage = * (4) Return Loss of Modified Planar Cross Dipole Antennas The simulated return loss plot obtained from Empire 3D and measured return loss plot obtained from VNA for the modified cross dipole antenna geometry are compared in Figure 8. Here also, from the compared return loss plots it can be seen that the measured and simulated values are in close agreement and satisfy the bandwidth specification. The simulated antenna gives a wide bandwidth of nearly 4 GHz (1.2 GHz -5.2 GHz) i.e., the bandwidth percentage is 125 %. The measured antenna also gives a wide bandwidth that includes the desired band. Return Loss of Comparison of Conformal and Planar Monopole Antennas The return loss plots of the simulated conformal monopole antenna and its planar geometry are given in Figure 9 and respectively. The results are obtained from HFSS. From the graphs it can be seen that both planar and conformal models give almost same return loss plots and its value is less than -10 db for the required operating bandwidth. where f C is the centre frequency. Figure 7 Return Loss Plot Comparison of Planar Monopole Antenna Return Loss of Planar Monopole Antenna The simulated return loss plot obtained from Empire 3D and measured return loss plot obtained from VNA for the modified planar monopole antenna geometry are compared in Figure 7. It is evident from the compared return loss plots that the measured and simulated values are in close agreement and satisfy the bandwidth requirement specified. As it can be seen the simulated antenna gives a wide bandwidth of nearly 4.6 GHz (1.1 GHz -5.7 GHz) i.e., the bandwidth percentage is 135%. The measured antenna also gives a wide bandwidth that includes the desired band. Figure 8 Return Loss Comparison Plot of Modified Cross Dipole Antenna Figure 9 Simulated Return Loss Plots Conformal Monopole Antenna Planar Monopole Antenna Return Loss of Comparison of Conformal and Planar Dipole Antennas Here the return loss plots of the simulated conformal cross dipole and its planar geometry are given in Figure 10 and respectively. The results are obtained from HFSS. From the return loss plots given in Figure 6.5 it can be seen that the both the planar and conformal cross dipole antenna conforms to the design specifications by giving an impedance bandwidth from 1.3 GHz to 2.5 GHz. 6.3 Radiation Pattern The far field radiation patterns of the simulated antenna designs are measured for the entire operating frequencies at three principal reference planes. For this purpose, antenna model to be simulated is kept inside a far field box. Radiation pattern gives the strength in db of the power radiated in different direction around the antenna. While the measurement of the radiation pattern SASTECH Journal 40 Volume 10, Issue 1, May 2011
the antenna is considered to be in receiving mode. The co-polarized component of radiation pattern ephi or GainPhi is of interest here. In a practical scenario power received by the antenna from a given direction depends on the orientation in which the antenna is mounted as well as on its polarization with respect to the transmitting antenna. The design iterations are aimed at obtaining omnidirectional radiation patterns when both the transmitting and receiving antennas are at the same polarization. Figure 10 Simulated Return Loss Plots Conformal Cross Dipole Antenna Planar Cross Dipole Antenna Radiation Patterns of Planar Antenna Geometries The planar monopole and the modified cross dipole geometries which are modelled and simulated in Empire 3D, give near omnidirectional radiation pattern in all the three reference planes consistently throughout the frequency range of operations. But for the frequencies above 2.5 GHz, the pattern no longer remains omnidirectional. The radiation pattern at 1.5 GHz, of the planar monopole and dipole at Phi =0, Phi= 90 and Theta= 90 planes are given in figure 11 and Figure 11Radiation Pattern Planar Monopole Antenna Planar Cross Dipole Antenna 7. CONCLUSION Development of conformal antennas has been attempted in prior by many antenna designers. The technical approach opted of achieving the desired specification is unique for the designed antennas. The primary objective of this paper is to design and develop a conformal antenna that gives omnidirectional radiation patterns in at least two principal planes and operating in the frequency band ranging from 1.3 GHz 2.5 GHz. The designed antenna geometries can be used in applications wherein sharp nulls in radiation pattern cannot be afforded. The following conclusions are drawn after carrying out the design simulations and testing the prototypes: The planar antenna geometries such as planar monopole and cross dipole were identified as suitable geometries to meet the design specification The planar and conformal geometries are giving wide bandwidth that includes the specified operating frequency range from 1.3 GHz to 2.5 GHz. The return loss measurements for the developed prototype antennas and their corresponding simulated geometries are in close agreement with each other. The designed antenna geometries can be used in applications wherein sharp nulls in radiation pattern cannot be afforded. It is seen that the co-polarized component of radiation pattern is nearly omnidirectional in the plane of the antenna consistently throughout the bandwidth.for the planar antenna designs however the average gain of the cross polarized component of radiation pattern is seen to be increased whereas the average gain of co-polarized component decreased, when the planar antenna geometries are made conformal. 8. REFERENCES [1] C.-C. Lin, L.-C. Kuo, and H.-R. Chuang, A Horizontally Polarized Omnidirectional Printed Antenna for WLAN Applications, IEEE Transactions on Antennas and Propagation, Vol.54, No.11, Nov 2006. [2] David Herold, Lance Griffiths and Tat Y. Fung, Lightweight, High-Bandwidth Conformal Antenna System for Ballistic Helmets, IEEE Xplore, Retrieved on Nov 11, 2009. [3] Jarrod Fortinberry, Zach Hood, and Erdem Topsakal, A Helmet Mounted GPS Antenna, Mississippi State University, IEEE Xplore, http://www.ece.msstate.edu, Retrieved on Nov 11 2009. [4] Dejan S. Filipovic and John L.Volakis, Multifunctional Conformal Antennas for Automobile Applications, EECS Dept., University of Michigan, USA, 2002. [5] Lars Josefsson and Patrik Persson, Conformal Array Antenna Theory and Design, Wiley- Interscience, New Jersey, 2006. SASTECH Journal 41 Volume 10, Issue 1, May 2011
[6] Joseph E. Kershaw, Conformal Helmet Antenna, US Patent No. 5503352, available at www.freepatentsonline.com/3977003.pdf, 24 th August 1976, Retrieved on 8 th Nov 2010. [7] Constantine A. Balanis, Antenna Theory Analysis and Design, 3 rd Edition, John Wiley and Sons, Inc., New Jersey, 2005. SASTECH Journal 42 Volume 10, Issue 1, May 2011