Undergraduate Research Opportunity Project (UROP ) DESIGN OF A DIPOLE ANTENNA USING COMPUTER SIMULATION Student: Nguyen, Tran Thanh Binh Schoo of Eectrica & Eectronic Engineering Nayang Technoogica University E-mai: binhnguyen@pmai.ntu.edu.sg Supervisor: A/P Hui Hon Tat Schoo of Eectrica & Eectronic Engineering Nayang Technoogica University Emai: ehthui@ntu.edu.sg Te: (65) 6794539 Abstract This project is to study the design of a dipoe antenna by using a sef-deveoped computer simuation program. The computer simuation program was deveoped by C anguage and the moment method was impemented. The design task was to investigate the changes of the input impedance and the radiation characteristic with the ength and the radius of the antenna wire. A commercia software was aso used in this project for the purpose of comparison and verification of the simuation resuts.. Theory.1 Dipoe antenna Wire antenna are the most famiiar antenna because they are seen virtuay everywhere. There are various shapes of wire antennas such as a straight wire (dipoe), oop and heix antenna. 1. Introduction 1.1 Background Nowadays, wireess communication has become more and more popuar in the word. An antenna is considered as the structure makes transition between a guided wave and a free space wave for any kind of wireess communication. Hence, it has been ong attracted and many kinds of antenna as aperture antenna, micro strip antenna, array antenna have been anayed. However, the coection of Input impedance and radiation pattern of dipoe antenna variation has not been in detai yet. 1. Purpose The project aim to review, summarie give the whoe thing from methodoogy to a numerica soution to find the Input impedance and radiation pattern of dipoe antenna. For that a C program was created and coection of parameters was gathered. 1.3 Scope Due to imited time (5-6 weeks) and no prior knowedge about antenna of the author, project just anaye dipoe antenna. Figure 1 Dipoe antenna.. Antenna fundamenta parameters a. Current distribution 1
Current distribution is the movement of eectron in the antenna. Current distribution is the most crucia parameter in antenna anaysis because input impedance and radiation pattern of antenna coud be cacuated after current distribution was found out. b. Input Impedance The Input impedance presented by an antenna at its terminas or the ratio of the votage to current at a pair of terminas or the ratio of the appropriate components of the eectric to magnetic fieds at a point. c. Radiation pattern It is defined as a graphica representation of the radiation properties of the antenna as a function of space coordinates. In most cases, the radiation pattern is determined in the farfied region and is represented as a function of the directiona coordinates..3 Numerica Soution Moment Method Appication of MOM to Antenna can be summaried in 8 steps 1. Estabishment of Green s function. Eectric fied equation 3. Boundary condition 4. Eectric fied integra equation 5. Discretie the antenna structure 6. System of equations (matrix equation) 7. Current distribution 8. Input impedance, radiation pattern..3.1 Green s function First, et assume that the antenna is made by a perfect conductor. Hence due to the skin depth effect, a of the current wi be on the surface of the antenna. Second, assume that the radius of antenna is much sma compare with the ength. We have the thin ine soution, that means a the of current around the antenna circe is the same. Green s function for dipoe antenna was given: E = jωµ r &&&& G Id (1) &&&& G. e Where &&&& is the free space matrix Green s function given by eo Gxx Gxy G x &&&& Ge = Gyx Gyy Gy Gx Gy G For the dipoe antenna, we have ony the component of the eectric fied Where ωµ. () E = j G Id (3) jkr e ( ') G = {[3+ j3 kr ( kr) ] + 4πkR R 3 ( kr) + [ 1 jk R+ ]} (4) R= R R' = x x' + y y' + ' ( ) ( ) ( ) k = ω µε (6).3. Eectric fied equation It is a fundamenta equation in eectromagnetic theory showing the reation between E and V V E d = inc..3.. Eectric fied equation It is a fundamenta equation in eectromagnetic theory showing the reation between E and V V E d = inc..3.3 Boundary condition The boundary condition for eectric fied states that the tangentia component of the tota eectric fied vanishes on the surface of the antenna wire. That is, for the dipoe antenna, Where E E E r E r inc + inc = is the -component of the scattered eectric fied is the -component of the incident eectric fied..3.4 Eectric fied integra equation (EFID) From (3), (8) and (9) V jωµ d GId =..4 Commercia software WIPL (7) (8) (9) (5) (1) In order to see whether the resuts obtained by C program are correct or not, we use a commercia software caed WIPL to test. 4. Resuts 4.1 When ength changed a. Current distribution
Since current distribution is ony mean to find input impedance and radiation pattern, we just show the graph of magnitude and phase of current for ength =.5 λ. 1 8 Input Impedance Variation Impedance (Ohm) 6 4 - -4-6 1 3 4 5 6 7 8 9 1 Dipoe ength (.1 amda) Resistance Reactance Figure 4 Input impedance variations with ength Figure Magnitude of current distribution for =.5 λ Both of vaues obtained from C program and WIPL are very cose, hence it confirms the accuracy of the written C program. The reactance impedance of antenna wi be when dipoe ength reaches near.5 and.8. At.5, antenna wi get resonant for its maximum radiation oeceiving. On the other hand, at.8 it has no practica appication because of so arge resistance. b. Radiation pattern vaue Since dipoe antenna is omni-directiona, so the radiation just different when θ changes. The foowing figures draw out the normaied radiation in db. We can image that the dipoe antenna is put horiontay, in -18 axis. Figure 3 Phase of current distribution for =.5 λ b. Input impedance characteristic 3
Figure 5 Radiation pane for =.1λ Figure 7 Radiation pattern pane for =1λ Figure 6 Radiation pattern pane for =.5λ Figure 8 Radiation pattern pane for = λ Resuts obtained by C program are very aike with commercia software resuts. Hence, it confirms the accuracy of C program. 4
When the ength gets coserλ, the radiation is more directiona. At = λ, radiation separate and has 4 obes.. When radius changed Notes: Since we are adopting thin ine soution. Radius changed must aways be much shorter than dipoe ength. Impedance (Ohm) 1 1 8 6 4 Input Impedance Variation 1 3 4 5 Radius thickness (amda/6) 6. Further work Due to time constrained, the project had stopped at the anaysis of dipoe antenna. Therefore, we recommend furtheesearch can make use of we-written C program to deveop soution for Other kinds of antenna anaysis ike heica antenna by just simpy change the geometry sub-routine of the program. Improve the speed of C program by changing the Sover subroutine, to make it fast. 7. Acknowedgement I woud ike to express deep gratitude to my supervisor, Assistant Professor Hui, who had guided me through the project with many vauabe materia, advice and review. 8. Reference [1] C. A. Baanis, Antenna Theory, Anaysis and Design, Harper & Row Pubishers, 1978. [] H. T. Hui, Anaysis of the Dieectric-Loaded Heica Antennas and the Dieectric-Loaded Sot Antennas, PhD thesis, City University of Hong Kong, 1998. [3] R. F. Harrington, Time-Harmonic Eectromagnetic Fieds, McGraw-Hi Book Company, New York, 1961. Series1 Series Figure 9 Radiation pattern pane when radius changed The graph shows that there is not much change in Input impedance when its radius changes. Hence, antenna radius just needs to be sma for economics and not too sma to keep the whoe structure firm. 5. Concusion The UROP has finished successfuy. It has achieved the requirement as it described at the beginning. The very cose resuts obtained from C program and commercia software has proved the accuracy and fexibiity of the buit program. In summary, Input impedance and radiation pattern of dipoe antenna changes significanty with the change of dipoe ength. The resonance comes when ength!.47 λ Input impedance not much depends on radius of antenna. For ength 1.5, the radiation is no onger maximiing at θ =9, instead it separate and create more obes. 5