Antenna Simulation and Design Exercise Student: Cagin Sari ID:10206074 Course: MEng Electrical and Electronic Engineering Lab Supervisor: Dr Laith Danoon Date: 18/11/2020 1 Table of Contents Introduction ............................................................................................................................ 2 Background Theory ............................................................................................................... 2 Procedures .............................................................................................................................. 4 Simulation result, discussion and analysis ............................................................................. 4 Conclusion ........................................................................................................................... 10 References ............................................................................................................................ 10 Introduction The report aims to discover how the impedance bandwidth, S-parameters, and input impedance of the dipole antenna changes with changing frequency and by using the background theory it also aims to understand the meaning of these changes for the antenna being modelled. To achieve these aims a simulation has been done by using CST Microwave Studio which is a specialist tool for simulating 3D Electromagnetic Radiation of high frequency components. Using the software package mentioned a model of dipole antenna has been built and simulation has been run across 1 GHz to 2 GHz. Field monitor is used to plot the electric, magnetic and far-field radiation pattern and the plots are analysed. S-parameters, input impedance parameters are also obtained from the simulation and used for analysis. Background Theory For a well matched resonant antenna where R in = Z0 the bandwidth of the antenna at any level of VSWR is given by the following equation: BW = VSWR-1 (1) √VSWRQ0 Impedance bandwidth is used to check the range of frequencies where the antenna has a good impedance matching Q0 is given by; 𝜔 Q0 = 2Z0 0 (2) Where 𝜔0 is the angular resonant frequency and Q0 is the unloaded Q-factor of the antenna, VSWR is the voltage standing wave ratio, it occurs when antenna is mismatched with the generator or the transmission line the smaller the VSWR the better since it is the measure of the mismatch. The mismatch causes the wave to reflect from the input of the antenna and the measure of the reflection of wave is provided by the reflection coefficient 𝛤 or S11 . 2 Bandwidth requirement of antennas is also given in terms of VSWR this can also be seen from the equation 1 and the bandwidth is also measured at VSWR=2 which corresponds to -10 dB. The VSWR and S11 is given by the following two equations: z −z S11 = zin +z0 (3) 0 in | 1+|S VSWR = 1−|S11| 11 (4) Other important equations involve Directivity and Gain. Directivity measures the power density at a particular direction and compares it to the isotrope radiation at a distant r at the far-field region [1]by this way one can calculate how concentrated antenna’s radiation pattern is in a particular direction. Directivity is given by the following equation: D= Sav,max Prad 4𝜋𝑟2 (5) Where Sav,max is the maximum instantaneous power density at the point and Prad is the maximum radiated power. Gain of an antenna heavily depends on how efficient the antenna is and it is defined with respect to the input power provided to the antenna[1]. G = 𝜂D G= 𝜂= Sav,max Pin 4𝜋𝑟2 Prad Pin (6) (7) (8) Where 𝜂 is the radiation efficiency of the antenna and Pin is the input power to the antenna[1]. 3 Procedures Figure 1 shows the modelled dipole antenna structure with dimensions The modelled antenna is shown in the figure 1 above the materials used for dipole is perfect electric conductor (PEC)[2]. PEC is selected because the Antenna is assumed to be lossless. For signal generation discrete port is used and frequency range is defined to be from 1 GHz to 2GHz. The length of the antenna wasn’t changed during the simulations, the only changing parameter in the structure of the antenna was radius. After the simulation is finished the Sparameters plot is analysed and resonant frequency is recorded by taking the minimum frequency of the curve and Z matrix is analysed to measure the impedance bandwidth of the antenna. Impedance bandwidth of the antenna is recorded by recording the frequency range at -10 dB than at – 6 dB. From the 1D plots section on Navigation Tree. By using the 2D/3D section of the navigation tree E-field, H-fields are observed and analysed and by using the far-field section far-field radiation patterns are observed and corresponding gain and directivity results are recorded. Further simulations are made by incrementing radius from 1 mm to 5 mm than to 10 mm to observe the changes it causes on the mentioned parameters that are being analysed. Simulation result, discussion and analysis Figure 2 shown above displays the S-parameters for different radius values ranging from 1mm to 10 mm over the frequency range from 1 GHz to 2 GHz, fresonant or fmin is marked on the figure. 4 The marked points given on the figure 2 shows the resonant frequencies and changes in the resonant frequency with the changing radius of the dipole. The simulation with 1mm radius reached S11=-15.803 dB and 1.317 GHz which means that most of the power is delivered and very small is reflected from the input of the antenna. Changing the radius to 5 mm caused a very little change in the results S11 =-15.963 dB and 1.243 GHz however increasing the radius to 10 mm changed the results tremendously S11=-9.0554 dB and fmin = 1.224 GHz, S11 being that low means that the antenna is mismatched and important proportion of the power is reflecting from the antenna input the reason for this might be the reactive part of the impedance. Since the capacitive reactance does not cancel out the inductive reactance the antenna does not reach the resonance so the maximum power transfer does not occur as can be seen from the graph results in a very large reflection coefficient[3]. Figure 3 shown above displays the fR given at R= 50 Ω Figure 3 above shows the real part of the input impedance against frequency and results with different dipole radius values. Results show that dipole with radius of 1 mm reaches 50 Ω at 1.211 GHz and dipole with radius 5 mm reaches 50 Ω at 1.2009 GHz the difference between these two frequencies is very small so changing the radius from 1 mm to 5 mm didn’t change the real part of the input impedance until 1.6 GHz. However for radius = 10 mm it never reached resistance of 50 Ω this is also a cause for the large reflection coefficient shown on the Figure 2 Figure 3 shown above displays the fX given at X= 0 Ω The figure 3 shows the change in the imaginary part of input impedance over frequency for different radius values. The one with radius= 1 mm reaches the 0 reactance at f= 1.3326 GHz this is the point where capacitive reactance and inductive reactance cancel each other out. The one with radius of 5 mm does not reach the 0 Ω but it gets very close to it however 10 5 mm radius never gets close to the 0 Ω thus this also proves the prediction made in the Sparameter analysis using figure 2; it was predicted that capacitive part of the reactance does not cancel the reactive part and as a result resonance can’t be reached so that large proportion of the power gets reflected from the input port of the antenna. Magnitude of S11 Radius of the dipole (mm) Bandwidth (GHz) Difference -6dB 1 -10dB 5 1.19851.0671.4781 1.5726 0.2796 0.5056 10 1 5 10 1.058- 1.251.13991.4684 1.3944 1.3825 none 0.4104 0.1444 0.2426 none Table 1 shows the bandwidths of the dipole antennas for different S11 magnitudes and different dipole radius measured from the values presented in the figure 2 As can be seen from the table 1 above the bandwidths of dipoles with radius 5 mm and radius of 10 mm at -6dB has broader bandwidth when compared to the dipole with radius 1 mm this might be useful when -6dB is enough for the reflection coefficient. When lower reflection coefficient is required the dipole antenna with 10 mm radius fails to reach -10 dB reflection coefficient for the the ones with 1 mm radius and 5 mm radius successfully operate at -10 dB line. The 5 mm radius dipole antenna has broader bandwith than 1 mm one so it might be useful for cases where broad bandwith is required. Radius(mm) 1 5 10 far-field radiation results directivity gain(IEEE) realised gain dBi dBi dBi 2.224 2.223 0.7427 2.151 2.15 2.039 2.138 2.138 1.562 Table 2 shows the change in the gain and realised gain when the radius is changed As can be observed from the table above for all of the radiusses gain and directivity is very similar so according to equation 6 efficiency is very high close to 100%, this is expected since the perfect electric conductor is used as the dipole antenna material. The gains and directivities of the antennas for all radiusses are very similar. It can be observed from the table 2 that for both 1 mm and 10 mm radius dipoles the realised gain is much different from the gain the reason for this is for realised gain the losses are also taken into account [4]. So this means that at radius 1 mm and 10 mm has lots of losses. 6 a) b) Figure 4 above shows the e-field of dipole with radius of 1mm at fmin at a) YoZ and b) XoY orientations The figure 4 shows shows how the electric field is formed by the 1mm radius dipole at Yoz and XoY frames a) b) Figure 5 above shows the h-field of dipole with radius of 1 mm at fmin at a) YoZ and b) XoY orientations 7 Again the figure 5 shows how the h-field is formed from two different frames for radius of 1mm. a) b) Figure 6 above shows the e-field of dipole with radius of 5 mm at fmin at a) YoZ and b) XoY orientations The figure 6 shows the e-field pattern variation at radius of 5mm when compared to the figure 4 it can be seen that at 5 mm radius the field is much more intense and this might be the result for significantly high realised gain which was indicated during the discussion of table 2. a) b) 8 Figure 7 above shows the h-field of dipole with radius of 5 mm at fmin at a) YoZ and b) XoY orientations The figure shown aove displays the images captured frm the h-field monitor from different orientation to show how the h-field is forming for radius of 5mm. a) b) Figure 8 above shows the e-field of dipole with radius of 10 mm at fmin at a) YoZ and b) XoY orientations The figure above showsthe e-field for the dipole with radius of 10 mm again when compared to the figure 5 this field looks much weaker. a) b) 9 Figure 9 above shows the h-field of dipole with radius of 10 mm at fmin at a) YoZ and b) XoY orientations Figure 9 above can be used to observe the changes in the h-field for the dipole with radius of 10 mm. H-field of the dipole with 10 mm radius looks the weakest compared to the other figures. Conclusion This report has helped to gain first hand experience about the practical modelling and analysis of dipole antennas by using CST microwave studio which is a very powerful tool for modelling electromagnetic radiation. The report also helped to understand the design requirements for a dipole antenna which is widely used in the industry, this report helped to combine the theoretical part of the lecture to practical modelling. In the discussion part, analysis of the figures and some research is done to reveal how the change in radius affects the bandwidth, and reflection coefficient directivity and gain. The report and the simulations have satisfied all of the aims and objectives and proved to be very valuable. References [1] L. Danoon, Antennas and Propagation 2020/2021. University of Manchester, 2020. [2] ‘CST MICROWAVE STUDIO - Workflow and Solver Overview.pdf’. Accessed: Nov. 19, 2020. [Online]. Available: https://online.manchester.ac.uk/bbcswebdav/pid12062319-dt-content-rid-52144752_1/courses/I3132-EEEN-60121-1201-1SE022594/CST%20MICROWAVE%20STUDIO%20%20Workflow%20and%20Solver%20Overview.pdf. [3] S. Rob, Transmission Lines. Manchester: University of Manchester, 2020. [4] Z. Lodro, ‘RE:Why there is a big difference in between realize gain and gain in my design?’, ResearchGate. https://www.researchgate.net/post/Why-there-is-a-bigdifference-in-between-realize-gain-and-gain-in-my-design (accessed Nov. 18, 2020). 10