CHAPTER 3 METHODOLOGY AND SOFTWARE TOOLS

Transcription

1 CHAPTER 3 METHODOLOGY AND SOFTWARE TOOLS Microstrip Patch Antenna Design In this chapter, the procedure for designing of a rectangular microstrip patch antenna is described. The proposed broadband rectangular microstrip patch antenna is designed for broadband application. The proposed geometry validated and simulated in IE3D Software Design Specifications The three essential parameters for the design of a rectangular Microstrip Patch Antenna: Frequency of operation (fo): The resonant frequency of the antenna is selected as per as application. The resonant frequency is 3.8 GHz. Dielectric constant of the substrate (ε r ): The dielectric materials selected for design is FR-4 which has a dielectric constant of 4.3. Height of dielectric substrate (h): The dielectric substrates is 1.5 mm for FR-4. Since geometry is stacked patch two dielectric substrates (FR-4) are used. Therefore, ε r = 4.3; tan δ = 0.019; h = 3 mm. METHODOLOGY Designing of Short Backfire Antenna For designing of any Short backfire antenna we can used mainly two parts: Mathematical Analysis: For designing of SBFA first we have to choose the substrate of the antenna. For numerical analysis various formulae s are used and last for input purpose different feeding methods are used; in this I used coaxial feed line technique. 53

2 Antenna Design by using IE3D software: For simulation part different software's are used, I used IE3D software Different steps are followed for designing of SBFA antenna Mathematical Analysis Mathematical analysis is necessary for knowing the exact dimension of the patch to be designed. By performing mathematical analysis we calculate width and length of the patch, ground plane & reflectors. For mathematical purpose main important point is how we can choose substrate. The bandwidth of the short backfire antenna is directly proportional to the substrate thickness (h). The bandwidth of the short backfire antenna is inversely proportional to the square root of substrate dielectric constant (εr). Substrate thickness is another important design parameter. Thickness of the substrate increases the fringing field at the patch periphery like low dielectric constant and thus increases the radiated power. It also gives lower quality factor and so higher bandwidth. The low value of dielectric constant increases the fringing field at the patch periphery and thus increases the radiated power. A small value of loss tangent is always preferable in order to reduce dielectric loss and surface wave losses and its increase the efficiency of the antenna. There are three essential parameters that should be known while performing mathematical analysis: Frequency of operation ( f 0 ): The resonant frequency of the antenna must be selected appropriately. Dielectric constant of the substrate ( ε r ):The dielectric material selected for my design for micro-strip patch & ground plane is and FR-4 and LTCC which has a dielectric constant of 4.3 and 7.4. Also an effective dielectric const ant (ε eff ) must be obtained in order to account for the fringing and the wave propagation in the line. Height of dielectric substrate ( h ): The height of the dielectric substrate is selected as 1.5 mm. 54

3 3.1.3 Formulae s Used for Mathematical Calculations: w = C 2 f ε r +1 2 (3.1) ε reff = ε r ε r h W 1 2 (3.2) L = 0.412h ε reff +0.3 W h ε reff W h (3.3) c 2f ε eff L = 2 L (3.4) L o = L + 6h (3.5) W o = W + 6h (3.6) where: f = Operating frequency ε r = Permittivity of the dielectric ε eff = Effective permittivity of the dielectric W = Patch s width L = Patch s length h = Thickness of the dielectric L 0 = Length of ground plate W 0 = Width of ground plate Effects of Substrate While performing mathematical analysis effect of substrate is also considered. As shown in Table 3.1. Some of the important points that needed to be considered are: The bandwidth of microstrip patch antenna is directly proportional to the substrate thickness (h) and inversely proportional to the square root of substrate dielectric constant (ε). 55

4 Thick substrate increases the fringing field at the patch periphery and thus increases the radiated power. A small value of loss tangent is always preferable in order to reduce dielectric loss and surface wave loss and it increases the efficiency of antenna. Low dielectric constant is used which has very low water absorption capability. Table 3.1 Data sheet of different substrate Properties LTCC FR4 Epoxy RT Duroid Dielectric constant Loss Tangent Breakdown voltage kv 55 kv >60 kv Proposed Antenna Designing IE3D is a full wave Simulator based on MOM (Method of Moment) and facilitates for designing antenna three essential parameters frequency; dielectric constant and the height of substrate are required. This is the basic design platform to get an analysis of patch antenna, on having this analysis a further step is moved for better performance. It is an integrated full wave electromagnetic simulation and optimization package for the analysis and design of 3-D microstrip antennas. The results of antenna designs such as the return loss and the radiation pattern can be obtained by using the EM Simulator IE3D (version 9.0) software. The results for the antenna simulation does not accurately give similar result as measured. Based on the simulations and measurements that haves been done, the operating frequency of the antenna fabricated are shifting to the lower frequency. There are two probabilities which affect the result. First is imperfection in the fabrication process and secondly the properties of the substrate used have some tolerance. In common we use a rectangular microstrip patch antenna which mainly includes a ground plate, a radiation unit and a power unit. The design of substrate is infinite ground plane, the size of radiation sticks slab L x W. some of the useful 56

5 results after simulation in IE3D software are: Return Loss (S11 parameter), VSWR, Directivity, Smith Chart, Gain, Current Distribution, Efficiency, Radiation Pattern etc. 3.2 Flow Chart of Methodology and Tools START INPUT f, h, ε r w = C 2 f ε r ε reff = ε r ε r h 2 2 W ε reff W L = h h ε reff W h c L = 2 L 2f ε eff L o = L + 6h W o = W + 6h END As shown in flow chart describe the complete designing of microstrip patch antenna. The frequency and substrate are selected and how this frequency and substrate mathematically analyze is easily understand by flow chart. 57

6 3.3. Design of short back fire antenna Design parameters of short backfire antenna By using equation (3.1)-(3.6) from above we find the length and width of rectangular microstrip patch of short backfire antennas, which is calculated as: Table Mathematical analysis for geometry 1 and geometry 2 S.no Parameters Values for geometry 1 Values for geometry 2 1 Resonant frequency(f 0 ) 6 GHz 3 GHz 2 Dielectric constant (ε r ) Height of substrate (h 1 ) 1.5mm 1.5mm 4 Loss tangent of FR Width of rectangular patch(w) mm mm 6 Length of rectangular patch(l) mm mm 7 Width of ground plane(w 0 ) mm mm 8 Length of ground plane(l 0 ) mm mm 9 Diameter of sub-reflector(d) 48mm 50mm 10 Height of sub-reflector(h 2 ) 53.3mm 25mm 11 Feed location: X f (Along Length) Y f (Along Width) Xf = 4.75 Yf = Xf = Yf =

7 3.3.2 Design Procedure The results of short backfire antenna designs such as the return loss, directivity and the radiation pattern can be obtained by using the EM Simulator IE3D (version 9.0) software. The results for the antenna simulation does not accurately give similar result as measured. Based on the simulations and measurements that haves been done, the operating frequency of the antenna fabricated are shifting to the lower frequency Steps Involved for Experimental Execution Step: 1 Click on Zealand program manager a window will open. Fig Zealand program manager window. Step: 2 Click on the m-grid icon a mgrid window will open. M-Grid window is basic platform for designer to model and realize their design work. Fig M-grid window. 59

8 Step: 3 Click on new then a basic parameter window will open. It is very essential to define the basic parameter such as dielectric constant of substrate(ε r ), thickness of metal layer(h 1 ), operational frequency(f 0 ) etc. for designing of an efficient antenna. Fig.3.3.3: Basic parameter window. Step: 4 Select Patch Width (w) =15.357mm, Length (L) = mm, substrate height (h) =1.5, dielectric constant for FR4(ε r ) = 4.3 Fig Top layer of the patch. The Fig shows the top layer of microstrip patch antenna. Which is design for 6 GHz and the details is mention in table The patch is a rectangular form and it is generally used for excitation of electromagnetic field. 60

9 Step: 5 Form H slot on the top layer by cutting the slots. Take mesh size = 0.5mm. Fig H-slot structure. Fig shows in details the geometry of the printed radiator as seen from above. The antenna takes up an area occupying * mm on the upper surface of the substrate. The radiator element is placed at a height D=1.5 mm from the Horizontal plane of the ground plane. The proposed structure is excited with a microstrip feed line of input impedance (Z0= 50 ohms). Step: 6 Formation of ground layer at z=0. With Length(L 0 ) and width(w 0 ). Fig Formation of ground layer. 61

10 Step: 7 Place a circular sub reflector of diameter (d) at z = 53.3mm. Fig Circular sub reflector at z= 53.3mm The geometry consists of two reflector one is rectangular patch and second is circular is as shown in Fig The circular patch with a diameter of 48mm at a height of 53.3mm is placed and it is work as a sub reflector. The used of sub reflector is to radiate electromagnetic field back to main reflector for enhancing the bandwidth. Step: 8 Provide feed location at X f along length and Y f along width. Fig Providing feed location along x and y axis. 62

11 For 50 Ohms impedance matching between ground and H slot exciting patch is calculated the Xf and Yf feed point. By proper controlling the Xf and Yf we control the impedance matching network. Step: 9 Simulation and optimization between desired range of frequencies. Fig Simulation and optimization window. TheFig show the different frequency simulation. Here we used total 40 frequency point at a step size Proposed Geometry of an antenna Fig Geometry1 of short backfire antenna. 63

12 The geometry consists of two reflector one is rectangular and another is circular in between them an excitation patch slot is connected Fig Geometry2 of short backfire antenna. For 50 Ohms impedance matching between ground and H slot exciting patch is calculated the Xf and Yf feed point. By proper controlling the Xf and Yf we control the impedance matching network. Fig D view of short backfire antenna geometry. 64

13 3D view of proposed antenna shows the sub reflector and main reflector in between that air is place the excited patch H slot generate the field which is reflected back from sub reflector so the back losses is reduced and maximum radiation is transfer into the free space. 3.4 SIMULATION RESULTS The rectangular microstrip patch short backfire antenna gave following results after simulation for Geometry 1 S11 Parameter (Return Loss) Curve Fig S11 parameter curve for geometry 1. The Fig shows that the return loss(s11) of the antenna is -45 db at the center frequency of 3.73 GHz. The bandwidth obtained from the return loss result is 37.5%. VSWR Curve Fig VSWR curve for geometry 1. 65

14 Fig dipicted that the VSWR is less than or approxitely equal to 1.5 at GHz. This shows that the maximum power is transmitted from source end thus there is less mismatch at source end. Directivity vs Frequency Curve Fig Directivity vs frequency curve for geometry 1. Fig shows the directivity of proposed antenna is 8dBi at 3.35 GHz. Which shows the proposed antenna is directional antenna. Gain vs Frequency Curve Fig Gain vs frequency curve for geometry 1. 66

15 Antenna Efficiency Curve Fig Radiating efficiency curve for geometry 1. Fig shows the proposed antenna radiation efficiency is approximately 90% at 3.4 to 4GHz. Which result that maximum radiation is obtain from proposed antenna Fig Antenna Efficiency curve for geometry 1. 67

16 Smith Chart Fig Smith chart for geometry 1. 3D Radiation Pattern Fig D radiation pattern for geometry 1. Fig dipict the radiation pattern in 3D domain which clearly shows that the field radiation only in forward directiion and there is very small radiation at back side so maximum radiation in forward direction and it is stable throughout the whole operating band 68

17 For Geometry 2 S11 Parameter (Return Loss) Curve Fig S11 parameter curve for geometry 2. The obtained minimum value of the antenna return loss is -20 db at the frequency of 5.5 GHz as shown in Fig Thus,the bandwidth obtained from the return loss result is 52.8% and a 4 GHz the bandwidth is 22.5%. VSWR Curve Fig VSWR curve for geometry 2. 69

18 Moreover, VSWR is a measure of how well matched antenna is to the cable impedance. A proposed antenna would have a VSWR of less than 2 for GHz. This indicates less power is reflected back from source. VSWR obtained from the simulation is less than 2 which is approximately equal to 1.1:1 as shown in figure. This considers a good value as the level of mismatch is not very high because high VSWR implies that the port is not properly matched. Directivity vs Frequency Curve Fig Directivity vs frequency curve for geometry 2. The maximum directivity of proposed antenna is 7.2dBi at 7.5GHz Gain vs Frequency Curve Fig Gain vs frequency curve for geometry 2. The maximum gain of proposed antenna is 6dBi at 7.5GHz 70

19 Antenna Efficiency Curve Fig Radiating efficiency curve for geometry 2. Fig Antenna Efficiency curve for geometry2. 71

20 Smith Chart Fig Smith chart for geometry 2. 3D Radiation Pattern Fig D radiation pattern for geometry 2. The 3D radiation pattern of the antenna at frequency 3.35 GHz (out of antenna frequency bandwidth) is shown in Fig It can be observed from this radiation that the design antenna has stable radiation pattern throughout the whole operating band. 72

21 3.5 Validation To ensure our graphically simulated results are reliable, we have also validated our results with experimental results obtained from other source reported by M.Javid et al.[19].it is evident from [19] that the VSWR bandwidth is 21.28% in simulation with the finite element method based software tool and 52.8% at the 4.6 to7.9ghz of the reported simulation result. Thus, it has depicted that the VSWR bandwidth spans more in simulated results as compared to reported results. The antena is validated by using Agilent Network Anylazer N9923A whose hardware result is approximately same as simulation results as shown in Fig Fig Side view of proposed antenna Fig Ground plane of proposed antenna Fig Hardware testing on agilent N9923A 73

22 The basic electrical characteristics of the proposed antenna are summarized in Table Table Basic electrical Characteristics of the proposed antenna S.no Parameters Value 1 Minimum S db at 5.5GHz 2 Bandwidth 52.8% between ( )GHz 3 VSWR <2.0 between ( )GHz 4 Maximum directivity 7.2dBi at 7.5 GHz 5 Maximum gain 6dBi at 7.5 GHz 6 Center frequency f 0 =0.5(f min +f max ) 6.25GHz A comparison between the dimension and some electrical characteristics of the classic SBFA with optimum dimension and the proposed antenna is accomplished in Table Table Comparison between the classic SBFA and the proposed antenna S.no Antenna Classic Proposed Dimension/Parameter SBFA Antenna 1 Center Wavelength λ 0 48 mm 2 Big reflector diameter D2 2 λ 0 50 mm (1.04 λ 0 ) 3 Small rectangular dimension D λ mm (0.755 λ 0 ) 4 Length of the antenna L A 0.5 λ mm (0.522 λ 0 ) 5 Distance source- small reflector d λ 0 1.5mm (0.031 λ 0 ) 6 Distance source- big reflector d λ 0 25mm (0.521 λ 0 ) 7 Maximum gain G 13dBi 6dBi 8 Bandwidth bw 4-5% 52.8% It is seen from Tables and that the used non optimum (smaller) dimension of the big reflector and non optimum placed of the source (feed) in the proposed 74

23 antenna shifts the centre frequency 2 times from the design frequency (from 3GHz to 6.25GHz) and decreases five times (with 7 db) its gain but the wideband microstrip excitation leads to a dramatic increase of the antenna bandwidth approximately 12 times (from 4.5% to 52.8%) 75