Chapter 2. Modified Rectangular Patch Antenna with Truncated Corners. 2.1 Introduction of rectangular microstrip antenna

Transcription

1 Chapter 2 Modified Rectangular Patch Antenna with Truncated Corners 2.1 Introduction of rectangular microstrip antenna 2.2 Design and analysis of rectangular microstrip patch antenna 2.3 Design of modified rectangular microstrip patch antenna with truncated corners. 2.4 Design of gap coupled truncated rectangular microstrip patch antenna 2.5 Discussion and conclusions.

2 Chapter 2 Modified Rectangular Patch Antenna with Truncated Corners 2.1 Rectangular Microstrip Antenna Introduction: Among the common shapes of microstrip patch geometry square and rectangular shape are most widely investigated due to their simplified mathematical modeling and associated boundary conditions. Extensive theoretical and measured analysis on these patch antennas may be seen in available literature on microstrip antennas. Derneyrd [1978] performed the theoretical investigation of microstrip antenna and reported that the radiation took place predominantly from the fringing end at the open circuited ends. Samras et al. [2004] theoretical investigated the changes of input impedance of a rectangular patch antenna with feed position. The basic rectangular patch antenna having width W and length L designed on a substrate having substrate thickness (h), relative permittivity (ε r ) and patch height (t) is supported by an infinite ground plane on the back side of the substrate. For a rectangular patch antenna length is normally 0.333λ < L < 0.5 λ, where λ is the wavelength of free space. And conductor patch thickness is given by t < < λ /10. From simple formula given below we can calculate length and width of RMSA. Take width W of the patch smaller or bigger than obtained value from equation. If w is lesser than gain and band width will decrease and if W is greater, than bandwidth increases due to the increase in the radiated fields. L 0.49 λ d = 0.49 λ ε r L = resonant length λd = wavelength in PC board λ = wavelength in free space εr = dielectric constant of the PC board material

3 C W = 2/ (ε r +1) 2f 0 The feed is used to excite the patch either by probe fed or by edge feed, a fringing field is developed between ground plane and underneath of the patch due to which antenna radiates. In the next section the performance of the conventional rectangular rmicrostrip patch antenna has been reported. This antenna is simulated and its performance is analyzed in free space. 2.2 Design and Analysis of Rectangular Microstrip Patch Antenna In the first step: a conventional rectangular microstrip patch antenna is simulated and designed by using IE3D simulator software. The basic requirements for any design of microstrip patch antenna (in this case rectangular patch antenna) are: a) Selection of the substrate (Єr): substrate material used in this design is FR4 substrate, having loss tangent tanδ =0.002 and relative permittivity Єr =4.4 to reduces the dimensions of the patch usually substrate of high dielectric constant is to be chooses. b) substrate height (h): antenna height should be kept as small as possible.the standard height for the available material FR4 is 1.59 mm so for this antenna the height is consider as h=1.6mm. c) (f 0 )- Resonant frequency: it should be choose appropriately for the proposed antenna. For rectangular patch it is selected as 1.5 GHz which lies in personal communication system band. 1.5 GHz to 5.2 GHz band of the frequency spectrum is referred as S band. Many satellites transmit at S band. d) Selection of feeding method: the antenna is fed through coaxial cable SMA connector of 50Ω. The (X f &X y ) feed location is optimized to excite the patch.

4 Fig2.1- Meshing in patch antenna geometry 37

5 Simulation of the antenna is done by IE3D simulator software. This software is based on methods of moment and divides the prototype geometry in small grids (meshes) as shown in Fig 2.3 the simulation accuracy depends upon the number of grids. There is a compromise between the desired level of accuracy, the amount of available computing resources and the size of the mesh. According to the size of the basic elements the accuracy of the solution depends. Solutions based on coarse meshes are not so accurate than Solutions based on fine meshes. To generate a precise description of the current each element of the mesh must occupy a region that is small enough for the current to be adequately interpolated from the normal value. However meshes with large number of elements require a significant computing memory and power. Therefore it is desirable to use a mesh that is fine enough to obtain an accurate current solution but not so fine that it exhausts the available amount of computing power and memory Antenna Design Using above equation and design parameter given above, the dimension of the antenna are calculated so that it can resonate at frequency 1.5 GHz and figure 2.4shows geometry of the proposed rectangular microstrip patch antenna.the substrate used for proposed antenna design was FR4 whose thickness is 1.6 mm and dielectric constant of 4.4. The geometrical parameters for proposed RMSA antenna are, length of rectangular patch L = 47mm and width of rectangular patch W = 62mm. The matching of input impedance of antenna with 50 ohms impedance of feed line is achieved by selecting an inset feed point. The antenna is fed from feed point (X f = 38mm, Y f = 26mm) through coaxial cable SMA connector of 50Ω.A trial and error mechanism is followed to analysis the reflection coefficient (S 11 ) minimum value at the resonant frequency. 38

6 Figure2.2 - rectangular microstrip patch antenna 39

7 2.2.2 Results and Discussion Fig. 2.5 represents the return loss for the proposed designed antenna.an excellent reflection coefficient approx -31dB has been achieved at the resonant frequency 1.49 GHz corresponds to dominant TE 11 mode of excitation. The simulated result has a bandwidth of 2.68 % across a range of freq 1.47 GHz to 1.51 GHz, below the -10 db RL at central frequency of 1.49 GHz. The change in simulated value of VSWR Vs frequency is shown in fig 4.4. VSWR presented by antenna across a bandwidth area is less than 2:1 value which is good for matching between feeding circuit and antenna. The simulate value of VSWR at resonant freq is Figure Variation of return loss Vs frequency 40

8 To match an antenna the impedance locus needs to be shifted as near as possible to the centre of the smith chart (matching point). As shown in fig2.7 the impedance matching point is very close to the centre of the smith chart. Figure 2.8 depict the simulated graph of input impedance of design antenna with freq.at resonance freq 1.49 GHz the simulated input impedance of antenna is 51+ j 2.77ohms which is in good agreement with the 50 ohms impedance of feeding network. Figure Variation of VSWR Vs. frequency 41

9 Figure Impedance Loci 42

10 Figure Graph of input impedance of antenna vs. frequency 43

11 The other radiation characteristics of antenna such as directivity, gain and efficiency are shown in fig 2.9, 2.10.and 2.11respectively. At resonance freq the total field directivity is 6.4 dbi as the typical value of directivity for microstrip antenna should be 5-8 dbi and the maximum gain of about 1.16 dbi at resonance frequency is obtained. at resonance freq antenna and radiation efficiencies of antenna is about 30% as shown in figure 2.11.the E-plane and H- plane elevation pattern and azimuth pattern of antenna at resonance freq 1.49ghz are shown in fig 2.12 and 2.13 respectively which indicates that the radiation intensity is maximum normal to the patch. Figure change in simulated value of directivity of antenna Vs frequency 44

12 Figure change in simulated value of gain of antenna Vs. frequency 45

13 Figure Variation of efficiency of antenna vs. frequency 46

14 Figure display of Elevation pattern of antenna 47

15 Figure display of Azimuth pattern of antenna 48

16 All the results presented above for a rectangular patch antenna suggests that the designed antenna present in this form is unsuitable for modern communication systems. Therefore we have to modify this antenna to improve its overall performances as reported in next section. 2.3 Design of Simple Truncated Rectangular Microstrip Patch Antenna A Rectangular patch antenna discuss in previous section is modified to achieve dual frequency/dual band performance. In these when two or more resonance frequencies of a microstrip antenna are close to each other, one gets broadband characteristics. The dual band antenna can be used in various applications like cellular systems, WLAN, radar, and radio frequency identification systems because of their advantages like low profile, light weight and reduced cost. Generally single layer dual band microstrip antenna are possible by utilizing the multi resonance characteristics of a single patch antenna by loading the patch with stub, using shorting post, introducing notches, corner chopped, and by loading slots.the detailed inspection of dual frequency microstrip antenna is available in open literature of antenna. Wong and Chen [1998] presented bow tie patch dual- frequency antenna by loading a pair of narrow slots. Gao et al. [2002] reported a rectangular microstrip patch antenna with a shorting pin and achieved large bandwidth and good reduction in antenna size. A.A. Heidari et al.[2009] presented a circularly polarized stub loaded microstrip patch antenna for G PS application. Wenquan et. al[2011] design a broadband circularly polarized microstrip antenna with a truncated corner patch using a single chip-resistor loading which gives effective axial ratio and wide bandwidth. The methods discuss above for obtaining dual frequency have their own merits and demerits. In this section a single layer rectangular microstrip antenna is modified by chopped the opposite corners of the patch antenna as this modification gives good impedance matching and better gain. It also provides a dual frequency behavior out of which one is similar to the resonant frequency of a conventional patch while the other is originating due to modification in geometry. This modification gives better performance as compare to simple rectangular patch.in further chapters dimensions of a rectangular patch are modified and various broadband techniques are 49

17 applied such that the resonance frequencies of the two orthogonal modes are close to each other to obtain broad bandwidth Antenna Design Figure shows the truncated RMSA, the design parameters for the proposed TRMSA design structure are length of patch (L) is 47mm, width of the patch (W) is 62 mm and two notches of 5 mm are introduces at two corners of rectangular microstrip patch.the dielectric constant (Єr) of the substrate is 4.4 mm and the thickness of the dielectric substrate is 1.6 mm. The patch is printed on inexpensive glass epoxy FR4 substrate. The 50-ohm coaxial cable with SMA connector is used for feeding. The proposed patch antenna gives dual resonance frequency f 1 = 1.16 GHz with impedance Bandwidth equal to 1.72 %and f2= 1.5 GHz with impedance Bandwidth equal to 2.66% over a range of frequency in between 1 GHz to 2 GHz, at appropriate feed point location( Xf=41mm,Yf= 60mm). A trial and error mechanism is followed to analysis the reflection coefficient (S 11 ) minimum value at the resonant frequency. The simulation of this design antenna is done by IE3D simulator software. For a good impedance matching across a wide range of frequency, notches are also introduced at the two corners of the rectangular patch antenna as shown. 50

18 Fig2.12- truncated rectangular microstrip patch antenna 51

19 2.3.2 Results and Discussion The proposed antenna resonating at two frequencies corresponding to different modes of excitation. Fig 2.15 represents the simulated return loss of the proposed design antenna. The first resonance freq is 1.16 GHz is due to notches and second resonance freq is 1.5 GHz which is similar to rectangular patch antennas studied in previous section. the simulated variation of VSWR presented by antenna at both the resonant frequency are display in fig 2.16 which indicates that VSWR bears values lower than 2:1 at both the frequencies this result confirms good matching of this antenna with the feed network. Figure 2.17 indicates the simulated graph of input impedance of design antenna with freq, the simulated values of input impedance of antenna at two resonance freq are j0.419 ohms and ( j1.572) the real parts of input impedances are in fair agreement with 50ohms impedance of feed line. Fig Variation of return loss Vs frequency 52

20 Fig Variation of VSWR Vs. frequency 53

21 Fig Graph of input impedance of antenna vs. frequency 54

22 Fig Impedance Loci 55

23 The other radiation characteristics of antenna such as directivity, gain and efficiency are shown in fig 2.19, 2.20 and fig 2.21 respectively. At resonance freq the total field directivity at 1.16 GHz is 6.19 dbi and at second resonant frequency of 1.5 GHz is 6.39 dbi and maximum gain of about dbi and 1.20 dbi at both resonance frequencies respectively. The directivity is somewhat unaffected over the frequency range and gain is marginally better in compare with that of previous case of rectangular antenna. The simulated elevation pattern and azimuth pattern of at both resonating freq are given in fig 2.22 and 2.23 respectively which indicates that the radiation intensity is maximum normal to the patch. Fig 2.17 change in simulated value of directivity of antenna Vs frequency 56

24 Fig 2.18 change in simulated value of gain of antenna Vs. frequency 57

25 Fig 2.19 Variation of efficiency of antenna vs. frequency 58

26 Fig 2.20a- display of Elevation pattern of antenna 59

27 Fig 2.20b- display of Elevation pattern of antenna 60

28 Fig 2.21a- display of Azimuth pattern of antenna 61

29 Fig 2.21b- display of Azimuth pattern of antenna 62

30 The modified antenna discussed above radiates at two frequency with good broad side radiation properties.however the impedance bandwidth of this antenna at both the freq are still narrow (of order of 1.72 % and 2.66 % corresponding to freq 1.16 and 1.5 GHz respectively) hence antenna in its present form is still unsuitable for communication systems and further improvement is required. 2.4 Design of gap coupled truncated rectangular microstrip antenna In following section an attempt is made to further improve the impedance Bandwidth of TRMSA (truncated rectangular microstrip antenna) as discussed in above section. For this purpose one horizontal slot parallel to non-radiating edge and two vertical slots parallel to radiating edge are applied in the radiating patch forming an H shape slot as shown in figure2.24.there fore the patch is divided in to six independent patches and gap coupled with one or more independent patches Antenna Design The optimized design parameter for the proposed antenna are, L1 = 47mm length of rectangular patch, W = 62 mm width of rectangular patch, notches n=5mm and slot s=1mm as shown in fig1.the dielectric constant (Єr) of the substrate is 4.4 mm and the thickness of the dielectric substrate is 1.6 mm. The patch is printed on inexpensive glass epoxy FR4 substrate. The 50-ohm coaxial cable with SMA connector is used for feeding. The proposed patch antenna gives wide bandwidth having resonance frequency f 0 = 2.2 GHz with impedance Bandwidth equal to 9.0%, over the range of frequency 2.1 GHz to 2.32 GHz, at appropriate feed point location Xf=38 and Yf=60. Simulation of the designed antenna is done by IE3D simulator software. A trial and error mechanism is followed to analysis the reflection coefficient (S 11 ) minimum value at the resonant frequency. For a good impedance matching over a wide range of frequency, notches are introduced on the two corners of the rectangular patch antenna as shown. 63

31 Fig2.22-Gap coupled truncated rectangular microstrip patch antenna. 64

32 2.4.2 Results and Discussion The simulated results for the return loss and parametric study for the proposed design are also studied. Fig represents the return loss for optimized proposed design. The simulated result has a bandwidth of 9.0 % across a range of freq 2.1 GHz to 2.32 GHz, below the -10 db RL at central frequency at 2.22 GHz. The change in simulated value of VSWR Vs frequency is shown in fig 2.26 VSWR presented by antenna across a bandwidth area is less than 2:1 value which is good for matching between feeding circuit and antenna. The simulate value of VSWR at resonant freq is Figure 4.5 depict the simulated graph of input impedance of design antenna with freq.at resonance freq 2.22 GHz the simulated input impedance of antenna is j 25 ohms which is in good agreement with the 50 ohms impedance of feeding network. Fig Variation of return loss Vs frequency 65

33 Fig Variation of VSWR Vs. frequency 66

34 Fig Graph of input impedance of antenna vs. frequency 67

35 Fig Impedance Loci 68

36 2.4.3 Parametric study and effect of gap slots between parasitic patches. By changing one parameter of geometry a parametric studies are presented while remaining parameters of geometry are fixed w.r.t reference design. Fig 2.29 represent the simulated graph of return loss for proposed design w.r.t freq for various values of spacing S between rectangular parasitic patches. From simulated results it is observed that if the gap spacing S decreases from the optimum value, there is decrease in Bandwidth with Slightly change of frequency range near to lower side of freq. it is observed too that by increasing gap of slot bandwidth is decreased approximately by 40%. The optimal performance is obtained for S = 1 mm as shown in figure 2.24 Fig Effects of variation of gap between parasitic patches. 69

37 The other radiation characteristics of antenna such as directivity, gain and efficiency are shown in fig 2.30, 2.31and fig 2.32 respectively. At resonance freq the total field directivity is 7.2 dbi and maximum gain of about 3.35dBi at resonance frequency. It is observed that simulated variation of gain, efficiency and directivity of antenna increases significantly as compare to that of simple rectangular and truncated antenna. The simulated elevation pattern and azimuth pattern of antenna are shown in fig 2.33 and 2.34 respectively which indicates that the antenna is strongly radiating normal to the patch. Fig Directivity VS. Frequency 70

38 Fig 2.29 gain vs. frequency 71

39 Fig efficiency vs. frequency 72

40 Fig display of Elevation pattern of antenna 73

41 Fig display of Azimuth pattern of antenna 74

42 The modified antenna discussed above radiates at resonance frequency of 2.2ghz with good broadside radiation properties having impedance bandwidth of 9.0%.The reported results suggest that the overall performance of this gap coupled truncated rectangular microstrip antenna a is improved considerably. In the next chapter same antenna is further modified to achieve further improvement in impedance bandwidth and other characteristics. 2.5 Discussion and Conclusion In this chapter, the investigation of a simple rectangular MSA and modified rectangular MSA with Truncated Corners are discussed. In first part, the characteristics of simple rectangular patch antenna is simulated and studied on single layer FR-4 substrate material. The useful bandwidth for the simple rectangular antenna is 2.68% which is too low for application in communication system. Further the antenna is modified by introducing two notches at the corners of rectangular patch which gives the dual frequency operation and better performance as compare to simple rectangular microstrip patch antenna. The impedance Band width of this antenna at two resonant frequencies is 1.72% and 2.66% which is also not suitable for application. In next section a gap coupled arrangement is used with the same parameters of truncated rectangular microstrip patch antenna by introducing horizontal and vertical slots such that the antenna is divided in to small patches and gap coupled with each other. By such arrangement the improved impedance bandwidth of 9.0% at resonance frequency 2.2 GHz is achieved and also there is a good improvement in gain, directivity and efficiency of the patch antenna. Simulated results verifying the application of such method for single layer antenna. In further chapters an efforts are made to increase the impedance bandwidth with same parameters by using different methods of bandwidth enhancement techniques.