Transcript
Substrate Permittivity Effects on Wideband and UWB Antennas
Chapter 4
Chapter 4
4. Substrate Permittivity Effects on Wideband and UWB Antennas In recent years, Wideband and Ultra wideband system designs and applications have become the focus of short range high speed wireless communication and military domains for its advantages of high speed data rate, high capability and low power consumption. The UWB covers the frequency range of 3.1 to 10.6 GHz according to the approval of Federal Communication Commission (FCC). UWB refers to the systems with very large bandwidth. This very large bandwidth offers several advantages like high time resolution, low cost implementation, obstacle penetration, resistance to interference, covert transmission, co-existence with narrowband systems and so on. Such advantages enable a wide range of applications of UWB to communications, radar, imaging and positioning. UWB also poses several challenges. Due to its extremely large bandwidth, the interference between UWB and narrowband system is a major concern. The design of UWB antennas is considerably more challenging than conventional antennas. Conventional wideband antennas cannot transmit UWB signals without distortion. It is also more difficult to characterise UWB antennas, as traditional narrowband antenna parameters are not directly useful to UWB. The design of UWB antennas is even more challenging for small mobile terminals. A vertical disc monopole can attain good bandwidth with nearly omnidirectional radiation pattern. This type of design is not a planar structure i.e. it requires ground plane perpendicular to the disc. Constructive difficulties are involved in this design, which limit its applications in practical case. To construct planar version of UWB disc monopole, either micro strip line or coplanar waveguide feeding structures are required. This section will cover the CPW fed antennas of different oriented structures of the wideband applications and substrate material effects on the performance of these antennas. 4.1 Circular Monopole Antenna A CPW fed monopole antenna is constructed on single layer metallic structure, as shown in Fig 4.1. Circular monopole with radius ‘r’ and a 50 ohm coplanar 41
Substrate Permittivity Effects on Wideband and UWB Antennas
Chapter 4
waveguide feed are taken on the same side of the substrate. Ws=26 mm and Ls=32 mm are the width and the length of the substrate material FR4 with εr=4.4 and height ‘h’ =1.6 mm. The feed line width is w=2.6 mm and gap between the feed line and ground is g=0.4 mm. Length of the feed line is L=10 mm. The overall dimension of the antenna is 32x26x1.6 mm. Fig 4.2 shows the fabricated prototype of the circular monopole antenna.
4.1.1 Design steps for Circular monopole Antenna Radius of the patch is calculated from the equation F 2h
R
(4.1)
F 1 eff F ln 1.7726 2h
Where
F
8.791*109 f r eff
(4.2)
fr = Resonant Frequency in Hz
εeff = Effective dielectric constant R = Radius of the patch h = Height of the substrate in mm Design a 50Ω CPW line on a substrate with permittivity εr .calculate εeff using εeff = (εr+1)/2 where εeff is the effective permittivity of the substrate. 42
Substrate Permittivity Effects on Wideband and UWB Antennas
Chapter 4
Width of the substrate Ws = 1.06 λc Length of the substrate Ls = 1.31 λc Length of the feed line L = 0.41 λc Feed line width w = 0.10 λc Gap between feed line and ground plane g = 0.016 λc Where λc is the wavelength corresponding to the centre frequency 4.1.2 Circular monopole antenna characteristics: From Fig 4.3 it is observed that simulated return loss is in good agreement with measured curve in range of the frequency band. The measured bandwidth ranges from 3.2 to 12.3GHz with impedance bandwidth of 117% in the desired band. Fig 4.4 showing the VSWR Vs frequency curve and it is been observed that 2:1 ratio is maintained in the desired band for the current design. Measured return loss obtained from the R&S ZNB 20 Vector network analyzer is shown in Fig 4.5. By doing parametric analysis with change in height of the substrate material, it has been observed from the Fig 4.6 that with 1.2 mm thickness an impedance bandwidth of 83% is obtained, except for the case of the 1.6mm thickness of all the remaining cases the impedance bandwidth is an average more than 75%.
Fig 4.3 Simulated and measured return loss Vs frequency curve for circular monopole antenna
Fig 4.4 VSWR curve for Circular monopole antenna
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Chapter 4
Fig 4.5 Measured S11 Parameter of circular monopole antenna on R&S ZNB 20 Vector Network analyzer
Fig 4.6 Parametric analysis of circular monopole antenna with change in substrate height
Fig 4.7 3D view of radiation pattern for CPW fed circular monopole antenna at 7.4 GHz
Fig 4.8 Radiation pattern at 7.4 GHz for circular monopole antenna in E&H Plane
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Chapter 4
Fig 4.7 giving three dimensional radiation view of the circular monopole antenna. From this Fig 4.7 we can observe that the direction of maximum in the xz-direction. The radiation patterns in polar coordinates for the antenna is shown in Fig 4.8 at 7.4 GHz. Omni directional radiation pattern can be observed in co-polarization and eight shaped orientation radiation in cross polarization. Fig 4.9 giving measured and simulated gain over the frequency band. It is been observed that both measured and simulated results are almost identical and a peak realized gain of 4 dB is attained in the measurement. Fig 4.10 shows simulated current distribution over the surface of the antenna and the corresponding current elements intensity with colour scaling. At lower frequency the current distribution is mostly concentrated on the feed line towards x direction. When we go to higher frequencies the current intensity is maximum at edges of ground plane and radiating element along with feed line. Gain Vs Frequency 6 Simulated Measured
5
G a in in d B
4 3 2 1 0
0
2
4
6
8 Frequency
10
12
14
16
Fig 4.9 Circular monopole antenna Gain Vs Frequency
Fig 4.10 circular monopole antenna current distribution at 4.3, 7.4 and 9.8 GHz
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Substrate Permittivity Effects on Wideband and UWB Antennas
Chapter 4
4.1.3 Parametric analysis of circular monopole with change in substrate permittivity
Fig 4.11 circular monopole antenna parametric analysis of reflection coefficient with change in permittivity
Parametric analysis with change in substrate permittivity is performed and presented. Fig 4.11 shows the simulated return loss Vs frequency plot with change in substrate permittivity. More or less except alumina, remaining materials are giving almost stable bandwidth in the desired band. Alumina based model is working like a multiband antenna rather than wideband antenna. Fig 4.12 shows the simulated VSWR Vs frequency plot with change in substrate permittivity. Table 4.1 shows the circular monopole antenna dimensions for different substrate materials.
Table 4.1 Circular monopole antenna dimensions (in mm) for different substrate materials Paramet er in mm Ls Ws L W G
RTduroid 5880 36.2 31.4 16 3.6 0.2
Arlon AD 250A
Ultralam 3850
Polyester
Plexiglass
FR4
Alumina
35.6 30.2 15 3.3 0.24
34.4 29.4 14 3.1 0.26
33.2 28.1 13.5 2.9 0.3
32.8 27.5 12 2.8 0.34
32 26 10 2.6 0.4
28 20 8 2.1 0.6
4.2 Circular Monopole with Tapered Step Ground The earlier design of circular monopole antenna is modified with tapered step grounded model as shown in the Fig 4.12. Bandwidth enhancement is achieved by adding the tapered steps in the ground plane. Antenna is printed on FR4 substrate with dimensions of 20X20X1.6 mm. The current model is operating in the wideband from 5.1 to 17 GHz. Simple design with coplanar waveguide feeding has been used in this model. A simple circular patch is fed by coplanar waveguide feed line and the ground plane is surrounded by the patch consisting of tapered steps. These tapered 46
Substrate Permittivity Effects on Wideband and UWB Antennas
Chapter 4
steps are giving new path for the current and enhancement in the bandwidth. The simulation is carried with FEM based HFSS tool and the fabricated model is printed on FR4 substrate with dielectric constant 4.4 and loss tangent 0.002. For decreasing the cost of fabrication and simplicity in the design, circular patch and the ground plane are printed on same side of the substrate. A simple circular patch with radius of 4 mm is fed by 50 ohm coplanar waveguide feed line. The width and length of the feed line are 2 mm and 4.65 mm respectively. The separation between the feed and the ground plane (G=0.5 mm) and feed width are chosen to reach 50 ohm impedance. The ground plane consisting of simple tapered steps and this newly created stepped path will leads to the impedance matching improvement and bandwidth enhancement. The dimensional characteristics of the antenna are G1=8 mm, G2=G3=2 mm, G4=1.5 mm, G5=8.2 mm, L=4.65 mm and W=2 mm respectively.
Fig 4.12 Circular monopole with tapered step ground
Fig 4.13 Reflection coefficient Vs frequency for circular monopole with tapered step ground
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Fig 4.14 Parametric analysis for circular monopole with tapered step ground antenna return loss curve for change in substrate height ‘h’
Fig 4.13 shows the return loss Vs frequency curve for the current model in the simulation and measurement. It has been observed from the S11 results that simulated and measured results are in good agreement with each other in the operating band. Fig 4.14 is showing the parametric analysis with change in the height of the substrate material. Fig 4.15 is giving S11 parameter for change in the feed gap between feed line and ground plane. Optimum results are obtained for the case of 0.5 mm thickness in the gap ‘G’. Fig 4.16 shows the three dimensional view of radiation at 13.3 GHz.
Fig 4.15 Parametric analysis for circular monopole with tapered step ground antenna return loss curve for change in feed gap ‘G’
Fig 4.16 3D view of radiation pattern for circular monopole with tapered step ground at 13.3 GHz
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Fig 4.17 Frequency Vs Gain and Efficiency for the circular monopole with tapered step ground
From Fig 4.17 it has been observed that the antenna is providing almost stable gain in the operating band. Peak realized gain of 4.2 dB is obtained at 9.5 GHz and 13.3 GHz. Efficiency of the antenna is also almost stable and above 80% in the operating range. Fig 4.18 shows the current distribution of the antenna at 13.3 GHz.
Fig 4.18 Current distribution on circular monopole with tapered step ground at 13.3 GHz
Fig 4.19 Radiation Pattern in E-plane and H-plane for circular monopole with tapered step ground at 9.5 GHz
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4.2.1 Design steps of the circular monopole with tapered step ground Frequency of operating band is taken into account, while deriving the design equations. Step by step procedure for the design of the antenna is paraphrased 1) Design a 50Ω CPW line on a substrate with permittivity εr .calculate εeff using εeff = (εr+1)/2 where εeff is the effective permittivity of the substrate. 2) Length of the feed line L= 0.60λc Length of the substrate Ls=1.10λc Width of the feed line W=0.11λc Width of the substrate Ws=1.10λc Gap between feed line and ground plane G=0.02λc Dimensions of the tapered step ground First tapered step slot on the ground plane G1=0.44λc Second and third tapered step slot on the ground plane G2=G3=0.11λc Fourth tapered step slot on the ground plane G4=0.08λc Fifth tapered step slot on the ground plane G5=0.46λc Where λc is the wavelength corresponding to centre frequency of operating band. 4.2.2 Parametric analysis of circular monopole with tapered step ground with change in substrate permittivity Table 4.2 Circular monopole with tapered step ground antenna dimensions (in mm) for different substrate materials Substrate material h εr εeff W G Ws LS G1 G2 G3 G4 G5 L
RT-duroid 5880 1.57 2.2 1.6 2.36 0.59 23.64 23.64 9.45 2.36 2.36 1.77 10.04 12.88
Arlon AD 250A 1.6 2.5 1.75 2.27 0.56 22.76 22.76 9.10 2.27 2.27 1.70 9.67 12.4
Ultrala m 3850 1.6 2.9 1.95 2.19 0.54 21.98 21.98 8.79 2.19 2.19 1.64 9.34 11.9
50
Polyester 1.6 3.2 2.1 2.13 0.53 21.32 21.32 8.53 2.13 2.13 1.59 9.06 11.62
Plexiglass 1.57 3.4 2.2 2.06 0.51 20.66 20.66 8.26 2.06 2.06 1.54 8.78 11.26
FR4 1.6 4.4 2.7 2 0.5 20 20 8 2 2 1.5 8.5 10.9
Alumina 1.6 9.2 5.1 1.83 0.45 18.34 18.34 7.33 1.83 1.83 1.37 7.79 9.99
Substrate Permittivity Effects on Wideband and UWB Antennas
Chapter 4
Fig 4.20 Parametric analysis for return loss with change in substrate permittivity of circular monopole antenna with tapered step ground
Table 4.1 shows the dimensions of the circular monopole antenna with tapered step ground on different substrate materials. From Fig 4.20 and 4.22 it is evident that even with change in substrate permittivity, there is not much difference we can observe in the operating band. Cross polarization in the E-plane is low and broadside pattern can be observed in both planes.
Fig 4.21 Parametric analysis for VSWR with change in substrate permittivity of circular monopole antenna with tapered step ground
Fig 4.22 Parametric analysis for Radiation pattern with change in substrate permittivity of circular monopole antenna with tapered step ground
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4.3 Elliptical Monopole with Tapered Step Ground In the earlier design, circular patch with tapered step ground is used in the antenna structure. By changing the patch shape from circular to elliptical, the antenna performance is recorded and presented in this section. Fig 4.23 shows the geometry of Elliptical Monopole antenna with tapered step ground. This model is also designed on FR4 substrate with dielectric constant 4.4 and the dimension of the antenna occupies 20x20x1.6 mm. Except the radiating element shape, the remaining dimensions are as usual like the previous circular monopole with tapered step ground antenna.
Fig 4.23 Elliptical monopole antenna with tapered step ground
By placing tapered step ground, the bandwidth improvement is achieved and Fig 4.24 is showing the return loss curve for with and without tapered step ground configuration results. It has been observed that with tapered step ground there is an improvement of 500MHz in the bandwidth. Fig 4.25 shows the VSWR of 2:1 ratio in the frequency range of 11.5-17 GHz.
Fig 4.24 Reflection coefficient of Elliptical monopole antenna with and without tapered step ground
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Fig 4.25 VSWR of Elliptical monopole antenna with tapered step ground
Fig 4.26 Impedance smith chart for Elliptical monopole antenna with tapered step ground
Fig 4.26 and Fig 4.27 shows the input impedance smith chart and 3D gain of the present model respectively. Fig 4.28 showing the current distribution on the antenna at 13GHz.
Fig 4.27 3D view of radiation pattern for elliptical monopole with tapered step ground at 13 GHz
Fig 4.28 Current distribution of Elliptical monopole antenna with tapered step ground at 13 GHz
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Substrate Permittivity Effects on Wideband and UWB Antennas
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Fig 4.29 Radiation Pattern of Elliptical monopole antenna with tapered step ground at 13 GHz
Radiation Pattern defines the variation of the power radiated by an antenna as a function of the direction away from the antenna. The power variation as a function of the arrival angle is found in the antenna’s far field. Fig 4.29 shows the Radiation Pattern of the Elliptical Monopole antenna with tapered step ground at 13GHz.
Fig 4.30 Gain and efficiency of Elliptical monopole antenna with tapered step ground
Fig 4.30 shows the Frequency Vs Gain and Frequency Vs Efficiency of the current antenna. From the figure it has been noted that almost constant gain between 2.6 to 3dB is attained in the desired band. Efficiency of the antenna is above 80% in the frequency band.
4.3.1 Design steps of the elliptical monopole with tapered step ground a) Design of elliptical monopole patch For the ellipse with major axis ‘a’ and minor axis ‘b’, the perimeter P =2aE (e)
---------
(34)
Where E (e) is a complete Elliptic integral of the second kind with elliptic modulus ‘e’. 54
Substrate Permittivity Effects on Wideband and UWB Antennas e=√1-(b/a) 2
Chapter 4
--------
(35)
If the lowest frequency in the impedance bandwidth of the antenna is fL (GHz) and the effective permittivity of the medium of radiation can be approximated by
εeff ≈ (εr+1)/2, fL=300/p√εeff
(36)
------------
Where perimeter unit is in the mm. Frequency of operating band is taken into account, while deriving the design equations. Step by step procedure for the design of the antenna is paraphrased
Design a 50Ω CPW line on a substrate with permittivity εr .calculate εeff using εeff = (εr+1)/2 where εeff is the effective permittivity of the substrate. b) Width of the substrate Ws and Length of the substrate Ls = 1.13 λc c) Tapered step ground plane dimensions G1 = 0.45 λc G2 = G3= 0.11 λc G4 = 0.08 λc G5 = 0.48 λc d) Length of the feed line L = 0.61 λc e) Gap between feed line and ground plane G = 0.02 λc f) Width of the feed line W= 0.11 λc 4.3.2 Parametric analysis of elliptical monopole with tapered step ground with change in substrate permittivity Table 4.3 Elliptical monopole with tapered step ground antenna dimensions (in mm) for different substrate materials Substrate material h Ԑr Ԑeff W G Ws
RT-duroid 5880 1.57 2.2 1.6 2.36 0.53 21.25
Arlon AD 250A 1.6 2.5 1.75 2.27 0.525 21.02
Ultralam 3850 1.6 2.9 1.95 2.19 0.517 20.68
55
Polyester
Plexiglass
FR4
Alumina
1.6 3.2 2.1 2.13 0.511 20.45
1.6 3.4 2.2 2.06 0.505 20.22
1.57 4.4 2.7 2 0.5 20
1.6 9.2 5.1 1.83 0.45 18.18
Substrate Permittivity Effects on Wideband and UWB Antennas Ls G1 G2 G3 G4 G5 L
21.25 8.5 2.12 2.12 1.59 9.03 11.5
21.02 8.40 2.10 2.10 1.57 8.93 11.45
20.68 8.27 2.06 2.06 1.55 8.78 11.27
20.45 8.18 2.04 2.04 1.53 8.69 11.14
20.22 8.09 2.02 2.02 1.51 8.59 11.02
Chapter 4 20 8 2 2 1.5 8.5 10.9
18.18 7.27 1.81 1.81 1.36 7.72 9.9
Fig 4.31 Return loss Vs Frequency for elliptical monopole with tapered step ground on different substrates
Fig 4.32 VSWR curve for elliptical monopole with tapered step ground on different substrates
Fig 4.33 Radiation pattern in E and H-Plane for elliptical monopole with tapered step ground on different substrates
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Table 4.3 shows the dimensional characteristics of the elliptical monopole with tapered step ground for different substrate materials. Fig 4.31 and 4.32 shows the return loss and VSWR for change in permittivity of the substrate respectively. Radiation pattern in E and H-plane also can be observed for different substrate materials in Fig 4.33. 4.4 Rectangular Monopole Antenna with Tapered Step Ground From previous design we observed that the lower and upper frequencies are sensitive to the variation in the antenna radiating patch shape. With elliptical monopole model it has been observed that, antenna does not satisfying the ultra wide band frequency range. The current model deals with the rectangular shaped radiating element with tapered step ground structure. Fig 4.34 shows the rectangular monopole antenna with tapered step ground. Except the radiating patch, the remaining dimensions are as-usual like previous tapered step models. Here length of the patch Lp = 5.65 and width Wp = 5.64.
Fig 4.34 Rectangular monopole antenna with tapered step ground
Fig 4.35 Reflection coefficient of Rectangular monopole antenna with and without tapered step ground
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Substrate Permittivity Effects on Wideband and UWB Antennas
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Fig 4.36 VSWR of Rectangular monopole antenna with tapered step ground
Reflection coefficient of rectangular monopole with and without tapered step ground is present in Fig.4.35. Without tapered step antenna is resonating in the band of 6-13 GHz and with tapered step antenna resonating between 6-15 GHz i.e., 2 GHz improvement in the bandwidth is attained from the current design. From Fig 4.36 VSWR<2 is attained in the desired band of 6-15 GHz.
Fig 4.37 Impedance smith chart for Rectangular monopole antenna with tapered step ground
Impedance relates the voltage and current at the input to the antenna. The impedance of the antenna will vary with frequency. Fig 4.37 shows the input impedance Smith chart of the current model and Fig 4.38 shows the 3D view of antenna radiation. Current distribution of the antenna at 9.2 GHz is shown in Fig 4.39.
Fig 4.38 3D view of radiation pattern for rectangular monopole with tapered step ground
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Substrate Permittivity Effects on Wideband and UWB Antennas
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Fig 4.39 Current distribution for Rectangular monopole antenna with tapered step ground at 9.2 GHz
From Fig 4.40 we observed that gain varies between 2.5 to 3.5 dB in the frequency range between 6-14 GHz and efficiency is almost greater than 70% in the frequency band. Radiation pattern of the antenna with low cross polarization in E-plane and omni directional pattern in co-polarization can be observed from Fig 4.41.
Fig 4.40 Frequency Vs Gain and Efficiency for Rectangular monopole antenna with tapered step ground
Fig 4.41 Radiation pattern of Rectangular monopole antenna with tapered step ground in E and H-plane at 9.2 GHz
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Substrate Permittivity Effects on Wideband and UWB Antennas
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4.4.1 Design steps of the rectangular monopole with tapered step ground Frequency of operating band is taken into account, while deriving the design equations. Step by step procedure for the design of the antenna is paraphrased Design a 50Ω CPW line on a substrate with permittivity εr .calculate εeff using
εeff = (εr+1)/2 where εeff is the effective permittivity of the substrate. b) Width of the substrate Ws and Length of the substrate Ls = 1.005 λc c) Tapered step ground plane dimensions G1 = 0.45 λc Second and third stage tapered step ground G2 = G3= 0.11 λc G4 = 0.08 λc G5 = 0.48 λc d) Length of the feed line L = 0.54 λc e) Gap between feed line and ground plane G = 0.02 λc f) Width of the feed line W = 0.11 λc g) Width of the patch Wp = 0.28 λc h) Length of the patch Lp = 0.28 λc 4.4.2 Parametric analysis of rectangular monopole with tapered step ground with change in substrate permittivity Table 4.4 Rectangular monopole with tapered step ground antenna dimensions (in mm) for different substrate materials Substrate material
Arlon AD 250A 1.6
Ultralam 3850
Polyester
Plexiglass
FR4
Alumina
h
RTduroid 5880 1.57
1.6
1.6
1.6
1.57
1.6
εr
2.2
2.5
2.9
3.2
3.4
4.4
9.2
εeff
1.6
1.75
1.95
2.1
2.2
2.7
5.1
W
4
3.8
3.55
3.35
3.2
2.54
1.23
G
0.58
0.54
0.53
0.52
0.51
0.5
0.46
Ws
23.5
21.8
21.5
21.0
20.4
20
18.7
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Substrate Permittivity Effects on Wideband and UWB Antennas
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Ls
23.5
21.8
21.5
21.0
20.4
20
18.7
G1
9.4
8.72
8.6
8.4
8.6
8
7.51
G2
2.35
2.18
2.15
2.1
2.04
2
1.87
G3
2.35
2.18
2.15
2.1
2.04
2
1.87
G4
1.76
1.63
1.61
1.57
1.53
1.5
1.4
G5
9.99
9.26
9.14
8.92
8.67
8.5
7.98
Wp
6.63
6.15
6.06
5.92
5.75
5.64
5.29
Lp
6.64
6.16
6.07
5.93
5.76
5.65
5.30
L
12.81
11.88
11.72
11.44
11.11
10.9
10.24
Fig 4.42 Return loss Vs Frequency for rectangular monopole with tapered step ground on different substrates
Table 4.4 shows the dimensions of rectangular monopole antenna with tapered step ground on different substrate materials. Fig 4.43 mirrors the return loss curve for change in substrate permittivity of rectangular monopole with tapered step ground. Alumina substrate material based model shows the lesser bandwidth compared to other substrate materials. From Fig 4.44 the radiation in E-plane for alumina material based antenna is somewhat quasi omni directional and remaining materials based models are omni directional with low cross polarization level in desired direction.
Fig 4.43 VSWR Vs Frequency for rectangular monopole with tapered step ground on different substrates
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Fig 4.44 Radiation pattern in E and H-Plane for rectangular monopole with tapered step ground on different substrates
4.5 Hexagonal Monopole Antenna with Tapered Step Ground There is always an increasing demand for small size, and greater capacities and transmission speeds, which will certainly require more operating bandwidth in the future. A compact hexagonal monopole antenna with tapered step ground is proposed in this design, which can be applicable for wideband applications. The proposed antenna not only occupies small size but also preserve a very single structure which is easy to be fabricated. Input impedance matching over a wide frequency range is achieved with the current design. It covers the wide frequency range (7-15 GHz) and satisfying the VSWR ≤ 2.
Fig 4.45 Hexagonal monopole antenna with tapered step ground
The configuration of the hexagonal monopole is presented in Fig 4.45. The radiating element is a hexagonal patch and the ground plane is placed on the same side of the substrate. The dimensions of ground plane substrate, feed line and port gap are similar to the previous models. The other dimensions of hexagonal shape are L1=1.99mm, L2=1.41mm, L3=1.99mm, L4=1.41mm respectively. Fig 4.46 shows the reflection coefficient of the antenna with and without tapered step ground. 62
Substrate Permittivity Effects on Wideband and UWB Antennas
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Tremendous improvement in the bandwidth towards lower frequency side has been observed by placing the tapered step ground. Without tapered step ground antenna is resonating between 10-16 GHz, where as with the tapered step ground antenna resonating between 7-15 GHz. Almost 2GHz improvement in the bandwidth is attained with tapered step ground model. From Fig 4.48 VSWR˂2 in the desired band is observed.
Fig 4.46 Reflection coefficient of hexagonal monopole with and without tapered step ground
Fig 4.47 VSWR of hexagonal monopole with tapered step ground
Fig 4.48 Impedance smith chart for hexagonal monopole antenna with tapered step ground
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Substrate Permittivity Effects on Wideband and UWB Antennas
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Fig 4.48 and Fig 4.49 showing input impedance smith chart and 3D view radiation for the current model. Average gain of 2.5 dB is obtained in the frequency range from Fig 4.50. More than 70% efficiency attained in the frequency band from the proposed model. To get further insight of the radiation mechanism of the proposed wideband structure, the surface current analysis is necessary. At 9.5 GHz the current distribution of hexagonal monopole is shown in Fig 4.51.
Fig 4.49 3D view of radiation pattern for hexagonal monopole with tapered step ground
Fig 4.50 Frequency Vs Gain and Efficiency for hexagonal monopole with tapered step ground
Fig 4.51 Current distribution at 9.5 GHz for hexagonal monopole antenna with tapered step ground
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Fig 4.52 Radiation pattern of hexagonal monopole with tapered step ground in E and H-plane at 9.5 GHz
4.5.1 Design steps of the hexagonal monopole with tapered step ground Frequency of operating band is taken into account, while deriving the design equations. Step by step procedure for the design of the antenna is paraphrased a) Design a 50Ω CPW line on a substrate with permittivity εr .calculate εeff using εeff = (εr+1)/2 where εeff is the effective permittivity of the substrate. b) Width of the substrate Ws and Length of the substrate Ls = 1.005 λc c) Tapered step ground plane dimensions G1 = 0.45 λc G2 = G3= 0.11 λc G4 = 0.08 λc G5 = 0.48 λc d) Length of the feed line L = 0.54 λc e) Gap between feed line and ground plane G = 0.02 λc f) Width of the feed line W = 0.11 λc g) Length L1 = 0.1λc h) Length L2 = 0.07λc i) Length L3 = 0.1λc j) Length L4 = 0.07λc
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4.5.2 Parametric analysis of hexagonal monopole with tapered step ground with change in substrate permittivity Table 4.5 Hexagonal monopole with tapered step ground antenna dimensions (in mm) for different substrate materials Substrate material h εr εeff W G Ws Ls G1 G2 G3 G4 G5 L L1 L2 L3 L4
RT-duroid 5880 1.57 2.2 1.6 4 0.58 23.5 23.5 9.4 2.35 2.35 1.76 9.99 12.81 2.34 1.65 2.34 1.65
Arlon AD 250A 1.6 2.5 1.75 3.8 0.54 21.8 21.8 8.72 2.18 2.18 1.63 9.26 11.88 2.17 1.53 2.17 1.53
Ultralam 3850 1.6 2.9 1.95 3.55 0.53 21.5 21.5 8.6 2.15 2.15 1.61 9.14 11.72 2.14 1.51 2.14 1.51
Polyester
Plexiglass
FR4
Alumina
1.6 3.2 2.1 3.35 0.52 21.0 21.0 8.4 2.1 2.1 1.57 8.92 11.44 2.09 1.48 2.09 1.48
1.6 3.4 2.2 3.2 0.51 20.4 20.4 8.6 2.04 2.04 1.53 8.67 11.11 2.03 1.43 2.03 1.43
1.57 4.4 2.7 2.54 0.5 20 20 8 2 2 1.5 8.5 10.9 1.99 1.41 1.99 1.41
1.6 9.2 5.1 1.23 0.46 18.7 18.7 7.51 1.87 1.87 1.4 7.98 10.24 1.87 1.32 1.87 1.32
Fig 4.53 Return loss Vs Frequency for hexagonal monopole with tapered step ground on different substrates
Fig 4.54 VSWR Vs Frequency for hexagonal monopole with tapered step ground on different substrates
66
Substrate Permittivity Effects on Wideband and UWB Antennas
Chapter 4
Fig 4.55 Radiation pattern in E and H-Plane for hexagonal monopole with tapered step ground on different substrates
Fig 4.53, 4.54 and 4.55 shows return loss, VSWR and radiation pattern of hexagonal monopole with tapered step ground on different substrates. Alumina material is showing multiband characteristics and other materials are showing wideband characteristics. 4.6 Trident Shaped Ultra Wideband Antenna Compact planar trident shaped antenna is designed for ultra wideband applications and its performance characteristics and analytical study based on substrate permittivity is presented. Fig 4.56 shows the configuration of the proposed ultra wideband antenna, which consisting of fork shaped slotted aperture patch on the top side of the model. The antenna which has the compact dimensions of 23X27X1.6 mm printed basically on FR4 substrate with permittivity of 4.4 and loss tangent of 0.02. The strip width W and gap G of the coplanar waveguide feed are derived using standard design equations for 50 ohm impedance. Besides, the structure of the antenna is symmetrical respect to the longitudinal direction. Ground plane and radiating patch are printed on same side of the substrate.
Fig 4.56 Trident Shaped CPW Fed Antenna, Table 4.6 Trident Shaped CPW Fed Antenna Dimensions
67
Substrate Permittivity Effects on Wideband and UWB Antennas
Chapter 4
Fig 4.57 Fabricated Trident shaped CPW fed Antenna
4.6.1 Results and analysis using FEM based HFSS Tool The simulation is performed using FEM based HFSS software. The result from Fig 4.58 clearly indicates that the current antenna covers wide frequency from 3 to 12 GHz (defined by return loss <-10 dB) with bandwidth of 9 GHz. The main concentration is based on the selection of substrate material for the proposed model and finding the antenna performance characteristics like return loss, VSWR, gain and radiation pattern etc. Initially this model was simulated on FR4 substrate material and after that different substrate materials are considered and all its parameters are applied in the tool to simulate the models.
Figure 4.58 Return Loss Vs Frequency of Trident shaped CPW fed Antenna
Fig 4.59 displays the input impedance smith chart curve and Fig 4.60 Shows the three dimensional radiation view. Fig 4.61 shows the gain curve with respect to frequency. Within the optimum design, a group of three adjacent resonant modes for the proposed antenna can be excited with suitable impedance matching, and a wide
68
Substrate Permittivity Effects on Wideband and UWB Antennas
Chapter 4
impedance bandwidth is formed. It is found that the gain is almost greater than 2.5 dB in the resonating band.
Figure 4.59 Input impedance smith chart of Trident shaped CPW fed Antenna
Fig 4.60 3D view of radiation for Trident shaped CPW fed Antenna
Figure 4.61 Gain Vs Frequency of Trident shaped CPW fed Antenna
Excellent bandwidth percentage is attained for the current model and it is calculated from the formula BW = ((f2 – f1)/√f1f2) X 100, where f1 and f2 are lowest and highest frequencies at which S11 is under -10 dB level. To reach a better performance and excellent design, the sharp edges of patch were calmed and sharp edges have been smoothed out. Peak gain of 4 dB is attained from the measurement results. 69
Substrate Permittivity Effects on Wideband and UWB Antennas
Chapter 4
Fig 4.62 Radiation Pattern in E-Plane and H-Plane at 3.1 and 4.3 GHz of Trident shaped CPW fed Antenna
Fig 4.62 shows the radiation patterns in the E-plane (yz-plane) and H-plane (xzplane) at frequencies 3.1 and 4.3 GHz. It can be seen that the radiation patterns in the xz-plane is nearly omnidirectional for two frequencies. Almost identical results obtained in the H-plane like E-Plane and eight shaped cross polarization attained.
Fig 4.63 E-Field distribution at 4.3 GHz for Trident shaped CPW fed Antenna
Fig 4.63 shows the E-Field distribution of the antenna at lower frequency side of 4.3 GHz. It has been observed that maximum intensity is focussed on the feed line and inner boundary of the tapered ground plane. Fig 4.64 reflects the current distribution of the antenna at three frequencies. At lower frequency most of the current elements are concentrated at feed line and at higher frequency in the band the current distribution is pointing towards the x-axis.
70
Substrate Permittivity Effects on Wideband and UWB Antennas
Chapter 4
Fig 4.64 Current distribution at 4.3, 8.1 and 9.8 GHz for Trident shaped CPW fed Antenna
Fig 4.65 Parametric analysis of return loss with change in length of L5 for Trident shaped CPW fed Antenna
Fig 4.66 Parametric analysis of return loss with change in substrate thickness for Trident shaped CPW fed Antenna
Fig 4.65 shows the parametric analysis of the return loss of antenna with change in ‘L5’ and Fig 4.66 shows the parametric analysis of return loss with change in substrate thickness. Fig 4.67 shows the parametric analysis of return loss with change in width of the ground plane. Optimum dimensions can be noted from this study and which will be useful for fabricating the model accordingly.
71
Substrate Permittivity Effects on Wideband and UWB Antennas
Chapter 4
Fig 4.67 Parametric analysis of change in width of ground plane width for Trident shaped CPW fed Antenna
4.6.2 Design steps of trident shaped CPW fed antenna Frequency of operating band is taken into account, while deriving the design equations. Step by step procedure for the design of the antenna is paraphrased a) Design a 50Ω CPW line on a substrate with permittivity εr .calculate εeff using εeff = (εr+1)/2 where εeff is the effective permittivity of the substrate. Length of the substrate material L1=0.99 Length of the tapered step ground inner boundary L2=0.44 Length of the trident patch right arm with base L3=0.30 Length of the trident patch left arm L4 =0.23 Length of the ground plane adjacent to feed line L5 =0.19 Width of the substrate material W1=0.84 Width of the ground plane adjacent to feed line W2=0.27 Width of the trident shaped patch including slots W3=0.21 Width of the tapered ground plane left side groom W4=0.36 Gap between feed line and adjacent ground plane G=0.01 Width of the feed line W=0.11
72
Substrate Permittivity Effects on Wideband and UWB Antennas
Chapter 4
4.6. 3 Parametric analysis of Trident shaped CPW fed antenna with change in substrate permittivity
Fig 4.68 Return loss of trident shaped CPW fed antenna with change in permittivity
Figure 4.68 shows the return loss curve for different materials. It has been observed that between 7 to 9 GHz some materials are not giving the pass band characteristics. Fig 4.69 shows the impedance matching of different materials based antenna in the operating frequency band.
Fig 4.69 Impedance of trident shaped CPW fed antenna with change in permittivity
4.7 CPW fed Broadband Antenna
Due to high bandwidth, low profile, uniplanar geometry and ease of integration with monolithic microwave integrated circuits, coplanar waveguide fed antennas has got much attention in these days. Wideband and broadband characteristics can be obtained by taking different configurations, slotted apertures and defected ground 73
Substrate Permittivity Effects on Wideband and UWB Antennas
Chapter 4
structures in these models. Circular polarization is becoming popular in wireless communications to enhance the system performance. The operation principle of circular polarization is to excite two orthogonal modes with equal amplitude but in phase quadrature. It can be achieved by introducing some symmetric and asymmetric perturbations into a wide slot antenna. These perturbations can be obtained and implemented by slot configurations or feed lines.
Axial ratio bandwidth can be improved by using different techniques. By implanting a pair of grounded strips or three inverted L-Shaped grounded strips, the axial ratio bandwidth can be improved. A novel simple structured and circularly polarized antenna model is proposed in this work. Asymmetric perturbation is introduced by placing a slot at lower side of the design. The current antenna is fed by a wide tuning stub can provide circular polarization and impedance bandwidth. Fig 4.70 showing basic models and proposed final model geometries.
Fig 4.70 CPW fed broadband antenna, (a) Slotted broadband monopole, (b) Slotted broadband rectangular monopole, (c) Slotted ground broadband rectangular monopole
Fig 4.71 Fabricated model of slotted ground broadband rectangular monopole antenna
74
Substrate Permittivity Effects on Wideband and UWB Antennas
Chapter 4
4.7.1 CPW fed Broadband Antenna Configuration The proposed antenna is printed on FR4 substrate with dielectric constant of 4.4 and thickness of 1.6 mm. A 50 ohm CPW feeding line is connected as shown in the Fig 4.70. In order for the CP operation, an open slot having an open width is used at lower side of the model. This is an open slot with a configuration which is open along the ground plane in X and the CPW feeding line in Y directions. The new technique of this slotted configuration can provide the perturbation with magnetic current distributions in X and Y directions. This can generate the Circular polarization operation by exciting two orthogonal modes in X and Y directions with equal amplitude but in phase quadrature.
It is obvious that the change in ground plane would sensitively influence the performance of Circular Polarization operation. A wide tuning stub with length L and width W is used to improve the circular polarization. If open slot is taken at lower right side of the feeding line then circular polarized waves of opposite sense will be produced. In order for the AR bandwidth enhancement a wide tuning stub is used in the model as shown in the figure. 4.7.2 CPW fed broadband antenna parameters Fig 4.70 shows the proposed coplanar waveguide fed circularly polarized slot antennas of 50 ohm CPW feeding with signal strip and gaps have the width of 4 and 0.35 mm. The first model is a slotted broadband monopole with strip length of 47 mm and the second model is the slotted rectangular broadband monopole with wider tuning stub. For wider impedance bandwidth and axial ratio bandwidth, an asymmetric ground plane is used in this design. The third model is the modified model of 2 with slots on the ground plane on either side to the feed line. The length of the feed line to tuned for good circular polarization bandwidth. Fig 4.71 shows the fabricated prototype of proposed model. Fig 4.72 shows the simulated return loss Vs frequency characteristics of the three models. It has been observed from the results that an impedance bandwidth of 70% (2.2-5 GHz) from the first model, impedance bandwidth of 100% (2.2-7 GHz) from second model and 102% (2.2-7.2 GHz) from the third model is attained. Fig 4.73 reflects the measured and simulation return loss of proposed model 3. Simulation and measurement results are in good agreement 75
Substrate Permittivity Effects on Wideband and UWB Antennas
Chapter 4
with each other. To achieve efficient excitation and good impedance matching, parametric analysis on open slot parameters and tuning stub are carried out with Ansys HFSS EM-Simulator and presented in this work. To accurately realize the influence of these parameters, only one parameter at a time is varied by keeping others constant.
Fig 4.72 Reflection coefficient curves of three models of broadband antenna
Fig 4.73 Measured and Simulated Return loss curves for proposed slotted ground broadband rectangular monopole antenna
Fig 4.74 shows the reflection coefficient of the antenna with change in slot length L1. Bandwidth is almost same for change in slot length L1 from 16 mm to 20 mm, but with L1=18 mm the reflection coefficient is stable in the lower frequency band. Fig 4.75 shows the axial ratio curve with change in L1. Impedance bandwidth of 21.3% from L1=16 mm, 34.2% from L1=18 mm, 31.6% from L1=19 mm and 40% from L1=20 mm attained from this result.
76
Substrate Permittivity Effects on Wideband and UWB Antennas Return Loss
0 .00
Chapter 4 CPW fed Broadband Antenna
ANSO FT
-5.00
-10 .00
s11
-15 .00
-20 .00
-25 .00
Curve Info dB(St(1,1)) Setup1 : Sw eep1 L1='16mm' dB(St(1,1)) Setup1 : Sw eep1 L1='18mm' dB(St(1,1)) Setup1 : Sw eep1 L1='19mm' dB(St(1,1)) Setup1 : Sw eep1 L1='20mm'
-30 .00
-35 .00
-40 .00 2.00
3.00
4.00
5 .00 Frequency [GHz]
6.00
7.00
7.50
Fig 4.74 Return loss Vs Frequency, Variations in length of slot L1=16mm, 18mm, 19mm, 20mm 4
A x ia l R a t io in d B
3.5 3 2.5 2
L1=16 mm L1=18 mm L1=19 mm L1=20 mm
1.5 1
3
3.5
4
4.5
Frequency in GHz
Fig 4.75 Frequency Vs Axial Ratio, Variations in length of slot L1=16mm, 18mm, 19mm, 20mm
Fig 4.76 shows the reflection coefficient of the antenna with change in slot width W1. Maximum bandwidth of 5400 MHz from W1=9 mm and minimum of 5000 MHz from W1=11 mm is attained from the current study result. Fig 4.77 shows the axial ratio curve with change in length of the slot W1 and an impedance bandwidth of 35% is attained for the case of W1=13 mm. Return Loss
0.00
CPW fed Broadband Antenna
ANSOFT
-5.00
-10.00
s11
-15.00
-20.00 Curve Inf o dB(St(1,1)) Setup1 : Sw eep1 W1='7mm'
-25.00
dB(St(1,1)) Setup1 : Sw eep1 W1='9mm' dB(St(1,1)) Setup1 : Sw eep1 W1='11mm' dB(St(1,1)) Setup1 : Sw eep1 W1='13mm'
-30.00
-35.00 2.00
3.00
4.00
5.00 Frequency [GHz]
6.00
Fig 4.76 Variations in width of slot W1=7mm, 9mm, 11mm, 13mm
77
7.00
7.50
Substrate Permittivity Effects on Wideband and UWB Antennas
Chapter 4
4
A x ia l R a t io in d B
3.5 3 2.5 2
W1=7 mm W2=9 mm W1=11 mm W1=13 mm
1.5 1
3
3.5
4
4.5
Frequency in GHz
Fig 4.77 Frequency Vs Axial Ratio, Variations in length of slot W1=7mm, 9mm, 11mm, 13mm Return Loss
0.00
CPW fed Broadband Antenna
ANSOFT
-5.00
-10.00
s11
-15.00
-20.00 Curv e Inf o dB(St(1,1)) Setup1 : Sw eep1 subH='0.8mm' dB(St(1,1)) Setup1 : Sw eep1 subH='1mm' dB(St(1,1)) Setup1 : Sw eep1 subH='1.4mm' dB(St(1,1)) Setup1 : Sw eep1 subH='1.6mm'
-25.00
-30.00
-35.00 2.00
3.00
4.00
5.00 Frequency [GHz]
6.00
7.00
7.50
Fig 4.78 Variations in substrate height sub H=0.8mm, 1mm, 1.4mm, 1.6mm
Fig 4.78 shows the reflection coefficient with change in thickness of the substrate material. Generally 1.6 mm thickness FR4 material is widely available, but the result shows the superior performance for 0.8 mm thickness, so we fabricated the model on 0.8 mm thickness FR4 material, which gives impedance bandwidth of 88% in the frequency range 2-7.25 GHz with centre frequency of 4.625 GHz. Fig 4.79 shows the parametric analysis for reflection coefficient with change in width of the ground plane slot. The simulation result shows that with the optimized dimension of 8 mm, the antenna is resonating in the wide band. Fig 4.80 shows the axial ratio Vs frequency of the proposed model with change in width of the ground plane slot. Return Loss
0.00
CPW fed Broadband Antenna
ANSOFT
-5.00
s11
-15.00
Curve Inf o dB(St(1,1)) Setup1 : Sw eep1 sl='1mm' sw ='8mm' dB(St(1,1)) Setup1 : Sw eep1 sl='1.5mm' sw ='8.5mm' dB(St(1,1)) Setup1 : Sw eep1 sl='2mm' sw ='9mm'
-25.00
-35.00
-45.00 2.00
3.00
4.00
5.00 Frequency [GHz]
6.00
7.00
7.50
Fig 4.79 Variations in ground plane slot width for modified model of 8 mm, 8.5 mm and 9 mm
78
Substrate Permittivity Effects on Wideband and UWB Antennas
Chapter 4
4
A x ia l R a tio in d B
3.5
3
2.5
8 mm 8.5 mm 9 mm
2
1.5
3
3.5
4
4.5
Frequency in GHz
Fig 4.80 Frequency Vs Axial Ratio, Variations in ground plane slot width 8, 8.5 and 9 mm
Fig 4.81 Current Distribution over the slotted broadband rectangular monopole and slotted ground broadband rectangular monopole at 3 GHz
Fig 4.82 3D Radiation plot for slotted broadband rectangular monopole and slotted ground broadband rectangular monopole at 3 GHz
When the slotted ground broadband rectangular monopole antenna is resonating at 3GHz, large current density can be observed along the feed line compared to slotted broadband rectangular monopole antenna in Fig 4.81. So from the result, it has been observed that strong surface currents are distributed around the feed line to produce the resonance mode. The radiation patterns at 3 GHz in the xz-plane (H-plane) and yz-plane (E-plane) are plotted in Fig. 4.83. The radiation patterns are omnidirectional in the E-plane and monopole-like in the H-plane. The radiation characteristic of the
79
Substrate Permittivity Effects on Wideband and UWB Antennas
Chapter 4
antenna is stable within the operating bands, and the cross-polarization radiation patterns are relatively small in E-Plane.
Fig 4.83 CPW fed broadband antenna radiation pattern in E and H-Plane at 3 GHz
4.7.3 Design steps of CPW fed broadband antenna 1) Design 50 ohm CPW line on a substrate with permittivity εr. Calculate Ere using εeff = (εr+1)/2 where εeff is the effective permittivity of the substrate. 2) Design Length of the rectangular patch L1=0.47 λc L2=0.70 λc L3=0.32 λc L4=0.60 λc L5=1.17 λc L6=0.12 λc L7=1.25 λc 3) Width of the rectangular patch W1=0.27 λc W2=0.025 λc W3=0.11 λc W4=0.12 λc W=0.10 λc Width of the substrate Ws=1.25 λc Length of the substrate Ls=1.25 λc Where λc is the wavelength corresponding to centre frequency of the operating band. 4.7.4 Parametric analysis with change in substrate permittivity of CPW fed broadband antenna In order to justify the design equations, the antenna parameters are computed for different substrates and tabulated in below table.
80
Substrate Permittivity Effects on Wideband and UWB Antennas
Chapter 4
Table 4.7 Antenna W and G Variation with respect to different laminates Antenna Antenna Antenna Antenna Antenna 5 Anten Antenna 7 1 2 3 4 na 6 Laminate RTArlon Ultralam Polyester Plexiglass FR4 Alumina duroid AD-250 3850 5880 h 1.56 1.6 1.56 1.56 1.5 1.6 1.2 εr 2.2 2.5 2.9 3.2 3.4 4.4 10.2 εeff 1.6 1.75 1.95 2.1 2.2 2.7 5.6 W 4 3.675 3.35 3.025 2.7 2.375 2.05 G 0.35 0.375 0.4 0.475 0.45 0.475 0.5
W1 (mm)
Table 4.8 Antenna dimensions for different substrate materials Antenna 1 Antenna 2 Antenna 3 Antenna 4 Antenna 5 Antenna 6
Antenna 7
W2 (mm)
13.257
12.636
12.258
11.905
11.232
11
10.368
W3 (mm)
1,2275
1.17
1.135
1.1025
1.04
1
0.96
W4 (mm)
5.401
5.148
4.994
4.851
4.576
4.5
4.224
W (mm)
5.892
5.616
5.448
5.292
4.992
5
4.608
L1 (mm)
4.91
4.68
4.54
4.41
4.16
4
3.84
L2 (mm)
23.077
21.996
21.338
20.727
19.552
19
18.048
L3 (mm)
34.37
32.76
31.78
30.87
29.12
28
26.88
L4 (mm)
15.712
14.976
14.528
14.112
13.312
13
12.288
L5 (mm)
29.46
28.08
27.24
26.46
24.96
24
23.04
L6 (mm)
57.447
54.756
53.118
51.597
48.672
47
44.98
Ws(mm)
5.892
5.616
5.448
5.292
4.992
5
4.608
Ls(mm)
61.375
58.5
56.75
55.125
52
50
48
W1 (mm)
61.375
58.5
56.75
55.125
52
50
48
Fig 4.84 shows the parametric analysis with change in substrate permittivity on the proposed model. Except for Arlon and Alumina, the other materials based model is showing wide bandwidth. Ultralam 3850 (Liquid Crystal Polymer) substrate is showing superior performance over the other materials based antenna.
Fig 4.84 Parametric Analysis of return loss for slotted ground rectangular monopole antenna with change in substrate permittivity
81
Substrate Permittivity Effects on Wideband and UWB Antennas
Chapter 4
Fig 4.85 Parametric Analysis of VSWR for slotted ground rectangular monopole antenna with change in substrate permittivity
Fig 4.86 Radiation pattern of slotted ground rectangular monopole antenna in E and H Plane with change in substrate permittivity
4.8 CPW fed Curved Elliptical Monopole Antenna Compact size with simple structure and omnidirectional radiation pattern are the attracting features for the UWB antennas, especially for indoor applications. Most of the slot models are for enhancing the upper frequency band and for improving the lower frequency of the band. Many designs in the literature for monopole antenna with multiband characteristics are employing slots and slits in the radiator, the ground plane and in the feeder to achieve this. A novel curved elliptical monopole antenna is proposed here with simple ellipticalshaped slot patch to enhance the impedance bandwidth. In this model, by employing a pair of ellipse-shape-combined design a proper control on the lower and higher frequencies of the band is achieved. By this combination in the patch, additional resonances are excited, and hence the bandwidth is increased, especially at higher band. Fig 4.87 shows the CPW fed configuration of curved elliptical shaped monopole antenna. Antenna is constructed on FR4 substrate with dielectric constant of 4.4 and thickness 1.6 mm. A gap of 0.2 mm is taken between feed line and ground plane in 82
Substrate Permittivity Effects on Wideband and UWB Antennas
Chapter 4
the model. The width and length of the substrate are W=40 and L=44 mm respectively. Width of the feed line W4=3 mm, width of the patch element W3 is 32 mm, W1=3 mm and W2=14.5 mm.
Fig 4.87 CPW fed Curved Elliptical Monopole Antenna
4.8.1 CPW fed curved elliptical monopole antenna parameters The measured results are showing that the antenna is operating over the frequency band between 2-20 GHz with impedance bandwidth of 163% defined by s11<-10 dB. Figure 4.88 is showing Reflection coefficient of CPW fed curved elliptical monopole antenna in both simulation and measurement. Fig 4.89 shows 2:1 VSWR in the desired band.
Fig 4.88 Reflection Coefficient of CPW fed Curved Elliptical Monopole Antenna
Fig 4.89 VSWR of CPW fed Curved Elliptical Monopole Antenna
83
Substrate Permittivity Effects on Wideband and UWB Antennas
Chapter 4
The impedance bandwidth of the proposed model has been improved by adjusting the dimensions of semi-ellipse-shaped patch structure. Half ellipse shaped slot is located on the patch to access the multi- operating bands of the wireless communications systems. In most of the designs, the modified ground plane acts as an impedance matching circuit. Careful selection of gap between radiator and ground plane will improve the impedance matching, especially at the upper frequency band.
Fig 4.90 3D View Radiation of CPW fed Curved Elliptical Monopole Antenna
Fig 4.91 Parametric analysis of CPW fed curved elliptical monopole antenna with change in radius of upper ellipse
Fig 4.92 Input Impedance smith chart of CPW fed Curved Elliptical Monopole Antenna
Fig 4.91 showing parametric analysis of the model with change in the radius of the upper patch and Fig 4.92 showing the input impedance smith chart for the curved elliptical monopole. 84
Substrate Permittivity Effects on Wideband and UWB Antennas
Chapter 4
Fig 4.93 Current distribution of CPW fed Curved Elliptical Monopole Antenna at 11 GHz
Fig 4.94 Radiation pattern of CPW fed elliptical monopole antenna in E and H-plane at 11 GHz
Current distribution of the proposed model at 11 GHz is shown in Fig 4.93. Radiation pattern of the antenna in E and H-plane is shown in Fig 4.94. Omni directional radiation pattern can be observed in E-plane with cross polarization level less than -36 dB. Fig 4.95 showing the Frequency VS Gain plot and it has been observed that, a peak gain of 3.5dB in simulation and 3dB in the measurement is obtained from the current model. Gain Vs Frequency 4 Simulated Measured
3.5
G a in in d B
3 2.5 2 1.5 1
0
2
4
6
8
10 Frequency
12
14
16
18
20
Fig 4.95 Frequency Vs Gain of CPW fed Curved Elliptical Monopole Antenna
85
Substrate Permittivity Effects on Wideband and UWB Antennas
Chapter 4
4.8.2 Parametric analysis of CPW fed curved elliptical monopole antenna with change in substrate permittivity Parametric analysis for reflection coefficient with the change in substrate permittivity is presented in Fig 4.96. Except for Alumina substrate material, antenna is showing wideband characteristics for remaining materials. Fig 4.97 shows the radiation pattern in E and H plane. Change in permittivity is causing variation in the radiation pattern has been observed from the current study.
Fig 4.96 Parametric Analysis of return loss for CPW fed curved elliptical monopole antenna with change in substrate permittivity
4.97 Radiation pattern of CPW fed curved elliptical monopole antenna in E and H Plane with change in substrate permittivity
86
Substrate Permittivity Effects on Wideband and UWB Antennas
Chapter 4
4.9 Comparision of Different Wideband Antennas Table 4.9 Comparision of Different Wideband Antennas S. No
Antenna Model
Dimension s in mm
Bandwidth in GHz
1
Circular Monopole Circular Monopole with Tapered Step Ground Elliptical Monopole with Tapered Step Ground Rectangular Monopole with Tapered Step Ground Hexagonal Monopole with Tapered Step Ground Trident Shaped Antenna
32x26x1.6
9.1
Impeda nce Bandwi dth % 117
20x20x1.6
11.9
107
20x20x1.6
5.5
20x20x1.6
CPW Fed Broadband Antenna Curved Elliptical Monopole
2
3
4
5
6
7
8
Resonating Frequencies or Band
Gain in dB
Efficiency %
Suitable Substrate Material
3.2-12.3 GHz 5.1-17 GHz
4
80
Plexiglass
4.2
80
Arlon AD-250
39
11.5-17 GHz
2.8
80
FR4
9
86
6-15 GHz
3
70
RTDuroid 5880
20x20x1.6
8
73
7-15 GHz
2.5
70
Arlon AD-250
23x27x1.6
9
120
3-12 GHz
4
80
50x50x1.6
5
106
2.2-7.2 GHz
3.2
85
RTDuroid 5880 Ultralam 3850
40x44x1.6
18
163
2-20 GHz
3
88
Polyester
Designed wideband antennas performance characteristics are compared with each other and presented in Table 4.9. As per bandwidth is concerned curved elliptical monopole antenna is covering wideband of 18 GHz compared to other models. Compactness with wide bandwidth can be attained by circular monopole with tapered step ground antenna. Suitable substrate materials are proposed based on the important parameter bandwidth. 4.10 Chapter Summary In this chapter coplanar waveguide fed wideband antennas are presented and their performance characteristics are investigated with change in substrate permittivity. The optimized models are prototyped and their parameters are analyzed. A coplanar waveguide fed circular monopole antenna is designed to operate with wide bandwidth of 9.1 GHz. Simple, compact structure with omni directional radiation pattern and an average gain of 2.4 dB in the desired band makes this model suitable 87
Substrate Permittivity Effects on Wideband and UWB Antennas
Chapter 4
for wideband applications. Further to enhance the bandwidth, a modified model of circular monopole with tapered step ground is proposed. This modification attained the success in improving the bandwidth to 11 GHz in the frequency band of 5-16 GHz. Peak realized gain of 4.2 dB, efficiency of more than 80% and omni directional radiation pattern with low cross polarization in E-plane is attained from the tapered step grounded circular monopole antenna. An elliptical monopole with tapered step ground is considered in the next model by keeping the overall dimension of the antenna constant with respect to the earlier model. An improvement in the bandwidth of 500 MHz is obtained with tapered step ground in the frequency range of 11.5 to 16.5 GHz by comparing with normal elliptical monopole antenna. Average gain of 2.8 dB, efficiency above 80% and omni directional radiation pattern with low cross polarization is achieved from this design. A rectangular monopole antenna with tapered step ground is considered in the next case and with this model, a bandwidth of 8 GHz (6-14 GHz) is attained. An average gain of 3 dB with more than 70% efficiency and good radiation characteristics are obtained from this model. A hexagonal monopole antenna with tapered step ground is proposed in the next case and obtained an improvement of 2 GHz in the bandwidth with this case when compared with hexagonal monopole antenna. An average gain of 2.5 dB with antenna efficiency more than 70% is attained in the desired band. In all the tapered step ground models, the overall dimension of the antenna is kept constant of 20x20x1.6 mm. A compact trident shaped ultra wideband antenna is designed on FR4 material with dimensions of 23x27x1.6 mm. This models covering the bandwidth of 9 GHz (3-12 GHz) and impedance bandwidth of 120% with average gain of 2.5 dB. Another model of circularly polarized CPW fed broadband antenna is designed to operate between 2.4 to 7.4 GHz. An impedance bandwidth of 102%, axial ratio less than 3 dB and good radiation characteristics are obtained from this model. After that a CPW fed curved elliptical monopole antenna is designed to operate in the wide frequency band of 2-20 GHz, which covers S, C, X and Ku bands. Peak realized gain of 3 dB and almost omni direction in radiation is obtained from this model.
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