CHAPTER 6 MICROSTRIP RECTANGULAR PATCH ARRAY WITH FINITE GROUND PLANE EFFECTS

Transcription

1 107 CHAPTER 6 MICROSTRIP RECTANGULAR PATCH ARRAY WITH FINITE GROUND PLANE EFFECTS 6.1 INTRODUCTION The finite ground plane effects of microstrip antennas are one of the issues for the wireless mobile communication applications given by Huang (1983). To achieve exactly the specified beamwidth and front-to-back ratio, especially for single element antennas or moderate size arrays, it is a difficult task for the antenna designer with low cost, from Gonca Cakir and Levent Sevgi (2005). It is very difficult to achieve the more accurate, efficient, and flexible computation methods including the finite ground plane diffraction. The spectral domain method of moment, from Ramesh and Yip (2003), formulations based on the Green s functions of an infinite grounded dielectric slab yield, in most cases, accurate results for input impedances and resonant frequencies. However, they will not allow us to predict the far field radiation pattern correctly if the microstrip structure is finite. The simulation package, IE3D (2003), uses the Methods of Moment (MOM) technique to evaluate the finite ground plane effects. In this chapter, the various microstrip arrays are designed with finite ground plane effects and compared with the infinite counterpart. From Lolit Kumar Singh et al (2012), the conventional microstrip antennas assume infinitely large ground plane dimensions and thus they are large in size. The size of ground plane is the limitation of antenna

2 108 characteristics, like distortion of radiation patterns, reduction of gain and compactness. Erik Lier and Kurt R. Jakobsen (1983) analyzed the rectangular microstrip patch antenna extensively with regard to variation in its input impedance and resonant frequency, both for infinite and finite ground plane dimensions. Bhattacharyya A.K. (1990) reported an analytical technique for the finite ground plane effect on radiation characteristics of a microstrip antenna. The simulation results show that the gain of a circular patch antenna varies with the ground plane size; also it is observed that the maximum gain is achieved when the radius of the ground plane is It was also found that the input impedance changes widely with the ground plane dimension and decreases with increase in the ground plane radius. The induced current on the ground plane, derived therein, is an approximate one and does not include the edge effects. Bhattacharyya A.K. (1991) reported the effects of ground plane truncation on the impedance of a patch antenna. The input impedance was found to vary widely with the ground plane dimensions, especially for electrically thick substrates. In this chapter, the effect of finite ground plane has considered for the linear and circular arrays with eight elements. 6.2 EM OPTIMIZATION WITH FINITE GROUND PLANES In IE3D, on MGRID, structures are not described as parameterized objects. All structures are described by polygons and polygons are described by vertices. To change the shape of a structure, we need to change the locations of vertices. It is necessary to identify which vertices to be adjusted to change the values L and D. IE3DLIBRARY has complete parameterized structure objects with equation-based dimensions. It is extremely flexible in optimizing structures. Using this approach, a single patch has been designed

3 109 and further the same is used to design the microstrip array of various configurations. Figure 6.1 Plot of reflection coefficient without optimization Figure 6.2 Plot of reflection coefficient of single patch with optimization Figures 6.1 and 6.2, shows the reflection coefficient (S 11 ) plot of single patch microstrip antenna without and with optimization respectively. It was observed that the reflection coefficient (S 11 ) of - 47 db has been achieved at 2.4 GHz by optimized patch antenna which can be used for WiMAX applications.

4 Optimization of Single Patch The Figure (6.3 and 6.4) shows the change in feed patch of the single rectangular patch after optimization. The Dielectric substrate glass epoxy (FR4) with r =4.6 and tan = is used here; hence its physical size is very small compared to other dielectrics. The loss tangent is a metric of the quantity of the electrical energy which is converted to heat by a dielectric. The lowest possible loss tangent maximizes the antenna efficiency. If the dielectric constant r is larger, the smaller element size to be achieved, by Odeyemi et al (2011). Figure 6.3 Single patch without optimization Figure 6.4 Single patch with optimization

5 111 The variable chosen here for optimization are length of the patch and the depth of the inset feed given to the patch, from Jagdish (2010). The corresponding vertices are selected and declared as variables for optimization in Optimization variable definition dialog. The value of length is varied from -100mils to 100mils from its default value. The value of inset depth is varied from -150mils to 150mils from its default value. The optimization set up is simulated for the resonant frequency 2.4 GHz with optimization goals as real and imaginary part of s-parameter. 6.3 ARRAY CONFIGURATIONS AND ITS SIMULATIONS USING IE3D The microstrip linear, planar and circular microstrip arrays with and without finite ground plane effects are simulated and the results were compared at 2.4 GHz frequency Linear Array An 8 element linear array having infinite ground plane with individual feed is designed using the optimized single patch, as shown in Figure 6.5, and simulated using IE3D. From Chapter 3, Figures 3.3 to 3.6, the spacing between the array elements with spacing greater than 0.5 reduces the spatial correlation effects. Since the spacing between the elements in this array is kept as 0.7, to reduce the mutual coupling between array elements. Figure 6.5 Linear array with infinite ground plane and individual feed

6 112 Figure 6.6 3D radiation pattern of 8-element linear array Figure 6.7 2D radiation pattern of 8-element linear array Figures 6.6 and 6.7 show the 3D and 2D radiation pattern of 8-element single feed linear array with infinite ground plane. The return loss of -38 db (from Figure 6.8) has been achieved with infinite ground plane.

7 113 Figure 6.8 S-parameter vs. frequency plot for 8-element linear array Figure 6.9 Total field gain vs. frequency plot for 8-element linear array

8 114 Figure 6.9 provides the variation of the gain (dbi) with respect to frequency (GHz) for the 8 element linear array. The total field gain of around 16.5 dbi is obtained at the design frequency 2.4 GHz. Since giving individual feeding to each element will increase the source cost, the previously designed 8-element array is modified to include a single feed, as shown in Figure 6.10, instead of individual feed. This type of feeding is called corporate feed from the literature Muhammad Mahfuzul Alam et al (2009). Figure 6.10 Linear 8 element array with corporate feed Figure D radiation pattern of linear array with corporate feed

9 115 Figure D radiation pattern of linear array with corporate feed The 2D and 3D radiation pattern of the 8 element linear array with corporate feed is shown in Figures 6.11 and 6.12 respectively. By comparison, the corporate feed linear array gain abruptly varies with respect to frequency (from Figure 6.14) and provides14.8 dbi gain, which is 2.5 dbi lesser than the individual feed array. Figure 6.13 S-parameter vs. frequency plot for 8-element linear array with corporate feed

10 116 Figure 6.14 Total field gain vs. frequency plot for 8 element linear array with corporate feed The return loss characteristics of the array as shown in Figure 6.13, and it was observed that the return loss of corporate feed is -17 db less than the individual feed. Hence achieving impedance matching is difficult in corporate feed than individual feed system. A microstrip patch or array with infinite ground plane is a theoretical antenna, but patch with finite ground plane is of practical interest. Here the previously designed 8 - element array with corporate feed, as shown in Figure 6.15, is modified to have a finite ground plane using IE3D. Figure 6.15 Linear 8 element array with corporate feed having finite ground plane

11 117 Figure D radiation pattern of linear 8 element array with corporate feed having finite ground plane Figure D radiation pattern of linear 8-element array with corporate feed having finite ground plane

12 118 The 2D and 3D radiation pattern of 8 element linear array with corporate feed under finite ground plane is shown in Figures 6.16 and 6.17 respectively. From the Figure 6.17, it was observed that there is a back radiation towards the ground plane and hence the return loss of db is achieved at GHz, but the designed frequency is 2.4 GHz. The finite ground plane effect reduces the reflection co-efficient (S 11 ) from -21 db to -17 db (from Figure 6.18), which requires an impedance matching network to reduce the reflections created by the ground plane. This array maintains the gain (14 dbi) but the radiation occurred at GHz instead of 2.4 GHz, as shown in Figure 6.19, hence the operating bandwidth of the antenna get reduced. Figure 6.18 S-parameter vs. frequency plot for linear 8 element array with corporate feed having finite ground plane

13 119 Figure 6.19 Total field gain vs. frequency plot for linear 8-element array with corporate feed having finite ground plane When comparing the gain of individual feed linear array and corporate feed linear array, there is a decrease of 2 dbi in the latter case. This is acceptable when compared with the source cost of both the arrays Planar Array Planar arrays are more versatile and can provide more symmetrical patterns with lower side lobes. In addition, they can be used to scan the main beam of the antenna towards any point in space. Here planar array having (8x2) 16 elements with corporate feed (2 feeds) is designed for 2.4 GHz and simulated using IE3D as shown in Figure 6.20 with infinite ground plane.

14 120 Figure 6.20 Planar 16 element array with corporate feed having infinite ground plane Figure D radiation pattern of planar 16 element array with corporate feed having infinite ground plane The simulation results of the (8x2) planar array with uniform spacing are depicted from the Figures 6.21 to It is noticed that the gain (18.7 dbi) of this array has been improved but numbers of side lobes are also increased when compared with 8 element linear array. The return loss of -21 db is obtained at 2.41 GHz (from Figure 6.23).

15 121 Figure D radiation pattern of planar 16 element array with corporate feed having infinite ground plane Figure 6.23 S-parameter vs. frequency plot for planar 16 element array with corporate feed having infinite ground plane

16 122 Figure 6.24 Total field gain vs. frequency plot for planar 16 element array with corporate feed having infinite ground plane Circular Array Here a circular array of 8 elements with individual feed and having finite ground plane, as shown in Figure 6.25, is designed for 2.4 GHz and simulated using IE3D by Loannides and Balanis (2005). Figure 6.25 Circular 8 element array with individual feed having finite ground plane

17 123 Figure D radiation pattern of circular 8 element array with Individual feed having finite ground plane Figure D radiation pattern of circular 8 element array with individual feed having finite ground plane Figures 6.26 and 6.27 shows the 2D and 3D radiation pattern of 8 element circular array with individual feed under finite ground plane respectively. From the Figure 6.27, it is observed that there is a back radiation towards the ground plane.

18 124 Figure 6.28 S-parameter vs. frequency plot for circular 8 element array with individual feed having finite ground plane Figure 6.29 Total field gain vs. frequency plot for circular 8 element array with individual feed having finite ground plane Hence the return loss of -34 db is achieved at 2.3 GHz, but the designed frequency is 2.4 GHz. The finite ground plane effect increases the reflection coefficient (S 11 ) to -34 db (from Figure 6.28), unlike linear arrays,

19 125 it does not require an impedance matching network due to its configuration. This array got the gain (12.5 dbi) but the radiation occurred at 2.5 GHz instead of 2.4 GHz, as shown in Figure 6.29, hence the operating bandwidth of the antenna get reduced. Table 6.1 Comparisons of optimized array configurations using IE3D with finite ground plane effects Properties Linear Array (Finite Ground) Uniform Planar Circular (Finite Ground) Frequency(GHz) Incident Power(W) Input Power (W) Radiated Power(W) Average Radiated Power (W/s) e Radiation Efficiency (%) Antenna Efficiency (%) Conjugate Match Efficiency (%) Voltage Source Efficiency % Total Field Properties Gain (dbi) Directivity(dBi) dB Beamwidth (deg) ( , ) ( , ) ( , ) Conjugate Match Gain (dbi) Voltage Source Gain (dbi) Radiated Power in Whole Space (w) Radiated Power in Upper Space (w) Radiated Power in Lower Space (w) e Radiation Efficiency in Whole Space (%) Radiation Efficiency in Upper Space (%) Radiation Efficiency in Lower Space (%) e

20 RESULTS AND DISCUSSION The optimized single patch has required return loss for the WiMAX application but has poor directivity and gain. From Table 6.1, it can be inferred as follows: The different microstrip array configurations discussed found to have the necessary gain and directivity (18 dbi) along with optimum return loss (below -25 db) for WiMAX application. The individually fed linear array with 8 elements has appreciable gain (8.9 dbi) and directivity (15.11 dbi) but has N-1 (7 numbers) side lobes. In individually fed linear array each element has to be excited separately with individual source. The linear 8-element array with corporate feed (single feed) is economical when compared with individually fed array in case excitation sources. The linear 8-element array with corporate feed on a finite ground plane is more practical in case of real time implementation. The gain (8.9 dbi) is comparable with previous case. Even though the linear array has higher directivity (15.11 dbi), it produces a flat beam (7.9 and 58.6 degrees, elevation and azimuth sides respectively) which cannot be used for smart WiMAX applications. The simulated 16-element planar array is found to have improved performance in case of gain (18.7 dbi) and

21 127 directivity (17.8 dbi) but the number of side lobes increased when compared with 8-element linear array. Circular configurations have highest radiated power (0.05 Watts) and radiation efficiency (78.4 %) than any other configurations. By comparing all the configurations, it is found that 8-element circular array has an optimum directivity (14.4 dbi) and beamwidth (27.7 and 30.6 degrees, elevation and azimuth sides respectively) which can be used for WiMAX.