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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

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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 0.6 0. 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

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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 (S11) plot of

single patch microstrip antenna without and with optimization respectively. It

was observed that the reflection coefficient (S11) of - 47 dB has been achieved at

2.4 GHz by optimized patch antenna which can be used for WiMAX applications.

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6.2.1 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 = 0.001 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

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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.

6.3.1 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

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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.

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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

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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 6.11 3D radiation pattern of linear array with corporate feed

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Figure 6.12 2D 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

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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

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Figure 6.16 3D radiation pattern of linear 8 element array with

corporate feed having finite ground plane

Figure 6.17 2D radiation pattern of linear 8-element array with

corporate feed having finite ground plane

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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 -17.5 dB is achieved at

2.465 GHz, but the designed frequency is 2.4 GHz. The finite ground plane

effect reduces the reflection co-efficient (S11) 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

2.465 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

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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.

6.3.2 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.

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Figure 6.20 Planar 16 element array with corporate feed having infinite

ground plane

Figure 6.21 3D 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 6.24. 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).

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Figure 6.22 2D 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

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Figure 6.24 Total field gain vs. frequency plot for planar 16 element

array with corporate feed having infinite ground plane

6.3.3 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

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Figure 6.26 3D radiation pattern of circular 8 element array with

Individual feed having finite ground plane

Figure 6.27 2D 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.

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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 (S11) to -34 dB (from Figure 6.28), unlike linear arrays,

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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) 2.4 2.4 2.4

Incident Power(W) 0.08 0.08 0.08

Input Power (W) 0.00328395 0.00307067 0.00643603

Radiated Power(W) 0.00239989 0.000488037 0.00505084

Average Radiated Power (W/s) 0.000190977 3.88368e-005 0.000401933

Radiation Efficiency (%) 73.0794 15.8935 78.4775

Antenna Efficiency (%) 23.9989 2.44019 6.31355

Conjugate Match Efficiency (%) 36.5397 7.94676 39.2388

Voltage Source Efficiency % 11.4004 14.7947 8.36868

Total Field Properties

Gain (dBi) 8.92185 18.74999 2.42232

Directivity(dBi) 15.1199 17.8758 14.4196

3dB Beamwidth (deg) (7.91308,

58.6623)

(10.0932,

44.7466)

(27.7746,

30.6283)

Conjugate Match Gain (dBi) 10.7476 6.87766 10.3567

Voltage Source Gain (dBi) 5.6891 9.57682 3.6461

Radiated Power in Whole Space (w) 0.00239989 0.000488037 0.00505084

Radiated Power in Upper Space (w) 0.00220806 0.000488037 0.00480238

Radiated Power in Lower Space (w) 0.000191827 2.14112e-013 0.000248458

Radiation Efficiency in Whole Space (%) 73.0794 75.8935 78.4775

Radiation Efficiency in Upper Space (%) 67.238 69.7263 74.6171

Radiation Efficiency in Lower Space

(%)

5.84133 6.97281e-009 3.86043

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6.4 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

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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.