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CHAPTER 7

DESIGN AND SIMULATION OF PIN FEED

MICROSTRIP PATCH ARRAY

7.1 INTRODUCTION

This chapter describes different feeding techniques applicable to

Microstrip patch antenna array which is one of the important aspects and the

effect of dielectric constant in Microstrip patch design.

The aim of this work is to design and simulate a Pin-fed rectangular

Microstrip Patch array antennae and study the effect of antenna dimensions,

Length (L), Width (W) and substrate parameters relative Dielectric constant

r) and substrate thickness (t) on the Radiation parameters of Gain, Return

Loss and Bandwidth.

The Simulations have been done by using the High Frequency

Electromagnetic Simulation Software – FEKO Suite 6.0 (2010). Here two

different substrates are considered for simulation with dielectric constant 2.2

and 4.4 and the simulation results were compared. The procedure for

designing a rectangular microstrip patch antenna is explained and a compact

rectangular microstrip patch antenna is designed to be used in handheld

devices like cellular phones for data communication. Finally, the results

obtained from the simulations are demonstrated.

For a rectangular patch, the length L of the patch is usually

0.3333 o< L < 0.5 o, where, o is the free-space wavelength. The patch is

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selected to be very thin such that t << o (where, t is the patch thickness). The

height ‘h’ of the dielectric substrate is usually 0.003 o h 0.05 o. The

dielectric constant of the substrate ( r) is typically in the range of 2.2 r 12.

7.2 MICROSTRIP PIN FEED

The Coaxial feed or pin feed is a very common technique used for

feeding Microstrip patch antennas.

Figure 7.1 Pin feed rectangular microstrip patch antenna

As seen from Figure 7.1, the inner conductor of the coaxial

connector extends through the dielectric and is soldered to the radiating patch,

while the outer conductor is connected to the ground plane.

Constantine Balanis (1997) discussed with respect to microstrip

feed techniques, the thickness of the dielectric substrate increases, surface

waves and spurious feed radiation also increases, which hampers the

bandwidth of the antenna. The feed radiation also leads to undesired cross

polarized radiation. The main advantage of Pin feed scheme is that the feed

can be placed at any desired location inside the patch, in order to match with

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its input impedance. This method of feed is easy to fabricate and has low

spurious radiation. It can be inferred from the above discussion that a thick

dielectric substrate, provides broad bandwidth.

7.3 MICROSTRIP PATCH ANTENNA DESIGN USING PIN

FEED TECHNIQUE

In this chapter, the procedure for designing a rectangular microstrip

patch antenna using PIN feed technique is explained. Next, a compact

rectangular microstrip patch antenna is designed for use in Wi-MAX

applications. The transmission line model is applicable to infinite ground

planes only. However, for practical considerations, it is essential to have a

finite ground plane.

Ghassemi and Sh Mohanna (2009) proved that similar results for

finite and infinite ground plane can be obtained if the size of the ground plane

is greater than the patch dimensions by approximately six times the substrate

thickness all around the periphery.

7.3.1 Design Specifications

The three essential parameters for the design of a rectangular

Microstrip Patch Antennas are the frequency of operation (f0), dielectric

constant of the substrate ( r) and the height of the substrate (h). The

parameters used in this work are described below:

Frequency of Operation (f0)

The Mobile Communication System (Wi-MAX) uses the frequency

range from 2100-5600 MHz. Hence the antenna designed must be able to

operate in this frequency range. The resonant frequency selected for the

proposed design is 3.5 GHz.

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Dielectric Constant of the Substrate ( r)

The dielectric material selected for our design is Teflon and Glass

Epoxy which has dielectric constant of 2.2 and 4.4 respectively. The

properties of the high frequency (2.1 GHz to 5.6 GHz) printed circuit board

material are given in Table 7.1.

Table 7.1 Properties of dielectric substrate materials

S.No. Properties (Units) Glass Epoxy

FR-4Teflon

1 Dielectric Constant 4.4 2.2

2 Loss Tangent 0.025 0.0004

3 Flex Modulus (GPa) 17 2.1

4 Flex Strength (MPa) 483 ---

5 Tensile Strength (MPa) 345 49

6 Density (g/cm3) 1.82 2.23

Table 7.1 shows the parameters which are important in microwave

circuit board substrates. From Thomas Laverghetta (2000), the substrate

materials must meet minimum physical characteristics, in general have a

flexural modulus of greater than 2.5 GPa and have a suitable machinability

for cutting and drilling small via holes. The dielectric constant and loss

tangent must both be low, below 3.0 and 0.002 respectively, as they determine

the energy loss and frequency range of the substrate.

The coefficient of thermal expansion must be low to prevent cracking

in via holes and to ensure the integrity of attachments to components. Also,

the material must be able to withstand the thermal strain of soldering

processes and high use temperatures. FR-4 (glass-epoxy) and reinforced

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Poly-tetrafluoroethylene (Teflon) composite is almost satisfies the electrical

and mechanical properties; hence this thesis considered FR-4 and Teflon as a

substrate material for designing MIMO antenna arrays using microstrip

antennas. Substrates that are used for high frequency circuits range in

dielectric constant from 2.17 to 5.2, and loss tangent from 0.0009 to 0.025.

For materials with low dielectric, the flexural modulus is low, just above 2

GPa, the density is high, and the cost is very high. In addition, in the case of

PTFE and other materials, special processing is required.

Height of Dielectric Substrate (h)

The thin dielectric substrates are desirable for microwave circuits

because they require tightly bond fields to minimize undesirable radiation and

coupling which lead to smaller element size. David B. Davidson (2005) stated

that it is possible to get the height of the dielectric substrates (FR4) from

1.6 mm through chemical etching techniques.

For the Microstrip patch antenna to be used in cellular phones, it is

essential that the antenna is not bulky. Also, the directivity is not a strong

function of the substrate height; as long as the height is maintained

electrically small (that is h/ 0). Hence, the height of the dielectric substrate is

selected as 2.87 mm. Hence, the essential parameters for the design are:

f0 = 3.5 GHz, r = 2.2 and 4.4, h = 2.87 mm

7.3.2 Design Procedure

Step 1: Calculation of the Width (W):

The width of the Microstrip patch antenna is given as:

r0

2

cW

12f

(7.1)

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Substituting, c = 3.00e+008 m/s,

r = 2.2 and f0 = 3.5 GHz, then

W = 0.0340 m = 34 mm

Step 2: Calculation of Effective dielectric constant ( reff):

The effective dielectric constant is:

r rreff

h1 12

W

1 1

2 2 (7.2)

Substituting, r = 2.2,

W = 34.0 mm and h = 2.87 mm, then

reff = 2.452

Step 3: Calculation of the Effective length (Leff):

The effective length is:

ff

0 reff

cLe

2f (7.3)

Substituting, reff = 2.452,

c = 3.00e+008 m/s and f0 = 2.4 GHz, then

Leff = 0.02736 m = 27.36 mm

Step 4: Calculation of the length extension ( L):

The length extension is:

reff

reff

W0.3 0.264

hL 0.412h

W0.258 0.8

h

(7.4)

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Substituting, reff = 2.452,

W = 34.0 mm and h = 2.87 mm, then

L = 1.4201 mm

Step 5: Calculation of actual length of patch (L):

The actual length is obtained by:

L = Leff - 2 L (7.5)

Substituting, Leff = 27.36 mm

L = 1.4201 mm, then

L = 0.0245 m = 24.50 mm

Thus the width and Length of the rectangular microstrip patch have

been calculated by using the above equations and tabulated (Table 7.2). The

Pin feed technique chosen for the patch design and the location of the feed

considered at 8.9 mm and the radius of the feed is 0.65mm.

Table 7.2 Comparison of patch parameters for different dielectric

materials

Patch Parameters

f0=3.5 GHz,

h=2.87 mm,

r = 2.2

f0=3.5 GHz,

h=2.87 mm,

r = 4.4

Patch Width (W) 34 26

Patch Length (L) 24.50 16.15

Substrate with lower dielectric constant leads to a larger antenna

size. From the Table 7.2, it is clear that a substrate with a high dielectric

constant ( r = 4.4) reduces the dimensions of the antenna by 49.5 %. To

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design a compact microstrip patch antenna, substrates with higher dielectric

constants must be used.

7.4 SIMULATION SETUP AND RESULTS

The software used to model and simulate the Microstrip patch

antenna is High Frequency Electromagnetic Simulation Software – FEKO

Suite 6.0. It has been widely used in the design of MICs, RFICs, patch

antennas, wire antennas, and other RF/wireless antennas. It can be used to

calculate and plot the S11 parameters, VSWR, current distributions as well as

the radiation patterns. Figure 7.2 shows the Pin fed Microstrip patch antenna

designed using FEKO suite 6.0.

Figure 7.2 Microstrip patch antenna (PIN feed technique) designed

using FEKO 6.0

7.4.1 Return Loss

The pin feed is designed at the distance of 8.9 mm and the radius is

0.65mm. A frequency range of 3.2 to 3.7 GHz is selected to obtain accurate

results. The center frequency is selected as the one at which the return loss is

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minimum. The bandwidth can be calculated from the return loss (RL) or

VSWR plot. The bandwidth of the antenna can be said to be those range of

frequencies over which the RL is greater than -9.5 dB (corresponds to a

VSWR of 2 which is an acceptable value) shown in Figure 7.3. For the

substrate Teflon ( r = 2.2) a Return Loss of -26 dB is obtained at the resonant

frequency of 3.5 GHz as shown in Table 7.2. But for the substrate Glass

Epoxy, Return Loss of -33 dB is obtained only at the frequency of 3.63 GHz

but which is better than value (-28 dB) obtained by Ghassemi and Mohanna

(2009).

Figure 7.3 S-parameter plot for return loss vs. frequency ( r = 2.2)

For the substrate Teflon, Figure 7.4 shows that the input impedance

of the port was matched with the normalized impedance value of 50 at the

frequency 3.5 GHz, which is the operating frequency selected for this

analysis. But the input impedance of Glass Epoxy is matched at 70 which is

not a desirable one.

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Figure 7.4 Z-parameter plot for input impedance ( r = 2.2)

7.4.2 Radiation Pattern Plots

Since a microstrip patch antenna radiates normal to its patch

surface, the elevation pattern for = 0 degrees and = 90 degrees would be

important.

Figures 7.5 and 7.6 below shows the gain of the antenna at

3.5 GHz for = 0 degrees and = 90 degrees for the substrate Teflon. The

maximum gain is obtained in the broadside direction and this is measured to

be 5.42 dBi for both, = 0 degrees and = 90 degrees.

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Figure 7.5 Elevation pattern for = 0 degrees and = 90 degrees for

Teflon ( r = 2.2) – Polar Plot

Figure 7.6 Elevation pattern for = 0 degrees and = 90 degrees for

Teflon ( r = 2.2) –Cartesian Plot

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Figure 7.7 Elevation pattern for = 0 degrees and = 90 degrees for

glass Epoxy ( r = 4.4)

As shown in Figure 7.7, which gives elevation pattern, the

maximum gain is obtained in the broadside direction for Glass Epoxy and is

measured to be 3.55 dBi for both, = 0 degrees and = 90 degrees.

7.4.3 VSWR Plot and Operating Bandwidth

When a transmitter is connected to an antenna by a feed line, the

impedance of the antenna and feed line must match exactly for maximum

energy transfer from the feed line to the antenna. When an antenna and feed

line do not have matching impedances, some of the electrical energy cannot

be transferred from the feed line to the antenna. Energy not transferred to the

antenna is reflected back towards the transmitter. It is the interaction of these

reflected waves with forward waves which causes standing wave patterns.

Ideally, VSWR must lie in the range of (1-2) which is achieved (Figure 7.8)

for the frequency range of 3.48 to 3.52 GHz.

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Figure 7.8 VSWR vs. frequency plot

The bandwidth of the antenna for these simulation results is

calculated as shown in Figure 7.8 to be 40 MHz for the substrate with r = 2.2,

and it is measured 50 MHz for the substrate with r = 4.4. The simulation

results are compared as per Table 7.3. The Gain get increased for the

microstrip patch array configurations.

Table 7.3 Comparisons of simulation results for different dielectric

materials

Patch

Parameters

Return Loss

S11(dB)

Gain

(dBi)

Bandwidth

(MHz)

Impedance

(Ohms)

Teflon

r = 2.2 -26 5.42 40 50

Glass Epoxy

r = 4.4 -33 3.55 50 70

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From the simulation results compared (Table 7.3), the low

dielectric constant substrate Teflon ( r = 2.2) generally needs to be preferred

for maximum radiation. Since this provides better efficiency, narrow

bandwidth and better radiation. By using Pin fed technique, feed can be

placed at any desired location inside the patch in order to match with its input

impedance. It provides Maximum gain and significant Return Loss. This Pin

fed method is easy to fabricate and has low spurious radiation.

7.5 PIN FED MICROSTRIP RECTANGULAR PATCH ARRAY

ANTENNAE

7.5.1 Introduction

Wi-MAX supports for MIMO adaptive antennae to provide good

Non-line-of-sight characteristics than SISO and MISO antennae systems. The

development of adaptive antennae includes the design of array antennae,

optimizing the array antennae parameters and the development of adaptive

array algorithms.

In this chapter, the linear and planar microstrip array configurations

were simulated using Matlab software for application at

3.5 GHz and the design process conceded using high frequency

electromagnetic simulation software FEKO. In this analysis, the parameters

such as gain, directivity and 3-dB beamwidth are compared quantitatively to

propose the array configuration which promises very narrow beamwidth and

leads to substantial improvement of capacity in wireless networks.

7.6 EXPERIMENTAL RESULTS USING MATLAB

The design and simulations of microstrip array configurations for

3.5 GHz with different element spacing are performed. The various array

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parameters namely, gain, half power beamwidth, number of side lobes and

main lobe to side lobe ratio are simulated and analyzed using Matlab 7.5

version platform. The simulation results for the Linear and Planar array are

given below: Each array configuration shown are with normalized array factor

plot and the corresponding polar plot.

7.6.1 Microstrip Linear Array

In this case, the desired angle is arriving at 30 degrees and the

interfering signal arriving at angle of 60 degrees. Figure 7.9 shows the

rectangular plot of linear array using array factor plot. Figure 7.10 shows the

polar plot for normalized array factor.

The maximum gain is at the angle of 30 degrees. While the angle of

desired user is makes the adaptive algorithm steer the beam towards the user,

adopted in the literature Sarkar (1989). So the continuous tracking can be

achieved. It produces the narrow beam towards the user so that the bandwidth

is also effectively utilized, from Hayssam Dahrouj and Wei Yu (2010).

Figure 7.9 Normalized array factor plot (Linear array)

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Figure 7.10 Polar plot for normalized array factor (Linear array)

7.6.2 Microstrip Planar Array

The simulation results for the planar array are shown in this section.

Figure 7.11 is drawn for the case where the desired signal is arriving at an

angle of 60 degrees and interference is at 30 degrees considered from Tsoulos

(1999). The array factor plot is shown Figure 7.12.

Figure 7.11 Polar plot for normalized array factor (Planar array)

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From the Figure 7.11, it is clear that the maximum gain is produced

at the desired angle 60 degrees, and nulls at the interference angle 30 degrees.

Here, the spacing between the elements is assumed to be half of one

wavelength. While the angle of desired user varies, the adaptive algorithm

steer the beam towards the user.

7.7 DESIGN AND SIMULATION OF MICROSTRIP PATCH

ARRAY

The single microstrip patch antenna has been designed and

simulated using high frequency electromagnetic simulation software - FEKO

suite 6.0. The low dielectric constant substrate Teflon ( r = 2.2) generally

needs to be referred for maximum radiation. Since this provides better

efficiency, narrow bandwidth and better radiation. Alka Verma and Neelam

Shrivastava (2011) discussed that the Pin fed method is easy to fabricate and

has low spurious radiations. The linear and planar microstrip array

configurations are designed using the array factor approach and simulated

using FEKO software.

7.7.1 Simulation Results Using FEKO

The Pin fed Rectangular Microstrip patch linear array have been

designed by using FEKO Suite 6.0 version (2010). The designed linear array

is shown in the Figure 7.12. The number of elements for the linear array is 5.

Figure 7.12 Microstrip patch linear array

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Planar array can be designed as rectangular grids of patches 2×2,

3×3, 4×4 etc. The 2×2 planar array has been designed with the same patch

width and patch length considered in linear array. The designed 2×2 planar

using FEKO is shown in Figure 7.13. The simulation results of linear array

and planar array are discussed in following subsections. Return loss, input

impedance and gain parameters are discussed for linear and planar arrays. The

simulation results are tabulated and compared, then discussed with the

simulation results obtained using Matlab.

Figure 7.13 Microstrip patch planar array

7.7.2 Return Loss and Gain

Return loss for linear and Gain for linear, planar arrays are shown

in Figures 7.14 and 7.15 respectively.

Figure 7.14 Return loss - Linear array

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Figure 7.15 Gain Plot- linear array

Figure 7.16 Gain Plot - Planar array

The return loss of linear array is optimum for fourth element is

-20 dB and for planar array is -14.5 dB. The return loss can be achieved more

if the numbers of patches are more. Thus the above Figure 7.15 and 7.16

shows the gain plots obtained for Linear and Planar arrays respectively.

Maximum gain achieved for the linear array of 5 patches to be 14 dBi. For the

2×2 planar array, the gain is measured to be 8.5 dBi.

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From these results, it is clear that the gain is directly proportional to

the number of patches used in the microstrip patch design. The consideration

here is 5 patches in linear array and 4 patches in planar array, more number of

patches to be designed for the practical implementations, from Michail

Matthaiou (2010). Similar manner the input impedance and the VSWR are

obtained approximately equals to the optimum values.

7.8 RESULTS AND DISCUSSION

From the simulation results compared as in Table 7.4, it is clear that

the linear array configuration gives narrow beamwidth and optimal gain

suitable for the Wi-MAX applications with the minimum number of side

lobes compared with Jagdish Rathod (2010). Since it gives narrow beam

towards the desired user, the available spectrum can be utilized efficiently.

Table 7.4 Comparisons of simulation results for microstrip array

configuration using Matlab

Microstrip

Array

Configurations

Gain

(dBi)

Half Power

Beamwidth

(degrees)

Number of

Side Lobes

Main Lobe to

Side Lobe

Ratio (dB)

Linear Array 9.80 15 More 12

Planar Array 18 17.5 Less 16

Table 7.5 Comparisons of simulation results for microstrip array

configuration using FEKO 6.0

Microstrip Array

Configurations

Return Loss

S11(dB) Gain (dB)

Impedance

Matching (Ohm)

Single Patch -26 5.42 50

Linear Array (N=5) -20 14 40

Planar Array (N=4) -14.5 8.5 40

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Return loss (-26 dB) of single patch is the optimum value which

can be achieved if the number elements are more than 8 for both the linear

and planar arrays. By Liang Sun et al (2009), the optimum value of gain

18 dBi is needed for Wi-MAX applications. The gain achieved here for linear

array with number of elements 5 is 14 dBi and it crosses 18 dBi for more than

8 patches. The 2×2 planar array shows the significant gain of 8.5 dBi for 4

patches also the impedance matching occurs closely to the resonant

frequencies. Hence while comparing Tables 7.4 and 7.5; the simulation results

of microstrip array configurations using FEKO are almost equal.

This research work has analyzed the design and simulations of

microstrip patch array configurations for 3.5 GHz with /2 element spacing.

The various array parameters namely, half power beamwidth, number of side

lobes and main lobe to side lobe ratio are simulated and analyzed using

Matlab 7.5 version platform and by high frequency electromagnetic

simulation software FEKO 6.0 version. The pin feed rectangular microstrip

patch array antenna was designed for the substrate Teflon. From the

simulation results compared, linear array gives the optimal results for all the

parameters than planar array and found suitable for Wi-MAX applications.