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CHAPTER 7

BEAMFORMING TECHNIQUES

7.1 INTRODUCTION

Smart antenna technology can be used to increase coverage of the

wireless system. Smart antennas have low power dissipation, improved link

quality and increased spectral efficiency (Ahmed El Zooghby 2005). These

enhancements are achieved by directing multiple radiation lobes towards

desired transmitters or receivers and nulls towards interferers (Rappaport

1998). Implementing such a system in real-time is complex and expensive. A

simpler, cheaper and sufficiently effective alternative is to use beamforming

technique. Beamforming finds its applications in Mobile communication for

pointing the beam towards a particular mobile terminal and receive the signal

from the mobile.

There are two types of beamforming namely analog and digital. In

digital beamforming the signals are propagated through individual channels

and are finally combined at the baseband level. In analog beamforming the

signals from various directions are combined in the front end of the receiver

and are given through a single channel for processing at the baseband level.

The beamformer combines energy over antenna aperture, obtaining a certain

antenna gain in a given direction while having attenuation in other directions.

In both analog and digital domains the common methods used to create

directional beams are either by time delay (time shift) or phase shift. The time

delay approach allows forming and steering the beam by adding adjustable

151

time delay steps. Time delay is independent from the operating frequency and

bandwidth. Since it is difficult to generate time delays in both the analog and

digital domains, they are used only strictly with large arrays or when the

bandwidth of the system is wide. In the case of phase shifting approach, a

phase is introduced instead of applying true time delays for each receiver and

it is simple to introduce such compensation as this works properly with

narrow band systems or small arrays.

Many receivers are connected to different antenna elements.

Having antenna elements as separate structures occupies more space.

Therefore antenna elements are combined to form array. In the antenna array

if the individual antenna elements are excited separately the array is Phased

Antenna Array. The Phased Array Antenna (PAA) is capable of steering the

beam electronically in a particular direction. The advantages of PAA are that

it gives very narrow beam width and minimum side lobe levels.

In this chapter, two methods of analog beamforming with Phased

Array Antenna (PAA) are proposed. One is an active beamforming system

and the other method is a passive beamforming system. The active

beamforming system uses PAA, LNA, variable attenuator and phase shifter.

The passive beamforming system uses PAA, Butler matrices, Single Pole

Eight Throw (SP8T) switches to form a directional beam. The steering of the

beam is accomplished by the Butler Matrices. This system has the advantages

of linearity, good dynamic range and is reciprocal in nature. The same

network can be used for transmitter and receiver RF stages.

7.2 LITERATURE SURVEY

Efficient beamforming algorithms are compared by Ronald Mucci

(1984). All the beamforming methods use individual antenna elements. Beam

selection performance analysis of multi-beam antenna system is discussed by

152

Matsumoto et al (1997). Jeon et al (2001) has proposed planar array smart

antenna system for hybrid analog-digital beamforming. Factors to be

considered in implementing smart antenna systems are proposed by Weon-

Ceol Lee et al (2005). Fakoukakis et al (2005) have proposed the

Development of an Adaptive and a Switched Beam Smart Antenna System

for Wireless Communications for 2.4GHz ISM band with separate antenna

systems. The design, working and performance of Microstrip antennas are

discussed by Bahl and Bhartia (1986), Hansen (1997) and James and Hall

(1989). Chao-Wei Wang et al (2007) have proposed a New Planar Artificial

Transmission Line and Its Applications to a Miniaturized Butler Matrix. A

Single-Layer Hollow-Waveguide 8-Way Butler Matrix has been discussed by

Shin-ichi Yamamoto et al (2005). Tayeb Denidni and Taro Eric Libar (2002)

have done Experimental Investigation of a microstrip Planar Feeding Network

for a Switched-Beam Antenna Array.

7.3 ACTIVE ANALOG BEAMFORMING WITH PAA

Active beamforming system for 2.4GHz is designed and

implemented. The active beamforming is implemented in three steps. First,

the design of PAA which produces a narrow beam and Right Hand Circular

Polarization (RHCP) is proposed and simulated. Second, the excitation

amplitudes for non-uniform amplitude distribution of antenna elements are

calculated. The excitation amplitudes are calculated to have low side lobes.

Third, the calculation of complex weights in terms of magnitudes and phase

to provide steering commands for beamforming in particular direction. The

beam can be steered in both azimuth and elevation planes with the help of

beamformer circuits designed at 2.4 GHz with a bandwidth of 20MHz. To

design PAA, first a single patch antenna element is designed and its

performance is studied. The specifications of the design are given in

Table 7.1.

153

Table 7.1 Specifications of PAA

Parameters Values

Frequency 2.4 GHz

Bandwidth 20 MHz

Axial ratio 3dB (maximum)

PolarizationRHCP

(Right Hand Circular Polarization)

Impedance 50 ohms

Axial ratio is the difference between the 'horizontal' and 'vertical'

(or otherwise orthogonal) components. In a pure circularly polarized wave

both electric field components have equal magnitude and the axial ratio is 1 or

0 dB. Axial Ratio of maximum 3 dB is acceptable (Bahl and Bhartia 1986).

7.3.1 Single Circularly Polarized Patch Design

The selection of substrate dielectric constant ‘ r’ and the substrate

thickness ‘h’ plays important role in the antenna design. Low dielectric

constant increases radiated power but at the cost of larger size. A thicker

substrate besides being mechanically strong will increase radiated power and

improves impedance bandwidth. Thicker substrates do have the disadvantages

of increasing weight and dielectric loss. Thicker substrates with low dielectric

constant increases radiated power, providing better efficiency and larger

bandwidth but at the expense of larger element size as given by Balanis

(1997). The dielectric chosen is RT Duroid 5870 with loss tangent, 0.0012;

height, 1.6 mm; and r, 2.32. Selection of these parameters is followed by

calculation of patch dimension. The value of the patch width W is calculated

with resonant frequency fr and the velocity of light, c as

154

r rW c/ 2f 2 / ( 1) (7.1)

The effect of fringing field increases the length (L) on both sides of

the patch. The increase in length by small factor due to fringing is called

effective length (Le). Effective length is calculated by including the effective

dielectric constant re, and is given as

e

r re

cL(2f )

(7.2)

The effective dielectric constant re, and extension in length L on

one side of the patch can be calculated as

1r r 2

re

( 1) ( 1) h[1 12 ]W2 2 (7.3)

re

re

W( 0.300 0.264)

hL 0.412hW

( 0.258 0.813)h

(7.4)

Length of the patch L is given by,

eL L 2 L (7.5)

The patch antenna works when excitation is given through a feed.

Coaxial line feed is used for the excitation of the patch antenna. It is better to

choose yf = W/2 if W L as proposed by Kara (1996). A simple method for

calculating xf that does not need radiation resistance is given by Bhal and

Bhartia (1986) and Kara (1996). The feed is not radiating and only the patch

is radiating.

155

The feed point F (xo, yo) is located at xo = xf and 0 yo = yf W,

where xf is the inset distance from the radiating edge given by (7.6).

f

re

Lx(2 (L))

(7.6)

where, re is the effective dielectric constant with fringing effects and is

given as

2/1rrre ]L/h121[2/)1(2/)1()L( (7.7)

Circular polarization can be achieved by dual feed or single feed

method. In this chapter, single point feed type is chosen because the antenna

should be compact and the bandwidth required is less than 1% of the

operating frequency (20 MHz). Hybrid coupler is not required in single feed

method. Single point feed can be of two types – Type A and Type B

depending on the feed location. It is type A feed when the feed location is on

the x or y axis and the feed is Type B if the feed is placed on the diagonal axis

of a patch as shown in Figure 7.1.

X

Y

F

X

Y

F

Type A Feed Type B Feed

Figure 7.1 Type A and Type B Feed

156

Type A feed is chosen for the design and the design equation given

by Ramesh Garg et al (2000) is chosen to design the feed. For the type-A feed

the design equation is given as

2/1Q/Ss o (7.8)

where, s , area of feed; S, area of the square patch and Qo, unloaded Q

factor. Using the above design equation along with dimensions and dielectric

thickness of single patch, the feed size is calculated. The patch antenna

dimensions with feed location after optimization is shown in Figure 7.2.

Figure 7.2 Dimensions of the patch in mm

Number 1 in the Figure 7.2 near the feed location denotes port

number. The plot of VSWR and return loss are shown in Figure 7.3a and 7.3b

respectively.

F (8.5, 0.7) mm

4.2 mm

40 mm

40 mm(0, 0)

157

(a) VSWR

(b) Return Loss

Figure 7.3 Parameters of Single Patch antenna

The VSWR and the return loss are low at the designed frequency.

The resonant frequency (fr) of the designed antenna is obtained as 2.412565

GHz. Simulation results at resonant frequency of the designed single patch are

158

as follows: VSWR, 1.005; return loss, -51.705dB; axial ratio, 0.145dB;

impedance at feed point, 49.8 + j 0.211 ; Gain, 6.6 dBi; Bandwidth, 20MHz;

resonant frequency, 2412.565 MHz; polarization, RHCP; antenna efficiency,

84.5865%. The radiation pattern at elevation plane and the 3D beam pattern

are shown in Figure 7.4a and 7.4b respectively.

a) Elevation Pattern (b) Main Lobe

Figure 7.4 Plots of antenna parameters.

The radiation properties of the patch antenna observed from the

radiation in elevation plane are given in Table 7.2.

Table 7.2 Radiation properties of patch antenna

Parameters Total fieldRHC

polarization

LHC

polarization

Gain (dBi) 6.64514 6.64483 -6.70265

Directivity(dBi) 7.37213 7.37183 -5.97566

3-dB beam width

, ) (degrees)87.2239, 88.5629 87.0395, 88.2769 18.6259, 147.825

From the Table 7.2 it is inferred that the patch antenna provides

RHC polarization. The gain and directivity are high for RHCP than LHCP. It

is clear from Figure 7.5 and Figure 7.6 that the input impedance obtained is

159

close to 50ohms and the gain is 6.6dBi at resonant frequency of 2.4125GHz

for a single patch antenna.

Figure 7.5 Impedance plot of single patch antenna

Figure 7.6 Gain of single patch antenna

7.3.2 Planar Array Design

The required specification of the array is to produce a beam width

of 15° in both elevation and azimuth planes; scannable to 45° in both planes

160

0.53

0.53

and side lobe should be lees than 25 dB than main lobe. The design procedure

is adopted from Robert Elliot (2000). A planar array of size 8 x 8 with

element spacing of 0.53 in both axes is designed to meet the specifications.

The structure of the designed planar array is shown in Figure 7.7.

Figure 7.7 Planar Array Geometry

The single patch is used as the array element for this planar array.

The simulation results of the designed 8 x 8 planar patch antenna array at

resonant frequency (fr) are as follows: VSWR, 1.0615; Return Loss, -30.49

dB; Axial ratio, 1.1dB; impedance at feed point, 49.3 - j 0.16 ; gain,

19.7dbi; bandwidth, 20MHz; resonant frequency, 2412.565 MHz;

Polarization, RHCP; 3 dB beam width: elevation plane, 15º, azimuth plane,

15º; side lobe level, -25dB.The plot of VSWR and return loss are shown in

Figure 7.8a and 7.8b respectively.

161

(a) VSWR

(b) Return Loss

Figure 7.8 Plots of antenna array parameters

The elevation plane radiation pattern and the main lobe are shown

in Figure 7.9a and 7.9b respectively.

162

(a) Elevation Pattern

(b) Main Lobe

Figure 7.9 Parameters of Array Antenna

163

E theta at 0°

E Theta at 90°

Axial Ratio (AR) is calculated by taking the difference of gain

between the E theta component at phi=0° and phi=90° for broad side case

from Figure 7.10.

(E theta component at phi=0°) - (E theta component at phi=90°) = 17.607 -

15.557 = 2.1 dB.

Figure 7.10 E theta components – Cartesian Plot

7.3.3 Array Beamsteering

In order to steer the beam electronically, necessary phase shifts

along with excitation amplitudes has to be applied. For M x N array, the phase

steering command at each element is given as,

mn m x n y (7.9)

164

where o ox kdxsin cos , sin sino o

y kdy , 2k , o and o are

scan angles in elevation and azimuth planes, n = 1, 2,…..M and m = 1,

2,….N,

The phase shift to be applied for the element at (m,n) position of

array is, x o o y o o

2mn md sin cos nd sin sin , where dx, dy are the element

spacing in x and y axis respectively.

The excitation amplitude applied to each element is

mnj

mn mnV V e where, Vmn is the excitation coefficient calculated from

non-uniform amplitude distribution.

In order to obtain a narrow main beam with lowered side lobe, non-

uniform amplitude distribution has to be used. Two methods of non-uniform

amplitude distribution, namely Taylor and Dolph-Chebyshev distributions are

used. The Taylor distribution array pattern has a wider main lobe than the

corresponding Dolph–Chebyschev distribution, but has low side lobes at the

expense of approximately 2° increase in half power beam width. Thus the

array with the smoothest amplitude distribution will have the smallest side

lobes and larger half power beam widths. Scan angle denotes the direction in

terms of ( , ) in which the beam has to be steered. A comparison of half

power beam widths and side lobe of the array at different scan angles for both

distributions is given in Table 7.3

165

Table 7.3 Beam width and side lobe of Taylor and Dolph – Chebyshev

distribution.

Taylor DistributionDolph – Chebyshev

DistributionScan Angle

(degrees) 3dB Beam

width (degrees)

Side Lobe

at (dB)

3 dB Beam width

(degrees)

Side Lobe

at (dB)

(0,0) (17.2,17.4) -15 (15.4,15.6) -5

(30,0) (21.6,19.3) -12 (19.6,17.4) -3

(45,0) (24.8,19.4) 0 (22.9,17.4) 4

(-45,30) (23.5,17.4) -30 (21.1,15.4) -25

(45,45) (23.3,18.8) -30 (20.9,17.1) -25

(-45, -45) (23.3,17.1) 0 (21.2,15.5) 4

The gain and directivity of the antenna array for different scan

angles for Taylor and Dolph-Chebychev distribution are given in Table 7.4.

Table 7.4 Gain and Directivity for different scan angles

Taylor DistributionDolph-Chebyshev

DistributionScan

Angle

(degrees) Gain (dBi)Directivity

(dBi)

Gain

(dBi)

Directivity

(dBi)

( 0, 0 ) 20.8 21.3 21.9 22.4

( 30, 0) 20.3 20.5 21.3 21.5

( 45, 0) 18.5 19 19.5 19.8

( -45, 30) 19.2 19.6 20.3 20.7

( 45, 45) 18.7 19.2 19.7 20.1

(-45,-45) 18.8 19.6 19.9 20.5

166

The radiation patterns of the array are shown for maximum and

minimum scan angles. The radiation pattern of the array, in elevation and

azimuth plane, for a scan angle of (-45°, -45°) is shown in Figure 7.11, with

dotted line showing Dolph-Chebyschev distribution and thicker line showing

Taylor distribution pattern. Similarly, the radiation pattern of the array for a

scan angle of (45°, 45°) is shown in Figure 7.12.

(a) Elevation Pattern

(b) Azimuth Pattern

Figure 7.11 Elevation and Azimuth Pattern of array at a scan angle of

(-45°, -45°)

167

(a) Elevation Pattern

(b) Azimuth Pattern

Figure 7.12 Elevation and Azimuth Pattern of array at a scan angle of

(45°, 45°)

-30

60

30

168

7.3.4 RF Beamformer

The Phased Array Antenna is composed of a group of similar

antennas, each with its variable attenuator, phase shifter and a summing

network, which gives a resulting signal representing a beam on an expected

location. Figure 7.13 shows the block diagram of the RF beamformer.

LNAVariable

Attenuator

Phase

Shifter

LNAVariable

Attenuator

Phase

Shifter

LNAVariable

Attenuator

Phase

Shifter

1

8

2

Figure 7.13 RF beamformer to combine the output of 8 columns of a row

8 RF beamformers are used to combine all 8 columns of each row

separately. The excitation coefficients are calculated in terms of magnitude

(dB) and phase (degrees) for different scan angles using Dolph-Chebychev

distribution and Taylor distribution. Magnitude of the excitation coefficients

using Taylor distribution for scan angles (45°, 0°), (45°, 45°) and (-45°, -45°)

is same and is given in Table 7.5. Magnitude of the excitation coefficients

using Dolph-Chebychev distribution for scan angles (45°, 0°), (45°, 45°) and

(-45°, -45°) is same and is given in Table 7.6.

169

Table 7.5 Magnitude in dB using Taylor distribution for Scan angle

(45°, 0°), (45°, 45°) and scan angle (-45°, -45°)

Row Column

1

Column

2

Column

3

Column

4

Column

5

Column

6

Column

7

Column

8

1 -67.042 -48.785 -38.814 -33.521 -33.521 -38.814 -48.785 -67.042

2 -48.785 -30.528 -20.557 -15.264 -15.264 -20.557 -30.528 -48.785

3 -38.814 -20.557 -10.587 -5.2936 -5.2936 -10.587 -20.557 -38.814

4 -33.521 -15.264 -5.2936 0 0 -5.2936 -15.264 -33.521

5 -33.521 -15.264 -5.2936 0 0 -5.2936 -15.264 -33.521

6 -38.814 -20.557 -10.587 -5.2936 -5.2936 -10.587 -20.557 -38.814

7 -48.785 -30.528 -20.557 -15.264 -15.264 -20.557 -30.528 -48.785

8 -67.042 -48.785 -38.814 -33.521 -33.521 -38.814 -48.785 -67.042

Table 7.6 Magnitude in dB using Dolph-Chebychev distribution for

Scan angle (45°, 0°), (45°, 45°) and (-45°, -45°).

Row Column

1

Column

2

Column

3

Column

4

Column

5

Column

6

Column

7

Column

8

1 1 1.5464 2.2296 2.6467 2.6467 2.2296 1.5464 1

2 1.5464 2.3913 3.4478 4.0927 4.0927 3.4478 2.3913 1.5464

3 2.2296 3.4478 4.9711 5.901 5.901 4.9711 3.4478 2.2296

4 2.6467 4.0927 5.901 7.0048 7.0048 5.901 4.0927 2.6467

5 2.6467 4.0927 5.901 7.0048 7.0048 5.901 4.0927 2.6467

6 2.2296 3.4478 4.9711 5.901 5.901 4.9711 3.4478 2.2296

7 1.5464 2.3913 3.4478 4.0927 4.0927 3.4478 2.3913 1.5464

8 1 1.5464 2.2296 2.6467 2.6467 2.2296 1.5464 1

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The phase of excitation coefficients for each antenna element due

to Taylor distribution and Dolph-Chebychev distribution is same for same

scan angle. Phase values for both distributions at scan angle (45°, 0°) is given

in Table 7.7.

Table 7.7 Phase values of elements of array in degrees for scan angle (45°, 0°)

Row Column

1

Column

2

Column

3

Column

4

Column

5

Column

6

Column

7

Column

8

1 -90 -180 90 0 -90 -180 90 0

2 -90 -180 90 0 -90 -180 90 0

3 -90 -180 90 0 -90 -180 90 0

4 -90 -180 90 0 -90 -180 90 0

5 -90 -180 90 0 -90 -180 90 0

6 -90 -180 90 0 -90 -180 90 0

7 -90 -180 90 0 -90 -180 90 0

8 -90 -180 90 0 -90 -180 90 0

The phase of excitation coefficients for each antenna element is

same for both distributions for scan angle (45°, 45°) is given in Table 7.8.

Table 7.8 Phase values of elements of array in degrees for scan angle

(45°, 45°)

Row Column

1

Column

2

Column

3

Column

4

Column

5

Column

6

Column

7

Column

8

1 -180 90 0 -90 -180 90 0 -90

2 90 0 -90 -180 90 0 -90 180

3 0 -90 -180 90 0 -90 -180 90

4 -90 -180 90 0 -90 -180 90 0

5 -180 90 0 -90 -180 90 0 -90

6 90 0 -90 -180 90 0 -90 -180

7 0 -90 -180 90 0 -90 -180 90

8 -90 -180 90 0 -90 -180 90 0

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The phase of excitation coefficients for each antenna element is

same for both distributions for scan angle (-45°, -45°) is given in Table 7.9.

Table 7.9 Phase values of elements of array in degrees for scan angle

(-45°, -45°)

Row Column

1

Column

2

Column

3

Column

4

Column

5

Column

6

Column

7

Column

8

1 0 90 -180 -90 0 90 -180 -90

2 -90 0 90 -180 -90 0 90 -180

3 -180 -90 0 90 -180 -90 0 90

4 90 -180 -90 0 90 -180 -90 0

5 0 90 -180 -90 0 90 -180 -90

6 -90 0 90 -180 -90 0 90 -180

7 -180 -90 0 90 -180 -90 0 90

8 90 -180 -90 0 90 -180 -90 0

The signal from the each antenna element is applied to attenuator

through the LNA. The signal from each element is processed in the attenuator

and phase shifter based on the excitation coefficients of the Taylor

distribution or Dolph-Chebychev distribution. The signal after the attenuator

is combined column wise. The power received is calculated column wise and

the total power received is calculated by combining the power from individual

columns. The procedure is repeated for different scan angles for both Taylor

distribution and Dolph-Chebychev distributions. The output power at each

column and total power from the array for Dolph-chebychev and Taylor

distributions are given in Table 7.10 for different scan angles.

172

Table 7.10 Output power from the array for Taylor and Dolph-

Chebychev distribution

Taylor Distribution (dB, Degrees)Dolph-Chebychev

Distribution (dB, Degrees)Scan

angle(45°, 0°) (45°, 45°) (-45°, -45°) (45°, 0°) (45°, 45°) (-45°, -45°)

Column 1 0.261, 90 0.155, 79 0.128, -10 12, -90 0.06, -162 0.06, 18

Column 2 2.274, 180 0.08, -178 0.086, 92 11, 179 0.09, 85 0.13, 48

Column 3 5.582, 90 0.485, 31 0.484, -122 8.64, 92 0.17, 37 0.17, -150

Column 4 8.647, 0 0.915, 49 0.906, -41 8.6, 1.8 0.08, -117 0.07, -63

Column 5 7.0468, 90 0.80, -127 0.799, 37 8.7, -92 0.12, 179 0.117, 10

Column 6 9.403, 180 0.421, 143 0.430, 127 8.7, 178 0.23, 126 0.21, 109

Column 7 2.161, 90 0.157, -2 0.139, -91 10.8, 91 0.07, -60 0.07, 120

Column 8 0.261, 0) 0.129, -73 0.133, -19 12, -.01 0.24, 126 0.05, 5.4

Total

power0.687,-34 0.258,-73 0.240, -16 0.1, -133 0.07, 131 0.06, 71

The signals from the different elements of the PAA are combined.

The power received after combining all the signals is found for the Taylor and

Dolph-Chebychev distributions for different scan angles. The signals received

from different directions are combined into a single unidirectional beam in a

particular direction.

7.4 PASSIVE ANALOG BEAMFORMING WITH PAA

Passive beamforming is a switched RF beamforming. Passive

beamforming is implemented using PAA, Butler matrix and Single Pole Eight

Throw (SP8T) switch. The PAA is designed and implemented as a 64 element

patch antenna array.

173

7.4.1 Phased Array Antenna Design and Implementation

A single square patch antenna is designed for 900MHz and the

dimensions of the patch are calculated. In the design procedure of single

patch, first the unloaded Quality factor (Qo) of the patch is determined. For

better accuracy Qo should be selected to ensure the patch radiation efficiency

greater than 90%. The single patch antenna is designed for frequency of 900

MHz producing Right Hand Circular Polarization (RHCP). For this design

with h = 1.6 mm, o =124 mm, h / o = 0.12 and efficiency of 90%, the Qo is

found to be 50. The calculated length is 108.8 mm. Hence the dimension of

square patch is 108.8 x 108.8 mm. The feed point coordinate is (35.9, 64.6)

mm, i.e. 35.9 mm from the edge towards the center of the patch in the x-axis

and at the middle along the y-axis. In this work, the dielectric chosen is RT

Duroid 5870 with loss tangent of 0.0012, height of 1.6 mm (1/16”) and the

dielectric constant of the substrate r 2.32 . The impedance is 50 at the

design frequency. The designed patch is shown in Figure 7.14.

Figure7.14 Single Patch with Dimensions and Feed point

174

The PAA is designed with this single patch antenna as the basic

element and the 8x8 array is formed by arranging the 64 single patch

antennas. The specifications for the PAA are same as for single patch. For

the given side lobe level of –25 dB the beam broadening factor is found to

be 1.06 and this factor is multiplied with the array length to get the actual

array length. Since the beam width required is 15° in both azimuth and

elevation plane and the scanning requirement for both planes are same, the

array length in terms of wavelength is same in both x and y-axis and is

found to be 4.24. To avoid multiple beams (grating lobes) during

scanning, the spacing between elements must be less than 0.59 . With this

condition the number of elements in both x and y-axis is found to be 7.2

which are rounded off to 8. Since the number of elements is rounded off to

eight the spacing between the elements becomes 0.53 . The PAA

structure without feed is shown in Figure 7.15 and its radiation pattern is

shown in Figure 7.16.

Figure 7.15 PAA with 64 elements

175

Figure 7.16 Radiation Pattern of PAA with 64 elements

The PAA is to be fed so that it can be connected to the Butler

matrix. There are two types of feed, series and parallel. The series feed and

the parallel feed are different in many aspects, the parallel feed provides a

larger bandwidth, while the series feed provides narrow bandwidths. The

main disadvantage of the parallel feed is that it suffers from more ohmic

losses since the structures used for feeding in parallel occupy more space. The

feed selected for PAA is combination of both series and parallel type of

feeding to achieve an acceptable tradeoff between bandwidth, radiation losses,

Ohmic losses and space.

The proposed antenna consists of 8 columns each one composed of

8 elements. The series feed of each column allows beam scanning in elevation

plane in the specified bandwidth. The design of the parallel feed provides the

power distribution necessary to maintain the side lobe level in azimuth below

the specifications for the entire bandwidth. The corporate feed provides a

power distribution that allows beam steering avoiding grating lobes. The

layout of corporate feed for the 64 element PAA is shown in Figure 7.17.

176

Figure 7.17 Layout of Corporate Feed for 64 Patches

The designed feed is connected to the PAA with 64 elements as

shown in Figure 7.18.

Figure 7.18 PAA with corporate feed

177

7.4.2 Butler Matrix

Butler Matrix is used to provide the necessary phase shift for a

linear antenna array. An N×N butler matrix can produce beams looking in

different directions with an N-element array (Neron and Jean-Sebastien

2003). A N×N butler matrix requires an (N/2) log2(N) 90° hybrids

interconnected by rows of (N/2)(log2(N)-1) fixed phase shifters to form the

beam pattern.

When a signal impinges upon the input port of the Butler Matrix, it

produces a different inter-element phase shifts between the output ports. The

Butler matrix consists of twelve 90° hybrids and eight cross couplers with two

ports and four cross couplers with four ports. When one of the input ports is

excited by an RF signal, all the output ports feeding the array elements are

equally excited but with a progressive phase between them. This results in the

radiation of the beam at a certain angle. The transmission line length and

width for various angles for impedance of 50 ohms is calculated using line

calculator in ADS tool. Line calculator is used to find the length and width of

the transmission line. The dimensions of the transmission line for various

impedances are given in Table 7.11.

Table 7.11 Dimensions of transmission line for various impedances

AnglesResistance Dimensions

22.5 45 67.5 90

Width(µm) 760.40 760.40 760.40 760.4050

Length(µm) 15247.60 30495.20 45742.00 60990.40

Width(µm) 1270.18 1270.18 1270.18 1270.1835

Length(µm) 14995.80 29991.60 44987.50 59983.30

Width(µm) 204.61 204.61 204.61 204.61100

Length(µm) 15839.90 31679.90 47519.80 63359.80

Width(µm) 425.56 425.56 425.56 425.5670.7

Length(µm) 15527.20 31054.30 46581.50 62108.70

178

7.4.2.1 Quadrature hybrid

The Hybrid coupler is often made of microstrip or stripline. These

couplers are 3 dB directional couplers with a 90° phase difference between

the outputs of the through and coupled lines. It is also known as a branch-line

hybrid. The schematic of hybrid coupler is shown in Figure 7.19.

Figure 7.19 Schematic of 3 dB directional couplers

The Layout of the coupler is shown in Figure 7.20. The length and

width of the coupler for various impedance values are shown in Table 7.12.

Figure 7.20 Layout of 3 dB directional couplers

179

Table 7.12 Dimensions of 3 dB directional couplers

Impedance 0Z =50 2/0Z =35

Length 60990.40 µm 59983.30 µm

Width 760.40 µm 1270.18 µm

The Table 7.13 gives the values of the substrate parameters used in

designing Quadrature hybrid and cross-coupler.

Table 7.13 Substrate parameters for microstrip structure

Substrate

parameter

Values

Er 2.2

Mur 1.0

H 254µm

Hu 3.9e+34 mil

T 17.50µm

Cond 4.1 e+7

TanD 0.0009

Rough 0.000mil

7.4.2.2 Cross-coupler

Cross-coupler also known as 0 dB couplers are an efficient means

of crossing two transmission lines with a minimal coupling between them. It

is observed that the planar implementation is made as a cascade of two hybrid

couplers (with slight modifications on line widths). Figure 7.21 is the

schematic design of Cross-coupler and Figure 7.22 is the layout design Cross-

coupler. The cross-coupler with two ports is used in the design of Butler

matrix.

180

Figure 7.21 Schematic of Microstrip Cross-coupler

a) Layout Design of Cross-coupler four Ports

b) Layout Design of Cross-coupler with two ports

Figure 7.22 Layout of cross couplers

181

7.4.2.3 Vertical Interconnection

The vertical interconnection shown in Figure 7.23 is used to

transmit a signal from one layer to the other by an electromagnetic coupling

only. There is no DC path between the 2 layers since they are AC coupled

only; a signal arriving on the upper transmission line (from the left)

eventually reaches the end of this transmission line. The discontinuity created

here radiates energy that is mostly coupled to the bottom line by the opening

in the ground plane (some power may radiate).

The following aspects must be considered to achieve an optimal

coupling between layers as well as a good matching of the transition:

1. A proper positioning of the line ends relative to the slot center

2. A proper dimensioning of the serial stub added at the end of

the transmission lines is necessary

3. The design of the slot in the ground plane: many shapes are

possible (circular, bowtie, rectangular, and so on), and the

dimensions should be carefully optimized

4. Orientation of the lines over the slot.

Figure 7.23 Layout of vertical interconnection

182

The layout of the Butler matrix is shown in Figure 7.24 and Figure

7.25. The integration of Butler matrix with one column is shown in

Figure 7.26.

Figure 7.24 Layout of Butler matrix without vertical interconnect

Figure 7.25 Layout Design of Butler Matrix with vertical interconnect

183

Figure 7.26 Integration of Butler matrix with Single Column

7.4.3 SP8T Switch

SP8T Switch is designed to select a single output form the eight

outputs of the Butler matrix. The SP8T switch is designed in four steps. In the

first two steps SPST switch and SP2T switch are designed. The two SP2T

switches are combined to form SP4T switch. Finally, two SP4T switches are

combined to form SP8T switch. A single-pole, eight-throw (SP8T) to provide

low insertion loss and input VSWR is designed. The design of a higher-order

multi throw switch requires the addition of a series capacitive element for

each throw at the junction. The advantage of this type is that the switch only

needs to be powered in one state, simplifying the biasing and potentially

increasing the switching lifetime of the device. The device specification for

design of the switch is as follows:

The diode chosen is di_tsb_lsv186_19930908 with UHF tuning,

Vr (max) =30v and S (2, 1) =-7.194 dB. The resistance chosen is of 560

ohm. Substrate is FR4 with dielectric constant 4.34 and and typical

thickness of 1.6 mm. The Loss tangent is 0.01 and Surface resistivity is

184

200K . The SP8T layout is shown in the Figure7.27. The insertion loss in

the SP8T switch is found to be -15dB.

Figure 7.27 Layout of SP8T Switch

7.4.4 Switched RF Beamforming System

The RF beamforming system has the advantage of easy

integration with existing architectures in base stations and offer reduced

complexity compared to IF Beamforming Systems. The front-end of

passive RF Switched beamforming system is shown in Figure 7.28.

185

Butler Matrix

8 x 8

S

P

8

T

S

P

8

T

S

P

8

T

1

8

Butler Matrix

8 x 8

Butler Matrix

8 x 8

Output

Figure 7.28 RF Switched Beamforming System

There are 64 antenna elements in the system that is realized in

the form of PAA with 8 elements in each row and 8 elements in each

column. The output of elements in 8 rows are combined column wise

using the 8×8 Butler matrix and SP8T switch and totally 8 outputs are

taken. This output of eight individual columns are further given to 8×8

Butler matrix and SP8T switch and combined as single output. The system

is implemented as shown in Figure 7.29.

186

Figure 7.29 Complete Layout of Switched RF Beamforming System

187

The results obtained for single cell, PAA without feed for 64

elements, PAA with feed for 64 elements and the complete system are given

in Table7.14.

Table7.14 Results of Passive Beamforming system

Parameters Single Patch

antenna

PAA without

feed for 64

elements

PAA with feed

for 64

elements

Switched RF

Beamforming

System

Power

radiated

(Watts)

4. 37096 e-5 1. 51257 e-5 5. 1301 e-6 0. 0166609

Directivity

(dB)

6. 94928 18. 5318 21. 61831 14. 8938

Gain

(dB)

5. 30524 11. 8807 10. 808 14. 3432

Radiation

Intensity

1.72 e -5 8. 58 e -5 5. 925 e-5 0. 04091

E max

(magnitude,

phase)

0.11, 83.89° 0.20, -3.03° 0.19, -1.4° 5.55, 125.45°

E max

(magnitude,

phase)

9.86x10-5

, -172.5° 0.14, 176.94° 0.08, 178.65° 0.00018, 51.34°

The results show that both the directivity and gain are improved in

the switched RF Beamforming system when compared with Single patch

antenna. PAA with feed and without feed gives more directivity but the gain

is decreased. The power radiated by the switched RF Beamforming system is

also larger. The maximum radiation is obtained for the maximum values of

Electric field intensity in the and directions. The maximum radiation after

combining the signals from different directions is obtained at an angle of

188

(125.45°, 51.34°) with maximum amplitude of Electric field intensity

5.55V/m in the direction.

7.5 CONCLUSION

Active beamforming circuit with PAA is implemented for 2.4GHz.

Design of Phased array antenna gives axial ratio (AR) of 2.1 dB maximum at

the desired scan angle of 45 . Two types of non-uniform amplitude

distributions namely, Dolph-Chebyshev distribution and Taylor distribution

are used for beamforming. Dolph-Chebyshev distribution gives narrow beam

width than Taylor distribution but the side lobes are constant. Taylor

distribution gives tapering side lobes with 2º broader beam width. This has

been observed at various scan angles. Also from the simulation results, it is

found that the beam broadening is more for scan angles greater than 45º.

Axial ratio (AR) also degrades when scanning away from the broadside.

The front end of Switched RF beamforming system is

implemented for 900MHz. Design and simulation of Sixty four cell patch

antenna has been implemented. Optimization of 64 cell array has been

done to obtain good response at the desired frequency by increasing the

number of sample points during simulation. Corporate feed design for the

phased array antenna is implemented and the antenna properties are

improved. An 8×8 Butler Matrix has been designed to change the

direction of radiated or received energy in a particular angle by the

transmitter or receiver. Butler matrix is interfaced with antenna array and

the radiation pattern is obtained with a phase shift of zero degree. Design

and implementation of Single pole eight throw switch is done and the

insertion loss is found to be -15 dB. The system works in the frequency

range of 900MHz and the results show that switched RF Beamforming

system provides good gain and directivity at 900MHz.