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Chapter-9

CONCLUSIONS AND FUTURE SCOPE

9.1 CONCLUSIONS

This thesis presented a detailed study about six different new antenna designs

developed to demonstrate the reconfigurable concept employing electrical

reconfiguration technique using PIN diodes. The new designs presented are the

Reconfigurable rectangular patch antenna (RRPA), Reconfigurable wheel antenna

(RWA), Reconfigurable meandered line antenna, Reconfigurable cavity backed

square spiral antenna, and Reconfigurable substrate integrated waveguide cavity

backed slot antenna and Reconfigurable diamond shape patch antenna.

A vast literature survey was conducted on the available reconfigurable antennas

starting from reconfigurable antennas implemented with mechanically movable

parts and arrays to microstrip reconfigurable antennas implemented with

mechanical and semiconductor switches. The survey revealed that conventional

mechanical switches are not practical for reconfigurable antenna applications due

to their large size and are not compatible with the printed circuit board. They are

preferred only for lower frequency and high power handling situations. Solid state

switches such as PIN diode and FET’s are most widely used to implement

reconfigurable antennas electrically, among these PIN diode switches can offer

promising characteristics for reconfigurable antennas. Therefore, in this work, all

reconfigurable antennas are designed and fabricated using PIN diodes.

Reconfigurable rectangular patch antenna single element for frequency

reconfiguration and 1X8 linear array for both frequency & pattern reconfiguration

using PIN diodes have been discussed in Chaper3. It has been shown that the

pattern can be steered by controlling the supply of PIN diodes in each iteration.

Therefore, the requirement of phase shifters in the phased array radar can be

eliminated thus reducing the system cost and complexity. Experimental data have

demonstrated the concepts of reconfigurable antenna by switching of PIN diodes

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for multiple radar frequencies. The simulated results are in good agreement with

the measured results. Fabrication accuracy can further improve the results of the

designed antenna array.

Simulated and experimental data presented in Chapter 4 demonstrated the

concepts of single element reconfigurable wheel antenna and its array by

switching OFF and ON of PIN diodes for multiple bands of frequencies. The

performance of RWA can be further improved by proper designing of driver

circuit in the antenna structure. The technique has taken the advantage of different

number of radiating lengths with the use of PIN diode switches, each

configuration resonating at different frequency, In array radiation pattern there is a

grating lobe within 35 deg for X-band, therefore the main beam can be steered

only within ±15 deg. For S-band there is no grating lobe as the inter element

spacing is less than a wavelength.

Multiband meander line antenna design for four states are presented in

Chapter 5.A total of 4 PIN diode switches were incorporated in to the antenna

geometry to achieve frequency reconfiguration, for experimental verification.

Fourth iteration has been fabricated and return loss, pattern measurements have

been carried out for the same. The simulated and experimental data have

demonstrated the concepts of multiband reconfigurable antenna by switching OFF

and ON of PIN diodes for multiple band frequencies. The technique has taken the

advantage of different number of radiating lengths with the use of PIN diode

switches, each configuration resonating at multiband frequencies.

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The development, design, simulation and measurement of the cavity backed

reconfigurable spiral antenna were presented in Chapter 6. The measured results

are in good agreement with the simulated one. In Band-II, the ripples are little high

because of the biasing circuit effect. This can be avoided by proper isolation

between RF and DC bias. The controlling of PIN diodes in real application can be

implemented using FPGA control to achieve the switching speed. The operational

frequency can be still further increased by multilayer spiral and proper broad band

matching.

Design and development of a SIW antenna with dual state and dual band for C-

band applications is discussed in Chapter 7.Two high performance PIN diode

switches were incorporated in to the new design to give dual band in both the

states, The corresponding biasing network of the diodes are also integrated in the

antenna geometry. The measured antenna performance was similar to the

predicted simulation performance and suggested that by using reconfigurable

multiband approach we can eliminate the bulky and expensive filters in modern

multi-band systems to improve the out-of-band noise rejection performance.

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A new microstrip antenna with triple-polarization diversity for C-band

applications is demonstrated with six discrete antenna states in Chapter 8. To

achieve the polarization reconfigurability, one SP3T switch to select the feed

location and 4 PIN diodes have been used to connect the truncated patches to the

main patch and the biasing network of the diodes are also integrated in the antenna

geometry. The types of achieved polarization are linear, circular and elliptical. The

purity of polarization has been estimated by measuring the axial ratio of the

developed proto type antenna, and it is found that it is less than 4dB for CP and

more than 30dB for linear. Table 9.1 provides a summary of the important

performance characteristics for the six antennas developed in this work.

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179

• Achieving the pattern reconfigurability without significant changes in the

operating frequency is somewhat difficult because of the relationship

between the source currents and the antenna structure. Here, antenna

structure has been modified to achieve the pattern thus resulting small

changes in the operating frequency.

• Pattern reconfigurability demonstrated with switches in this work is similar

to that achieved with traditional phased arrays but without the inherent

costs of phase shifters.

9.2 FUTURE SCOPE

The reconfigurable antenna designs using PIN diodes reported in this dissertation

may be extended by using the RF micro electro mechanical systems (MEMS)

switches which give the superior performance than the PIN diodes with respect to

bandwidth, linearity, power consumption, insertion loss and isolation. The specific

disadvantage of this PIN diode is that, it is unsuitable for reconfigurable antenna

design where a large number of switches may be employed and individual device

losses have a cumulative impact on overall antenna performance. Additionally, the

non-linear nature of solid-state semiconductor switches always has the potential to

introduce undesirable inter-modulation products into the RF signal path.

The controlling of PIN diodes/MEMS can be made programmable. In order to

control more number of switches, a switch matrix can be preloaded into a memory

as a look-up table and that memory will be recalled by a simple embedded

program.

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AGILENT ADS MOMENTUM

Momentum is a part of Advanced Design System and gives the simulation tools

need to evaluate and design modern communications systems products. The key

features of momentum as follows.

• An electromagnetic simulator based on the Method of Moments

• Adaptive frequency sampling for fast, accurate, simulation results

• Optimization tools that alter geometric dimensions of a design to achieve

performance specifications

• Comprehensive data display tools for viewing results

• Equation and expression capability for performing calculations on simulated data

• Full integration in the ADS circuit simulation environment allowing EM/Circuit

Co-simulation

Momentum is an electromagnetic simulator that computes S-parameters for

general planar circuits, including microstrip, slot line, stripline, coplanar

waveguide, and other topologies. Vias and air bridges connect topologies between

layers, so we can simulate multilayer RF/microwave printed circuit boards,

hybrids, multichip modules, and integrated circuits. Momentum gives a complete

tool set to predict the performance of high-frequency circuit boards, antennas, and

ICs. Momentum optimization extends momentum capability to a true design

automation tool. The momentum optimization process varies geometry parameters

automatically to help us achieve the optimal structure that meets the circuit or

device performance goals. Momentum visualization is an option that gives users a

3-dimensional perspective of simulation results, enabling us to view and animate

current flow in conductors and slots, and view both 2D and 3D representations of

far-field radiation patterns.

The following section describes the overview of the momentum.

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MOMENTUM OVERVIEW

Momentum commands are available from the Layout window. The following

steps describe a typical process for creating and simulating a design with

Momentum:

1. Create a physical design. You start with the physical dimensions of a planar

design, such as a patch antenna or the traces on a multilayer printed circuit board.

There are three ways to enter a design into Advanced Design System:

• Convert a schematic into a physical layout

• Draw the design using Layout

• Import a layout from another simulator or design system. Advanced Design

System can import files in a variety of formats.

2. Choose Momentum or Momentum RF mode. Momentum can operate in two

simulation modes: microwave or RF. You can select the mode based on your

design goals. Use Momentum (microwave) mode for designs requiring full-wave

electromagnetic simulations that include microwave radiation effects. Use

Momentum RF mode for designs that are geometrically complex, electrically

small, and do not radiate. You might also choose Momentum RF mode for quick

simulations on new microwave models that can ignore radiation effects, and to

conserve computer resources.

3. Define the substrate characteristics. A substrate is the media upon which the

circuit resides. For example, a multilayer PC board consists of various layers of

metal, insulating or dielectric material, and ground planes. Other designs may

include covers, or they may be open and radiate into air. A complete substrate

definition is required in order to simulate a design. The substrate definition

includes the number of layers in the substrate and the composition of each layer.

This is also where you position the layers of your physical design within the

substrate, and specify the metal characteristics of these layers.

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4. Solve the substrate. Momentum calculates the Green’s functions that

characterize the substrate for a specified frequency range. These calculations are

stored in a database, and used later on in the simulation process.

5. Assign port properties. Ports enable you to inject energy into a circuit, which

is necessary in order to analyze the behavior of your circuit. You apply ports to a

circuit when you create the circuit, and then assign port properties in Momentum.

There are several different types of ports that you can use in your circuit,

depending on your application.

6. Add a box or a waveguide. These elements enable you to specify boundaries

on substrates along the horizontal plane. Without a box or waveguide, the

substrate is treated as being infinitely long in the horizontal direction. This

treatment is acceptable for many designs, but there may be instances where a

boundaries need to be taken into account during the simulation process. A box

specifies the boundaries as four perpendicular, vertical walls that make a box

around the substrate. A waveguide specifies two vertical walls that cut two sides

of the substrate.

7. Create Momentum components. Momentum components can be used in the

schematic design environment in combination with all the standard ADS active

and passive components to build and simulate circuits including the parasitic

layout effects. The Momentum engine is automatically invoked to generate an S-

parameter model for the Momentum component during the circuit simulation.

8. Set up and generate a circuit mesh. A mesh is a pattern of rectangles and

triangles that are applied to a design in order to break down (discretize) the design

into small cells. A mesh is required in order to simulate the design effectively.

You can specify a variety of mesh parameters to customize the mesh to your

design, or use default values and let Momentum generate an optimal mesh

automatically.

9. Simulate the circuit. You set up a simulation by specifying the parameters of a

frequency plan, such as the frequency range of the simulation and the sweep type.

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When the setup is complete, you run the simulation. The simulation process uses

the Green’s functions computed for the substrate, plus the mesh pattern, and the

currents in the design are calculated. S-parameters are then computed based on the

currents. If the Adaptive Frequency Sample sweep type is chosen, a fast, accurate

simulation is generated, based on a rational fit model.

10. View the results. The data from Momentum simulation is saved as S-

parameters or as fields. Use the Data Display or Visualization to view S-

parameters and far-field radiation patterns.

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ANTENNA MEASUREMENTS

General Requirements of Antenna Measurement Procedures

The ideal condition for measuring the far field characteristics of an antenna

is its illumination by a uniform plane wave. This is a wave, in which has a plane

wave front with the field vectors being constant across it. If Dmax is the maximum

dimension of the antenna under test (AUT), a distance Rmin from the source of a

spherical wave is given by

Rmin = 2D2/λ

This will ensure that the maximum phase difference between a plane wave

and the spherical wave at the aperture of the AUT is |cjÕ ¸ 22.5� often many

antennas, because of their complex structural configuration and excitation method

cannot be investigated analytically. Experimental results are needed soften to

validate theoretical data.

Experimental investigations suffer from a number of drawbacks such as:

1. For pattern measurements, the distance to the far field region (m × �Øv² ) is

too long even for outside ranges. It also becomes difficult to keep unwanted

reflections from the ground and the surrounding objects below acceptable

levels.

2. In many cases, it may be impractical to move the antenna from the

operating environment to the measuring site.

3. For some antennas, such as phased arrays, the time required to measure the

necessary characteristics might be enormous.

4. Outside measuring systems provide an uncontrolled environment, and they

do not possess an all- weather capability.

5. Enclosed measuring systems usually cannot accommodate large antenna

systems (such as ships, aircrafts and large spacecrafts).

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6. Measurements techniques in general are expensive.

Some of the above shortcomings can be overcome by using special

techniques such as the far-field pattern prediction from near-field measurements

scale model measurements, and automated commercial equipment specifically

designed for antenna measurements and utilizing computer assisted techniques.

Because of the accelerated progress made in aerospace / defense related systems

(with increasingly small design margins), more accurate measurement methods

were necessary. To accommodate these requirements improved instrument and

measuring techniques were developed which include tapered anechoic chambers,

compact ranges, near field probing techniques and swept frequency measurements,

indirect measurements of antenna characteristics and automated test system’s

performance are the pattern (amplitude and phase), gain, efficiency, impedance,

etc.

Antenna Test Ranges

The testing and evaluation of antennas are performed in antenna ranges.

Typically there exist indoor and outdoor ranges and limitations are associated with

both of them. Outdoor ranges are not protected from environmental conditions

whereas indoor facilities are limited by space restrictions. Because some of the

antenna characteristics are measured in the receiving mode and require far field

criteria, the ideal field incident upon the test antenna should be a uniform plane

wave. To meet this specification a larger space is usually required and it limits the

value of indoor facilities. The classification of the test ranges is shown in Fig.1.

Figure.1 Different Types of Antenna Test Ranges

Advantages of outdoor ranges:

1. Large antennas can be tested.

2. Very low frequency antennas can be tested.

3. No absorbers are required.

4. No need to do complicated near field to far field conversion.

Limitations:

1. Interference from external environment.

2. High power transmitters due to long distances.

Advantages of Indoor ranges

1. No interference from external environment.

2. Accurate results by implementation of near field transformation.

3. Transmitting power is limited.

4. Availability of quit zone in indoor ranges.

Limitations:

1. Large antennas cannot be tested, far field is very large.

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Figure.1 Different Types of Antenna Test Ranges

Advantages of outdoor ranges:

Large antennas can be tested.

low frequency antennas can be tested.

No absorbers are required.

No need to do complicated near field to far field conversion.

Interference from external environment.

High power transmitters due to long distances.

Advantages of Indoor ranges:

No interference from external environment.

Accurate results by implementation of near field transformation.

Transmitting power is limited.

Availability of quit zone in indoor ranges.

Large antennas cannot be tested, far field is very large.

No need to do complicated near field to far field conversion.

Accurate results by implementation of near field transformation.

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2. For more accurate results proper grounding and shielding of chamber is

necessary.

Outdoor Test Ranges

Elevated Ranges

Elevated ranges are usually designed to operate mostly over smooth

terrains. The antennas are mounted on towers or roofs of adjacent buildings. These

ranges are used to test physically large antennas. A geometrical configuration is

shown in the Fig.2.

The contributions from the surrounding are usually reduced or eliminated by

1. Carefully selecting the directivity and side lobe level of the antenna.

2. Clearing the line of sight between the antennas.

3. Redirecting or absorbing any obstacles from the range surface and/or

from any obstacles that cannot be removed.

4. Utilizing special signal processing techniques such as modulation

tagging of the desired signal by using short pulses.

Tx Antenna Direct Ray Rx Antenna

Reflected Ray

Figure.2 Elevated range

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Ground Reflection Ranges

In general, there are two basic types of antenna ranges, the reflections and

the free-space. The reflection ranges can create a constructive interference in the

region of the test antenna, which is referred to as the “quite zone”. This is

accomplished by designing the ranges so that secular reflections from the ground,

as shown in the Fig.3 combine constructively with direct rays.

Tx Antenna Rx Antenna

Direct Ray

Reflected Ray

Figure.3 Reflection range

Usually it is desired for the illuminating field to have small and symmetric

amplitude taper. This can be achieved by adjusting the transmitting antenna height

while maintaining constant that of the receiving surface and they are usually

employed in the UHF region for measurements of patterns of moderately broad

antenna. They are also used for operating in the UHF.

Slant Ranges

Slant ranges are designed so that the test antenna, along with its positioner,

is modulated at a fixed height on a non conducting tower while the source

(transmitting) antenna is placed near the ground, as shown in the Fig.4. The source

antenna is positioned so that the pattern maximum, of its free space radiation is

Reflecting

surface

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oriented towards the center of the test antenna. The first null directed toward the

ground spectral reflection point to suppress reflected signals. Slant ranges in

general are more compact than elevated ranges as they require less land

Test Antenna

Source Antenna

Figure.4 Slant Range

Indoor Test Ranges

Anechoic Chamber

The Anechoic Chambers are the most popular antenna measurement sites

especially in microwave frequency range. They provide convenience and

controlled EM environment. However, they are very complex and expensive

facilities. An Anechoic Chamber is typically a large room whose walls, floor,

ceiling are first EM isolated by steel sheet. Besides, all inner surfaces of the

chamber are lined with RF/Microwave absorbers.

Absorbing materials are with much improved characteristics proving

reflection coefficients as low as -50 dB at normal incidence for a thickness of

about four wave lengths are used in the chamber. Reflections increases with

increase in angle of incidence. A typical absorbing element has the form of

pyramid or a wedge shape. Pyramids are designed to absorb best the waves in

normal incidence, while they do not perform well at large angles of incidence.

Their resistance gradually decreases as the pyramid’s cross section increases.

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The pyramidal and wedge shaped absorbers are shown in Fig.5 and Fig.6

these absorbers are used in the anechoic chamber for the better results.

Figure.5 Pyramidal shaped absorbers

Figure.6 Wedge shaped absorbers

Wedges, on other hand, perform much better than pyramids for waves,

which travel nearly parallel to their ridges.

There are two types of anechoic chamber designs:

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1. Rectangular Chambers

It is usually designed to stimulate free space conditions. High quality

absorbing material such as carbon impregnated poly eurithrene pyramidal

absorbers are used, on surfaces that reflect energy directly towards the test region

in order to reduce the reflected energy level as shown in Fig.7. Even though the

sidewalls, floor and ceiling are covered with absorbing material, significant

specular reflections can occur from these surfaces, especially for the case of large

angles of the incidence. One precaution that can be taken is to limit the angles of

incidence to those for which the reflected energy is for below the level consistent

with the accuracy required for the measurements to be made in the chamber.

Often, for the high quality absorbers, this limit is taken to be a range of incidence

angles of 00 to 70

0 (as measured from the normal to the wall). For the rectangular

chamber this leads to a restriction of the overall width or height of the chamber.

The actual width and height chosen shall depend upon the magnitude of the errors

that can be tolerated and upon the measured characteristics of the absorbing

material used to line the walls. Additionally the room width and the size of the

source antenna should be chosen such that no part of the main lobe of the source

antenna is incident upon the side walls, floor and ceiling.

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Figure.7 Rectangular anechoic chamber

2. Tapered Chambers

The design of both chambers is based on geometrical optics considerations,

whose goal is to minimize the amplitude and phase ripples in the test zone, which

are due to the imperfect absorption by the wall lining. The Tapered chamber has

the advantage of turning by moving the source antenna closer to (at higher

frequencies) or further closer (at lower frequencies) the apex of the taper. Thus,

the reflected rays are adjusted to produce nearly constructive interference with the

directed rays at the test location. Simple anechoic chambers are limited by

distance requirements of the far-field measurements of large antennas or

scatterers. There are two type basic approaches to overcome this limitation. One is

presented by the compact Antenna Test Ranges (CATRs), which produce a nearly

uniform plane wave in a very short distance via a system of reflectors. Another

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approach is presented by techniques based on near-field zone or in Fresnel zone of

AUT. The tapered anechoic chamber is shown in the Fig.8.

Compact Antenna Test Ranges

Microwave antenna test measurements often require that the radiator

under test be illuminated by a uniform plane wave. This is usually achieved only

in the far field region, which in many cases dictates very large distances.

Feed

Figure.9 Compact ranges

Figure.8 Tapered anechoic chamber

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The compact ranges are shown in this Fig.9. The requirement of the plane

wave illumination can be achieved by techniques that require smaller distances

and the use of a reflector. To accomplish this source antenna is used as an offset

feed that illuminates a paraboloidal reflector. The illuminated reflector converts he

impinging spherical waves into plane waves. The geometrical arrangement is

shown in figure. This techniques lead to far field pattern simulation. It requires

smaller distances than conventional methods and it is referred to as a ‘compact

range’. Usually the linear dimensions of the reflector are three to four times

greater than those of the test antenna.

Network Analyzer

RF or microwave energy can be viewed as a light wave. The energy is

either reflected from or transmitted through the test device. By measuring the

amplitude ratios and phase differences between the incident and the two (reflected

and transmitted) new waves we can determine the reflection (impedance) and

transmission characteristics of the device. There may be many names for these

measurements, some use magnitude information only (scalar), others include both

magnitude and phase information (vector). All names can be classified under the

general headings of transmission and reflection.

Figure.10 Internal architecture of network analyzer

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Here a HP-8722D vector network analyzer has been used for VSWR

measurements all developed antennas

A network analyzer measurement system can be divided into four major parts.

1. A signal source providing the incident signal.

2. Signal separation devices to separate the incident, reflected and transmitted

signals.

3. A receiver to convert the microwave signals to a lower intermediate (IF)

signal.

4. Signal processor/display sections to process the IF signals and display the

information on CRT.

The signal source (RF or microwave) produces the incident signal used to

simulate the test device. The test device responds by the reflecting part of the

incident and transmits the remaining part. By sweeping the frequency of the

source the frequency response of the test device can be determined.

The next step in the measurement process is to separate the incident, the

reflected and the transmitted signals. Once separated, their individual magnitude

and phase differences can be measured. This can be accomplished through the use

of directional couplers, bridges, power splitters or even high impedance probes.

Reflection measurements require a directional device. Separation of the

incident and reflected signals can be accomplished using either a dual directional

coupler or a power splitter with a single directional coupler or bridge. The receiver

provides the means of converting the RF or microwave voltages to a lower IF or

DC signal to allow for a more accurate measurement. Lastly, the IF signals must

be measured and processed before the relevant information can be displayed in an

appropriate format on the CRT.

Source Antennas for Antenna Ranges

With the reception of a few highly specialized installations, antennas test

ranges are designed to operate over wide band of frequencies. This means that

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they shall be equipped with a family of source antennas and signal sources

covering the entire band. The antennas shall, of course, have the beam widths and

polarization properties consistent with the measurements to be performed on the

range. For frequencies above 400 MHz families of parabolas with broadband feeds

are most often used and for frequencies above 1 GHz horn antennas are used. A

pyramidal horn antenna is being used as a source antenna for measurements.

Signal Sources

The selection of the transmitter depends upon several system

considerations. There are a number of types of signal sources available such as

triode cavity oscillators, klystrons, magnetrons, backward wave oscillators and

various solid-state oscillators. Whatever type of signal is chosen, the following

performance requirements apply:

Frequency control: A means shall be available to tune the signal source to the

desired frequency. For the case of oscillators that can be electrically tuned, an

adjustable, regulated power supply is required.

Frequency stability: Since the antennas and their associated radio-frequency

circuitry are highly frequency sensitive, it is necessary that the signal-source

frequency remain constant over the measured period, which may be in excess of

30 minutes.

Spectral purity: Some types of oscillations are rich in harmonics, which if

transmitted, would contaminate the desired signal. In some cases spurious or non-

harmonically related signals are generated. Hence the source selected must have

degree of spectral purity.

Power level: The required power output of the signal source for a particular

measurement is dependent upon the source and the test antenna gains, the receiver

sensitivity, the transmission loss between the two antennas, and the dynamic range

required for the measurement. Accordingly power level must be chosen.

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Modulation: For some systems amplitude modulation is required, hence the

signal sources should have that capability. There are cases where special pulse

shaping is required to reduce the distortion of the pulse spectrum.

Receiving Systems

The receiving subsystem used in the antenna’s amplitude/pattern

measurement system may be simply a crystal detector (usually mounted directly

on the test antenna or in the case of a scale model, inside the model) and its

associated amplifier, the output of which supplies the signal to the recorder. With

this system the transmitter is usually modulated. For high sensitivity even the

mixers can be used in conjunction with receivers.

Antenna Pattern Recorder

The Antenna-pattern recorder provides a means of obtaining a visual

display of the antenna pattern. It is used to plot the relative signal strength

received by the test antenna as a function of the angular position of the antenna.

The signal to be plotted is obtained from the output of a receiver or directly from a

microwave detector, depending upon the type of receiving system used. The

position information is normally obtained from synchro transmitters or digital

encoders geared to the positioned axes.

Typical antenna-pattern recorders are electro-mechanical devices

employing servo systems to drive the recorder axes. A chart servo system usually

positions the recording paper as a function of the angular position of the antenna.

A pen servo system positions a recording pen in response to the amplitude of the

input signal. Ink-writing systems are mostly used in preference to electric, thermal,

pressure-sensitive or photographic systems because of the high quality, high

writing speed, reproducibility, economy and simplicity of an ink system.

The antenna pattern may be recorded in either polar or rectangular format.

The polar form is often preferred for plotting patterns of antennas that are not

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highly directional. The polar format is particularly useful for visualizing the power

distribution in space. In the rectangular format the signal amplitude is the y-axis

(ordinate) and the position angle is the x-axis (abscissa). The rectangular format

permits narrow beam patterns to be recorded in finer detail because the pattern

does not become crowded in regions of relatively low gain as it does in a polar

graph. To provide adequate resolution in a rectangular display of patterns of

different beam widths, selectable chart scales are required.

Data Processing and Control Computers

An on-line instrumentation minicomputer provides a natural solution to the

data gathering, control and data-processing requirements of an automatic antenna-

measurement system. Instrumentation computers can be equipped with a variety of

input-output devices, depending upon the requirements of the particular

measurement program. Computer plotters can be employed to provide a variety of

visual displays of antenna patterns such as contour plots and three-dimensional

plots. For lengthy measurement programs or for programming convenience, a

larger central computer at the user’s facility can process the recorded data.

Measurement of the Antenna

Testing of the antenna includes the measurement of return loss and

radiation pattern. From the obtained radiation patterns gain and beam widths are

calculated.

1. RETURN LOSS MEASUREMENT

In the measurement of return loss HP-8722D vector network analyzer has been

used.

Measurement Procedure

1. Adjust the sweep oscillator RF power level so that the reference channel

level is in operate position of the scale. This ensures that there is enough

power for phase locking.

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2. Select sweep frequency range by selecting start and stop frequency.

3. Select one port s11 for calibration measurement.

4. Select log amplitude mode on display.

5. Calibrate the network analyzer by connecting the standard short circuit,

open circuit and matched loads at the test port. Observe the trace on the

display to get a solid reference line.

6. Remove the standards and connect the antenna and observe the shift in the

trace of the display. The display can be changed for obtaining the return

loss, reflection coefficient, and impedance over the selected frequency

band.

Return loss in dB=20log (ρ), where ρ is the reflection coefficient and

Calibration of the network analyzer is done by using the standard loads

supplied by the manufacturer. All the measurements are carried out carefully by

not disturbing the cable setup, which is necessary for accurate measurement.

2. RADIATION PATTERN MEASUREMENT IN AN ANECHOIC

CHAMBER

Measurement Procedure

1. Mount the antenna under test on the antenna positioned as shown in Fig.11.

2. Mount the transmitting antenna, which is connected to a signal source.

3. Transmit the signal of the desired frequency from the transmitting antenna.

4. Receive the signal from the crystal detector that in turn is applied to the

spectrum analyzer.

5. Adjust the attenuation of the spectrum analyzer to ensure that the signal is

within the range of the spectrum analyzer.

ρρ

−

+=

1

1VSWR

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6. To obtain the pattern in orthogonal plane, rotate the test antenna by 90° and

repeat step 5.

Figure.11 Setup for antenna radiation pattern measurement in an anechoic

chamber

For the test antenna the radiation pattern measurements were carried out for both

horizontal and vertical polarizations.

Beam width

Beam width is calculated from the radiation pattern measured on the

calibrated chart. The half power beam width is equal to the angular width between

directions where the gain decreases by 3dB (the radiated field reduces to 1/√2 if

the maximum value.).

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Gain

The power gain of an antenna is 4π times the ratio of the power radiated per

unit solid angle in the direction of maximum radiation to the net power accepted

by the antenna from its generator. Two general categories of gain measurement

methods exist. These are the “absolute-gain measurements” and the “gain transfer

measurements”. The first method is used when extremely high accuracies are

necessary and is usually employed in laboratories that specialize in the calibration

of standards. Here we used the second method in which the gain of the antenna

under test is measured by comparing it to that of the standard gain antenna.

Gain of the antenna is measured by comparing gain pattern of the antenna

under test to that of the standard linear isotropic antenna. Radiation pattern of the

test antenna and standard gain antenna are measured with the same transmitting

antenna. The difference between the measured power levels of the standard gain

antenna and test antenna gives the gain of the test antenna. The gain measurements

require essentially the same environment as the pattern measurements, although

they are not so much sensitive to reflections and EM interference. To measure the

gain of the antennas operating above 1 GHz, usually, free-space ranges are used.

Between 0.1 GHz and 1 GHz, ground reflection ranges are used.

Procedure

1. Fix vertical polarization of the transmit antenna

2. Transmit signal for known frequency

3. Mount the test antenna in azimuth plane rotate the antenna through 360o

and record the power received on the recorder. Note down the power

output of the transmitter at each frequency.

4. Replace the test antenna with standard gain horn antenna and record the

power received in the spectrum analyzer without any change at transmit

or receive end. Make sure that the test power output of the transmitter is

same as that at step3.

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5. For gain at various frequencies repeat steps 1 to 4

Gain Calculations

1. Calculate the gain of the antenna under test using the following procedure

2. From the Gain vs. frequency plot of the standard gain horn, calculate the

Gain of the standard Gain horn (say X dB)

3. For the same frequency find the difference in dB between the amplitude of

the test antenna and standard Gain horn (say Y dB)

4. The gain of the antenna under test for the frequency is given by G=(Y-

X+A) dB

3. RADIATION PATTERN MEASUREMENT USING OUTDOOR

ANTENNA TEST RANGE

This facility is used at Astra Microwave Products Limited, Hyderabad

There are three outdoor antenna test ranges installed in Astra Microwave products

Limited. These are 22m, 120m, and 1km. with the following salient features. The

range that was selected to perform the testing of the various antennas discussed in

this thesis was the 22m outdoor elevated range.

• Ranges: 3 outdoor ranges (22m,120m, and 1km)

• Frequencies: 100 MHz to 18 GHz

• Measurement type: amplitude

• Dynamic ranges: 80 dB

• Sensitivity: -124 dBm

• Maximum size of the antenna 6m

• Positioner: azimuth over elevation

Measurement of Directional Pattern

Measurement of the directional pattern of the antenna reveals a lot about the

functioning of the antenna and gives an overview about its performance. The

pattern is plotted in both the horizontal as well as the vertical plane of the antenna

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by using a transmitting antenna operating in the same frequency band of the AUT.

There are three outdoor antenna test ranges installed in Astra Microwave products

Limited. These are 22m, 120m, and 1km. The range that was selected to perform

the testing of the various antennas in this thesis was the 22m outdoor elevated

range.

Figure.12 Antenna test set up

The transmitting signal is generated by a sweep oscillator at the transmit antenna.

The transmitted signal is approximately amplified with a suitable gain to

overcome the path losses that are especially prominent in the microwave

frequencies. The received signal is fed to a network analyzer and later to the

digital pattern recorder that plots the received pattern at various points.

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Gain Measurement Procedure

The method of comparison was employed to measure the gain of the AUT.

In this method, a standard antenna of known gain is connected to the receiver and

the transmit antenna is pointed in the direction of maximum signal intensity. The

input to the transmitting antenna is adjusted to a convenient level, and the readings

are noted. The difference in the readings of the two antennas is calculated. This

value is either subtracted from or added to the gain of the standard gain antenna

depending on whether the AUT signal is lower than or higher than the standard

gain antenna signal respectively. This final value gives us the gain of the AUT.

Test procedure for measurement of Antenna Beam width

• Set the center frequency of the antenna in the signal source.

• Align the direction of both the transmitting and the receiving

antennas on the same angle of elevation.

• Rotate the antenna under test through 360º in the azimuth with the

help of positioner.

• Plot the radiation pattern by using the digital pattern recorder. repeat the

steps for the entire band of frequencies for the antenna under test.

Test Set Up for the Radiation Pattern and Gain measurements

• Mount the standard gain antenna of known frequency on the positioner.

• Keep it on axis direction.

• Set the center frequency band in the transmitter.

• Record the plot of the standard gain antenna with the help of the spectrum

analyzer for on the axis of the standard gain antenna on the

center frequency.

• Dismount the standard gain antenna and place the antenna under test in its

place.

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• Now rotate the antenna in 360 degrees in azimuth with the help of the

positioner and record the antenna gain with the help of the DPR.

• Find the difference of the gains of the standard gain antenna and antenna

under test and add the known gain of the standard gain antenna to

the difference. The final result so obtained gives the gain at the center

frequency.

• Repeat the above steps in the entire band of frequencies of the frequency

band.

Axial Ratio Measurement

Axial ratio is the ratio of major axis to the minor axis of the

polarization ellipse. The axial ratio is determined as a function direction by

using the rotating source method. The method consists of continuously

rotating a linearly polarized source antenna (a pyramidal horn antenna is being

used for the measurements) as the direction of observation of the test antenna

is changed. This method is of greatest value for testing nearly circularly

polarized antennas. The rotating source antenna causes the tilt angle tw of the

incident field to rotate at the same rate. Care shall be taken to ensure that the

time response of the recording system can adequately follow the excursions in

tw.

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Figure.13 Test set up for radiation pattern and gain measurements

SNO EQUIPMENT QUANTITY

1 Synthesized micro sweeper 1

2 Spectrum Analyzer 1

3 Azimuth over elevation positioner 1

4 Flam & Russell Positioner controller 1

5 Positioner cables 1

6 ACORN Digital pattern recorder

Flam & Russell Inc-944 (version 2)

a) CPU

b) Color monitor

c) Laser printer

1

7 Rotary joint 1

TABLE.1 List of Equipments used in Test set up

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PHOTOGRAPHS OF THE ACTUAL MEASUREMENT SET UP USED

Figure.14 Network analyzer kit

Figure.15 Anechoic chamber

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Figure.16 Test set up for radiation pattern, gain measurement

Figure.17 Transmitting antenna used