Post on 21-Apr-2018
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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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• 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