anechoic chamberself

7/31/2019 Anechoic Chamberself

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A PROJECT REPORT

ON

MEASURENT OF RCS OF METALLIC WIRES USING

VECTOR NETWORK ANALYSER IN ANECHOIC

CHAMBER

SUBMITTED TO : SUBMITTED BY:

Dr. PRASHANT VASHISTHA ABHISHEK GAUR(JIET COLLEGE)

D.R.D.O BHISHM NARAYAN CHOUHAN (MECRC)RAHUL GAUR(MECRC)

BHUPENDRA CHOUDARY(GOVNT. COLLEGE

KOTA)


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ACKNOWLEDGEMENT

With great pleasure and respect we express our

thanks to Dr. S.R. Vadera , Director, Defence

laboratory, jodhpur for providing us an

opportunity to accomplish training at this

institute.

We are grateful to our Training guide Dr.Prasant

Vasistha for his benevolent help and support

through out the training period, without whom

things could not have proceeded as smoothly as

they did.

We are also indebted to our parents for their

valuable and constant encouragement.


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CONTENTS:

1. Introduction about Anechoic Chamber2. Types

3. Instrumentation

I. Vector network analyzer

a. Definition

b. Block diagram

c. S-parameters

d. Various applications

4. Radar cross section (RCS)

I. Definition

II. Dependence

III. Various measurement

5. RCS measurement of metallic wire

6. Preparation of models7. Measurement and result


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ANECHOIC CHAMBER:-

DEFINITION:

An anechoic chamber (an-echoic or non-echoing) is a room designed to stop

reflections of either sound or electromagnetic waves. They are also insulated from

exterior sources of noise.

Anechoic chambers were originally used in the context of acoustics (sound waves) to

minimize the reflections of a room. More recently, rooms designed to reduce

reflection and external noises in radio frequencies have been used to test antennas,

radars, or electromagnetic interference.

Anechoic chambers range from small compartments to ones as large as aircraft

hangars. The size of the chamber depends on the size of the objects to be tested

and the frequency range of the signals used, although scale models can sometimes

be used by testing at shorter wavelengths.


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TYPES OF ANECHOIC CHAMBER:

Acoustic anechoic chambers:

Anechoic chambers are commonly used in acoustics to conduct experiments innominally "free field" conditions. All sound energy will be traveling away from the

source with almost none reflected back. Common anechoic chamber experiments

include measuring the transfer function of a loudspeaker or the directivity of noise

radiation from industrial machinery.

Semi-anechoic chambers:

Full anechoic chambers aim to absorb energy in all directions. Semi-anechoic

chambers have a solid floor that acts as a work surface for supporting heavy items,

such as cars, washing machines, or industrial machinery, rather than the mesh floor

grille over absorbent tiles found in full anechoic chambers. This floor is damped and

floating on absorbent buffers to isolate it from outside vibration or electromagnetic

signals. A recording studio may utilize a semi-anechoic chamber to produce high-

quality music free of outside noise and unwanted echoes.

Radio-frequency anechoic chambers:

An RF anechoic chamber.

The internal appearance of the radio frequency (RF) anechoic chamber is

sometimes similar to that of an acoustic anechoic chamber, however, the interior

surfaces of the RF anechoic chamber are covered with radiation absorbent material

(RAM) instead of acoustically absorbent material. The RF anechoic chamber is

typically used to house the equipment for performing measurements of antenna
http://en.wikipedia.org/wiki/Antenna_%28radio%29http://en.wikipedia.org/wiki/File:Radio-frequency-anechoic-chamber-HDR-0a.jpghttp://en.wikipedia.org/wiki/Antenna_%28radio%29


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radiation patterns, electromagnetic compatibility (EMC) and radar cross section

measurements.

RADIATION ABSORBENT MATERIAL (RAM):

The RAM is designed and shaped to absorb incident RF radiation (also known as

non-ionising radiation), as effectively as possible, from as many incident directions

as possible. The more effective the RAM is the less will be the level of reflected RF

radiation. Many measurements in electromagnetic compatibility (EMC) and antenna

radiation patterns require that spurious signals arising from the test setup, including

reflections, are negligible to avoid the risk of causing measurement errors and

ambiguities.

Types of RAM:

(A) Iron ball paint:

One of the most commonly known types of RAM is iron ball paint. It contains tiny

spheres coated with carbonyl iron or ferrite.

Radar waves induce molecular oscillations from the alternating magnetic field in this

paint, which leads to conversion of the radar energy into heat. The heat is then

transferred to the aircraft and dissipated. The iron particles in the paint are obtained

by decomposition of iron pentacarbonyl and may contain traces of carbon, oxygen

and nitrogen.

(B) Foam absorber:

Foam absorber is used as lining of anechoic chambers for electromagnetic radiation

measurements. This material typically consists of a fireproofed urethane foam

loaded with carbon black, and cut into long pyramids. The length from base to tip of

the pyramid structure is chosen based on the lowest expected frequency and the

amount of absorption required..


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(C) Jaumann absorber:

A Jaumann absorber or Jaumann layer is a radar absorbent device. Being a

resonant absorber (i.e. it uses wave interfering to cancel the reflected wave), the

Jaumann layer is dependent upon the /4 spacing between the first reflective surface

and the ground plane and between the two reflective surfaces (a total of /4 + /4).

One of the most effective types of RAM comprises arrays of pyramid shaped pieces,

each of which is constructed from a suitably lossy material.

To work effectively, all internal surfaces of the anechoic chamber must be entirely

covered with RAM. Sections of RAM may be temporarily removed to install

equipment but they must be replaced before performing any tests. To be sufficiently

lossy, RAM can neither be a good electrical conductor nor a good electrical insulator

as neither type actually absorbs any power. Typically pyramidal RAM will comprise a

rubberized foam material impregnated with controlled mixtures of carbon and iron.

The length from base to tip of the pyramid structure is chosen based on the lowest

expected frequency and the amount of absorption required.

For low frequency damping, this distance is often 24 inches, while high frequency

panels are as short as 34 inches. Panels of RAM are installed with the tips pointing

inward to the chamber. Pyramidal RAM attenuates signal by two effects: scattering

and absorption. Scattering can occur both coherently, when reflected waves are in-

phase but directed away from the receiver, or incoherently where waves are picked

up by the receiver but are out of phase and thus have lower signal strength. This

incoherent scattering also occurs within the foam structure, with the suspended

carbon particles promoting destructive interference. Internal scattering can result in

as much as 10dB of attenuation. Meanwhile, the pyramid shapes are cut at angles

that maximize the number of bounces a wave makes within the structure. With each

bounce, the wave loses energy to the foam material and thus exits with lower signal

strength.

An alternative type of RAM comprises flat plates of ferrite material, in the form of flat

tiles fixed to all interior surfaces of the chamber. This type has a smaller effective

frequency range than the pyramidal RAM and is designed to be fixed to good

conductive surfaces. It is generally easier to fit and more durable than the pyramidal


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type RAM but is less effective at higher frequencies. Its performance might however

be quite adequate if tests are limited to lower frequencies (ferrite plates have a

damping curve that makes them most effective between 301000 MHz).There is also

a hybrid type, a ferrite in pyramidal shape. Containing the advantages of both

technologies the frequency range can be maximized while the pyramid remains

small (10 cm).

CLOSE UP OF A PYRAMIDAL RAM


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INSTRUMENTATION OF ANECHOIC CHAMBER:-

(1) NETWORK ANALYZER:-

Anetwork analyzer is an instrument that measures the network parameters of

electrical networks. Today, network analyzers commonly measure sparameters

because reflection and transmission of electrical networks are easy to measure at

high frequencies, but there are other network parameter sets such as y-parameters,

z-parameters, and h-parameters. Network analyzers are often used to characterize

two-port networks such as amplifiers and filters.

ZVA40 VECTOR NETWORK ANALYZER

Network analyzers are used mostly at high frequencies; operating frequencies can

range from 9 kHz to 110 GHz. Special types of network analyzers can also cover

lower frequency ranges down to 1 Hz. These network analyzers can be used for

example for the stability analysis of open loops or for the measurement of audio and

ultrasonic components.


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TYPES OF VNA:

The two main types of network analyzers are

Scalar Network Analyzer (SNA) measures amplitude properties only

Vector Network Analyzer (VNA) measures both amplitude and phase

properties.

Generalized Network Analyzer:

Here is a generalized block diagram of a network analyzer, showing the major

signal-processing sections. In order to measure the incident, reflected and

transmitted signal, four sections are required: Source for stimulus

Signal-separation devices

Receivers that down convert and detect the signals

Processor/display for calculating and reviewing the results

Fig 1. Block diagram of generalized network analyzer


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PARTS OF GENERALIZED NETWORK ANAYZER:

Source:

The signal source supplies the stimulus for our stimulus-response test system. The

sources were either based on open-loop voltage-controlled oscillators (VCOs) which

were cheaper, or more expensive synthesized sweepers which provided higher

performance, especially for measuring narrowband devices.

Signal Separation:

The next major area is the signal separation block. Thehardware used for this

function is generally called the test set.There are two functions that our signal-

separation hardware mustprovide. The first is to measure a portion of the incident

signal toprovide a reference for ratioing. This can be done with splitters or directional

couplers. Splitters are usually resistive. They are non-directional devices (more on

directionality later) and can be very broadband. Directional couplers have very low

insertion loss (through the main arm) and good isolation and directivity. They are

generally used in microwave network analyzers. The second function of the

signalsplitting hardware is to separate the incident (forward) and reflected(reverse)traveling waves at the input of our DUT.

Receiver:

The receivers make the measurements. For the SNA, the receiver only measures

the magnitude of the signal. A receiver can be a detector diode that operates at the

test frequency. The simplest SNA will have a single test port, but more accurate

measurements are made when a reference port is also used.

For the VNA, the receiver measures both the magnitude and the phase of the signal.

It needs a reference channel to determine the phase, so a VNA needs at least two

receivers. The phase may be measured with a quadrature detector. A VNA requires

at least two receivers, but some will have three or four receivers to permit

simultaneous measurement of different parameters.


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Processor/Display:

The last major block of hardware in the network analyzer is the display/processor

section. This is where the reflection and transmission data is formatted in ways that

make it easy to interpret the measurement results. Most network analyzers have

similar features such as linear and logarithmic sweeps, linear and log formats, etc.

BLOCK DIAGRAM OF VNA:-


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Architecture of Vector network analyzer

Signal generator

Device under test

Receiver

1.Signal Generator:-

- In old days external function generator is used to produce reference signal

which is then incident to the device under test or to the target.Now a days there is

a inbuilt signal sources .

- networkanalyzers today have integrated,

synthesized sources, providing excellent

frequency resolution and stability.

2. Device under test :-

- The reference signal is applied to the device under

test..therefernce signal is alterd in magnitude and phase.

- It will change the magnitude component of the reference signal

because of its resistive nature..

- It will change the phase component of the reference signal

because of its reactive nature.

3.Receiver/ Detector:-

- generally receiver contains the comparators and conversionoscillator.

- comparators are used to compare the original reference signal andaltered signal which is obtained by passing the original signal through thedevice under test.

- basically reference signals have high frequencies and thecomparators of VNA operates at lower frequencies..so for VNA to have highfrequency range, some type of frequency conversion scheme is used toconvert the reference and altered signal to the frequency range of the VNAcomparators..


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Detector Types There are two basic ways of providing signal detection innetwork analyzers.

1. Diode detectors

2. Tuned Receivers

1.Diode Detectors-:

- Diode detector convert the RF signal level to a proportional DClevel.

- If the stimulus signal is amplitude modulated, the diode strips the RFcarrier from the modulation (this is called AC detection).

- Diode detection is inherently scalar,as phase information of the RFcarrier

is lost.

(a) Broadband Diode Detection:-

- The two main advantages of diode detectors are that they provide broadbandfrequency coverage ( < 10 MHz on the low end to > 26.5 GHz at the high end) andthey are inexpensive compared to a tuned receiver.

- Diode detectors provide medium sensitivity and dynamic range: they canmeasure signals to60 dBm or so and have a dynamic range around 60 to 75dB,depending on the detector type.

- One application where broadband diode detectors are very useful ismeasuring frequency-translating devices, particularly those with internal LOs.

Fig:-broadband diode detection


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Narrowband DetectionTuned Receiver:-

- The tuned receiver uses a local oscillator (LO) to mixthe RF down to a lower intermediate frequency (IF).The LO is eitherlocked to the RF or the IF signal so that the receivers in the networkanalyzer are always tuned to the RF signal present at the input.The IFsignal is bandpassfiltered,which narrows the receiver bandwidth andgreatly improves sensitivity and dynamic range.

- Modern analyzers use an analog-to-digital converter (ADC) anddigital-signal processing (DSP) to extract magnitude and phase information from theIF signal.The tuned-receiver approach is used in vector network analyzers andspectrum analyzers.

Fig:- narrowband detection tuned receiver


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4. Processor/Display:-- The last major block of hardware in the network analyzer is the

display/processor section.

- This is where the reflection and transmission data is formatted in

ways that make it easy to interpret the measurement results.

- Most network analyzers have similar features such as linear and

logarithmic sweeps, linear and log formats, polar plots, Smith charts, etc.Other

common features are trace markers, limit lines, and pass/fail testing.

Fig:-processor or display


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SCATTERING PARAMETERS:

Scattering parameters or S-parameters (the elements of a scattering matrix or S-

matrix) describe the electrical behavior of linear electrical networks when undergoing

various steady state stimuli by electrical signals.

The S-parameters are members of a family of similar parameters, other examples

being: Y-parameters, Z-parameters, H-parameters, T-parameters or ABCD-

parameters. They differ from these, in the sense that S-parametersdo not use open

or short circuit conditions to characterize a linear electrical network; instead matched

loads are used. These terminations are much easier to use at high signal

frequencies than open-circuit and short-circuit terminations. Moreover, the quantities

are measured in terms of power.

S-parameters are readily represented in matrix form and obey the rules of matrix

algebra.

Two-Port S-Parameters:
http://en.wikipedia.org/wiki/Linearhttp://en.wikipedia.org/wiki/Linear


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The S-parameter matrix for the 2-port network is probably the most commonly used

and serves as the basic building block for generating the higher order matrices for

larger networks. In this case the relationship between the reflected, incident power

waves and the S-parameter matrix is given by:

() () (

)

Expanding the matrices into equations gives:

and

Each equation gives the relationship between the reflected and incident power

waves at each of the network ports, 1 and 2, in terms of the network's individual S-

parameters, , ,, and. If one considers an incident power wave at port 1() there may result from it waves exiting from either port 1 itself () or port 2 ().However if, according to the definition of S-parameters, port 2 is terminated in a load

identical to the system impedance () then, by the maximum power transfertheorem, will be totally absorbed making equal to zero. Therefore

Similarly, if port 1 is terminated in the system impedance then becomes zero,giving

Each 2-port S-parameter has the following generic descriptions:

is the input port voltage reflection coefficient

is the reverse voltage gainis the forward voltage gain


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is the output port voltage reflection coefficient

S-Parameter properties of Two-port networks:

An amplifier operating under linear (small signal) conditions is a good example of a

non-reciprocal network and a matched attenuator is an example of a reciprocal

network. In the following cases we will assume that the input and output connections

are to ports 1 and 2 respectively which is the most common convention. The nominal

system impedance, frequency and any other factors which may influence the device,

such as temperature, must also be specified.

(A) Complex linear gain:

The complex linear gain G is given by

That is simply the voltage gain as a linear ratio of the output voltage divided by the

input voltage, all values expressed as complex quantities.

(B) Scalar linear gain:

The scalar linear gain (or linear gain magnitude) is given by

|| ||That is simply the scalar voltage gain as a linear ratio of the output voltage and the

input voltage. As this is a scalar quantity, the phase is not relevant in this case.

(C)Scalar logarithmic gain:

The scalar logarithmic (decibel or dB) expression for gain (g) is

||dB

This is more commonly used than scalar linear gain and a positive quantity is

normally understood as simply a 'gain'... A negative quantity can be expressed as a

'negative gain' or more usually as a 'loss' equivalent to its magnitude in dB. For

example, a 10 m length of cable may have a gain of - 1 dB at 100 MHz or a loss of 1

dB at 100 MHz.


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(D) Insertion loss:

In case the two measurement ports use the same reference impedance, the insertionloss (IL) is the dB expression of the transmission coefficient. It is thus given by.

||dB

It is the extra loss produced by the introduction of the DUT between the 2 reference

planes of the measurement. Notice that the extra loss can be introduced by intrinsic

loss in the DUT and/or mismatch. In case of extra loss the insertion loss is defined to

be positive.

(E) Input return loss:

Input return loss (R) is a scalar measure of how close the actual input impedanceof the network is to the nominal system impedance value and, expressed in

logarithmic magnitude, is given by

||dB

By definition, return loss is a positive scalar quantity implying the 2 pairs of

magnitude (|) symbols. The linear part,|| is equivalent to the reflected voltagemagnitude divided by the incident voltage magnitude.

(F) Output return loss:

The output return loss (R) has a similar definition to the input return loss butapplies to the output port (port 2) instead of the input port. It is given by

||dB

(G) Reverse gain and reverse isolation:

The scalar logarithmic (decibel or dB) expression for reverse gain () is: ||dB.

Often this will be expressed as reverse isolation () in which case it becomes apositive quantity equal to the magnitude of and the expression becomes:


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||||dB.

(H) Voltage reflection coefficient:

The voltage reflection coefficient at the input port () or at the output port () areequivalent to and respectively, so

As and are complex quantities, so are and.

(I) Voltage standing wave ratio:

The voltage standing wave ratio (VSWR) at a port, represented by the lower case 's',

is a similar measure of port match to return loss but is a scalar linear quantity, the

ratio of the standing wave maximum voltage to the standing wave minimum voltage.

It therefore relates to the magnitude of the voltage reflection coefficient and hence to

the magnitude of either for the input port or for the output port.

At the input port, the VSWR () is given by

|| ||

At the output port, the VSWR () is given by

||

||


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APPLICATIONS OF VECTOR NETWORK ANALYZER:-

(A) RADAR CROSS SECTION (RCS) :-

Radar cross section (RCS) is a measure of how detectable an object is with a radar.

A larger RCS indicates that an object is more easily detected.

An object reflects a limited amount of radar energy. A number of different factors

determine how much electromagnetic energy returns to the source such as:

Material of which the target is made;

Absolute size of the target;

Relative size of the target (in relation to the wavelength of the illuminatingradar);

The incident angle (angle at which the radar beam hits a particular portion of

target which depends upon shape of target and its orientation to the radar

source);

Reflected angle (angle at which the reflected beam leaves the part of the

target hit, it depends upon incident angle);

Strength of the radar emitter;

Distance between emitter-target-receiver.

Radar cross section is used to detect planes in a wide variation of ranges. For

example, a stealth aircraft (which is designed to have low detectability) will have

design features that give it a low RCS (such as absorbent paint, smooth surfaces,

surfaces specifically angled to reflect signal somewhere other than towards the

source), as opposed to a passenger airliner that will have a high RCS (bare metal,

rounded surfaces effectively guaranteed to reflect some signal back to the source,

lots of bumps like the engines, antennae, etc.).

4.1 DEFINITION:

Informally, the RCS of an object is the cross-sectional area of a perfectly reflecting

sphere that would produce the same strength reflection as would the object in

question. (Bigger sizes of this imaginary sphere would produce stronger reflections.)

Thus, RCS is an abstraction: The radar cross-sectional area of an object does not


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necessarily bear a direct relationship with the physical cross-sectional area of that

object but depends upon other factors.

More precisely, the RCS of a radar target is the hypothetical area required to

intercept the transmitted power density at the target such that if the total intercepted

power were re-radiated isotropically, the power density actually observed at the

receiver is produced. This is a complex statement that can be understood by

examining the monostatic (radar transmitter and receiver co-located) radar equation

one term at a time:

where

Pt= power transmitted by the radar (watts)

Gt= gain of the radar transmit antenna (dimensionless)

r= distance from the radar to the target (meters)

= radar cross section of the target (meters squared)

Aeff= effective area of the radar receiving antenna (meters squared)

Pr= power received back from the target by the radar (watts)

The term in the radar equation represents the power density (watts per meter

squared) that the radar transmitter produces at the target. This power density is

intercepted by the target with radar cross section , which has units of area (meterssquared).

Thus, the product has the dimensions of power (watts), and represents a

hypothetical total power intercepted by the radar target. The second term

represents isotropic spreading of this intercepted power from the target back to the


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radar receiver. Thus, the product represents the reflected power

density at the radar receiver (again watts per meter squared).

The receiver antenna then collects this power density with effective area Aeff, yielding

the power received by the radar (watts) as given by the radar equation above.

The scattering of incident radar power by a radar target is never isotropic (even for a

spherical target), and the RCS is a hypothetical area. In this light, RCS can be

viewed simply as a correction factor that makes the radar equation "work out right"

for the experimentally observed ratio of Pr / Pt. However, RCS is an extremely

valuable concept because it is a property of the target alone and may be measuredor calculated. Thus, RCS allows the performance of a radar system with a given

target to be analysed independent of the radar and engagement parameters. In

general, RCS is a strong function of the orientation of the radar and target, or, for the

bistatic (radar transmitter and receiver not co-located), a function of the transmitter-

target and receiver-target orientations. A target's RCS depends on its size,

reflectivity of its surface, and the directivity of the radar reflection caused by the

target's geometric shape.

4.2 FACTORS THAT AFFECT RCS:

4.2.1 Size:

As a rule, the larger an object, the stronger its Radar reflection and thus the greater

its RCS. Also, Radar of one band may not even detect certain size objects. For

example 10 cm (S-band Radar) can detect rain drops but not clouds whose droplets

are too small.

4.2.2 Material:

Materials such as metal are strongly radar reflective and tend to produce strong

signals. Wood and cloth (such as portions of planes and balloons used to be

commonly made) or plastic and fiberglass are less reflective or indeed transparent to

Radar making them suitable for radomes. Even a very thin layer of metal can make

an object strongly radar reflective. Chaff is often made from metallised plastic or


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glass (in a similar manner to metallised foils on food stuffs) with microscopically thin

layers of metal.

4.2.3 Radar Absorbent Paint:

This consisted of small metallic-coated balls. Radar energy is converted to heat

rather than being reflected.

4.2.4 Shape, Directivity and Orientation:

Some surfaces are designed to be flat and much angled. This has the effect that

Radar will be incident at a large angle (to the normal ray) that will then bounce off at

a similarly high reflected angle; it is forward-scattered. The edges are sharp to

prevent there being rounded surfaces. Rounded surfaces will often have some

portion of the surface normal to the Radar source. As any ray incident along the

normal will reflect back along the normal this will make for a strong reflected signal.

4.2.5 Smooth surfaces:

The relief of a surface could contain indentations that act as corner reflectors which

would increase RCS from many orientations. This could arise from open bomb-bays,

engine intakes, ordnance pylons, joints between constructed sections, etc.

4.3 MEASUREMENTS OF RADAR CROSS SECTION(RCS):

Measurement of a target's RCS is performed at a radar reflectivity range or

scattering range.

The first type of range is an outdoor range where the target is positioned on a

specially shaped low RCS pylon some distance down-range from the transmitters.

Such a range eliminates the need for placing radar absorbers behind the target,

however multi-path interactions with the ground must be mitigated.

An anechoic chamber is also commonly used. In such a room, the target is placed

on a rotating pillar in the center, and the walls, floors and ceiling are covered by

stacks of radar absorbing material. These absorbers prevent corruption of the


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measurement due to reflections. A compact range is an anechoic chamber with a

reflector to simulate far field conditions.

Near and Far Field:

The near field and far field regions of an isolated source of electromagnetic radiation

are generally used terms in antenna measurements and describe regions around the

source where different parts of the field are more or less important.

The boundary between these two regions depends on the geometric dimensions of

the source and the emitted by the source dominant wavelength .

In the region of near field of an antenna the angular field distribution is dependent

upon the distance from the antenna. The different parts of energy emitted by

different geometric regions of the antenna have got a different running time and the

resultant field cannot be constructively interfered to an evenly wave front.

A point like isotropic source cannot have a near field. This near field occurs, if the

geometric dimension of the source lies near the wavelength at least.

The two regions are defined simply for mathematical convenience, enabling certain

simplifying approximations of the Maxwells equations. In the near field region there

is a region, into an antenna collect a part of the just emitted energy too.


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Near field region of an antenna array


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Near field region of an antenna array

This figure shows an antenna array of four in phase feeded elements. Every element

emits an electromagnetic field. These partially fields are combined to a common

field. Although the figure is drawn large, all shown distances are near field region.

To show the far field, the width of this figure must be fourfold, to show the far field

region, to show, that the magnitudes of the electromagnetic fields are added to a

coherent wave front. In the far field, the shape of the antenna pattern is independent

of distance from the source.

For small antennas (radiators width is smaller than the wavelength) the near field is

the region within a radius r > .

Al larger antennas (antenna arrays or using a big reflector, like parabolic dish

antenna) the boundary between the two regions can be roughly calculated as:

Where

D = Geometrical dimension

= wavelength


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The far-field region is sometimes referred to as the Fraunhofer region, and the near-

field region is sometimes referred to as the Fresnel region.

CALCULATION OF RCS:

Quantitatively, RCS is calculated in three-dimensions as

||||

Where is the RCS, Si is the incident power density measured at the target, and

Ssis the scattered power density seen at a distance raway from the target.

In electromagnetic analysis this is also commonly written as

||||

whereEs and Ei are the far field scattered and incident electric field intensities,

respectively.

In the design phase, it is often desirable to employ a computer to predict what theRCS will look like before fabricating an actual object. Many iterations of this

prediction process can be performed in a short time at low cost, whereas use of a

measurement range is often time-consuming, expensive and error-prone

The linearity of Maxwell's equations makes RCS relatively straightforward to

calculate with a variety of analytic and numerical methods, but changing levels of

military interest and the need for secrecy have made the field challenging,

nonetheless.

The field of solving Maxwell's equations through numerical algorithms is called

computational electromagnetics, and many effective analysis methods have been

applied to the RCS prediction problem. RCS prediction software are often run on

large supercomputers and employ high-resolution CAD models of real radar targets


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(E) 4 REDUCTION OF RCS:

RCS reduction is chiefly important in stealth technology for aircraft, missiles, ships,

and other military vehicles. With smaller RCS, vehicles can better evade radar

detection, whether it be from land-based installations, guided weapons or other

vehicles.

Reduced signature design also improves platforms' overall survivability through the

improved effectiveness of its radar counter-measures.Several methods exist. The

distance at which a target can be detected for a given radar configuration varies with

the fourth root of its RCS.

Therefore, in order to cut the detection distance to one tenth, the RCS should be

reduced by a factor of 10,000. Whilst this degree of improvement is challenging, it is

often possible when influencing platforms during the concept/design stage and using

experts and advanced computer code simulations to implement the control options

described below.

4.5 METHODS OF RCS REDUCTION:

(A) Purpose shaping:

With purpose shaping, the shape of the targets reflecting surfaces is designed such

that they reflect energy away from the source. The aim is usually to create a cone-

of-silence about the targets direction of motion. Due to the energy reflection, this

method is defeated by using Passive (multi-static) radars.

(B) Active cancellation:

With active cancellation, the target generates a radar signal equal in intensity but

opposite in phase to the predicted reflection of an incident radar signal (similarly to

noise canceling ear phones). This creates destructive interference between the

reflected and generated signals, resulting in reduced RCS. To incorporate active

cancellation techniques, the precise characteristics of the waveform and angle of

arrival of the illuminating radar signal must be known, since they define the nature of

generated energy required for cancellation.


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(C) Passive Cancellation:

Passive cancellationrefers to RCS reduction by introducing a secondary scattering to

cancel with the reflection of the primary target. The target with the scattering element

is called the loaded body, as opposed to the bare target, which is the unloaded body.

Consequently, this method is also known as impedance loading, and it is essentially

the same approach as that used in the design of the Salisbury screen and

Dallenbach layer. As with any cancellation or tuning method, this technique is

effective over only a narrow frequency band and is usually limited to a small spatial

sector. If large parasitic elements are to be avoided, the magnitude of RCS that can

be canceled is relatively small. Thus passive cancellation is used to supplement

shaping andabsorbers. The exception to this is the treatment of traveling waves, in

which case passive cancellation by parasitic structures is often the primary means of

RCS reduction.

(D) Radar absorbent material:

With radar absorbent material (RAM), it can be used in the original construction, or

as an addition to highly reflective surfaces. There are at least three types of RAM:

resonant, non-resonant magnetic and non-resonant large volume. Resonant but

somewhat 'lossy' materials are applied to the reflecting surfaces of the target. The

thickness of the material corresponds to one-quarter wavelength of the expected

illuminating radar-wave (a Salisbury screen). The incident radar energy is reflected

from the outside and inside surfaces of the RAM to create a destructive wave

interference pattern. This results in the cancellation of the reflected energy. Deviation

from the expected frequency will cause losses in radar absorption, so this type of

RAM is only useful against radar with a single, common, and unchanging frequency.

Non-resonant magnetic RAM uses ferrite particles suspended in epoxy or paint to

reduce the reflectivity of the surface to incident radar waves. Because the non-

resonant RAM dissipates incident radar energy over a larger surface area, it usually

results in a trivial increase in surface temperature, thus reducing RCS at the cost of

an increase in infrared signature.


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(E) Optimization methods:

Thin non-resonant or broad resonance coatings can be modeled with a Leontovich

impedance boundary condition (see also Electrical impedance). This is the ratio of

the tangential electric field to the tangential magnetic field on the surface, and

ignores fields propagating along the surface within the coating. This is particularly

convenient when using boundary element method calculations. The surface

impedance can be calculated and tested separately. A perfect electric conductor has

more back scatter from a leading edge for the linear polarization with the electric field

parallel to the edge and more from a trailing edge with the electric field perpendicular

to the edge, so the high surface impedance should be parallel to leading edges and

perpendicular to trailing edges, for the greatest radar threat direction, with some sort

of smooth transition between.

To calculate the radar cross section of such a stealth body, one would typically do

one dimensional reflection calculations to calculate the surface impedance, then two

dimensional numerical calculations to calculate the diffraction coefficients of edges

and small three dimensional calculations to calculate the diffraction coefficients of

corners and points. The cross section can then be calculated, using the diffraction

coefficients, with the physical theory of diffraction or other high frequency method,

combined with physical optics to include the contributions from illuminated smooth

surfaces and Fock calculations to calculate creeping waves circling around any

smooth shadowed parts.Optimization is in the reverse order. First one does high

frequency calculations to optimize the shape and find the most

important features, then small calculations to find the best surface impedances in the

problem areas, then reflection calculations to design coatings. One should avoid

large numerical calculations that run too slowly for numerical optimization or distract

workers from the physics, even when massive computing power is available.

Thin non-resonant or broad resonance coatings can be modeled with a Leontovich

impedance boundary condition (see also Electrical impedance). This is the ratio of

the tangential electric field to the tangential magnetic field on the surface, and

ignores fields propagating along the surface within the coating. This is particularly

convenient when using boundary element method calculations. The surface


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impedance can be calculated and tested separately. A perfect electric conductor has

more back scatter from a leading edge for the linear polarization with the electric field

parallel to the edge and more from a trailing edge with the electric field perpendicular

to the edge, so the high surface impedance should be parallel to leading edges and

perpendicular to trailing edges, for the greatest radar threat direction, with some sort

of smooth transition between.

To calculate the radar cross section of such a stealth body, one would typically do

one dimensional reflection calculations to calculate the surface impedance, then two

dimensional numerical calculations to calculate the diffraction coefficients of edges

and small three dimensional calculations to calculate the diffraction coefficients of

corners and points. The cross section can then be calculated, using the diffraction

coefficients, with the physical theory of diffraction or other high frequency method,

combined with physical optics to include the contributions from illuminated smooth

surfaces and Fock calculations to calculate creeping waves circling around any

smooth shadowed parts.Optimization is in the reverse order. First one does high

frequency calculations to optimize the shape and find the most

important features, then small calculations to find the best surface impedances in the

problem areas, then reflection calculations to design coatings. One should avoid

large numerical calculations that run too slowly for numerical optimization or distract

workers from the physics, even when massive computing power is available.


34/36

PREPARATION OF MODEL

OBJECT:measurement of rcs of metallic wires ( aluminium coated glass fibres oraluminium fiber) using vna in anechoic chamber

MATERIAL USED:

1.Imported fiber (aluminium coated glass fibres 28mm each)

2.DLJ fiber(aluminium fibers 28mm each)

PREPARATION:

To calculate the rcs of metallic wires two kind of model is prepared one in which

the metallic fibers are arranged in a array and another in which metallic fiber are

randomly arranged.

1. First kind of model is prepared by using metallic wire (either imported fibers or

DLJ fibers).Metallic wires model is having 40 elements arranged in a 58 array .Each

element is having spacing of (5.6 cm) side by side.

2. Another kind of model is prepared by using metallic wire (either imported fibersor DLJ fibers).Metallic wires model is having 40 elements randomly arranged .Each

element is having spacing of (5.6 cm) side by side.

MEASUREMENTS:

Measurement is done in anechoic chamber(which avoids reflection of radio waves

back to the transmitter).first the E-M waves i.e radio waves is incident on the model

by the transmitter(microwave generator) and we know that E-M is reflected by themetallic object hence the model having metallic wires reflect the waves back to the

radar which is then measured and rcs of the object is calculated.


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Fig: RCS v/s Aspect angle at 4.3 GHz


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Fig: RCs v/s Frequency sweep

3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8-50

-40

-30

-20

-10

0

10

20

Frequency (GHz)

ReturnLoss(dBsm)

anechoic chamberself

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