ANTENNA for Communication Systems-2008cep

8/12/2019 ANTENNA for Communication Systems-2008cep

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ANTENNAS FOR MODERN

COMMUNICATION SYSTEMSBy

BI NAY K. SARKAR.

I SRO CHAI R PROFESSOR

( Lecture prepared for delivering at the Summer School on Recent Trend in AntennaTechnology to be held at KIIT , Bhubaneswar on Aug.05,2008 )

Department of Electrical and Electronic Communication Engineering

Indian Institute of Technology, KharagpurKharagpur-721302


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1 INTRODUCTION:

Antenna:A usually metallic device (as a rod or wire) for radiating or receiving radio waves.

An antenna can be any conductive structure that can carry an electrical current. If it carries a time

varying electrical current, it will radiate an electromagnetic wave, may be not efficiently or in a

desirable manner but it will radiate.

Antennas are the connecting linkbetween RF signals in an electrical circuit, such as between a

PCB and an electromagnetic wave propagating in the transmission media between the transmitter

and the receiver of a wirelesslink.

In the transmitter, the antenna transforms the electrical signal into an electromagnetic wave byexciting either an electrical or a magnetic field in its immediate surroundings, the near field.

Antennas that excite an electrical field are referred to as electrical antennas; antennas exciting a

magnetic field are called magnetic antennas.

The oscillating electrical or magnetic field generates an electromagnetic wave that propagates

with the velocity of light c. The speed of light in free space cois 300000 km/s. If the wave travels

in a dielectric medium with the relative dielectric constant r, the speed of light is reduced to:

Co/(r)1/2
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Concept of Radiation:

A typical radiating system will compose of a source, a transmission line and antenna as shown

below


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If the antenna system is not properly designed, the transmission line could act as an energy

storage device. Zg

RL

RA

Antenna

Transmission line

Source

Losses

LineAntenna

VSWR

Minimization Losses

Transmission line matching

Reduce loss resistance, RLof antenna


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Two Wires:


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Applying voltage across two-conductor transmission line creates an electric field between

the conductors.

Electric field is associated with it electric lines of force which are tangent to the electricfield at each point and their strength is proportional to the electric field intensity.

Electric line of forceacts on free electron in the conductorforce them to displace

movement of charge creates electric currentthat in turn creates magnetic field intensity

associated with magnetic field intensitymagnetic lines of force which are tangent to

magnetic field.


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T- period: Lines of force created

between arms of a small center fed

dipole in time when charge reached

maximum value & lines have

traveled .

During next original lines travel another (total

) and charge density on conductors begin to

diminish.

This can be accomplished by introducing opposite

charges which at the end of first half of the periodhave neutralized the charge on the conductors.

Lines of force created by opposite charges shown

by dashed line and travel by distance.

4

2

4


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2. Types of Antennas

Various antennas used in radar and Communication systems:

1 ) Dipole

2 ) Monopole

3 ) Sleeve Monopole

4 ) Loop

5 ) Helix6 ) Yagi- Uda.

7 ) Log-Periodic

8 ) Horn

9 ) Slotted waveguide

10 ) Microstrip antennas.

11 ) Parabolic reflector12 ) Cassegrain reflector

13 ) Array

14) Special Antennas

15) MEMS Antennas


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Dipole Antenna

Half -Wave Dipole

Half -Wave Dipole Antenna.The half-wave dipole antenna (Figure ) is the basis of many other antennas and is also

used as a reference antenna for the measurement of antenna gain and radiated power

density.

At the frequency of resonance, i.e., at the frequency at which the length of the dipole

equals a half-wavelength, we have a minimum voltageand a maximum current at the

terminations in the center of the antenna, so the impedance is minimal.
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Therefore, we can compare the half-wave dipole antenna with a series RLC resonant circuit

. For a lossless half-wave dipole antenna, the series resistance of the equivalent resonant

circuit equals the radiation resistance, generally between 60 and 73 , depending on the

ratio of its length to the diameter.

The bandwidth of the resonant circuit (or the antenna) is determined by the L-to-C ratio. A

wire with a larger diameter means a larger capacitance and a smaller inductance, which

gives a larger bandwidthfor a given series resistance. That's why antennas made for

measurement purposes have a particularly large wire diameter.

As opposed to the (only hypothetical) isotropic radiator, real antennas such as the half-wave

dipole have a more or less distinct directional radiation characteristic.

The radiation pattern of an antenna is the normalized polar plot of the radiated powerdensity, measured at a constant distance from the antenna in a horizontal or vertical plane.
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Figure below shows the radiation pattern of a half-wave dipole antenna.

Radiation Pattern of a Half-Wave Dipole Antenna

Since the dipole is symmetrical around its axis, the three-dimensional radiation pattern rotatesaround the wire axis.


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The isotropic gain of a half-wave dipole antenna is 2.15 dB. Therefore, in the direction

perpendicular to the wire axis, the radiated power density is 2.15 dB larger than that of the

isotropic radiator.

There is no radiation in the wire axis. The half-wave dipole produces linear polarization with the

electrical field vector in line with, or in other words parallel to, the wire axis.

Because the half-wave dipole is often used as a reference antenna for measurements, sometimesthe gain of an antenna is referenced to the radiated power density of a half-wave dipole instead of

an isotropic radiator.

Also the effective radiated power (ERP), which is the power delivered to an ideal dipole thatgives the same radiation density as the device under test, is used instead of the EIRP. The

relations Gdipole = Gisotropic - 2.15 dB and ERP= EIRP - 2.15 dB can be applied.
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The half-wave dipole needs a differential feed because both of its terminations have the same

impedance to ground. This can be convenient if the transmitter outputor the receiver inputhave

differential ports.

A balun will be used along with the half-wave dipole in case of single-ended transmitters or

receivers, or if an antenna switch is used.

For external ready-manufactured half-wave dipoles, the balun is visually built-in to the antenna

and provides a single-ended interface.

The half-wave dipole is an electrical antenna. This means that it is easily detuned by materials

with a dielectric constant larger than 1 within its reactive near field.

If, for instance, the housing of a device is in the reactive near field, the housing has to be present

when the antenna is matched.

The human body has a large dielectric constant of approximately 75. As a result, if an electrical

antenna is worn on the body or held in the hand, it can be strongly detuned.
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If the antenna is built as two traces on a PCB, the dielectric constant of the PCB material has to

be considered. The electrical field in the reactive near field region spreads out partially into the

PCB material, partially into the surrounding air. This gives an effective dielectric constant effbetween that of the air and the PCB material :

Where h is the thickness of the PCB material, w is the trace width of the dipole arms. Therequired length of the half-wave dipole is then:

Underneath the dipole and within the reactive near field, no ground plane is allowed.


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A


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Monopole Antennas

Quarter-Wave Monopole

In many cases, the half-wave dipole is just too large. Also, the needed differential feed is often a

disadvantage. If we replace one branch of the dipole antenna by an infinitely large ground plane,

due to the effect of mirroring, the radiation pattern above the ground plane remains unaffected.

This new structure is called a monopole antenna.

Building Up the Quarter-Wave Monopole.

Because all the radiated power is now concentrated in the half-space above the ground plane, thegain of the monopole is 3 dB larger than the gain of the dipole.

Often a large ground plane is not feasible. The Marconi antenna replaces the (not realizable)

infinitely large ground plane by several open-ended /4-Stubs, called the counterpoise.

A f h d i l b i h l k lik b di l Wh


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A further reduction to only one stub gives a structure that looks like a bent dipole antenna. When

designing a monopole antenna, the radiator should go as long as possible perpendicular to the

ground stub or the ground plane. Bends close to the feeding point reduce the radiation resistance

and the efficiency of the antenna.

The ideal quarter-wave monopole has a linear polarization with the vector of the electrical field in

the wire axis. If the ground plane becomes unsymmetrical, the direction of polarization will be

tilted towards the larger part of the ground plane, but still remains linear.

The radiation resistance of an ideal quarter-wave monopole is half of that of a dipole; dependingon the ratio of length to diameter of the radiator between 30 and 36.5 .

Like the half-wave dipole, the quarter-wave monopole is an electrical antenna. It is influenced by

the dielectric constant of the material in the reactive near field.

The same formulas for the effective dielectric constant and the required length as for the half-

wave dipole hold for the quarter-wave monopole.

T bl 1 i h l h f h lf di l d l i f d


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Table 1gives the length of half-wave dipoles and quarter-wave monopoles in free space and on a

PCB for commonly used short-range frequencies. For the PCB antennas, a PCB thickness of h =

1.5 mm and a trace width of w = 1 mm has been assumed; the PCB material is FR4 with r= 4.2.

This gives an effective dielectric constant of r= 2.97.

It has to be mentioned that parasitic components, such as capacitance to ground, inductance

introduced by bends in the antenna as well as the influence of the package, alter the antenna

impedance.

F l t th d l i ti ll th t l th t


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For monopole antennas, the ground plane is sometimes smaller than a quarter-wave length or not

perpendicular to the radiator. In practice, the exact length of the dipole or the monopole has

therefore to be determined by measuring the feed impedance with a vector network analyzer.

Sometimes the available space limits the length of an antenna. The antenna is made as long as the

geometry permits, which can be smaller than one quarter wavelength. A monopole shorter than a

quarter-wave length can be considered as a quarter-wave monopole, which is used at a frequency

lower than the frequency of resonance.

According to the equivalent schematic of an antenna , the input impedance at the frequency of

operation will then be a series connection of a resistor and a capacitor.

The series capacitance can be resonated out by a series inductor. A monopole antenna shorter than

l/4 with a series inductor is also referred to as a loaded stub antenna.


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The radiation resistance of a loaded stub decreases with decreasing length.

The smaller radiation resistance and the larger L-to-C ratio increase the quality factor and make

the bandwidth smaller than for a quarter-wave monopole. Approximations for the radiation

resistance of a monopole antenna are:

At the frequency of operation (i.e., resonance), the impedance of the short stub will be that of a

small resistor (radiation resistance plus loss resistance) with a series capacitor.

F th S ith Ch t i Fi th t t hi t 50 b hi d b


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From the Smith Chart in Figure we can see that matching to a 50 source can be achieved by a

series inductor and a parallel capacitor.

Matching of a Short Loaded Stub Antenna

Figure below shows an example of a loaded stub PCB antenna with matching components.

Loaded Stub PCB Antenna With Matchin Com onents

The series ind ctor and the parallel capacitor transform the antenna impedance to 50 the input
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The series inductor and the parallel capacitortransform the antenna impedance to 50 , the input

impedance of the filter (FIL1).

For dipole or monopole antennas, the component values for the series inductor and the parallel

capacitor (CP) have to be determined by measuring the feed impedance at point A in Figure 10

(with LS = 0 resistor and CPleft unpopulated) with a vector network analyzer.

Once this is determined, we can use a Smith Chart to assist in matching the antenna to 50 usingLSand CP.

Derivatives of the monopole are the inverted-L and inverted-F antennas as shown in Figure

below.

Inverted-L Antenna and Inverted-F Antenna

In the inverted-L antenna, the monopole does not run perpendicularly to the ground plane over its

whole length but is bent parallel to the ground plane after some distance. This helps to save

space, but decreases the radiation resistance because the radiator comes closer to the ground

plane. An additional matching circuit is needed to match the low-feed impedance to the usual

transmission line impedance of 50 .

If we proceed from the feed point of the inverted L antenna to the end we notice that the voltage


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If we proceed from the feed point of the inverted-L antenna to the end, we notice that the voltage

increases (while the current decreases) from a maximum voltage value at the feeding point to

almost zero at the end.

This means, that the antenna impedance has its minimum if we feed the

antenna as shown in Figure a) and increases if we move the feeding point towards the end.

The inverted-F antenna in Figure bis an inverted-L antenna with a feeding tap that gives larger

antenna impedance.

If the antenna is tapped at the right location, no additional matching circuit is required.

The structure of inverted-F antennas and, in particular, the location of the tap, is usually

determined by electromagnetic simulations.


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Monopole Discone Antenna


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Widebandth10:1 range.

Omnidirectiional in horizontal plane.

Vertically polarized.

Gain is similar to a dipole. Z approaches 50 ohms.

Application: RX scanner antenna for VHF and UHF.

Can also be used for TX.

PCB Monopole Antenna Module


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PCB Monopole Antenna Module

Figure shows the layout of the PCB monopole antenna. In order to save PCB space the

monopole has been bent by 90 degree. In the PCB layout, the monopole was made longer than

calculated according to Table 1. This should give some room for possible manufacturing

tolerances.

Layout of the PCB Monopole Antenna.

Using a vector network analyzer, we measured the antenna impedance on the upper pad of L1

and cut back the monopole until real antenna impedance was achieved.

The antenna impedance in resonance is 35 5 which is within the theoretical value range of 30
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The antenna impedance in resonance is 35.5 , which is within the theoretical value range of 30

to 36.5 . The mismatch loss to 50 is as low as 0.13 dB in this case. No further matching

components have been used; inductor L1 was replaced by a 0 resistor. C2 was left unpopulated.

The radiation characteristic of the antenna module was measured in an anechoic chamber with

the test module upright (see Figure ) and flat (see Figure ) on the turntable.

The outer boundary of the radiation patterns given in this report correspond to an effective

radiated power (dipole related) of ERP= + 10 dBm; the scale is 20 dB/division.

25. Vertical Radiation Pattern o the Stub Module U ri ht .

The radiation pattern is almost angle-independent the maximum ERP is + 6 5 dBm
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The radiation pattern is almost angle-independent, the maximum ERP is + 6.5 dBm,

corresponding to EIRP = 6.5 dBm + 2.15 dB = +8.65 dBm. The TRF4903 delivers +8 dBm of

output power. The maximum antenna gain is therefore +0.65 dB.

As expected, the horizontal radiation pattern has a more pronounced radiation characteristic:

. Horizontal Radiation Pattern of the Stub Module (Flat).

The maximum ERP is +10.85 dBm, corresponding to EIRP = +13 dBm. With +8-dBm transmit

power, this gives an antenna gain of 5 dB.

Electrical antennas are sensitive to detuning by dielectric material in their reactive near field.

Figure shows the horizontal radiation pattern of the same stub module attached to the arm of atest erson.


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Horizontal Radiation Pattern of the Stub Module Close to the Human Body.

The maximum ERP is -4.4 dBm, compared to +10.85 dBm measured on the free stub module.

The loss due to detuning and absorption is as large as 15.25 dB in the direction of maximum

radiation.


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3. Sleeve Monopole antenna


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3. Sleeve Monopole antenna

4. Loop antennas:


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4. Loop antennas:

Small Loop Antennas


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S oop e s

Small Loop Antenna With Differential Feed

The loop antenna shown in Figure has a differential feed. Often a ground plane is made part ofthe loop, giving a single-ended feed as shown in Figure .

Single-Ended Loop Antenna

The small arrows indicate the current flow through the loop. On the ground plane, the current is

mainly concentrated on the surface. The electrical behavior of the structure in Figure is thereforesimilar to that of the loo with differential feed shown in Fi ure .


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The following considerations on small loop antennas are based on [4] and assume that the current

is constant over the loop. This means that the circumference must be smaller than one tenth of a

wavelength.

Although this is rarely the case, the given formulas describe the principal behavior and can beused as a starting point for the loop antenna design.

If the current is constant over the loop, we can consider the loop as a radiating inductor with

inductance L, where L is the inductance of the wire or PCB trace. Together with the capacitor C,

this inductance L builds a resonant circuit. Often a resistor Ratt is added to reduce the quality

factor of the antenna and to make it less sensitive to tolerances. Of course, this resistor dissipatesenergy and reduces the antenna's efficiency.

The following calculations hold for circular loops with the radius a for square loops with the side

length a. A rectangular antenna with the sides a1 and a2 will be approximated by an equivalent

square with the side length:

The length (circumference) of the wire building the loop will be called U, where U = 2a for a

circular loop, or 4a for a square loop.

For the calculation of the inductance, the wire radius b, where b is 1/2 the diameter of the actual


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Figure belowshows the equivalent schematic of a small loop antenna.

, ,

wire used to fabricate the antenna, is needed. In the frequent case where a loop antenna is

realized by a trace on a PCB, b = 0.35.d + 0.24.w can be used, where d is the thickness of the

copper layer and w is the trace width.

Equivalent Schematics of the Small Loop Antenna

The radiation resistance of loop antennas is small, typically smaller than 1 . The loss resistance


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p , yp y

Rloss describes the ohmic losses in the loop conductor and the losses in the capacitor.

Usually, the losses in the capacitor cannot be neglected. Interestingly, the thickness of the copper

foil is not needed for the calculation of the loss resistance because due to the skin effect, the

current is confined on the conductor surface.

Together with the loop inductance L, which is the inductance of the wire, the capacitor C builds a

series resonant circuit.

In practice, the L-to-C ratio of this resonant circuit is large giving a high quality factor (Q). This

would make the antenna sensitive to tolerances. That's why often an attenuating resistor Ratt isadded to reduce the Q.

To describe the influence of Ratt on the loop antenna, we make a parallel to series conversion and

use the equivalent series resistance Ratt_trans. The resistance value of Ratt_trans is determined

by the acceptable tolerance of the capacitor and the geometry of the loop.

The maximum usable quality factor is calculated from the capacitance tolerances C/C:

The series transformed attenuation resistance then will be:


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This gives the efficiency of the loop antenna:

In most cases, the radiation resistance is much smaller than the loss resistance and the

transformed attenuation resistance, giving a poor efficiency. In this case, the approximation:

can be used. Rr is determined by the loop area, which is a2 for circular loops, a2for square loops,

and a1a2for rectangular loops.

Figure in next slideshows the efficiency of small circular loop antennas versus their diameter

for an assumed tolerance of 5 percent. The trace width has been assumed as 1 mm, the copper

thickness as 50 um; but both values have only a minor influence on the efficiency, which is

mainly determined by the attenuation resistance Ratt. As expected, the efficiency increases with

increasing diameter.


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In both cases, matching to 50 will be difficult. That's why the loop antenna is often tapped,


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giving an impedance in between the too small and the too large values. Figure belowshows an

example:

Example of a Tapped PCB Loop Antenna.

A series feed (in the lower right corner) would give a small impedance. A parallel feed (directly at

the capacitors) would give a much too large impedance.

The tap provides an impedance close to 50 in this example. The loop capacitor has been split

into two series capacitors C1 and C2. This makes it possible to realize capacitance values. R1 is

the damping resistor which de-Qs the circuit, thus increasing the bandwidth and subsequently

Unfortunately, there are no easy formulas that describe the tapped structure and give the right


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location for the tap.

The line from the antenna termination to the tap is not a transmission line and will disturb the

field in the antenna. Therefore, we have to find out the optimal structure by electromagnetic

simulations.

Often a trial and error procedure is used as an alternative.

For example, using a vector network analyzer, we determine the capacitance value that gives the

best return loss and the largest resistance value that gives the required bandwidth.

The loop antenna gives a linear polarization with the vector of the electric field oscillating in the

plane built by the loop.

In contrast to all of the antennas discussed so far, loop antennas are magnetic antennas.

This means that they are not detuned by the dielectric constant of the material in their reactivenear field. That's why loop antennas are often used for body-worn or hand-held equipment.

Loop Antenna Module


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Figure belowhas the layout of the loop antenna module. The loop capacitoris a series

connection of the two capacitors C40 and C44; this allows realizing small capacitance values in

fine steps. R11 is the attenuation resistor; L5 and C46 should help to improve the matching to 50

experimentally, through an iterative approach.

PCB Loop Antenna

The loop width is 25 mm, the height 11.5 mm, the trace width 1.5 mm with a copper thickness of

50 m.

According to the formulas in Figure 15, the inductance is L = 40.9 nH and the radiation

resistance Rr = 0.22 .

The calculated capacitance needed for resonance at 915 MHz is 0.74 pF.


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Usual tolerance values for low cost capacitors are around five percent. For the small capacitance

needed here, the effective tolerance would be larger because the parasitic capacitance of the

damping resistor, the PCB pads, and even the solder material contributes to the total capacitance

uncertainty. We assumed a total tolerance of 20 percent, which gives a maximum quality factor:

The impedance of the loop inductance is ZL = 2.x 915 MHz x 40.9 nH = 235 . The damping

resistance must therefore be not larger than Ratt = 10.5 x 235 = 2.47 k.

In the test module, a 2.2-k resistor was chosen, giving Q = 2.2 k/235 = 9.36. The

transformed series attenuation resistance is then Ratt_trans = ZL/Q = 235 / 9.36 = 25.1 .

Compared to that, we can neglect the loss resistance of the copper trace on the PCB.

The theoretical antenna efficiency is then:

The loop antenna has been tapped to increase the antenna impedance. The position of the tap was


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determined empirically by electromagnetic simulations.

The antenna impedance was measured on the assembled PCB on the upper pad of C51; L5 was

replaced by a 0- resistor at first.

We varied the loop capacitors C40 and C44 to bring the loop into resonance. A series connection

of 0.8 pF and 0.5 pF gives the reflection coefficient of 0.22.ej35, corresponding to a mismatch

loss of 0.2 dB. No further matching was required, so C46 was left unpopulated.

The capacitance of the series connection of the 0.5-pF and 0.8-pF capacitors is 0.3 pF and thussmaller than the calculated value of 0.74 pF. One explanation is that the parasitic capacitance of

the resistor also contributes to the loop capacitance. Also, the parasitic inductance of the

capacitors themselves makes their capacitance look larger at high frequencies than their nominal

value.

Figure next slidehas the horizontal radiation pattern of the loop antenna module arranged flat ona turntable.


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Radiation Pattern of the Loop Antenna Module.

The maximum ERP is +4.15 dBm, corresponding to EIRP = +6.3 dBm. This gives an antenna

gain of -1.7 dB, much more than calculated. The reason is that the circumference of the loop isnot negligible with respect to the wavelength. All calculations were made under the assumption

that the circumference is smaller than one tenth of the wavelength. The geometrical

circumference of the loop antenna in the test module is 73 mm, the wavelength in free space for

915 MHz is 327 mm. Assuming the effective dielectric constant of a trace on FR4 material of

2.97 , the wavelength on the PCB is:


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The circumference is therefore 73 mm / 190 mm = 0.38 times the wavelength. The loop does not

act like a purely magnetic antenna any more; it will also excite the electrical field in its reactive

near field region. As a result, the behavior is in between that of a small loop and an electrical

antenna.

For a loop antenna, a smaller influence of dielectric material in the reactive near field on the

tuning and the radiation characteristic is expected. Figure below shows the radiation pattern of

the loop module attached to the forearm of a test person.

Radiation Pattern of the Loop

Antenna Module Close to the HumanBody.

The ERP is -9.75 dBm, corresponding to EIRP = -7.6

dBm. Given the transmitter power of +8 dBm, the

antenna gain is -15.6 dB, almost 14 dB worse than infree space.

Note that the gain is still larger than the theoretical

value of -20.6 dB. This again comes from the large

dimensions which make the antenna more similar to

an electrical radiator.

In order to achieve greater independence from the

influences of the surrounding material, the size of the

loop antenna must be made smaller. This reduces the

radiation resistance and L-to-C ratio and decreases

the efficiency.


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Small loop antennas are insensitive to varying dielectric conditions in their reactive near field.

Th b d l i f bl d h d h ld d i b h h l ffi i


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They can be a good solution for portable and hand-held devices but have a much lower efficiency

than electrical antennas.

Only antennas with a circumference smaller than one tenth of a wavelength can be considered as

purely magnetic antennas.

Larger loops have a higher gain but also a higher sensitivity to the environmental conditions.

Size matters: Always keep in mind that Chu's and Wheeler's limit determines the product of the

bandwidth and the efficiency for a given antenna dimension.

An extremely small antenna cannot be efficient and tolerance-insensitive at the same time

5. Helix.


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Transversal Mode Helical Antenna

Another option to reduce the size of a monopole is to coil it up into a helix as shown in Figure bel

Helix Antenna on a Ground Plane

When the coil circumference and the spacing between adjacent turns are comparable to the wavele

the antenna radiates a circular polarized beam in the axis of the helix. These antennas are called axmode helicals.

In small short-range applications, the helix diameter and the spacing between turns are much small

than a wavelength. So, the result is a normal mode helical antenna.

The radiation pattern of a normal mode helix is similar to that of a monopole; the maximum

di ti di l t th h li i


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radiation occurs perpendicular to the helix axis.

Due to the shape and the size of the ground plane, radiation patterns of practical antennas can

show deviations from this idealized form.

The radiation from a normal mode helix is elliptically polarized. Usually the component having

the electrical field vector parallel to the antenna axis is stronger than the component which is

parallel to the ground plane.

The exact calculation of transversal mode helical antennas is not as simple as for dipole and

monopole antennas.

Usually they are designed empirically: start with a wire that is half a wavelength long, wind it up

into a helix, and measure the antenna impedance using a vector network analyzer. Then, cut it

back until nearly real input impedance at the frequency of operation is obtained.

Real input impedance means that the antenna is in resonance. Fine-tuning of the frequency of

i ibl b i t t hi th h li


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resonance is possible by compressing or stretching the helix.

Even if the antenna is in resonance, it will not be matched to 50 yet.

The input impedance will be the sum of the radiation and loss resistances, usually smaller than 50

. For the design of the needed additional matching circuit, we can use the Smith Chart as

described above.

Chu's and Wheeler's [1,2] limit on the bandwidth for a given dimension also holds for helix

antennas.

A small transversal mode helix, therefore, has tight bandwidth and is sensitive to tolerances of thematching components.


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As opposed to the monopole, underneath and around the antenna there is a ground plane. The

white dots in the previous Figure are vias connecting the top side ground with the internal


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white dots in the previous Figure are vias connecting the top side ground with the internal

ground layer of the PCB.

The antenna impedance was measured on the upper pad of C7. Without matching components

and with L1 replaced by a 0- resistor, the measured reflection coefficient was = 0.91.e-143.

L1 = 5.6 nH and C2 = 16 pF give an acceptable reflection coefficient of = 0.35.e147. Figure

29has the radiation pattern. The test module was placed flat on the turntable.

Horizontal Radiation Pattern of the Helix Module

The maximum ERP is -10.6 dBm, corresponding to EIRP = -8.45 dBm. With +8-dBm transmit

power follows a gain as low as 16 45 dB


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power follows a gain as low as -16.45 dB.

The small dimensions of the antenna lead to a small radiation resistance and a strong effect of

losses in the PCB and the matching components. Also, the size of the PCB ground plane is not

large compared to the wavelength. That's why the shape and the size of the ground plane have an

effect on the radiation pattern.

Figure below shows the radiation pattern under the same conditions but with the helix module

attached to the forearm of a test person.

The PCB ground plane was between the helix and the arm; the forearm was held in a horizontalposition.Horizontal Radiation Pattern of the

Helix Module Close to the Human Body

Despite the lower efficiency in the

upper right corner (which comes from

the absorption by the chest of the test

person), the radiation pattern is identical

to that of the free helix module.

The PCB ground plane acts as a shield

and makes the radiation patternindependent on the tissue underneath it.

6. Yagi - Uda


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Array antennas can be used to increase directivity.

Parasitic array does not require a direct connection to each element by a feed network.

The parasite elements acquire their excitation from near field coupling by the driven

element.

A Yagi-Uda antenna is a linear array of parallel dipoles.

The basic Yagi unit consists of three elements:

1. Driver or driven element

2. Reflector

3. Director


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Develops an endfire radiation pattern.

Optimum spacing for gain of a reflector and driven element is 0.15 to 0.25 wavelengths

Director to director spacings are 0.2 to 0.35 wavelengths apart.

Reflector length is typically 0.5 wavelengths or 1.05 that of the driven element.

The driven element is calculated at resonance without the presence of parasitic

elements.

The directors are usually 10 to 20% shorter than at resonance.


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Gain is related to boom length and number of directors.

Max directivity of a 3 element Yagi is 9 dBi or 7dBd.

Addition of directors up to 5 or 6 provides significant increase in gain. Addition of more

directors has much less impact on gain.

Increasing N from 3 to 4 results in 1 dB increase.

Adding a director to go from 9 to 10 presents a 0.2 dB gain improvement.

Adding more reflectors has minimal impact on gain however does impact on feedpoint Zand the backlobe.


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Metal booms can be implemented because voltage is at zero midway through the

element.

Other factors that effect resonant lengths:

1. A comparatively large boom will require parasitic elements to increase their

length.

2. Length to diameter ratio of the elements.


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7. Log-Periodic


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lengthselementrespectiveisLwhere

L

L

L

L

L

L......

4

3

3

2

2

1

.

.sin

.....

1

4

3

3

2

2

1

shortestisD

themgcloangleofapexand

elementsbetweenspacingsrepresentsDwhere

D

D

D

D

D

D

2tan

2 1

1 DL

Alpha is the angle of the apex of tapered elements and is typically 30 degrees.

. :


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Horn is widely used as a feed element for

large radius astronomy dish

satellite tracking dish communication dish

feed for reflector and lenses

common element for phased arrays

universal standard for calibration

Horn is

simple in construction

easy to excite

versatile

large gain good overall performance

Types of Horn


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yp

1. E-plane sectorial horn

2. H-plane sectorial horn

3. Pyramidal

4. Conical

5. Corrugated

6. Aperature matched

7. Multimode

8. Dielectric loaded


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2

P E H

0 1 12

D D D32ab

1 4G ( a b ).2

Conical hornSupport any polarization .


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Corrugated horn

9. Parabolic antenna


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i) Parabolic cylinder

Feed: linear dipole linear array or slotted waveguide.

ii) Parabolic (parabola of revolution).

Feed: pyramidal or conical horn. F/d ratio, spill over.


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10. Cassegrain reflector

To improve the performance of large ground based microwave reflector antennas for satellitetracking and communication, it has been proposed that a two reflector system be utilized.

This is known as cassegrain dual reflector system. The use of second reflector, a hyperboloid

known as sub reflector give an additional degree of freedom for achieving good performance.

Cassegrain antenna is usually attractive for applications that requires gain of 40 dB or greater.


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11. Slotted waveguide antenna


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is widely used as radar antenna because its simple construction and compact shape .


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12.Microstrip Antennas


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-

Basic Characteristics:

Microstrip antennas consist of very thin (t


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MICROSTRIP LINE:


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In a microstrip line most of the eletric field lines are concentrated underneath the

microstrip.

Because all fields do not exist between microstrip and ground plane we have a

different dielectric constant than that of the substrate. It is less, depending on

geometry.

The electric field underneath the microstrip line is uniform across the line. It is

possible to excite an undesired tranverse resonant mode if the frequency or line

width increases. It now behaves like a resonator consuming power.

A standing wave develops across its width as it acts as a resonator. The electric fieldis at a maximum at both edges and goes to zero in the centre.

Microstrip discontinuities can be used to advantage.


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Abrupt truncation of microstrip lines develop fringing fields storing energy and

acting like a capacitor because changes in electric field distribution are greater than

that for magnetic field distribution.

The line is electrically longer than its physical length due to capacitance.

For a microstrip patch the width is much larger than that of the line the fringingfields also radiate.

An equivalent circuit for a microstrip patch illustrates a parallel combination of

conductance and capacitance at each edge.

Radiation from the patch is linearly polarized with the E field lying in the same

direction as path length.


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Endfire radiation can also be accomplished by judicious mode selection.For


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Endfire radiation can also be accomplished by judicious mode selection.For

rectangular patch, usually

The strip (patch) and the ground plane are separated by a dielectric sheet (substrate)

The substrates that are most desirable for antenna performance are thick

substrates whose dielectric constant is lower because they provide better efficiency,larger bandwidth, loosely bound fields for radiation into space but at the expense of

larger element size.


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tconsdielectricrelative

wavelengthspacefreeiswhere

re

o

re

o

tan

2/112

1115.0

/5.0

W

H

L

rreffr

reo

13. Arrays.- beam shaping scanning, large gain


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The total field of the array is determined by the vector addition of the fields radiated by the

individual elements.

In an array of identical elements, there are five controls that can be used to shape the overall

pattern of the antenna.

( i) the geometrical configuration of the overall array (linear, circular, rectangular, etc)

(ii) relative displacement between the elements

(iii) excitation amplitude of the individual elements

(iv) excitation phase of the individual elements

(v) the relative pattern of the individual element


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MICROMACHINED ANTENNAS


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Size of the antenna gets smaller as the frequency increases demandinghigher precision manufacturing which can be overcome by micro

machining technique

Incorporation of micro machined actuators within the antenna itself tofacilitate special features to the antenna such as beam shaping and

reconfigurability

Microstrip antennas


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Circuit operation demand higher dielectric constant substrate whereasantenna operation requires low dielectric constant substrate.


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Effective dielectric constant is reduced by drilling small holes in the

antenna substrate


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Antenna performance can be improved by removing materials of the

substrate below the antenna .

Improve antenna characteristics achieved by forming trenches below

the radiating edges (instead of forming cavity below the patch) so that

the conductor of the patch was overhanging.


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40% improved in BW and marked improvement inradiation pattern achieved at 13.8GHz


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Mutual coupling between antenna array elements is reduced by makingcavities below the patch radiators. Surface waves which contribute to the


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mutual coupling are excited on the substrate above a cut off frequency

determined by its thickness.

More than 2/3 of the power is lost as surface waves on a 200m thick

substrate.

Micromachived Reconfigurable Antennas

Antennas capable of adaptively changing their characteristics arecalled reconfigurable antennas.

Patch radiator of a micro machined antenna is rotated about a

fulcrum to get beam steering capabilities


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The antenna is patterned on 100m fuzed quartz atop 500 m

ili b t t d d b i f t i i


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silicon substrate, suspended by a pair of torsion springs.

The rotation is achieved by electrostatic forces activating fixed

electrode on the substrate.

MEMS actuators used to adjust the angles of the arms of a Ve

antennas operating at 17.5GHz

Reconfigurable multiband phasedarray antennas are receiving a lot

of attention lately due to the emergence of RF MEMS (microelectro


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y g (

mechanical systems) switches [1-6].

A MEMS- switched reconfigurable multi band antenna is one that

can be dynamically reconfigured within a few microseconds to serve

different applications at drastically different frequency bands, such as

communications at L band (1-2 GHz) and synthetic aperture radar(SAR) at Xband (8-12.5 GHz).

They can be used both for ground and airborne moving target

indication (GMTI/AMTI) at these frequencies in order to detect moving

targets such as vehicles on the ground and low observables in the air.


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The RF MEMS switch is attractive because it shows of achieving

execellent switching characteristics over an extremely wide band (DC40 GHz and upwards).

These switches can also be used to develop wideband phase shifters .

Although there is currently a tremendous amount of research in RF

MEMS devices, reliability and packaging of the switches continue to be

problematic.

The switches are also limited in their power handling capability.

Reconfigurable patch Module (RPM):

In the design and fabrication of a dual L/X band reconfigurable


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In the design and fabrication of a dual L/X band reconfigurable

antenna, microstrip antenna elements were chosen due to their inherent

low profile, which is suitable for satellite and UAVapplications.

RT/ duroid 5880 material with a dielectric constant of 2.2 and a loss

tangent of 0.0009 at 10 GHz is usually used.

The material thickness must be chosen carefully, since it controls both

the bandwidth and array scanning performance.

The thicker the material, the more bandwidth, particularly at the low

frequency end.

However, if the substrate becomes too thick, surface waves are generated

and array scanning performance and efficiency are lost.


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Figure 1 shows a picture of the 3x3RPM fabricated on 0.125 duroid.

The patches are 0.370square and separated by 0.590on center.

Interconnecting tabs are 0.050wide and 0.085long.

The reconfigurable antenna was actually fabricated as two separated

prototypes (OPEN and CLOSED configurations) for testing in thelaboratory.


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Figure 1: Picture of 3x3 array prototypes fabricated on 0.125 thick substrate in (a)

OPEN configuration; and (b) CLOSEDconfiguration.

1.2% impedance bandwidth for the Lband configuration and greaterthan 7% bandwidth at Xband are achieved.


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This bandwidth was limited primarily by the substrate thickness.

Computer simulation results for both the return loss and radiationpatterns agree well with the measurements.

Reconfigerable Antenna at C- band

In a number of radar applications it is desired to have


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In a number of radar applications, it is desired to have

limited antenna beam scanning and also different beam

patterns (like pencil beam, fan beam, etc.)

Microelectromechanical system (MEMS) technology is made

it possible to realize such antenna with simple configuration.

In one configuration, the elements of a phased array antennaare switched using RF MEMS switches so that aperture area

and / or antenna elements sizes are changed.

In another configuration which uses V-antenna, the arms ofthe V-antenna is moved using MEMS actuators so that the

beam can be scanned and also the beam can be shaped.

In many applications in proximity fuse, there is a requirementof small light weight antenna with direction of beam along the


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axis of the fuse.

If one get the facility of beam scanning, it provides good

system flexibility.

Usually, scanning beam requires either costly phase shifters orcumbersome mechanical arrangement.

This system eliminates the need of phase shifter and

cumbersome mechanical arrangement yet it provides beamscanning.

The overall system is light weight, simple and cheap.

The basic antenna is a Vee-antenna on silicon substrate, fabricated usingMEMS (microelectromechanical system) technology and for beam

scanning electrostatic actuators is used as shown in fig 1


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scanning electrostatic actuators is used as shown in fig.1..

When the angles between the arms of the antenna is changed, the beam

shape is altered .

When both the arms rotated together in one direction, the beam is

scanned.


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Fig-1: MEMS Reconfigurable Vee- Antenna


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Fig2: Top view and details of a MEMS reconfigurable Vee-

antenna.

The system is capable of not only beam scanning, but also beamshaping. The characteristics of Vee-antenna depends on the included

angle 2 of V dipole and the length l of the arms The optimum included


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angle,2of V dipole and the length, l of the arms. The optimum included

angles and maximum directivities are given by

3 2

0 2

149.3 603.4 809.5 443.6

for 0.5 1.52

13.39 78.27 169.77

for 1.5 3

0

2.94 1.15 for 0.5 3lD

By changing the angle, 2othe beam shape may be modified.

For scanning the beam, it requires the arms of V to be moved


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simultaneously keeping the same included angle of V.

The arms of the V-antenna is movable through pulling or pushing byactuators.

One end of the antenna arm is held by rotational hinge locked on the

substrate which allows the arms to rotate with the hinge as the center of

the circle.The antenna arms are pulled or pushed by support bars connected to the

actuators with movable rotation hinges on both ends.

The movable rotation hinges translate the lateral movement of the

actuators to circular movement of the antenna arms which are electrically

separated from the support bar by dielectric material.

Fig.2 shows the system and Fig. 3 shows the performance at 3.00 GHz

reported (1) in the literature.


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Fig 3 : E-plane co-pol and cross-pol patterns compared with the

theoretical co-pol patterns for the 3-GHZ model.

The actuations used for moving the arms of the antenna is usually ofelectrostatic type. Which consists of a capacitor arrangement, where one

of the plate is movable by the application of bias voltage This produces


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of the plate is movable by the application of bias voltage. This produces

displacement.

However , when a voltage is applied across the system, one plate moves

towards the other (are plate is kept fixed), resulting in a net gap

d = d0-x

The capacitance with the plates in new position is

0

0

AC d

1

000

0 0 0 0 0

1

( ) ( ) ( ) ( ) ( )( ) 1

A A xC C

dd d x

Q t x t Q t Q t x t V t

C d C C d

The displacement x(t ) changes with the change of voltage V(t).


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Specifications (Typical)

Frequency : 5.0 GHZ

Beamwidth : 3

Scanning angle : 45

Size : 5.5x 5.5 cm2


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Multiple Frequency Antennas


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3. FUNDAMENTAL PARAMETERS OF ANTENNASRadiation Pattern

Radiation Power density.

Radiation Intensity


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y

Directivity

Gain

EfficiencyBeamwidth

Beam efficiency

Bandwidth

Polarization

Input impedance

Antenna equivalent areas.Antenna Radar Cross section

Antenna Temperature.

Radiation Pattern:

Defined as a mathematical functions or graphical representation of the radiation properties of

the antenna as a function of space coordinatesIn general, the pattern of an antenna is three- dimensional. Because it is impractical to measure a

three dimensional pattern, a no. of two dimensional patterns are measured.

ii) The minimum no. of two dimensional pattern is two and they are E

and H- plane patterns.

Two dimensional pattern is obtained by fixing one of the angle ( and ) while varying other.


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Power Pattern: -A trace of the received power at a constant radius

Amplitude F ield Pattern: -A graph of the spatial variation of the electric (or magnetic)

field along a constant radius.


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IsotropicRadiatorA hypothetical lossless antenna having equal radiation in all directions.Directional Antennahaving the property of radiating or receiving electromagnetic waves more

effectively in some directions than in others.


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OmniDirectionalhaving an essentially nondirectional pattern in a given plane and a

directional pattern in any orthogonal plane.

Principal Patternfor linearly polarized antenna, performance is described in terms of principal

Eand H-plane patterns.

E- plane- the plane containing the electric field vector and the direction of maximum radiation.

H-Plane- the plane containing the magnetic field vector and the direction of maximum radiation.

Field region:-

There are three main components to the radiated electromagnetic fields surrounding an antenna.

Two near field regions and far field region.

In the reactive near field, reactive field components predominate over the radiated field. Thismeans that any variations in the electrical properties (for electrical antennas) or magnetic

properties (for magnetic antennas) have a strong influence on the antenna's impedance at the

antenna feed point. The distance from the antenna to the boundary of the reactive near field


138/231

region is commonly assumed as:

In the radiating near field, the radiated field predominates, and the antenna impedance is only

slightly influenced by the surrounding media in this region. But the dimensions of the antenna

cannot be neglected with respect to the distance from the antenna. This means that the angular

distribution of the radiation pattern is dependent on the distance. For measurements of the

radiation pattern, the distance from the antenna should be larger than the radiating near fieldboundary, otherwise the measured pattern will be different from that under real life conditions.

The diameter of the radiating near field is

with D as the largest dimension of the antenna.For distances larger than R2, the radiation pattern is independent of the distance, meaning we are

in the far field region. In a practical application, the distance between transmitter and receiver

antennas is usually in this region.

Radiating near fieldRadiating near field (fresnel region)

Far Field (Fraunhofer

i )Radiating near field


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2

2

2DR

3

1 0.62 D

R (fresnel region)

region)Radiating near field

(fresnel region)

Radiating near field(fresnel region)

The field strengths of the near field components decreases rapidly with the increasing

distance from the antenna, one component being inversely related to distance squared and the

other distance cubed.

Far field-radiated field is TEM. Wave front is practically plane

22 /R D


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Relativepower

dB

sinD

U

24 /R D R=

R=

R=

Calculated radiation patterns of a paraboloid antenna.

Radiation & Steradian:

One radian is defined as the plane angle with its vertex at the center of a circle of radius r that

is subtended by an arc length, r.


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Steradian:

-as the solid angle with its vertex at the center of a sphere of radius, r that is subtended by a

spherical, surface area equal to that of a square of each side of length, r

Surface area = 4r2

The infinitesimal area dA on the surface of a sphere of radius, r is

dA = r2sindd(sr)

Therefore, the element of solid angle d of a sphere can be written as

( sr)2 sindA

d d d

r

Isotropic Radiator

The concept of the isotropic radiator is often used to describe radiated power and antenna gain.

The isotropic radiator is a hypothetical antenna, which radiates the supplied RF power equally in

ll di i h d i di f h i i di i h f h


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all directions. The power density at a distance rfrom the isotropic radiator is therefore the

supplied power divided by the area of a sphere with the radius r.

1. Isotropic Radiator

If we measure the power density in some distance from a device under test, the effective isotropicradiated power (EIRP) is the power which we would have to supply to an isotropic radiator in

order to get the same power density in the same distance. The EIRP describes the power radiation

capability of a device (including its antenna).


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From the EIRP, we can calculate the electrical field strength at a given distance from the radiator,which is specified in some government and regional regulations. The density of the radiated

power D (in W/m2) measured in the distance rfrom an isotropic radiator radiating the total

power EIRP is the radiated power divided by the surface area of the sphere with the radius r:

f we measure the power density in some distance from a device under test, the effective isotropicradiated power (EIRP) is the power which we would have to supply to an isotropic radiator in

order to get the same power density in the same distance. The EIRP describes the power radiation

capability of a device (including its antenna).

From the EIRP, we can calculate the electrical field strength at a given distance from the radiator,which is specified in some government and regional regulations. The density of the radiated

power D (in W/m2) measured in the distance rfrom an isotropic radiator radiating the total

power EIRP is the radiated power divided by the surface area of the sphere with the radius r:


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The relationship between the electrical field strength and the power density is the same asbetween voltage and power in an electrical circuit.

With the impedance of free space Z0 = 377 = 120 , the rms value of the electrical field

strength is then:

This gives:

Or:

Taking the logarithm on both sides gives the EIRP value in dBm:

EIRP[dBm]=E[dBV/m]+20 logr[meters]-10log30-90 dB


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In standard test setups, the electrical field strength is often measured at a distance of 3 m. In thiscase we can use the simple formula:

EIRP[dBm] = E[dBV/m] " 95.23 dB

As opposed to the hypothetical isotropic radiator, real antennas exhibit more or less distinct

directional radiation characteristics. The radiation pattern of an antenna is the normalized polar

plot of the radiated power density, measured at a constant distance from the antenna in a

horizontal or vertical plane.

The isotropic gain Giso of an antenna indicates how many times the power density of thedescribed antenna in the main direction of propagation is larger than the power density from an

isotropic radiator at the same distance.

Antenna gain does not imply an amplification of power; it comes only from the bundling of the

available radiated power in certain directions.


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Radiation Power Density:The power associated with the electromagnetic wave is described by Poynting vector.

W H


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W = instantaneous Poynting vector(W/m2)

= inst. Electric field intensity (V/m)H = inst. Magnetic field intensity (A/m)

W is power density. The total power crossing a closed surface can be obtained by integrating the

normal component of the Poynting vector over the entire surface.

Time average pointing vector (average power) can be written as.

Wav(x,y,z)=[w(x,y,x,t)]av = Re[EH*]. W/m2

*

* 2

. .

( , , , ) Re[ ( , , ) ]

( , , , ) Re[ ( , , ) ]

1Re( )

21 1

Re[ ]. Re[ ]2 2

s s

jwt

jwt

jwt jwt jwt

j wt

P W ds W n da

x y x t E x y z e

H x y x t H x y z e

Ee Ee E e

W H E H E He

s

*

P rad=Pav= . . da

1= Re[ ]

sWrad ds wav n

E H ds


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Re[ ].2 s

E H ds

Radiation Intensity:

Power radiated from an antenna per unit solid angle

U - radiation intensity (W/solid angle)

W radradiation density (W/m2)

The radiation intensity is also related to the far zone electric field of an antenna by

The total power is obtained by integrating radiation intensity over the entire solid angle

of 4.

2 U r W rad

2 2

, ( , ,2

rU E r

2

0 0P rad = U d sinU d d


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Where d= element of solid angle = sindd

2

y

x

For an isotropic source, U is independent of angles &

or Radiation intensity of an isotropic source is

0 0 0rad= 4P U d U d U

0

rad

4

PU


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2

0

0 0

P rad= U , ( , )sin

,4

d B F d d

FD


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2

0 0

0 2

0 0

2

0 0

, 4

, sin

, max4

, sin

4

=, sin , max

4 = A- beam solid angle

A

D

F d d

FD

F d d

F d d F

The beam solid angle A is defined as the solid angle through which all the power of

the antenna would flow if its radiation intensity is constant (and equal to the maximum

value of U) for all angles with A

Gain:

Absolute gain: Ration of the intensity, in a given direction, to the radiation intensity that would be

obtained if the power accepted by the antenna were radiated isotropically.


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Relative gainthe ratio of the power gain in a given direction to the power gain of a reference

antenna in its reference direction.Reference antenna usually a dipole, horn or any other antenna. Whose gain can be calculated or

known.

4 ( )

U , = 4

Pin

Radiation Intensity

gain total input accepted power

0 0

4 ( , )

( )

P rad= lcd Pin

,G( , ) 4

rad

G( , ) ,

, max , max D

UG

Pin lossless isotropic source

Ulcd

P

lcdD

G G lcdD lcd


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Antenna Efficiency

The radiation resistance is part of the impedance of the antenna at its feed point. Additionally, we

have the loss resistance Rloss which accounts for the power dissipated into heat as well as

reactive components L and C Figure 2 has an equivalent circuit that describes the antenna
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reactive components L and C. Figure 2has an equivalent circuitthat describes the antenna

around its resonant frequency.

2. Antenna Equivalent Circuit

The inductor and the capacitorin the equivalent circuit build a series resonant circuit. The

antenna impedanceZis:

At the frequency of resonance,

the reactances of the capacitor and the inductor cancel each other out soonly the resistive part of
http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=circuit&x=&y=http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=capacitor&x=&y=http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=capacitor&x=&y=http://www.rfdesignline.com/encyclopedia/defineterm.jhtml?term=circuit&x=&y=


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the reactances of the capacitor and the inductor cancel each other out, soonly the resistive part of

the antenna impedance remains.

The inductance L and the capacitance Cin the equivalent schematic are determined by the

antenna geometry. If we want to build an antenna for a given frequency, we have to find a

geometry that is resonant at the frequency of operation, such as a wire with a certain length.

At the frequency of resonance, the antenna inputimpedance equals Rr + Rloss. The antenna

efficiency hin resonance is the ratio of the radiated power to the total power accepted by theantenna from the generator:

At frequencies other than the resonant frequency, the antenna input impedance is either capacitiveor inductive. This phenomenon is why it is possible to tune an existing antenna by adding a series

capacitor or inductor.

The L-to-C ratio determines thebandwidthof the antenna for given radiation and loss resistances.

For the same resistance values, a larger L-to-C ratio means a higher quality factor Q and a

smaller bandwidth.

The values of L and C in the equivalent schematic depend on the antenna geometry.Often we candeduct intuitively how a variation of the geometry can influence L and C. The quality factor is

influenced by a contribution Qrad from the radiation resistance and Qloss from the loss

resistance. The overall Q of the antenna is:
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Chu [1] and Wheeler [2] gave the theoretical limit for the quality factor Q and the fractional

bandwidth of a lossless antenna as:

with aas the radius of the smallest circumscribing sphere surrounding the antenna.

The selectivity of the antenna can help to suppress unwanted out-of-band emissions; but not

always a small bandwidth is desirable. A small bandwidth means stringent requirements on thetolerances of the matching components and the antenna itself. For a given dimension of a small

antenna, we can only increase the bandwidth if we introduce intentional losses. The bandwidth of

an antenna with the efficiency is then:

-Antenna Efficiency takes into account the losses at the input terminal and within antenna

structure.


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1. Reflection loss due to mismatch between transmission line and antenna

2. I2R losses (conduction and dielectric)

Voltage reflection coeff. at the input terminals of the antenna.

Zininput impedance of the antenna

Z0Characteristic impedance of the transmission line.

.

0

in 0

in 0

Z =

Z

r c dl l e e

Z

Z

2

01l lcd

lcd = lc ld = antenna radiation efficiency.


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Half Power Beamwidth:

Important figure of merit. Often traded off between it and sidelobe level. Beamwidth

decreases as sidelobe level increases and viceversa. Resolution capability of two targets or

radiation sources depends on beamwidth.

Resolution capability of an antenna to distinguish two sources is equal to half the first nullbeamwidth (FNBW/2) which is usually used to approximate the half power beamwidth.

Beam efficiency: (BE)

2FNBW HPBW

1( )

( )

Power transmitted received within cone angleBE

Power transmitted received by the antenna

1 half angle of the cone within which the percentage of the total power is to be found.

12

0 0

2

( , )sinU d d

BE


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0 0

( , )sinU d d

If 1 is chosen as the angle where first null or minimum occurs, then the beam efficiency will

indicate the amount of power in the major lobe compared to the total power.

A BE required for Radar, Radiometry and other applications.

Bandwidth:

The range of frequencies within which the performance of the antenna with respect to

some characteristics conforms to a specified standard.

Polarization

Polarization describes the trace that the tip of the electrical field vector builds during the

propagation of the wave. In the far field, we can consider the electromagnetic wave as a plane

wave.


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wave.

In a plane electromagnetic wave, the electrical and the magnetic field vectors are orthogonal tothe direction of propagation and also orthogonal to each other. In the general case, the tip of the

electrical field vector moves along an elliptical helix, giving an elliptical polarization. The wave

is called right-hand polarized if the tip of the electrical field vector turns clockwise while

propagating; otherwise it is left-hand polarized.

If the two axis of the ellipse have the same magnitude, the polarization is called circular. If one of

the two axis of the ellipse becomes zero, we have linear polarization. Similarly, polarization is

vertical if the electrical field vector oscillates perpendicularly to ground, and it is horizontal if its

direction of oscillation is parallel to the ground plane.

A transmission system has the best performance (ideal case) when the polarization of the

transmitter and the receiver antenna are identical to each other. Circular polarization on one end

and linear polarization on the other gives 3-dB loss compared to the ideal case. If both antennas

are linearly polarized but 90 turned to each other, theoretically no power is received. The same

phenomenon happens if one antenna is right-hand circularly polarized and the other one is left-

hand circularl olarized In an indoor environment, reflections in the transmission path may change the

polarization, which makes the polarization of the received wave difficult to predict. If

one of the antennas is portable, we have to make sure that it works in any position.

Circular polarization at one end and linear polarization at the other end results in a

principal loss of 3 dB, but avoids the case of a total blackout, where no power is


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p p , , p

received.

Input-Impedance:

The impedance presented by an antenna at its terminals or the ratio of voltage to current at a

pair of terminals or the ratio of appropriate components of the electric to magnetic fields at a

point.

Generator Radiated wave


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( )g g g

g

t A g r L g A g

V V VIZ Z Z R R R j X X


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1

2 22( )

g

g

r L g A g

V

IR R R j X X

The power delivered to the antenna for radiation is given by.

And that dissipated as heat by

The remaining power is dissipated as heat on the internal resistance Rg of the generator.

22

2 2

1

2 2

g rr g

r L g A g

V RP I Rr

R R R X X

22

2 2

1

2 2

g LL g L

r L g A g

V RP I R

R R R X X

2

2 22

g g

g

V RP

R R R X X


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r L g A g R R R X X

Maximum power is delivered to the antenna when we have conjugate matching i.e

Rr+RL=Rg

XA = -Xg

For this case2

2

2

2

2

8 ( )

8 ( )

8

L

g rr

r L

g

Lr L

g

g

g

V RP

R R

RV

P R R

VP

R

Power supplied by the generator during conjugate matching.

2

01 1

2 4

g

S g g

r L

VP V I

R R


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Half of the power dissipated as heat in the internal resistance of the generator and the other half isdelivered to the antenna. Of the power that is delivered to the antenna, part is radiated through the

mechanism provided by the radiation resistance and the other is dissipated as heat.

Antenna Vector Effective Length and Equivalent Areas:

Vector effective length: The effective length of an antenna whether it be linear or anaperture antenna, is a quantity that is used to determined the voltage induced on the open-circuit

terminals of the antenna when a wave impinges upon it.

` VocOpenCircuit voltage at antenna terminalsEiincident electric field

LeVector effective length

r L

i

oc eV = E . L

Antennas Equivalent Areas:

These are used to describe the power capturing characteristics of the antenna when a wave

impinges on it. It is defined as.

The ratio of the available power at the terminals of a receiving antenna to the power flux

density of a plane wave incident on the antenna from that direction, the wave being polarization


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matched to the antenna.

Aeeffective aperture m2

PTPower delivered to the load (W)

WiPower density of incident wave (W/m2)

All the power that is intercepted, collected or captured by an antenna is not delivered to the load.

2

2T TTe

i i

I RPAW W

Under conjugate matching only half of the captured power is delivered to the load, the other half

is scattered and dissipated as heat.

To account for the scattered and dissipated power, scattering loss and capture equivalent areas are

defined.

Capture area = Effective area + Scattering area + loss area

Maximum effective areaAperture efficiency =

Physical area

1, for aperture antenna

>1, for wire antenna


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L A

A

L A

Z Z

Z Z


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s s s

A

t

I 1E (ZL)=E (o) - 1 I 2

tE

To separate out structural and antenna mode scattering terms, assume the antenna is loaded with

conjugatematched impedance (ZL=Z*A)

*s s * s

L A

t

*

*

*

I 1 E (Z )=E (Z ) -

I 2 2

t

A

L A

L A

ZAE

R

Z Z

Z Z

- electric field scattered by the antenna with conjugatematched load.s *

AE (Z )

* * *( ) 2s A m A A A mI Z I Z Z R I I*mscattering current when antenna is conjugatematched (ZL=Z*A)

*I


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The total radar cross section of the antenna terminated with a load ZLcan be written as

* *s s tmL At

IE Z E Z E

I

2

1 s a j rA e

s-RCS due to structural term

a-RCS due to antenna mode term

r-relative phase between the structural and antenna mode terms

-Total RCS with antenna terminated with ZLIf the antenna short circuited (A=-1) then

short=sIf the antenna open circuited (A=-1), then

2

2s a j ropen e

If the antenna, matched (A=-0),then

2s a j r

matched e


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4. ANTENNA MEASUREMENTS

The radiation characteristics of the antennas can be analyzed by methods like

GTD

Moment Method

Finite-Difference Time Domain (FDTD)

Fi i El

ANTENNA for Communication Systems-2008cep

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