Antennas and Propagation 10EC64

Dept of ECE, SJBIT 1

QUESTION PAPER SOLUTION

Unit- 1: Antenna Basics

1. Explain Radiation pattern (june/july08)

A radiation pattern defines the variation of the power radiated by an antenna as a function of

the direction away from the antenna. This power variation as a function of the arrival angle is

observed in the far field.

This is an example of a donut shaped or toroidal pattern. In this case, along the z-axis, which

would correspond to the radiation directly overhead the antenna, there is very little power

transmitted. In the x-y plane (perpendicular to the z-axis), the radiation is maximum. These

plots are useful for visualizing which directions the antenna radiates.

2. Explain Field Regions(june/july07)

A pattern is "isotropic" if the radiation pattern is the same in all directions. These antennas

don't exist in practice, but are sometimes discussed as a means of comparison with real

antennas. Some antennas may also be described as "omnidirectional", which for an actual

means that it is isotropic in a single plane (as in Figure 1 above for the x-y plane). The third

category of antennas are "directional", which do not have a symmetry in the radiation pattern.

The far field region is the most important, as this determines the antenna's radiation pattern.

Also, antennas are used to communicate wirelessly from long distances, so this is the region

of operation for most antennas. We will start with this region.

Far Field (Fraunhofer) Region

The far field is the region far from the antenna, as you might suspect. In this region, the

radiation pattern does not change shape with distance (although the fields still die off with

1/R^2). Also, this region is dominated by radiated fields, with the E- and H-fields orthogonal

to each other and the direction of propagation as with plane waves.

If the maximum linear dimension of an antenna is D, then the far field region is commonly

given as:

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This region is sometimes referred to as the Fraunhofer region, a carryover term from optics.

In the immediate vicinity of the antenna, we have the reactive near field. In this region, the

fields are predominately reactive fields, which means the E- and H- fields are out of phase by

90 degrees to each other (recall that for propagating or radiating fields, the fields are

orthogonal (perpendicular) but are in phase).

The boundary of this region is commonly given as:

Radiating Near Field (Fresnel) Region

The radiating near field or Fresnel region is the region between the near and far fields. In this

region, the reactive fields are not dominate; the radiating fields begin to emerge. However,

unlike the Far Field region, here the shape of the radiation pattern may vary appreciably with

distance.

The region is commonly given by:

Note that depending on the values of R and the wavelength, this field may or may not exist.

Finally, the above can be summarized via the following diagram:

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3. Explain DIRECTIVITY(june/july07)

Directivity is a fundamental antenna parameter. It is a measure of how 'directional' an

antenna's radiation pattern is. An antenna that radiates equally in all directions would have

effectively zero directionality, and the directivity of this type of antenna would be 1 (or 0

dB).

An antenna's normalized radiation pattern can be written as a function in spherical

coordinates:

Because the radiation pattern is normalized, the peak value of F over the entire range of

angles is 1. Mathematically, the formula for directivity (D) is written as:

This equation might look complicated, but the numerator is the maximum value of F, and the

denominator just represents the "average power radiated over all directions". This equation

then is just a measure of the peak value of radiated power divided by the average.

4. Explain EFFICIENCY AND GAIN (june/july09)

The efficiency of an antenna relates the power delivered to the antenna and the power

radiated or dissipated within the antenna. A high efficiency antenna has most of the power

present at the antenna's input radiated away. A low efficiency antenna has most of the power

absorbed as losses within the antenna.

The losses associated within an antena are typically the conduction losses (due to finite

conductivity of the antenna) and dielectric losses (due to conduction within a dielectric which

may be present within an antenna). Sometimes efficiency is defined to also include the

mismatch between an antenna and the transmission line, but this will be discussed in the

section on impedance.

The efficiency can be written as the ratio of the radiated power to the input power of the

antenna:

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The term Gain describes how much power is transmitted in the direction of peak radiation to

that of an isotropic source. Gain is more commonly quoted in a real antenna's specification

sheet because it takes into account the actual losses that occur.

A gain of 3 dB means that the power received far from the antenna will be 3 dB (twice as

much) higher than what would be received from a lossless isotropic antenna with the same

input power.

Gain is sometimes discussed as a function of angle, but when a single number is quoted the

gain is the 'peak gain' over all directions. Gain (G) can be related to directivity (D) by:

The gain of a real antenna can be as high as 40-50 dB for very large dish antennas (although

this is rare). Directivity can be as low as 1.76 dB for a real antenna, but can never

theoretically be less than 0 dB. However the peak gain of an antenna can be arbitrarily low

because of losses. Electrically small antennas (small relative to the wavelength of the

frequency that the antenna operates at) can be very inefficient, with gains lower than -10 dB

(even without accounting for impedance mismatch loss).

5. Explain BEAM WIDTH AND SIDE LOBES: (june/july07)

The main beam is the region around the direction of maximum radiation (usually the region

that is within 3 dB of the peak of the main beam). The main beam in Figure 6 is centered at

90 degrees.

The sidelobes are smaller beams that are away from the main beam. These sidelobes are

usually radiation in undesired directions which can never be completely eliminated. The

sidelobes in Figure 6 occur at roughly 45 and 135 degrees.

The Half Power Beamwidth (HPBW) is the angular separation in which the magnitude of

the radiation pattern decrease by 50% (or -3 dB) from the peak of the main beam. From

Figure 2, the pattern decreases to -3 dB at 77.7 and 102.3 degrees. Hence the HPBW is 102.3-

77.7 = 24.6 degrees.

Another commonly quoted beamwidth is the Null to Null Beamwidth. This is the angular

separation from which the magnitude of the radiation pattern decreases to zero (negative

infinity dB) away from the main beam. From Figure 2, the pattern goes to zero (or minus

infinity) at 60 degrees and 120 degrees. Hence, the Null-Null Beamwidth is 120-60=60

degrees.

Finally, the Sidelobe Level is another important parameter used to characterize radiation

patterns. The sidelobe level is the maximum value of the sidelobes (away from the main

beam). From Figure 6, the Sidelobe Level (SLL) is -14.5 dB.

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6. Explain Antenna's impedance(Dec/Jan 07)

An antenna's impedance relates the voltage to the current at the input to the antenna. Letan

antenna has an impedance of 50 ohms. This means that if a sinusoidal voltage is input at the

antenna terminals with amplitude 1 Volt, the current will have an amplitude of 1/50 = 0.02

Amps. Since the impedance is a real number, the voltage is in-phase with the current.

Let's say the impedance is given as Z=50 + j*50 ohms (where j is the square root of -1). Then

the impedance has a magnitude of

and a phase given by

This means the phase of the current will lag the voltage by 45 degrees. To spell it out, if the

voltage (with frequency f) at the antenna terminals is given by

then the current will be given by

So impedance is a simple concept, which relates the voltage and current at the input to the

antenna. The real part of an antenna's impedance represents power that is either radiated away

or absorbed within the antenna. The imaginary part of the impedance represents power that is

stored in the near field of the antenna (non-radiated power). An antenna with a real input

impedance (zero imaginary part) is said to be resonant.

7. Explain BANDWIDTH(june/july07)

Bandwidth describes the range of frequencies over which the antenna can properly radiate or

receive energy. Often, the desired bandwidth is one of the determining parameters used to

decide upon an antenna. For instance, many antenna types have very narrow bandwidths and

cannot be used for wideband operation.

Bandwidth is typically quoted in terms of VSWR. For instance, an antenna may be described

as operating at 100-400 MHz with a VSWR<1.5. This statement implies that the reflection

coefficient is less than 0.2 across the quoted frequency range. Hence, of the power delivered

to the antenna, only 4% of the power is reflected back to the transmitter. Alternatively, the

return loss S11=20*log10(0.2)=-13.98 dB.

8. Explain Polarization (Dec/Jan 08)

Polarization is a fundamental characteristics of any antenna.

Linear Polarization

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A plane electromagnetic (EM) wave is characterized by travelling in a single direction (with

no field variation in the two orthogonal directions). In this case, the electric field and the

magnetic field are perpendicular to each other and to the direction the plane wave is

propagating. As an example, consider the single frequency E-field given by equation (1),

where the field is traveling in the +z-direction, the E-field is oriented in the +x-direction, and

the magnetic field is in the +y-direction.

In equation (1), the symbol is a unit vector (a vector with a length of one), which says that

the E-field "points" in the x-direction.

9. Explain ANTENNA TEMPERATURE

Antenna Temperature ( ) is a parameter that describes how much noise an antenna

produces in a given environment. This temperature is not the physical temperature of the

antenna. Moreover, an antenna does not have an intrinsic "antenna temperature" associated

with it; rather the temperature depends on its gain pattern and the thermal environment that it

is placed in.

For an antenna with a radiation pattern given by , the noise temperature is

mathematically defined as:

10. Explain EFFECTIVE APERTURE(june/july09)

A useful parameter calculating the receive power of an antenna is the effective area or

effective aperture. Assume that a plane wave with the same polarization as the receive

antenna is incident upon the antenna.

Then the effective aperture parameter describes how much power is captured from a given

plane wave. Let W be the power density of the plane wave (in W/m^2). If P represents the

power at the antennas terminals available to the antenna's receiver, then:

Hence, the effective area simply represents how much power is captured from the plane wave

and delivered by the antenna. This area factors in the losses intrinsic to the antenna (ohmic

losses, dielectric losses, etc.). This parameter can be determine by measurement for real

antennas.

A general relation for the effective aperture in terms of the peak gain (G) of any antenna is

given by:

Antennas and Propagation 10EC64

Dept of ECE, SJBIT 7

Effective aperture will be a useful concept for calculating received power from a plane wave.

To see this in action, go to the next section on the Friis transmission formula.

11. Explain FRIIS transmission formula(Dec08/jan09)

Consider two antennas in free space (no obstructions nearby) separated by a distance R:

Assume that Watts of total power are delivered to the transmit antenna. For the moment,

assume that the transmit antenna is omnidirectional, lossless, and that the receive antenna is

in the far field of the transmit antenna. Then the power p of the plane wave incident on the

receive antenna a distance R from the transmit antenna is given by:

If the transmit antenna has a gain in the direction of the receive antenna given by , then

the power equation above becomes:

The gain term factors in the directionality and losses of a real antenna. Assume now that the

receive antenna has an effective aperture given by . Then the power received by this

antenna ( ) is given by:

Since the effective aperture for any antenna can also be expressed as:

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The resulting received power can be written as:

This is known as the Friis Transmission Formula. It relates the free space path loss, antenna

gains and wavelength to the received and transmit powers. This is one of the fundamental

equations in antenna theory, and should be remembered (as well as the derivation above).

12. Explain Reciprocity (Dec07/jan08)

An antenna’s electrical characteristics are the same whether it is used for transmitting or

receiving. Because this is always true, throughout this lecture, we will consider antennas as

transmitting antennas.

13. Explain Polarization(Dec07/jan08)

Polarization is the orientation of the electric field vector of the electromagnetic wave

produced by the antenna. For most antennas, the orientation of the antenna conductor

determines the polarization. Polarization may be vertical, horizontal or elliptical.

The diagram above shows vertical and horizontal polarization. If the radio wave's electric

field vector points in some other direction, it is said to be obliquely polarized.

If the electric field rotates in space, such that its tip follows an elliptical path, it is elliptically

polarized.

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14. Explain Wavelength (Dec07/jan08)

This is the length of one RF wave. It can be computed by either of the following formulas,

depending on the units required:

(in m) = 300/f(in MHz) or (in ft) = 984/f(in MHz)

Unit- 2: Point Sources and Arrays

1. Explain Isotropic radiator (june/july08)

An isotropic radiator is a theoretical point source of waves which exhibits the same

magnitude or properties when measured in all directions. It has no preferred direction of

radiation. It radiates uniformly in all directions over a sphere centred on the source. It is a

reference radiator with which other sources are compared. Isotropic radiators obey Lambert's

law.

2. Explain ANTENNA ARRAYS (Dec 08/Jna09)

An antenna array (often called a 'phased array') is a set of 2 or more antennas. The signals

from the antennas are combined or processed in order to achieve improved performance over

that of a single antenna. The antenna array can be used to:

increase the overall gain

provide diversity reception

cancel out interference from a particular set of directions

"steer" the array so that it is most sensitive in a particular direction

determine the direction of arrival of the incoming signals

to maximize the Signal to Interference Plus Noise Ratio (SINR)

An antenna array is an antenna that is composed of more than one conductor. There are two

types of antenna arrays:

Driven arrays – all elements in the antenna are fed RF from the transmitter

Parasitic arrays – only one element is connected to the transmitter. The other elements are

coupled to the driven element through the electric fields and magnetic fields that exist in the

near field region of the driven element

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3. Explain COLLINEAR ARRAY( Dec 07/jan 08)

The collinear array consists of /2 dipoles oriented end-to-end. The center dipole is fed by the

transmitter and sections of shorted transmission line known as phasing lines connect the ends

of the dipoles as shown below.

The length of the phasing lines are adjusted so that the currents in all the dipole sections are

in phase, as shown below.

The input impedance of a collinear array is approximately 300 ohms. The directivity of a

collinear array slowly increases as the number of collinear sections is increased.

4. Explain BROADSIDE ARRAY (june/july08)

A broadside array consists of an array of dipoles mounted one above another as shown below.

Each dipole has its own feed line and the lengths of all feed lines are equal so that the

currents in all the dipoles are in phase.

Rows of broadside arrays can be combined to form a two dimensional array as shown below:

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The two-dimensional array is used in high performance radar systems. The amplitude and

phase of each input current is adjusted so that the antenna radiates its RF in a narrow beam.

By making changes to the input phase and amplitude, the beam can be made to scan over a

wide range of angles. Electronic scanning is much faster than mechanical scanning (which

uses a rotating antenna) and permits rapid tracking of large numbers of targets.

A special type of phased array consisting of 2 or more vertical antennas is widely used in

AM broadcasting. Consider an AM transmitter located in a coastal city such as Charleston,

SC. It would make no sense to radiate a signal in all directions; there is only water to the east

of city. Two or more antennas could be used to produce a directional pattern that would

radiate most of the signal to the west.

The design and analysis of phased arrays is quite difficult and will not be covered further in

this unit.

Unit- 3: Electric Dipoles and Thin Linear Antennas

1. Write a note on short dipole antenna and describe the fields of short dipole

antenna(june/july07)

The short dipole antenna is the simplest of all antennas. It is simply an open-circuited wire,

fed at its center as shown in Figure 1.

The words "short" or "small" in antenna engineering always imply "relative to a wavelength".

So the absolute size of the above dipole does not matter, only the size of the wire relative to

the wavelength of the frequency of operation. Typically, a dipole is short if its length is less

than a tenth of a wavelength:

If the antenna is oriented along the z-axis with the center of the dipole at z=0, then the current

distribution on a thin, short dipole is given by:

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The fields radiated from this antenna in the far field are given by:

2. Explain the fields of thin linear antenna (june/july08)

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Unit- 4: Loop, Slot, Patch and Horn Antenna

1. Describe LOOP ANTENNAS (june/july10)

All antennas discussed so far have used radiating elements that were linear conductors. It is

also possible to make antennas from conductors formed into closed loops. There are two

broad categories of loop antennas:

1. Small loops, which contain no more than 0.085 wavelengths (~/12) of wire

2. Large loops, which contain approximately 1 wavelength of wire.

SMALL LOOP ANTENNAS

A small loop antenna is one whose circumference contains no more than 0.085 wavelengths

of wire. In such a short conductor, we may consider the current, at any moment in time to be

constant. This is quite different from a dipole, whose current was a maximum at the feed

point and zero at the ends of the antenna. The small loop antenna can consist of a single turn

loop or a multi-turn loop as shown below:

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2. Explain the radiation pattern of small loop antenna

The radiation pattern of a small loop is very similar to a dipole. The figure below shows a 2-

dimensional slice of the radiation pattern in a plane perpendicular to the plane of the loop.

There is no radiation from a loop

3. Explain slot antenna (june/july10)

A slot antenna consists of a metal surface, usually a flat plate, with a hole or slot cut out.

When the plate is driven as an antenna by a driving frequency, the slot radiates

electromagnetic waves in similar way to a dipole antenna. The shape and size of the slot, as

well as the driving frequency, determine the radiation distribution pattern. Slot antennas are

often used instead of line antennas when greater control of the radiation pattern is required.

Slot antennas are often found in standard desktop microwave sources used for research

purposes.

A slot antenna's main advantages are its size, design simplicity, robustness, and convenient

adaptation to mass production using PC board technology.

Slot antennas are used typically at frequencies between 300 MHz and 24 GHz. These

antennas are popular because they can be cut out of whatever surface they are to be mounted

on, and have radiation patterns that are roughly omnidirectional (similar to a linear wire

antenna, as we'll see). The polarization is linear. The slot size, shape and what is behind it

(the cavity) offer design variables that can be used to tune performance.

4. Explain Babinets principle (June/July09)

Antennas and Propagation 10EC64

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Babinet's principle relates two antennas. The first result states that the impedance of the slot

( ) is related to the impedance of its dual antenna ( ) by the relation:

In the above, is the intrinsic impedance of free space. The second major result of

Babinet's/Booker's principle is that the fields of the dual antenna are almost the same as the

slot antenna (the fields components are interchanged, and called "duals"). That is, the fields

of the slot antenna (given with a subscript S) are related to the fields of it's complement

(given with a subscript C) by:

Hence, if we know the fields from one antenna we know the fields of the other antenna.

Hence, since it is easy to visualize the fields from a dipole antenna, the fields and impedance

from a slot antenna can become intuitive if Babinet's principle is understood.

Note that the polarization of the two antennas are reversed. That is, since the dipole antenna

on the right in Figure 2 is vertically polarized, the slot antenna on the left will be horizontally

polarized.

5. Explin Horn antenna (june/july10)

A horn antenna is used for the transmission and reception of microwave signals. It derives its

namefrom the characteristic flared appearance. The flared portion can be square, rectangular,

or conical. The maximum radiation and responsecorresponds with the axis of the horn. In this

respect, the antenna resembles anacoustic horn. It is usually fed with a waveguide.

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In order to function properly, a horn antenna must be a certain minimum size relativeto the

wavelength of the incoming or outgoing electromagnetic field. If the horn istoo small or the

wavelength is too large (the frequency is too low), the antenna will not work efficiently.

Horn antennas are commonly used as the active element in a dish antenna. The horn is

pointed toward the centerof the dish reflector. The use of a horn, rather than a dipole antenna

or any other type of antenna, atthe focal point of the dish minimizes loss of energy (leakage)

around the edges of thedish reflector. It also minimizes the response of the antenna to

unwanted signalsnot in the favored direction of the dish.

Horn antennas are used all by themselves in short-range radar systems, particularlythose used

by law-enforcement personnel to measure the speeds of approaching or retreatingvehicles

6. Explain PATCH antenna (Dec 08/jan09)

A patch antenna is a wafer-like directional antenna suitable for covering single-floor small

offices, small stores and other indoor locations where access points cannot be placed

centrally. Patch antennas produce hemispherical coverage, spreading away from the mount

point at a width of 30 to 180 degrees.

Patch antennas are also known as panel, flat panel or microstrip antennas. They are formed

by overlaying two metallic plates, one larger than the other, with a dielectric sheet in the

middle. This type of antenna is usually encased in white or black plastic, not only to protect

the antenna, but also to make it easy to mount. Because they are flat, thin and lightweight,

patch antennas are often hung on walls or ceilings where they remain visually unobtrusive

and blend easily into the background.

Antennas and Propagation 10EC64

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Unit- 5 & 6: Antenna Types

1. Explain Helical antenna (june/july07)

A helical antenna is a specialized antenna that emits and responds to electromagnetic fields

with rotating (circular)polarization. These antennas are commonly used at earth-based

stations in satellite communications systems. This type of antenna is designed for use with an

unbalanced feed line such as coaxial cable. The center conductor of the cable is connected to

the helical element, and the shield of the cable is connected to the reflector.

The length of the helical element is one wavelength or greater. The reflector is a circular or

square metal mesh or sheet whose cross dimension (diameter or edge) measures at least 3/4

wavelength. The helical element has a radius of 1/8 to 1/4 wavelength, and a pitch of 1/4 to

1/2 wavelength. The minimum dimensions depend on the lowest frequency at which the

antenna is to be used. If the helix or reflector is too small (the frequency is too low), the

efficiency is severely degraded. Maximum radiation and response occur along the axis of the

helix.

The most popular helical antenna (often called a 'helix') is a travelling wave antenna in the

shape of a corkscrew that produces radiation along the axis of the helix. These helixes are

referred to as axial-mode helical antennas. The benefits of this antenna is it has a wide

bandwidth, is easily constructed, has a real input impedance, and can produce circularly

polarized fields. The basic geometry is shown in Figure .

The parameters are defined below.

- Diameter of a turn on the helix.

- Circumference of a turn on the helix (C=pi*D).

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- Vertical separation between turns.

- pitch angle, which controls how far the antenna grows in the z-direction per turn, and

is given by

- Number of turns on the helix.

- Total height of helix, H=NS.

2. Explain LOG PERIODIC DIPOLE ARRAY (june/july07)

The log periodic dipole array (LPDA) is one antenna that almost everyone over 40 years old

has seen. They were used for years as TV antennas. The chief advantage of an LPDA is that it

is frequency-independent. Its input impedance and gain remain more or less constant over its

operating bandwidth, which can be very large. Practical designs can have a bandwidth of an

octave or more.

Although an LPDA contains a large number of dipole elements, only 2 or 3 are active at any

given frequency in the operating range. The electromagnetic fields produced by these active

elements add up to produce a unidirectional radiation pattern, in which maximum radiation is

off the small end of the array. The radiation in the opposite direction is typically 15 - 20 dB

below the maximum. The ratio of maximum forward to minimum rearward radiation is called

the Front-to-Back (FB) ratio and is normally measured in dB.

Log-Periodic Dipole Array

The log periodic antenna is characterized by three interrelated parameters, andas well

as the minimum and maximum operating frequencies, fMIN and fMAX. The diagram below

shows the relationship between these parameters.

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Unlike many antenna arrays, the design equations for the LPDA are relatively simple to work

with. If you would like to experiment with LPDA designs, click on the link below. It will

open an EXCEL spreadsheet that does LPDA design.

3. Explain YAGI-UDA ARRAY (june/july10)

The Yagi-Uda array, named after the two Japanese physicists who invented it, is the most

common antenna array in use today. In contrast to the other antenna arrays that we have

already looked at, the Yagi has only a single element that is connected to the transmitter,

called the driver or driven element. The remaining elements are coupled to the driven

element through its electromagnetic field . The other elements absorb some of the

electromagnetic energy radiated by the driver and re-radiate it. The fields of the driver and

the remaining elements sum up to produce a unidirectional pattern. The diagram below

shows the layout of elements in a typical Yagi.

Behind the driven element is a single element that is approximately 5% longer. This is the

reflector. It prevents radiation off the back of the array. In front of the director are a series of

elements that are shorter than the driven element. These are the directors. They help focus the

radiation in the forward direction. Together the reflector and directors can reduce the

radiation off the back of the antenna to 25 - 30 dB below the forward radiation. As more

directors are added, the forward gain increases.

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4. Explain CORNER REFLECTORS (Dec 08/jan09)

To increase the directivity of an antenna, a fairly intuitive solution is to use a reflector. For

example, if we start with a wire antenna (lets say a half-wave dipole antenna), we could place

a conductive sheet behind it to direct radiation in the forward direction. To further increase

the directivity, a corner reflector may be used, as shown in Figure . The angle between the

plates will be 90 degrees.

5. Explain LENS ANTENNAS (Dec 08/jan09)

With a LENS ANTENNA you can convert spherically radiated microwave energy into a

plane wave (in a given direction) by using a point source (open end of the waveguide) with a

COLLIMATING LENS. A collimating lens forces all radial segments of the spherical

wavefront into parallel paths. The point source can be regarded as a gun which shoots the

microwave energy toward the lens. The point source is often a horn radiator or a simple

dipole antenna.

Waveguide Type The WAVEGUIDE-TYPE LENS is sometimes referred to as a

conducting-type. It consists of several parallel concave metallic strips which are placed

parallel to the electric field of the radiated energy fed to the lens, as shown in figure 3-10A

and 3-10B. These strips act as waveguides in parallel for the incident (radiated) wave. The

strips are placed slightly more than a half wavelength apart. Figure.—Waveguide lens.

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6. Explain Parabolic reflector (June/july09) The most well-known reflector antenna is the parabolic reflector antenna, commonly known

as a satellite dish antenna. Examples of this dish antenna are shown in the following Figures.

Parabolic reflectors typically have a very high gain (30-40 dB is common) and low cross

polarization. They also have a reasonable bandwidth, with the fractional bandwidth being at

least 5% on commercially available models, and can be very wideband in the case of huge

dishes (like the Stanford "big dish" above, which can operate from 150 MHz to 1.5 GHz).

The smaller dish antennas typically operate somewhere between 2 and 28 GHz. The large

dishes can operate in the VHF region (30-300 MHz), but typically need to be extremely large

at this operating band.

The basic structure of a parabolic dish antenna is shown in Figure It consists of a feed

antenna pointed towards a parabolic reflector. The feed antenna is often a horn antenna with a

circular aperture.

Unlike resonant antennas like the dipole antenna which are typically approximately a half-

wavelength long at the frequency of operation, the reflecting dish must be much larger than a

wavelength in size. The dish is at least several wavelengths in diameter, but the diameter can

be on the order of 100 wavelengths for very high gain dishes (>50 dB gain). The distance

between the feed antenna and the reflector is typically several wavelenghts as well. This is in

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contrast to the corner reflector, where the antenna is roughly a half-wavelength from the

reflector.

7. Explain Paraboloid (Dec 08/jan09)

The parabola is completely described by two parameters, the diameter D and the focal length

F. We also define two auxilliary parameters, the vertical height of the reflector (H) and the

max angle between the focal point and the edge of the dish ( ). These parameters are

related to each other by the following equations:

Unit- 7 & 8: Radio Wave Propagation

1. Explain GROUND WAVE PROPAGATION (Dec 08/jan09)

Ground Waves are radio waves that follow the curvature of the earth. Ground waves are

always vertically polarized, because a horizontally polarized ground wave would be shorted

out by the conductivity of the ground. Because ground waves are actually in contact with the

ground, they are greatly affected by the ground’s properties. Because ground is not a perfect

electrical conductor, ground waves are attenuated as they follow the earth’s surface. This

effect is more pronounced at higher frequencies, limiting the usefulness of ground wave

propagation to frequencies below 2 MHz. Ground waves will propagate long distances over

sea water, due to its high conductivity.

Ground waves are used primarily for local AM broadcasting and communications with

submarines. Submarine communications takes place at frequencies well below 10 KHz,

which can penetrate sea water (remember the skin effect?) and which are propagated globally

by ground waves.

2. Explain SPACE WAVE PROPAGATION (june/july09)

Space Waves, also known as direct waves, are radio waves that travel directly from the

transmitting antenna to the receiving antenna. In order for this to occur, the two antennas

must be able to “see” each other; that is there must be a line of sight path between them. The

diagram on the next page shows a typical line of sight. The maximum line of sight distance

between two antennas depends on the height of each antenna. If the heights are measured in

feet, the maximum line of sight, in miles, is given by:

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Because a typical transmission path is filled with buildings, hills and other obstacles, it is

possible for radio waves to be reflected by these obstacles, resulting in radio waves that arrive

at the receive antenna from several different directions. Because the length of each path is

different, the waves will not arrive in phase. They may reinforce each other or cancel each

other, depending on the phase differences. This situation is known as multipath propagation.

It can cause major distortion to certain types of signals. Ghost images seen on broadcast TV

signals are the result of multipath – one picture arrives slightly later than the other and is

shifted in position on the screen. Multipath is very troublesome for mobile communications.

When the transmitter and/or receiver are in motion, the path lengths are continuously

changing and the signal fluctuates wildly in amplitude. For this reason, NBFM is used almost

exclusively for mobile communications. Amplitude variations caused by multipath that make

AM unreadable are eliminated by the limiter stage in an NBFM receiver.

An interesting example of direct communications is satellite communications. If a satellite is

placed in an orbit 22,000 miles above the equator, it appears to stand still in the sky, as

viewed from the ground. A high gain antenna can be pointed at the satellite to transmit

signals to it. The satellite is used as a relay station, from which approximately ¼ of the

earth’s surface is visible. The satellite receives signals from the ground at one frequency,

known as the uplink frequency, translates this frequency to a different frequency, known as

the downlink frequency, and retransmits the signal. Because two frequencies are used, the

reception and transmission can happen simultaneously. A satellite operating in this way is

known as a transponder. The satellite has a tremendous line of sight from its vantage point in

space and many ground stations can communicate through a single satellite.

3.Describe SKY WAVES (june/july08)

Propagation beyond the line of sight is possible through sky waves. Sky waves are radio

waves that propagate into the atmosphere and then are returned to earth at some distance

from the transmitter. We will consider two cases:

ionospheric refraction

tropospheric scatter

4. Explain IONOSPHERIC REFRACTION (june/july09

This propagation mode occurs when radio waves travel into the ionosphere, a region of

charged particles 50 – 300 miles above the earth’s surface. The ionosphere is created when

the sun ionizes the upper regions of the earth’s atmosphere. These charged regions are

electrically active. The ionosphere bends and attenuates radio waves at frequencies below 30

MHz. Above 200 MHz the ionosphere becomes completely transparent. The ionosphere is

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responsible for most propagation phenomena observed at HF, MF, LF and VLF. The

ionosphere consists of 4 highly ionized regions

The D layer at a height of 38 – 55 mi

The E layer at a height of 62 – 75 mi

The F1 layer at a height of 125 –150 mi (winter) and 160 – 180 mi (summer)

The F2 layer at a height of 150 – 180 mi (winter) and 240 – 260 mi (summer)

The density of ionization is greatest in the F layers and least in the D layer Though created by

solar radiation, the ionosphere does not completely disappear shortly after sunset. The D and

E layers disappear almost immediately, but the F1 and F2 layers do not disappear; rather they

merge into a single F layer located at a distance of 150 – 250 mi above the earth.

Recombination or charged particles is quite slow at that altitude, so the F layer lasts until

dawn.

The critical frequency varies from place to place, and it is possible to view this variation by

looking at a real-time critical frequency map

The critical frequency varies from 1 to 15 MHz under normal conditions. Most

communications is done using radio waves transmitted at the horizon, to get the maximum

possible distance per hop. The highest frequency that can be returned when the takeoff angle

is zero degrees is called the MUF, maximum usable frequency. The MUF and critical

frequency are related by the following formula:

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5. Write a note on ionospheric propagation (Dec08/jan09)

The ionosphere also attenuates radio waves. The amount of attenuation is roughly inversely

proportional to the square of the frequency of the wave. Thus attenuation is a severe problem

at lower frequencies, making daytime global communications via sky wave impossible at

frequencies much below 5 MHz.

The properties of the ionosphere are variable. There are 3 periodic cycles of variation:

diurnal (daily) cycle

seasonal cycle

sunspot cycle

The daily cycle is driven by the intensity of the solar radiation ionizing the upper atmosphere.

The D and E layers form immediately after sunrise, and the F layer splits into two layers, the

F1 and F2. The density of the layers increases until noon and then decreases slowly

throughout the afternoon. After sunset, the D and E layers disappear and the F1 and F2

merge to form the F layer. Take another look at the real-time MUF map and notice the

difference between the MUF numbers in the day and night regions. If you aren't sure which

region is the daytime region, it has a small yellow sun icon in its center. The thick gray lines

indicate the location of the terminator - the division between day and night.

Seasonal variation is linked to the tilt of the earth’s axis and the distance between the earth

and sun. The effects are complex, but the result is that ionospheric propagation improves

dramatically during the for the northern hemisphere during their winter, while seasonal

variation in the southern hemisphere is much smaller.

The 11 year sunspot cycle exerts a tremendous effect on the atmosphere. Near the peak of the

cycle (the last peak occurred in December 2001) the sun’s surface is very active, emitting

copious amounts of UV radiation and charged particles, which increase the density of the

ionosphere. This leads to a general increase in MUF’s and attenuation at lower frequencies.

When the sun becomes extremely active, or a major solar flare occurs, the ionosphere can

become so dense that global ionospheric communications are disrupted.

The maximum distance that can be covered by a single hop using ionospheric propagation is

about 2500 miles. Greater distances can be covered using multi-hop propagation, in which

radio waves are reflected by the ground back up to the ionosphere.

The ionosphere is not uniform and different regions refract RF differently. Multipath

propagation is the result. This leads to rapid variations in the received signal amplitude

known as fading.

One of the consequences of ionospheric propagation is that reception of signals on the AM

broadcast band varies greatly from day to night.

6. Explain SPORADIC-E PROPAGATION (Dec08/jan09)

For reasons that are not clearly understood, clouds of densely ionized gases appear in the E -

layer of the ionosphere. The clouds are generally relatively small and can happen at any time

of day. These clouds are formed throughout the year, but are most common in the summer

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months. Because these clouds are so densely ionized, they can support ionospheric

propagation at frequencies well above the normal MUF. Sporadic E propagation has been

observed at frequencies as high as 144 MHz, and is relatively common at 50 MHz.

The E-layer is lower than the F-layer and as a result, the distance covered by a sporadic-E

hop is approximately 1000-1300 miles, depending on the cloud's height. The sporadic-E

clouds drift through the E-layer, adding to the unpredictability of sporadic-E propagation.

Sporadic-E propagation is not generally useful because of its unpredictability. Its main

impact is negative, causing VHF-TV and FM broadcasters in different markets to interfere

with each other.

7. Explain TROPOSPHERIC SCATTER (june/july09)

Regional over the horizon communications are possible through a sky wave technique called

tropospheric scatter (troposcatter or just tropo). As shown in the diagram below, the

troposphere, which is the layer of the atmosphere closest to the ground, has pockets or cells

of air within it that have a different water vapor content and therefore a different refractive

index for radio waves. As a result, radio waves are scattered by the cells over the horizon.

This scatter occurs at frequencies of 0.3 – 10 GHz. Operation above 10 GHz is not possible

because water vapor in the air strongly absorbs the signals This scattering process is not

efficient and very little of the transmitted signal is scattered in the direction of the receiver.

High power transmitters and sensitive receivers are required.

The troposphere contains almost all of the earth’s weather patterns, which makes the

troposphere’s properties quite variable. This makes troposcatter communications subject to

weather induced fading and communications blackouts. To improve the reliability of

troposcatter links, a technique called diversity operation is used. There are three types of

diversity:

Frequency Diversity – two frequencies simultaneously transmit the same signal

Polarization Diversity – radio waves of both polarizations are transmitted simultaneously

Space Diversity – pairs of widely separated antennas are used for transmitting and receiving

Diversity operation greatly increases the reliability of troposcatter links, but it comes at a

significant cost, because at least double the amount of equipment is needed at each

installation.