1 fundamentals of antenna

8/3/2019 1 Fundamentals of Antenna.

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Q.1.What is meant by radiation pattern?

Ans. The radiation pattern is the representation of the radiation properties

of the antenna as a function of space coordinates. The trace of the spatial variation of the received! radiated power at a

constant radius from the antenna is called the power pattern and if the trace

is due to field strength then is called as field strength pattern.

Q. 2. What is polarization ? What are its types?

Ans. Polarization of EM fields describes the time variations of the time

harmonic field vectors at a given points. In other words, it describes the way

the direction and magnitude of the field vectors change in time. Polarizationis a time harmonic field characterises. It is the figure traced by the extremity

of the time-varying field vector at a given observation point.Polarization is of three types.1. Linear2. Circular3. Elliptical

Q. 3. Define effective aperture of an antenna.

Ans. The Effective Antenna aperture is the ratio of the available power at

the terminal of the antenna to the power flux density of a plane wave

incident upon the antenna which polarization matched of the antenna.


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Q. 4. Define directive gain of antenna. Write expression of radiation

resistance of a dipole antenna. Directive Gain G is

Ans. The gain is also called as directive gain when the antenna radiatepower in a particular direction relative to the average power radiated by the

Antenna.

Q. 5. Draw the two dimensional top view and side view of omni

directional and isotropic radiation pattern.

Ans.Radiation Pattern (2D)Omnidirectional


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Q. 6. Define the term Directivity?

Ans. Directivity of an antenna is the ratio of the radiation intensity in a

particular direction and the radiation intensity averaged over all directions.


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It can also be defined as the ratio of the radiation intensity of the antenna in

a given direction and the radiation intensity of an isotropic radiator fed by

the same amount of power.

Q. 7.Define antenna Beamwidth?

Ans.This is the range of frequencies, within which the antenna

characteristics confirm to &tam specifications.

Q. 8. Differentiate Radian and Steradian.

Ans..Radian is the plane angle with its vertex at the centre of a circle ofradius r which is subtended by an arc of length. In a circle of circumference

2irr there are 2t radians. Steradian is the solid angle with its vertex at thecentre of a sphere of radius r, which is subtended by a spherical surface area

equal to that of a square with each side of length r. In a closed surface,

there are 4.ir steradian.

Q 9. Explain half-power beam width?

Ans.Power Beam width (HPBW) is the angle between two vectors,

originating at the patterns origin and passing through these points of the

major lobe where the radiation intensity is half its maximum.


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The HPBW is the best parameter to describe the antenna resolution

properties.

It is defined as the ratio of the Main beam area to the total beam area

Q.10. What is the relationship between directivity and maximum

effective area?

Ans. Consider two Antenna A1 and A2A1 is transmitting antennaA, Receiving antenna.Distance between two antenna be R.Power density generated by A1 at A2.


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Q. 11. State the uniqueness theorem?

Ans. Any solution to Laplaces equation or Poissons equation which also

satisfies the boundary conditions must be the only solution that exists. It isunique i.e. it is the only possible solution.

Q. 12. What is antenna reciprocity?

Ans. According to reciprocity theorem if Antenna A1 is a transmitting

antenna and antenna A2 is the receiving one; the ratio of transmitted to

receive Power


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will not change if A1 becomes the receiving antenna and A2 becomes the

transmitting one.

Q. 13. What factors affect the directional pattern of an antenna?

Ans. Directional Antenna: Gain and directivity are intimately related in

antenna. The directivity of an antenna is a statement of how the RF energy

of focussed in one or two directions. Because the amount of RF energy

remains the same, but is distributed over less area, the apparent signalstrength is higher. This apparent increase in signal strength is the antenna

gain. The gain is measured in decidable over either a dipole (dBd) or

a theoretical construct called an isotropic radiator (dBi). The isotropicradiator is a spherical signal source that radiates equally well in all

directions. One way to view to omindirectional pattern is that it is a slice

taken horizontally through the three dimensional sphere. Local installation

factors can affect the radiation pattern. free space, i.e. the antenna is

installed at great distance from the surfaces of the Earth, trees, houses,wiring and so forth, the pattern will be nearly perfect. But in practical

situations, the two lobes might not be equal, or the minima might be less

distinct.

Q. 14. How is antenna efficiency calculated?

Ans.


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Q. 15. What is dipole?

Ans. A short linear conductor is so short that current may be assumed to be

constant throughout its lengths . Such short linear conductor is called aDipole.

Q. 16. What is the difference between isotropic and non isotropic

sources?


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Ans. Isotropic. An omnidirectional antenna can be practically implemented

but an isotropic antenna cannot be implemented practically. An isotropic

antenna resembles an ideal antenna with ideal values for all parameters.Some antenna sources radiate energy equally in all directions. Radiation of

this type is known as ISOTROPIC RADIATION.Non Isotropic Radiators. Virtually all types of antennas are nonisotropic

sources.That is that they tend to radiate more energy in a particular direction..

Q. 17. Define antenna efficiency.

Ans. The antenna efficiency accounts for all losses in the antenna prior toradiation.The losses may be due to mismatch of the input terminals, conduction

losses, dielectric losses and aperture illumination losses.Taking into consideration the radiation efficiency of an antenna, we canexpress relationship between the antennas total radiated power and the

total power input as:Power Radiated = (Antenna Radiation Efficiency) (Power Input)


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In the above formula, antenna radiation efficiency only includes conduction

efficiency and dielectric efficiency and does not include reflection efficiency

as part of the total efficiency factor.Moreover, the IEEE standards state that gain does not include losses arising

from impedance mismatches and polarization mismatches

Q. 18. What is Brewsters angle?

Ans. Brewsters angle: (also known as the polarization angle) is an angle of

incidence at which light with a particular polarization is perfectly transmittedthrough a surface, with no reflection.

Q. 19. Explain the concept of polarization in antenna what are the

conditions for different type of polarizations ? Also show that

curricular polarization is a conductor of elliptical polarization.

Ans. Polarization or plane of polarization of a radio wave can be defined bythe direction in which the electric vector E is aligned during the passage of

one full cycle.Since electric vector E and magnetic vector H are mutually perpendicularand this EM wave propagate in the perpendicular direction as shown in

figure.


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The magnetic field surround the wire and perpendicular to it, it means the

electric field is parallel to the wire. Polarization refers to the physicalorientation of the radiated EM wave in space. A EM wave is said to be

linearly polarized if they all have the same alignment in space. If E is verticalor lies in the vertical plane, the wave is said to be vertically polarized.Besides linear polarization, antenna may also radiate circularly or elliptically

polarized wave. If two linearly polarized waves are simultaneously produced

in the same direction from the same antenna provided that the two linearpolarization are mutually perpendicular to each other with a phase difference

of 90, then circularly polarized waves are produced.Circular polarization may be right handed or left handed upon the sense of

rotations i.e. phase difference is positive or negative.Circular polarization results only when the amplitudes of two linearly

polarized waves are equal. If the amplitudes are not equal, then combination

of two linearly, polarized waves will produce elliptically polarized wave.

Q. 20. Explain self and mutual impedance. What is the importance of

impedance in antenna?

Ans. Self impedance of an antenna is defined as its input impedance when

all other antennas are completely removed i.e. away from it. The input


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impedance is the impedance at the point where the transmission

line carrying RF power from the transmitter is connected. Also, at this

impedance the transmission line operate. So this know as driving point

impedance.Self Impedance can be represented as

Mutual impedance is defined as the negative ratio of the voltage induced in

one circuit to the current in the second circuit, with all the circuits opencircuited excepting the second circuit.

The mutual impedance happens with all antennas present in the nearby. Theimpedance is generally derived to supply maximum available power from

transmitter to the antenna or to extract maximum amount received energy

from the antenna.

Q. 21. Obtain the fundamental relation of electromagnetic radiation.

Ans. The time varying condition Maxwells equation predict the radiation ofEM energy from current source. The antenna is designed to radiate as

effectively as possible.


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We analyse the radiation fields obtained from typical antenna sources shown

in figure.

At first the E and B fields are described in terms of the auxiliary scalar and

vector potential V and A respectively which in turn satisfy wave equation.A solution of the wave equation in A is then obtained in the form of anintegral over the antenna current. The electric and magnetic fields of charge

and current sources are related to potential functions as

The potential fields at any point in free space is expressed by integration

over the charge and its current source as


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Q. 22. Explain the following terms1. Field radiation pattern2. Power radiation pattern3. Effective length4. Radiation resistance

Ans.

1.Field radiation pattern : The graphical representation of an antenna as a

function of direction is given the name radiation pattern of the antenna.2. Power radiation pattern : The trace of the spatial variation of the

received/ radiated power at a constant radius from the antenna is called the

power pattern.The radiation pattern is measured in the far field region, where the spactial

distribution of the radiated power does not depend on the distance.3. Effective length: Effective length is the parameter of antenna which is

related to its apertures. It represents the effectiveness of an antenna to

radiate or collect power.

Radiation resistance is defined as that fictitious resistance which when

substituted in series with the antenna will consume the same power as is

actually radiated.


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Q. 23. Write notes on the following.(a) Equivalent noise temperature.(b) SNR(c) Antenna Temperature

Ans. (a) Equivalent noise temperature is defined as that fictional

temperature at the input of the network which would account for the noise

at the output. Delta N is the additional noise introduced by the network

itself.

(b) The signal to noise ratio is given by

Where

is input signal and noise power

(c) For a lossless antenna, antenna temperature has nothing to do with the

physical temperature of the antenna but is related to the temperature ofdistant region of space coupled to the antenna via radiation resistance.


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Q.24.What do you mean by(i) Effective Area(ii) Beam solid area(iii) Directive gain.

Ans. (i) The effective area (Aperture) is the ratio of the available power at

the terminals of the antenna to the power flux density of a plane wave

incident upon the antenna, which is polarization method to the antenna.

(ii) Beam solid angle of an antenna is the solid angle through which all thepower of the antenna would flow if its radiation intensity were constant and

equal to the maximum radiation intensity U0 for all angles within beam solid

area

(iii) Directive Gain : Gain is the ratio of the radiation intensity U in a givendirection and the radiation intensity that would be obtained if the power fed

to the antenna were radiated isotropically.


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The gain may also be called the directive gain when the antenna radiate

power in a particular direction relative to the average power radiated by the

antenna.

Q. 25. Explain half power beamwidth and first null beamwidth.

Ans. Antenna Beamwidth is ati angular width in degrees of the major lobe

between points where the power radiated is at half its maximum value.Half power Beamwidth (HPBW) : Is the angle between two vectors,

originating at the Patterns origin and passing through these points of the

major lobe where the radiation intensity is half its maximum.First Null Beamwidth (FNBW) Is the angle between two vectors, originating

at the Patterns origin and tangent to the main beam at its base.

Q. 26. Write a note on the antenna impedances.


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Ans. Antenna Impedance : Technically, antenna Impedance is the ratio at

any given point in the antenna of voltage to current at that point. Depending

upon height above ground, the influence of surrounding objects and otherfactors, our quarter wave antenna with a near perfect ground exhibits a

nominal input impedance of around 36 ohms. A half wave dipole antenna is

normally 75 ohms while a half wave folded dipole antenna is nominally 300ohms.Impedance matching is the electronics design practice of setting the inputimpedance (Z5) of an electrical load equal to the fixed output impedance

(ZL) of the signal source to which it is ultimately connected, usually in order

to maximize the power transfer and minimize reflections from the load. This

only applies when both are linear devices.

Q. 27. Define polarization. Explain different types of polarization in

detail.

Ans Antenna Polarisation : Depending upon how the antenna is orientated

physically determines its polarisation. An antenna erected vertically is saidto be vertically polarised while an antenna erected horizontally is said (notso surprising) to be horizontally polarised. Other specialised antenna exist

with cross polarisation, having both vertical and horizontal components

and we can have circular polarisation.The polarization is the figure traced by the extremity of the time-varying

field vector at a given observation point.According to the shape of the trace, three types of polarization exist for

harmonic fields, linear, circular and elliptical.


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In the most general case, elliptical polarization is defined If electric field magnitude is not same or the phase angle is other than 0,

90, 180.


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The polarization of a radiated wave (polarization of a radiating antenna) at a

specific point in the far one is the polarization of the locally plane wave. The

polarization of a received wave (polarization of a, receiving antenna) is the

polarization of a plane wave, incident from a given direction, and havinggiven power flux density, which results in maximum available power at the

antenna terminals.

1 fundamentals of antenna

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