4)What is duality of antenna?
It is defined as an antenna is a circuit device with a resistance and temperature on the one hand and the space device on the other with radiation patterns, beam angle,directivity gain and aperture.
5)Define Side Lobe Ratio
Side Lobe Ratio is defined as the ratio of power density in the principal or main lobe to the power density of the longest minor lobe.
6)List the arrays used for array tapering
Binomial Array: Tapering follows the coefficient of binomial seriesDolphTchebycheff Array: Tapering follows the coefficient of Tchebycheff polynomial.
7)Define a traveling wave antenna?
Traveling wave or non resonant antenna are those in which there is no reflected wave, i.e., only incident traveling wave travel in the antenna.
8)What is beverage or wave antenna?
A single wire antenna terminated in its characteristic impedance may have essentially a uniform traveling wave. This type of antenna is referred to as beverage antenna.
9)State Huygen’s Principle?
Huygen’s principle states that each point on a primary wave front can be considered to be a new source of a secondary spherical wave that a secondary wave front can be constructed as the envelope of these secondary waves.
10)What is Slot Antenna?
The slot antenna is an opening cut in a sheet of a conductor, which is energized through a coaxial cable or wave guide.
11)Which antenna is complementary to the slot dipole?
The dipole antenna is the complementary to the slot antenna. The metal and air regions of the slot are interchanged for the dipole.
12)What is the relationship between the terminal impedance of slot and
dipole antenna?
Z
s
Z
d
= h
o2/4 Where Z
sis the terminal impedance of the slot antenna Z
d
is the terminal impedance of the dipole antenna
h
o
is the intrinsic impedance of the free space.
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13)What is the difference between slot antenna and its complementary
dipole antenna?
• Polarization are different
• The electric field be vertically polarized for the slot and
horizontally polarized for its complementary dipole
• Radiation form the backside of the conducting plane of the slot antenna has the opposite polarity from that of the dipole antenna.
• 14)How spherical waves are generated?
When a voltage V is supplied at the input terminals of a biconical antenna, it will produce outgoing spherical waves. The biconical antenna acts as a guide for spherical waves.
15)What do you meant by sectoral horn?
If flaring (opened out) is done only in one direction, then it is called as a sectoral horn.
16)What do you meant by pyramidal horn?
If flaring is done along both the walls( E & H), then it is called as a pyramidal horn.
17)What is back lobe radiation?
Some radiation from the primary radiator occurs in the forward direction in addition to the desired parallel beam. This is known as back lobe radiation.
18)What are the various feeds used in reflectors?
• Dipole antenna
• Horn feed
• End fire feed
• Cassegrain feed
• 19)What are secondary antennas? Give examples?
Antennas that are not radiators by themselves are called secondary antennas. For example ,Cassegrain, Hyperbolic antennas.
20)StatePoynting theorem.
It states that the vector product of electric field intensity vector E and the magnetic filed intensity vector H at any point is a measure of the rate of energy flow per unit area at that point.The direction of power flow is perpendicular to both the electric field and magnetic field components.
21)Define beam width of maMor lobe?
It is defined the angle between the first nulls (or) itis defined as twice the angle between the first null and themaMor lobe maximum direction.
22)State Uniqueness theorem?
It states that for a given set of sources and boundary conditions in a lossy medium the solution to Maxwell’s equation is unique.
23)Define Image theory?
The image theory states that a given charge configuration above an infinite grounded perfect conducting plane may be replaced by the charge configuration itself, its image and an equipotential surface in plane of the conducting plane.
24)State the method of feeding slot antennas?
The slot antenna can be energized with a coaxial transmission line or by a waveguide.
25)What are the applications of corner reflector?
• In Television
• Point to point communication
• Radio Astronomy applications.
26)What is Flat Reflector?
It has a large flat sheet near a linear dipole antenna. It provides backward radiation. It provides substantial gain in the forward direction by reducing the space between the antenna and the sheet.
27)What is Corner Reflector?
It is a retro reflector consisting of 2 or 3 mutually perpendicular, intersecting flat surfaces which reflects the EM waves back towards the source.
28)What is Active Corner Reflector?
When the driven element is used in conMunction with the corner reflector, it is called as active corner reflector.
29)What is Passive Corner Reflector?
When thecorner reflector without driven element is called as passive corner reflector.
30)Uses of Corner Reflector?
• Used on the vessel’s mask at a height of atleast 4.6m above sea
level.
• Used on ships as Radar Reflectors.
• Used in automobile and bicycle tail lights.
31)Define V antenna?
An antenna having a V shaped arrangement of conductors fed by a balanced line at the apex is called V-antenna.
32)Types of V-antenna?
• Resonant V antenna
• 1on-Resonant V antenna
UNIT-4 SPECIAL ANTENNAS AND ANTENNA MEASUREMENTS
1)What are the parameters to be considered for the design of an helical
antenna?
The parameters to be considered for the design of an helical antenna are:
1. Bandwidth 2. Gain 3. Impedance 4. Axial Ratio
2)What are the types of radiation modes of operation for an helical
antenna
The two types of radiation modes of operation possible for an helical antenna are:
1. 1ormal mode of operation 2. Axial mode of operation
3)List the applications of helical antenna
The applications of helical antenna are:
• It became the workhouse of space communications for telephone, television anddata, being employed both on satellites and at ground stations
• Many satellites including weather satellites, data relay satellites all
have helicalantennas
• It is on many other probes of planets and comets, including moon and mars, being used alone, in arrays or as feeds for parabolic reflectors, its circular polarization and highgain and simplicity making it effective for space application.
4)What is a normal mode of helix antenna?
Radiation field is maximum in the direction normal to the helix axis &wave is nearly or exactly circularly polarized wave. Mode is said to benormal if the dimensions of helix is small compared with the wavelength.
5)What is a axial mode of helix antenna?
Radiation field is maximum in the end fire direction(ie) along the helix axis & polarization is circular or nearly circular.
6)How the axial mode is improved?
When the helix circumference D & spacing S of order of one wavelength. Axial mode of radiation is produced by raising helix circumference (c/G) of order of one wavelength & spacing is approximately of G/a.
7)What is the error introduced in direction finding of loop antenna?
Antenna or Vertical effect, 1ight effect or Aeroplane effect or polarization error.
8)What is an “array factor”?
It is apparent that the total field of an array is equal to equal field of asingle element positioned at the origin multiplied by a factor which is widelyreferred as array factor.
9)What are the advantages of binomial arrays?
• As HPBW increases and hence directivity decreases.
• For design of a large array, larger amp ratio of sources is required.
10)What is the disadvantage of a binomial array?
A maMor practical disadvantage of binomial array is the wide variations between the amplitudes of the different elements of an array,especially for an array with a large no of elements. This leads to very low efficiencies, and makes this method not very desirable in practice.
11)What is a binomial array?
The coefficients of binomial expansion represent relative amplitudes of the elements. Since the coefficients are determined from the binomialseries, expansion the array is known as binomial array.
12)Name and draw a frequency independent antenna
Log periodic antenna is a frequency independent antenna.It includes active region and reflective region.
13)What is YagiUda antenna?
It is an array of a driven element, a reflector and one or more directors.
14)What do you mean by parasitic element?
The passive elements which are not connected directly connected to the transmission line but are electrically coupled are called as parasitic elements.
15)What do you mean by driven elements?
Driven elements are an active element where the power from the transmitter is fed or which feeds the received power to the receiver.
16)Why folded dipole antenna is used in yagi antenna?
The folded dipole has high input impedance. If the distance between the driven and parasitic element is decreased, it will load the driven element so input impedance of driven element reduces. But this will be compensated.
17)What is beam antenna?
If three-element array are used then such a type of yagiuda is referred to as beam antenna.
18)What is a frequency independent antenna?
An antenna in which the impedance, radiation pattern and directivity remain constant as a function of frequency is called as frequency independent antenna. Eg, Log periodic antenna.
19)What is LPDA?
LPDA means log periodic dipole array. It is defined as an antenna whose electrical properties repeat periodically with logarithm of the frequency.
20)What are the different regions in log periodic antenna and how are they
differentiated?
• Inactive region ± L< l
• Active region ± L» l
• Inactive reflective region ± L>l
21)What are the applications of log periodic antenna?
• HF communication
• Television reception
• All round monitoring
22)What are the applications of Rhombic antenna?
• HF transmission and reception
• point to point communication.
23)Define rhombic antenna.
An antenna which consists of four straight wires arranged in the shape of diamond, suspended horizontally above the surface of the earth is called as a rhombic antenna. It is otherwise called as diamond antenna or traveling wave antenna.
24)What are the two types of rhombic antenna design?
• Alignmentdesign
• Maximum field intensity design
25)What are the limitations of rhombic antenna?
• It needs a larger space for installation
• Due to minor lobes transmission efficiency is low. 26)Define lens antenna?
An antenna, which collimates the incident divergent energy to prevent it from spreading in undesired directions, is called as lens antenna.
27)What are the different types of lens antenna?
• dielectric lens or H plane metal plate lens
• E plane metal plate lens antenna
28)What is a dielectric lens antenna?
Dielectric lens antennas are the antennas in which the traveling wave fronts are delayed by lens media
29)What are the drawbacks of lens antenna?
• Lens antennas are used only at higher frequencies (above 3 GHz) because at lower frequencies they become bulky and heavy. Lens antennas have excessive thickness at low frequencies.
• Costlier for the same gain and beam width in comparison with
reflectors.
30)What is biconical antenna?
The biconical antenna is a double cone antenna which is driven by potential, charge or an alternating magnetic field at the vertex. In this antenna both the cones face in the opposite direction.
31)What is Lunenburg lens?
The Lunenburg lens is a spherical symmetric delay type lens formed from a dielectric with index of refraction ‘n’ which varies as a function of radius .
32)What are the advantages of lens antenna?
• the lens antenna, feed and feed support do not block the aperture as
the rays are transmitted away from the feed
• It has greater design tolerance
• It can be used to feed the optical axis and hence useful in applications where a beam is required to be moved angularly with respect to the axis
33)Mention the uses of lens antenna?
• Unstepped dielectric lens is a wide band antenna as its shape does not depend on the wavelength and hence it can be used over a wide frequency range, however this is not true for the dielectric lens antenna which is frequency sensitive.
• Both reflectors and lens antenna are commonly used above 1000 MHz. Lens antenna is a microwave device. So it is preferred to be usually above 3000 MHz and not below it.
UNIT-5 RADIO WAVE PROPAGATION
1)Define Sky wave.
Waves that arrive at the receiver after reflection in the ionosphere is called sky wave.
2)Define Tropospheric wave.
Waves that arrive at the receiver after reflection from the troposphere region is called Tropospheric wave.(ie 10 Km from Earth surface).
3)Define Ground wave.
Waves propagated over other paths near the earth surface is called ground wave propagation.
4)What are the type of Ground wave.
Ground wave classified into two types.
• Space wave • Surface wave.
5)What is meant by Space Wave?
It is made up of direct wave and ground reflected wave. Also includes the portion of energy received as a result of diffraction around the earth surface and the reflection from the upper atmosphere.
6)What is meant by Surface Wave?
Wave that is guided along the earth’s surface like an EM wave is guided by a transmission is called surface wave. Attenuation of this wave is directly affected by the constant of earth along which it travels.
7)What is meant by fading?
Variation of signal strength occur on line of sight paths as a result of the atmospheric conditions and it is called .It can not be predicted properly.
8)What is meant by diversity reception?
To minimize the fading and to avoid the multi path interference the technique used are diversity reception. It is obtained by two ways.
• Space diversity reception.
• Frequency diversity reception.
• Polarization diversity.
9)Define Space diversity Reception.
This method exploits the fact that signals received at different locations do not fade together. It requires antenna spaced at least 100 l apart are referred and the antenna which high signal strength at the moment dominates.
10)Define frequency diversity Reception.
This method takes advantage of the fact that signals of slightly different frequencies do not fade synchronously. This fact is utilized to minimize fading in radio telegraph circuits.
11)Define polarization diversity reception.
It is used in normally in microwave links, and it is found that signal transmitted over the same path in two polarizations have independent fading patterns. In broad band dish antenna system, Polarization diversity combined with frequency diversity reception achieve excellent results.
12)What is meant by Faraday’s rotation?
Due to the earth’s magnetic fields, the ionosphere medium becomes anisotropic and the incident plane wave entering the ionosphere will split into ordinary and extra ordinary waves/modes.When these modes re-emerge from the ionosphere they recombine into a single plane wave again.Finally the plane of polarization will usually have changed, this phenomenon is known as Faraday’s rotation.
13)What are the factors that affect the propagation of radio waves?
• Curvature of earth.
• Earth’ s magnetic field.
• Frequency of the signal.
• Plane earth reflection.
14)Define gyro frequency.
Frequency whose period is equal to the period of an electron in its orbit under the influence of the earth’s magnetic flux density B.
15)Define Magneto-Ions Splitting.
The phenomenon of splitting the wave into two different components (ordinary and extra-ordinary) by the earth’s magnetic field is called Magneto- Ions Splitting.
16)Define LUHF?
The lowest useful HF for a given distance and transmitter power is defined as the lowest frequency that will give satisfactory reception for that distance and power.It depends on
• The effective radiated power
• Absorption character of ionosphere for the paths between transmitter
and receiver.
• The required field strength which in turn depends upon the radio
noise at the receiving location and type of service involved.
17)Define Refractive index?
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18)Define maximum Usable Frequency.
The maximum Frequency that can be reflected back for a given distance of transmission is called the maximum usable frequency (MUF) for that distance.
MUF = f
c
secF
i
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