Chapter 1

1.Define antenna.

An antenna is a structure that has been designed to radiate or receive electromagnetic waves.

2. Define radiation pattern.

An antenna radiation pattern is defined as a mathematical function or a graphical

representation of the radiation properties of the antenna as a function of space coordinates.

The radiation properties include power flux density,radiation intensity, field strength,

directivity, phase or polarization.

3. Draw the thevenin’s equivalent of the antenna system and name the terms used.

Vg – Voltage generator

Zg – Generator impedance

RL – Less resistance

Rr – Radiation resistance

XA – Antenna reductance

4. What are the two types of radiation pattern.

i) Field pattern

If the radiation from the antenna is expressed in terms of the field strength(either E or H),

then the radiation pattern is called as field pattern.

ii) Power pattern

If the radiation from the antenna is expressed in terms of power per unit area,then the

resultant pattern is power pattern.

5. Define Beam area or Beam solid angle.

6. Distinguish between radian and steradian.

The measure of a plane angle is radian. The measure of a solid angle is steradian.

1 steradian= 1 radian2

7. Define radiation intensity.

The power radiated from an antenna per unit solid angle is defined as radiation intensity. The

unit of radiation intensity is watts/steradian.

8. Define antenna beam efficiency.

The ratio of main beam area to the total beam area is called antenna beam efficiency.

εM =ΩM/ΩA

where ΩM = Main beam area; ΩA = Total beam area;

ΩA = ΩM + Ωm

9. Define stray factor.

The ratio of minor lobe area Ωm to the beam area ΩA is called stray factor.

εM =ΩM/ΩA

10.Define antenna gain.

Gain is defined as the ability of the antenna to concentrate the radiated power in a given direction

or to absorb effectively the incident power from that direction.

Gain,G = Max. radiation intensity from test antenna / Max. radiation intensity from reference

antenna

11. Define directivity.

Directivity is defined as the ratio of the maximum radiation intensity U(θ,Φ)max to the average

radiation intensity Uave.

D=U(θ,Φ)max/ Uaverage

= S(θ,Φ)max/ Saverage

12.Give the relationship between effective aperture and directivity.

13. Define effective aperture.

The effective aperture is defined as the ratio of power received at the antenna load terminal to

the power density of the incident wave.

Ae =PT/S m2.

14. Define scattering aperture.

It is defined as the ratio between the reradiated power and the power density of the incident

wave.

15. Define loss aperture.

The loss aperture is defined as the area of the antenna which dissipates power as heat.

16.What is collecting aperture?

The collecting aperture is obtained by adding effective, loss aperture and scattering aperture.

Ac = Ae + As + AL

17. What is aperture efficiency?

The ratio of effective aperture to physical aperture is the aperture efficiency.

Εap = Ae/ Ap ; 0< εap <1.

18. Define HPBW.

HPBW is the angular width measured on the major lobe of radiation pattern between points

where the radiated power has fallen to half of its maximum value. IT is otherwise called as 3

db beamwidth.

19. Define antenna efficiency.

Antenna efficiency is defined as the ratio of power radiated by the antenna to the total input

power supplied to the antenna. It is denoted by k. It lies between 0 and 1.

20.Define FBR.

Front to back ratio is defined as the ratio of the power radiated in desired direction to the power

radiated in the opposite direction.

Higher the FBR, better the antenna performance.

21.Define radiation resistance.

22.State reciprocity principle.

If a current I1 at the terminals of antenna no.1 induces an emf E21 at the open terminals of antenna

no.2 and a current I2 at the terminals of antenna no.2 induces an emf E12 at the open terminals of

antenna no.1 then E12 = E21 provided I1 = I2.

1. Define antenna array.

A radiating system composed of several spaced and properly phased radiator is called as antenna

array.

2. What are the advantages of the antenna array?

We go for antenna array to get greater directivity in a desired direction.

3.Define uniform linear array.

An uniform linear array is one in which the elements are fed with a current of equal magnitude

with uniform phase shift along a line.

4.Define broadside array.

Broadside array is defined as an arrangement in which the principle direction of radiation is

perpendicular to the array axis and also to the plane containing the array element. The phase

difference between adjacent elements is zero.

5.Define endfire array.

End fire array is defined as an arrangement in which the principle direction of radiation coincides

with array axis.

6.What is array factor?

The factor by which the array increases the field strength over that of single element radiating the

same total power.

The normalized value of the electric field is given by E--------------------

The above expression is called as antenna array factor.

7.Define BWFN.

Beam width between first nulls (BWFN) is defined as the angle between first nulls or the double

the angle between first null and major lobe maximum directions.

8.Differentiate broadside and end fire array.

Sl.no Broadside array End fire array

1. Array elements are fed with the currents Array elements are fed with the

of equal amplitude and in phase. Currents of equal amplitude

and out of

Phase,

2. Maximum radiation is perpendicular to the Maximum radiation is directed

along

Direction of array axis. the array axis.

3. BWFN =± 2λ/nd radians BWFN=± 2 √2λ/nd

radians.

9.Define the principle of pattern multiplication.

The total field pattern of an array of non isotropic but similar point sources isthe

multiplication of the individual source patterns and the pattern of an array of isotropic point

sources, each located at the phase centre of individual source and having the same relative

amplitude and phase while the total phase pattern is the addition of the phase pattern of the

individual sources and the array of isotropic point sources.

10.State image theory.

The image theory states that a gives charge configuration above an infinite grounded ,perfect

conducting plane may be replaces by the charge configuration itself, its image and an

equipotential surface in plane of the conducting plane.

12.Distinguish between active and passive arrays.

Active array consists of elements of which driven by physical load. Passive array consists of

one element actively driven others couple so it electromagnetically through the near field.

13.What are the various types of phased arrays?

1. Frequency scanning array 2.Retro array 3. Adaptive array.

14. Define phased array.

Phased array is an array of many elements with the phase of each element being a variable,

providing control of the beam direction and pattern shape.

15. What is a retro array?

If a wave incident on an array is received and transmitted back in the direction, the array act

as a retro array.

1. Define loop antenna.

The loop antenna is a radiating coil of any convenient cross section of one or more turns carrying

RF currents.

2. Name the different Shapes of loop antenna.

3. Name an antenna that produces circularly polarized wave.

Helical antenna produces circularly polarized waves.

4. Define pitch angle. What happens when α =0⁰ & α =90⁰?It is the angle between a line tangent to the helix wire and the plane normal to the helix axis.

Pitch angle, α = tan¯¹(S/∏D)

Where, S is helix turn spacing, D is the diameter of helix.

When, α=0⁰, helix becomes a loop; when, α=90⁰, helix becomes a linear conductor

5. What are the various modes of operation of helical antenna?

I) Normal mode ii) Axial mode

6.What are the important parameters to be considered for the design of an helical antenna?

The important design parameters are

1. Beam width 2. Gain 3. Axial ratio 4. Impedance.

7.List out the applications of helical antenna?

Helical antenna is used in

1. VHF transmission such as satellite communication

2.Space telemetry link with ballistic missiles, satellites,etc.

8.What are the advantages of helical antenna?

1.Very simple 3. Higher directivity

2. Wideband operation is possible 4.Circular polarization is obtained.

9.What are the disadvantages of loop antenna?

1. It is not suitable for high frequency applications

2.Transmission efficiency is low

3. Spurlous induced voltages are produced if near by loops are present.

1. Define retardation time.

It is the time required for the wave to propagate over the distance r. It is given by r/c where c is

3*108 m/s.

2. Define Hertzian dipole.

Hertzian dipole is defined as a short linear conductor or short electric dipole whose length is very

short compared to wave length (l<<λ ) and current is assumed to be constant throughout its

length l.

3. Write down the expression for radiation resistance of a Hertzian electric dipole.

Radiation resistance, Rr =80∏2(L/λ)2 ohms (or) Rr =790(L/λ)2 ohms.

4. What are the field components present around a Hertzian dipole?

Er, Eθ and EΦ are the various field components present around a Hertzian dipole.

5. What is a half wave dipole? (or) What is a resonant length antenna?

Any linear antenna of λ/2 long is called as half wave dipole antenna or resonant length antenna.

6. What is the radiation resistance of a half wave dipole?

Radiation resistance of a half wave dipole antenna i.e., λ/2 antenna is given by Rr =73Ω.

7. Define induction field (or near field)

The field which predominates at the points closer to the current element where r is small is

known as induction field. The near field is inversely proportional to square of the distance (1/r2).

It is of less importance.

8. What is radiation field?

The radiation field is far field and it varies inversely with distance [1/r]. This field contributes to

the flow of energy away from the source. This radiation field or far field is of great importance at

a large distance.

9. What is a quarter wave monopole antenna?

The quarter wave monopole antenna is an antenna whose length is one half of a half wave dipole

antenna and is located on a conducting ground plane.

10. What is a capacitance hat?

The capacitance hat is circular in shape with mast at the centre of the circle. There are number of

horizontal conducting wires with their ends joined together by means of a ring. The capacitance

hats are used to increase the electrical length of low frequency antennas.

11. Define Top loading.

Top loading is a method to increase the effective capacitance at the top of the antenna. This is

accomplished by mounting one or more horizontal conductors at the top of the antenna.

12. What is retarded current?

Since current flowing through a short dipole is assumed to be constant throughout its length, the

effect of this current is not felt instantaneously in a distant point P, but only after an interval

equal to the time required for the wave to propagate over distance r. This current is called as

retarded currents.

Retarded current [I] = Iθ ejω(t-r/c) , where r/c –retardation time.

1. What is yagi uda antenna?

It is an array of a driven element, a reflector and one or more directors.

2. What do you mean by parasitic elements?

The passive elements which are not connected directly to the transmission line that are

electrically coupled are called as parasitic elements.

3. What do you mean by driven elements?

Driven element is an active element where the power from the transmitter is fed or where feeds

the received power to the receiver.

4. What is beam antenna?

If three elements array i.e., one reflector , one driven element and one director are used,then such

type of yagi-uda antenna is referred toas beam antenna.

5.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.

6.Why log periodic antenna is named so far?

The geometry of log periodic antenna is so chosen that electrical properties must repeat

periodically with logarithm of the frequency.

7. What is the condition for an antenna to be frequency independent?

The condition for frequency independent is,

----------------------

Where f(θ) is any function of

8. 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.

9.What are the application of log periodic antenna?

1. HF communication, 2. Television reception, 3. All round monitoring.

10.What are application of rhombic antenna?

1. HF transmission and reception 2. Point to point communication.

11.Define rhombic antenna.

An antenna which consists of four straight wires, arranged in the shape of diamond, suspended

horizontally above the surface of earth is called as rhombic antenna. It is otherwise called as

diamond antenna or travelling wave antenna.

12.What are the limitations of rhombic antenna?

1. It needs a larger space for installation.

2. Due to minor lobes, transmission efficiency is low.

13.Define a traveling wave antenna.

Traveling wave or non-resonant antennas are those antennas in which there is no reflected wave

i.e., only incident traveling wave travel on the antenna.

14.What is the main advantages of traveling wave antenna?

It provides larger bandwidth.

15.What is a 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 a beverage antenna.

16.What are the type of radiation pattern produced when a wave travels in a wire? Draw the

pattern.

Unidirectional radiation pattern is produced when a wave travels in a wire.

1. What are the 3 modes of radio wave propagation ?

1. Ground wave or surface wave propagation. (up to 2MHZ)

2. Sky wave or ionosphere wave propagation. (Between 2 to 30 MHZ)

3. Space wave propagation. (above 30MHZ)

2. Define ground wave ?

A wave that is guided along the surface of the earth is called as ground wave or

surface wave.

3. What are the factors that cause attenuation in ground wave ?

Frequency, surface irregularities, permittivity, conductivity, diffraction and tilt in

the wave front are the various factors that cause attenuation in ground wave.

4. What are the two components present in space wave ?

Direct wave: It reaches directly from transmitting antenna to receiving antenna.

Indirect wave: It reaches receiving antenna after reflection from ground.

5. Define troposphere?

The portion of the atmosphere which extends up to 16km from the earth’s surface is

called as troposphere.

6. Define LOS distance?

Line of sight distance is a distance in which both the transmitting and receiving

antenna can usually see each other.

7. Define ionosphere?

The upper part of the atmosphere where the ionizations possible is known as

ionosphere.

8. Define fading?

It is the fluctuation in the received signal strength at the receiver or a random

variation in the received signal.

9. How can we minimize fading?

The most common method to minimize fading is diversity reception. In this

method, some Part of the signal is duplicated and even if one part experiences a deep

fade, the other may not.

10. What are the various types of diversity reception?

Time diversity reception

Frequency diversity reception

Space diversity reception

11. Define critical frequency.

The critical frequency fc of an ionized layer is defined as the highest frequency

which can be reflected by a particular layer at vertical incidence. It is different for

different layer.

Fc=9 √N m

Nm MHz

Wher Nm → maximum electron density in the layer.

12. Which is virtual height?

Virtual height is defined as the height to which a short pulse of energy send vertically

upward and travelling with speed of light would reach taking the same to ways travel

time does the actual pulse reflected from the layer.

13. Define MUF.

Maximum usable frequency (MUF) is dfdefined as the frenquency whfich can be

reflected back to earth for some specific angel of incident

FMUF = FC sec i

FC →critical frequency

I → angel of incidence

14.write down the expression for the refractive index

refractive index=µ √(1-(81N/f2))

N → number of electron per cubic meter

f→frequency in Hz.

15.what are the factor that affect propagation of radio waves.

The factors that affect the propagation of radio waves are:

1. Earth magnetic felid

2. Frequency of the signal.

3. Curvature of earth.

4.permittivity and conductivity

16.what is OWF or OTF?

Optimum working frequeny or optimum traffic frequency is frequency that is

normally used for ionospheric transmission. It is chosen to be about 1.5% less than MUF.

17.what is skip distance or skip Zone?

Skip distance defined as the minimum distance from the transvitter at which a sky wave

of given frequency is retured to earth by the ionosphere.

18. define duct propagation

Ducts are found between the layer of different tempatures andwater vapour contents in air. Duct

guides EM waves between its wall.the higher frequencies or microwaves are continuously

refracted in the duct and reflected by the ground.

So that they propagate around the curvature for beyond the line of sight.this special refraction of

EM waves is called the duct propagation.

19.what is multihop propagation.

The converage of transmission distance between transmission and receiver in more than one hop

is known as multihop propagation.

1. What are the three methods used for measuring polarization?

1. Polarization pattern method.

2. Linearly component method

3. Circular component method.

2. Define polarization ratio.

The ratio of maximum to minimum field intensity is known as polarsation ratio

or ellipticiy.

3. What is an anechoic chamber?

It is an indoor chamber. It is popular for antenna mearsuments. There exists no

electromagnetic interference(EMI) in anechoic chamber walls, ceiling and floor are filled

with RF energy absorbers.

1. State Huygen’s principle or field equivalent principle.

Huygen’s state that each point on a primary wave front can be considered to be

a new source of a secondary spherical wave and that a secondary wave front can be

constructed as the envelope these secondary waves.

2. What is a slot antenna?

The slot antenna is an opening cut in a sheet of a conductor which is energized

via a coxial cable or waveguide.

3. Which antenna is complementary to the slot antenna?

The dipole antenna is complementary to slot antenna. The etal and air regions

of the slot are interchanged for the dipole.

4. Define lens antenna.

An antenna which collimates the incident divergent energy to prevent it from

spreading in undesired is called as lens antenna.

5. What are the different types of lens antenna?

Lens can be divided into 2 types.

1. Dielectric lens or H plane plate lens

2. E plane eta plate lens antenna.

6. What are the drawback of lens antenna.

1. Lens antenna are used only at higher frequencies (above 3GHz)because at

lower frequencies they became bulky and heavy. Lens antenna have

excessive thickness at low requencies.

2. Thickness, t=λμ−1=

cf (μ−1)

3. Costler for the same gain and beamwidth in comparison with reflectors.

7. What are the advantages of stepped dielectric lens antenna?

1. It is mechanically strong

2. Reduced weight

3. Less power dissipation

8. What are advantages of lens antenna?

1. In lens antenna, feed and feed support do not block the aperture as the rays

are transitted away from the feed.

2. It has greater design tolerance. i.e, large amount of wrapping and twisting is

possible in lens antenna as wave enters one side and emerges at the other

side, maintaining the electricialpath length.

3. It can be used to feed at off the optical axis and hence useful in

applications where beam is required to be oved angularly with respect to

axis.

9. What is luneberg lens?

The luneberg len is a spherically symmetric delay type lens formed from a

dielectric with index of refraction n which varies as a function of radius given by,

n=√2−(r /R) r= radial distance from center of sphere. R= radius of sphere.

10. What are different types of horn antenna?

1. Sectoral horn

2. Pyramidal horn

3. Concial horn

4. Blconical horn

11. What are the secondary antenna? Give the examples.

Antenna that are not radiators by themselves are called as secondary

antenna.eg., cassegrain,hyperbolic.

12. Define E plane metal plate lens antenna.

It is an antenna in which the travelling wavefronts are spaced by the lens

medium.

13. Define refractive index of lens antenna.

refractive index,μ=Velocity of wave∈air

velocity of wave∈lensm edium

14. What are the different methods of feeding slot antennas?

a) waveguide fed slot

b) boxed in slot

c) coaxial transmission lines

15. What are the advantages of patch or microstrip antenna?

1. Small size and weight

2. Ease of installation

3. Linear and circular polarizations are possible

4. Narrow beam of radiation

16. What are the application of microstrip antenna?

1. Used in spacecraft and aircraft systes

2. Used for beam scanning and beam steering purposes.