Q. 1) Explain in details the radiation mechanism of antenna with suitable diagram.

(6 Marks)
Answer:
The radiation from the antenna takes place when the Electromagnetic field generated by the source is
transmitted to the antenna system through the Transmission line and separated from the Antenna into
free space.
A conducting wire fundamentally radiates electromagnetic wave because of time varying current or an
acceleration (or deceleration) of charges, if there is no charge motions in wire then no radiation occurs
because there is no current flow, the radiation will happen if the charge is oscillating with time. Fig. 1
shows the mechanism of radiation in an antenna;

Fig.1. Radiation Mechanism of antenna

To create radiation (electric field), there must be a time-varying current dI/dt or an acceleration (or
deceleration) a of a charge q.
 If the charge is not moving, a current is not created and there is no radiation.
 If a charge is moving with an uniform velocity, there is no radiation if the wire is straight, and
infinite in extent there is radiation if the wire is curved, bent, discontinuous, terminated or
truncated , If the charge is oscillating in a time-motion, it radiates even if the wire is straight
So, it is the current distribution on the antennas that produce the radiation.
As shown in figure 1; when a voltage is applied across the terminals of two wire transmission line an
electric field is generated between the conductors. The electric field has associated with it electric lines
of force which are tangent to the electric field at each point and their strength is proportional to the
electric field intensity. The electric lines of force have a tendency to act on the free electrons (easily
detachable from the atoms) associated with each conductor and force them to be displaced. The
movement of the charges creates a current that in turn creates magnetic field intensity. Associated with
the magnetic field intensity are magnetic lines of force which are tangent to the magnetic field.
Magnetic field lines always form closed loops encircling current-carrying conductors because
physically there are no magnetic charges. The creation of time-varying electric and magnetic fields
between the conductors forms electromagnetic waves which travel along the transmission line, as
shown in Figure 1.

Q. 2) Derive the fundamental equation for free space propagation. (4 Marks)

Answer:
Consider the transmitter power (Pt) radiated uniformly in all the directions (isotropic), the power
received at a distance (r) from the isotropic source is given by,

Pr = Pt / 4πr2 (W/m2) … (1)

Where,
Pt = Transmitter power (Watts)
Pr = Received power (Watts)
r = Distance between the transmitting (and) receiving antenna.

The power density (PD) along the maximum radiation for the directive antenna is given by,
PD = Gt * Pr … (2)
Where,
Gt = Gain of the transmitting antenna
Pr = Received powe
PD = Power density.

For antenna with an effective aperture area (Ae), received power is given by,

Pr = PD * Ae … (3)

Substitute equations ( l ) and (2) in equation (3), we get,

Pr = PD *Ae
Pr= (Gt *Pt / 4πr2) *Ae

Pr = (Gt *Pt / 4πr2) *Ae (W/m2) … (4)

We know that, relation between gain and maximum effective aperture is given by,

G = (4π / λ2) Ae
Gr = (4π / λ2) Ae … (5)

Where,
Gr = Gain of the receiving antenna
Ae = Effective aperture area.
From equation (5),
Ae = (λ2 /4π) Gr

Substituting equation (6) in equation (4), we get,

 Pr = (Gt *Pt / 4πr2)Ae
Pr = (Gt * Pt / 4πr2) (λ2 /4π) Gr

Pr = PtGtGr λ2/ (4πr) 2

The above equation is general expression for free space propagation.

Here,
Pr = Received power in watts
Pt = Transmitter power in Watts
Gt = Gain of transmitting antenna
Gr = Gain of receiving antenna
λ = Wave length (m)
r = Distance between the transmitting (and) receiving antenna.

The above equation written in another form is,

Pr = PtGtGr / Ls
Where,
Ls = (4πr / λ) 2

Q. 3) Explain the following characteristics of antenna in detail: (5 Marks)

i) Radiation Pattern
ii) Efficiency
Answer:
1) Radiation Pattern: It is defined as “graphical representation of the radiation field of the
antenna as a function of space coordinates.
Since the radiation pattern is the variation over a sphere centered on the antenna with a
constant radius (r), so we have only two parameters variations, they are θ and φ variations.
The plane where θ is varies called the zenith plane, whereas the plane where φ is varies called
the azimuth plane. The angles contained in these planes are called zenith and azimuth angles
respectively. The azimuth angle is the angle around the vertical axis, it varies from 0 degree
to 360 degree in the clockwise direction, the zenith angle is the vertical angle between the
horizon and any position, and it varies from 0 degree to 180 degree. Fig.2 shows the radiation
pattern of an antenna in spherical coordinate system.
 Fig.2. Antenna Radiation pattern in spherical coordinate system

2) Efficiency: The antenna efficiency can be defined as “ratio of radiated power by antenna
to the net power accepted from transmission line”.
There are two types of losses:
 Losses from reflections (occurs due to the mismatch between an antenna impedance
and transmission line impedance).
 Conduction and dielectric losses.
So the efficiency of an antenna can be written as:
etotal = er*ecd
where:
etotal: The total antenna efficiency.
er : The reflection efficiency.
ecd : The conduction and dielectric efficiency.

Q. 4) Enlist the different types of antennas. Explain any two types in detail. (6 Marks)
Answer:
Following are various types of antenna;
1) Wire antenna
2) Aperture antenna
3) Reflector antenna
4) Lens antenna
5) Microstrip antenna
6) Array antenna

1) Wire antenna:
Wire antennas are antennas that use conductive wires to transmit and receive radio signals.
These antennas are widely used in broadcasting, wireless communication, and amateur radio
applications. Wire antennas are easy to install, transmit, and receive signals over long
distances.
They are popular among amateur radio operators because they are easy to build and can be
customized. Wire antennas are also used in military and defense applications for long-range
communication and surveillance. Half wavelength dipole, folded dipole, monopole, loop
antenna, helical antenna are examples of wire antennas. Overall, wire antennas are versatile
and one of the most common types of antennas used.

Fig.3 (a) Half wavelength dipole (b) Monopole antenna (c) Helical antenna (d) loop antenna

2) Aperture antenna:
An Antenna including an aperture at the ending is known as an Aperture antenna. The best
example of this antenna is Waveguide. Here, the aperture may be square, cone, rectangular or
circular. Antenna aperture is the area around an antenna where power can be derived from
electromagnetic field effects.

Fig.4. Horn Antenna

Aperture antennas are used in various operations like satellite communication, radar systems,
wireless communication, etc. They are popular because they can handle high frequencies and
give high gain, making them useful in long-distance communication.

Aperture antennas are different from other types of antennas because they use the size and
shape of the opening or aperture to control the signals. The aperture can be changed to
control the strength and direction of the signal, making them a versatile choice for different
operations.

Q.5) The radiation resistance of an antenna is 72Ω and loss resistance is 8Ω. Calculate
directivity in dB if power gain is 16. (5 Marks)
Answer:

Q.6) A communication link is to be established between two stations using half

wavelength antenna for maximum directivity gain 1.64. The distance between
transmitter and receiver is 100km and transmitter power is 1 KW. Frequency of
operation is 100MHz. What is the maximum power received by receiver. (5 Marks)

Answer:
Given, Transmitted power = 1 KW = 1000 W, Frequency = 100 MHz = 100*106 Hz, r = 100
km = 100*103 m, Maximum Directivity = 1.64 (Maximum Directivity is said to be treated as
gain of antenna).
The power received by an antenna can be calculated as;
Pr = PtGtGr λ2/ (4πr) 2

𝝀 = c/f
𝝀 = [3*108 / 100*106]
𝝀 = 3 meter
Hence;

Pr = (1000*1.64*1.64*32) / (4*𝝅*103)2
Pr = 1.53*10-8 W

Q.7) Calculate the power density reaching the moon’s surface from 1MW pulse
transmitter located on the earth. The antenna gain is 55 db. The distance between the
moon and earth 4,00,000 km. (4 Marks)

Answer:
Given, PT = 1 MW = 1*106 W; GT = 55 dB, r = 400000 km = 400*106 m

GT (dB) = 10log10 (GT)

55 = 10log10 (GT)
(GT) = Antilog (5.5)
GT = 0.3162*106
The power density can be formulated as;
WD = (PT* GT) / (4πr) 2
WD = (1*106*0.3162*106) / (4*𝝅*400*106)2
WD = 0.15734 μW