UNIT – 2

RADIATION
PROPAGATION
TRANSMISSION LINES

Generally the radiating antenna is located away from the transmitter.

In such cases the energy from the transmitter output is carried to the
radiating antenna by means of a pair of parallel conductors or coaxial
cables know as transmission lines .
WAVE GUIDES

A hollow conducting tube used for the

transmission of electrical energy is known
as a wave guide. The function of wave guide
is the same as that of the transmission
lines. However, these wave guides provide a
better alternative at ultra high and
microwave frequencies. At these
frequencies electric energy in the form of
EM wave is guided along the length of the
tube and hence the name "wave guide"
In order to determine the EM field
configuration within the guide and to
know how these waves are transmitted
along the tube, Maxwell's equation
should be solved subject to appropriate
boundary conditions at the walls of the
guide. Such solutions give rise to a
number of field configuration. Each
such configuration is known as a
"mode".
TRANSVERSE ELECTRIC MODE (TE MODE)
In this mode (or field configuration), the electric field is everywhere
transverse to the axis of the tube and has no component in the direction of
axis. However the magnetic field has component in the direction of the guide
axis.

TRANSVERSE MAGNETIC MODE (TM MODE)

In this case the magnetic field is everywhere transverse to the axis of the
guide while the associated electric field has components in the direction of
the guide axis.
TRANSVERSE ELECTROMAGNETIC WAVE (TEM mode)

In this mode of wave propagation, both the field components i.e., electric and
magnetic fields are totally transverse to the direction of wave propagation. It is
abbreviated as TEM mode.

It is to be noted here that, TEM mode is not supported in waveguides. As for

the TEM mode, there is a need for the presence of two conductors and we
already know that a waveguide is a single hollow conductor.
These various mode travelling along the guide are attenuated and have
phase velocity. The propagation constant is given by

Where is the attenuation constant per unit length

the phase constant per unit length
Another important property of the wave guide is it's filtering action. It behaves
like a high pass filter for each mode. That is, each mode has its own cut off
frequency fc depending on the dimensions of the guide. That particular mode
will be propagated with zero attenuation only if it's frequency is greater than its
critical frequency fc. This cut off frequency is different for different modes. The
mode for which fc is greater is known as "dominant mode“

A wave guide having rectangular cross section is known as "rectangular wave

guide" and that having circular cross section is known as "circular wave guide".
Practical wave guides are usually rectangular or circular cylinders. Wave guides
of other cross sectional shapes are also possible, yet they are not in use because
they offer no electrical advantage over the first two. Moreover these wave
guides are very expensive to manufacture.
RADIO WAVE PROPAGATION
Radio signals are transferred from one location to another inside the Earth's
atmosphere or through open space using a process known as radio propagation.
Radio wave propagation studies how radio waves behave as they move from one
end to the other. It is also sometimes referred to as electromagnetic wave
propagation or just wave propagation. Information can be transferred from one
end to the other using radio waves.
Three categories of radio wave propagation are distinguished based on
frequencies,
i. Ground wave propagation
ii. Sky wave propagation and
iii. Space propagation
GROUND WAVE or SURFACE WAVE PROPAGATION

The word itself denotes those electromagnetic waves in ground wave propagation
go from one end of the ground to the other by gliding over its surface. We are
aware that electromagnetic wave propagation is a type of wireless communication
in which the signal is spread out throughout the area between the transmitting
and receiving antennas. Therefore, in order to facilitate signal propagation, the two
ends of the device must contain a transmitting and receiving antenna.
The receiving antenna gathers the electromagnetic wave that the transmitting
antenna sends out into the world. As a result, we can state that a receiving
antenna must be the first element at the receiving end and a transmitting
antenna must be the last element at the broadcasting end for electromagnetic
waves to propagate. Therefore, in the case of ground wave propagation, the two
antennas (i.e., transmitting and receiving) buried in the ground permit the signal
to travel through the earth’s surface in the form of electromagnetic waves.
Ground waves are efficient for frequencies between 50 kHz and 250 kHz and
travel over the earth’s surface. At these low frequencies, the signals have a long-
range and can travel hundreds or even thousands of kilometres.
APPLICATIONS OF GROUND WAVE PROPAGATION

Following are the applications of ground wave propagation:

i. Ground wave propagation is typically used, particularly by radio broadcast
stations needing to cover a specific location, to offer local radio
communications coverage.
ii. As they reach a substantial depth in the sea, ground waves can be
employed for one-way communication from the military to submerged
submarines.
iii. Ground waves can be used to broadcast via AM, FM, and television.
On these frequencies throughout the day, ground wave propagation of radio
signals is appropriate for relatively short distance propagation.
ADVANTAGES & DISADVANTAGES OF GROUND WAVE PROPAGATION

Advantages Disadvantages
Only atmospheric noise causes interference because it The transmitter (Tx) and receiver (Rx) antennas should
operates at lower frequencies. Additionally, there is less not be too far apart; otherwise, ground and air
EM wave absorption at lower frequencies. Therefore, it absorptions would cause a significant reduction in the
can travel farther. However, as the distance from the received signal intensity. As a result, the two stations
transmitter grows, the path loss increases. Therefore, the cannot establish communication. Repeaters must
distance between Tx and Rx ought to be ideal. frequently be used between Tx and Rx for this. The
system’s overall cost goes up as a result.

To avoid an electric field (E) component short circuit, Electric field components short circuit with the ground if
they are vertically polarised. there is a change in the polarisation of the ground wave
SPACE WAVE PROPAGATION
Space wave propagation is the transmission of radio waves directly from the
transmitter to the receiver through the troposphere region of the atmosphere. It is
a type of line of sight communication.
It allows for transmission of the waves using a high frequency. These waves can
travel in the troposphere region which is almost 20km above ground level. Thus
most of the space wave propagation occurs in the troposphere layers. They are also
known as tropospheric communication. Since these waves propagate like any other
electromagnetic wave in free space, they are called space waves.
Space Wave Propagation Frequency Range:

Space waves propagate in the frequency range of 30 MHz to 300 MHz,

which is considered as ultra high frequency (UHF) bands. Due to their high
frequency, they have less wavelength thus they cannot be transmitted to
long distances restricting them to the line of sight characteristics. Their high
frequency allows these waves to carry more energy.
APPLICATION OF SPACE WAVE PROPAGATION

Space wave propagation has different applications such as,

i. Line of sight communication
ii. Television broadcast
iii. Radar communication including both general communication by radio
waves or transmission at specific channels and frequencies
iv. Microwave linking or the transmission of radio waves in the
microwave frequencies to transmit videos, audios, etc.
ADVANTAGES AND DISADVANTAGES OF SPACE WAVE PROPAGATION

Advantages Disadvantages

Absorption of the space waves is almost Due to their straight path of propagation, the
negligible due to them being propagated at radius of the Earth becomes a limitation to their
very high frequencies. range.

It is a simple method of communication. There is a requirement for big-sized antennas

for an improved range of communication.
SKY WAVE PROPAGATION
Sky wave propagation, also known as ionospheric
propagation, is the mode of propagation in
which electromagnetic waves emitted from an
antenna and directed upward at great angles are
reflected back to earth by the ionosphere.
In the case of sky wave propagation, the permitted
frequency range ranges from
3 MHz to 30 MHz. Basically, the ionosphere reflects
electromagnetic radiation between 3 and 30 MHz.
However, despite reflection, signals with frequencies
greater than 30 MHz are penetrated. Therefore, only
this particular frequency range is acceptable for sky
wave propagation.
Applications of Sky Wave Propagation

• Short-wave (SW) radio services use skywave propagation. Radio transmission

over vast distances is conducted on medium and high frequencies.
• Skywave propagation can be utilized for extraordinarily long-distance
communication because it employs a high frequency that experiences several
internal reflections between the earth and ionosphere.
• Skywave propagation is used for satellite communication since it depends on
the upper atmospheric conditions.
• Radar systems and mobile communication services are also based on sky wave
propagation.
Advantages & Disadvantages of Sky Wave Propagation
Advantages:
i. It is the most straightforward mode of transmission and offers constant support for
electronic communications system because it makes use of the reflective feature of
the ionosphere that is present above the planet at higher frequencies.
ii. Large-distance propagation is supported. The operating frequency range is very
broad.
iii. Less attenuation is caused by atmospheric factors.
Disadvantages
i. Ionospheric proximity varies depending on whether it is day or night. Sky waves can
therefore cover greater or lesser distances. Before a transmitter signal reaches a
receiver, it makes several hops. If there are greater gaps between the transmitter and
receiver antennas, the signal intensity is significantly reduced.
ii. Large-sized antennas are required for long-distance propagation.
iii. There are differences in signal transmission between day and night due to the
ionosphere’s existence during night and day, respectively.
IONOSPHERE AND ITS LAYERS

The upper part of the earth's atmosphere

is ionised by gamma rays, UV rays, as well
as, by cosmic rays from outer space. The
region of space in the upper atmosphere
where the ionisation is appreciable is
known as "ionosphere". Though it has no
sharp boundaries, it extends from about
80 km above the earth's surface.
It must be pointed out here that the ion
density is not uniform throughout the
ionosphere. The levels at which the
electron density reaches a maximum are
termed as layers. The prominent day time
layers are known as E, F1 and F2 layers.
The position of E and F1 are relatively
constant at a height of 110 km and 220 km
respectively. But the position of F2 layer
varies between 250 and 350 km. In
addition to these, there is D region below E
layer which lies between 50 and 90 km.
This region is responsible for most of the
day time attenuation of high frequency
waves
At night F1 layer fades out while F2 layer
descends with the result that these two
layers combine to form a single F2 layer.
The density of E layer gets weakened and
the D region is absent at night.
In the absence of these ionised layers the
radio waves would not reach the receiver
R from the transmitter T
SKIP DISTANCE
In the sky wave propagation, for a fixed
frequency, the shortest distance between the
point of transmission and point of reception
along the surface is known as the skip distance.
When the angle of distance is large for the ray R1
as shown in diagram, the sky wave returns to the
ground at a long distance from the transmitter. As
this angle is slowly reduced naturally the wave
turns closer and closer to the transmitter as
shown by the rays R2 and R3. If the angle of
incidence is now made significantly less than that
of ray R3, the ray will be very close to the normal
to be returned to the earth. If the angle of
incidence is reduced further the radio waves
penetrate through the layer as shown by the rays
R4 and R5.
For a particular angle of incidence, the distance
between point of transmission and point of
reception is minimum. The minimum distance
between the transmitter and ray like R3 which
strikes the earth is called the skip distance. The
region between the points where there is no
reception of ground waves at the point where
the Sky waves received is known as skip zone.
In the skip zone there is no reception at all.