ACO Ch6.pptx

1. CH.6
Antennas
( 16 Marks)
Visit for more Learning Resources

2. Antenna Fundamentals
Definition:
An antenna is generally a metallic object often a wire or a
group of wires used to convert the high frequency current flowing
through it into electromagnetic waves and vice-versa.
Functions of Antenna:
(i)It couples the transmitter output to the free space or the
received input to the receiver.
(ii)It must be capable of radiating or receiving the electromagnetic
waves.
(iii)It converts high frequency current into electromagnetic waves.
If RF current flows in a wire conductor it is found that the energy
applied at one end is not exactly same at the other end.
Some of the energy ‘escapes’ i.e. radiated.
It is possible to calculate the amount of energy escaped, its
direction using the Maxwells equation.
In short, an antenna or aerial as it is sometimes called, is one or
more electrical conductors of a specific length that radiate radio
waves generated by a transmitter or that collect radio waves at the
receiver.
There are hundreds of different types of antennas in use today.

3. Terms and Definitions
Radiation Pattern
All transmitting antennas are not isotropic radiators
they can transmit ‘more’ energy in some directions
than other directions.
Definition:
A graph or diagram which tells us about the
manner in which an antenna radiates more power
in different directions is known as the “radiation
pattern of antenna”.
For a receiving antenna the diagram is known as the
directional pattern of the antenna.

4. Radiation Pattern(Continued…)
Fig. 6.1: Radiation Pattern of An Antenna

5. Radiation Pattern(Continued…)
As shown in Fig. 6.1,
(i) This antenna radiates maximum energy
in the direction of 180. The radiated energy then
gradually decreases with increase in the angle on
both the sides of 180 direction.
(ii) The radiation pattern has been drawn
for the constant distant.
(iii) The antenna having this type of
radiation pattern is called ‘directional antenna’. Thus,
the directional antennas do not radiate equally in all
directions.

6. Antenna Gain
We know that, the directional antennas radiate more power in
certain direction than other.
Also the omnidirectional antenna radiate equally in all directions.
Another way of looking at this concentration of the radiation is to
say that antennas have ‘gain’ (in decibels).
(i) Directive Gain
Directive gain is defined as the ratio of the power density in
a particular direction of one antenna to the power density that
would be radiated by isotropic antenna in the same direction.
Directive Gain =Power density radiated / Power density radiated
in a particular direction /by isotropic radiator in the
same direction
For practical antennas, Directive gain > 1.
The power density of both types of antenna is measured at a
specified distance, and a comparative ratio is established.

7. Antenna Gain(Continued…)
Two sets of characteristics can be obtained:
1.The longer the antenna, the higher the directive gain.
2.Non-resonant antennas have higher directive gain
than resonant antennas.
(ii)Directivity:
Directivity is defined as the maximum directive
gain which is obtained in only one direction in which
the radiation is maximum.
Directivity =Maximum Directive Gain
(iii) Power Gain:
The power gain of an antenna is defined as the
ratio of power fed to an isotropic antenna to the power
fed to a directional antenna, to develop the same field
strength at the same direction.

8. Antenna Gain(Continued…)
Isotropic Radiator:
An isotropic radiator is a point source antenna that
radiates equally in all directions shown in Fig. 6.2.
Fig. 6.2: Isotropic Radiator

9. Antenna Gain(Continued…)
The EM waves spread uniformally in all directions in a space.
The radiation pattern of isotropic antenna is a sphere shown in Fig.
6.2.
Power gain = Power fed to the Isotropic Antenna
Power fed to the directional antenna
A(dB) = 10log (P2/P1)
where,
A(dB) = Antenna gain in decibels
P1 = Power of directional antenna
P2 = Power of isotropic antenna
Relation between power gain and directive gain is,
Ap= ɳ D
where,
Ap = Power gain
D = Directivity
 = Antenna efficiency = 1 for
ideal lossless antenna

10. Antenna Resistance
(i) Radiation Resistance:
Radiation resistance is the ratio of the power radiated
by the antenna to the square of current at the feed point.
Radiation resistance =Power radiated by Antenna
Square of current at the feed point
Rr = Pt/I²
where, Rr is an a.c. resistance.
(ii) Antenna losses and efficiency:
Antenna losses can be caused by:
(i) Power dissipated in the antenna and ground
resistance.
(ii) Losses due to corona effects, imperfect
dielectric near the antenna.
(iii) Energy loss due to eddy currents induced into
nearby metallic objects, and I2R losses in the antenna itself.

11. Antenna Resistance(Continued…)
(ii)Antenna losses and efficiency:
Antenna losses can be caused by:
(i) Power dissipated in the antenna and
ground resistance.
(ii) Losses due to corona effects, imperfect
dielectric near the antenna.
(iii) Energy loss due to eddy currents
induced into nearby metallic objects, and I2R losses
in the antenna itself.

12. Antenna Resistance(Continued…)
Antenna efficiency:
Antenna efficiency is defined as the ratio of power
radiated to the total input power supplied to the antenna.
=Power radiated/total input power
= Rrad  100%
(Rrad + Rd
where, Rd = Antenna resistance
Rrad = Antenna radiation resistance
Low and medium-frequency antennas are least efficient
because of difficulties in achieving the proper physical
lengths.
These antennas can approach efficiencies only 75 to 95%.
Antennas at higher frequencies can easily achieve values
approaching 100%.
Radiation resistance values may vary from a few ohms to
several hundred ohms depending on the choice of feed
points, physical and electrical characteristics.

13. Bandwidth, Beamwidth and Polarization
Bandwidth, beamwidth and polarization are three
important terms dealing with the operation frequency
range, the degree of concentration of the radiation
pattern, also the space orientation of the radiated
waves.
(i) Bandwidth:
The term bandwidth refers to the range of
frequencies over which the operation of the antenna is
satisfactory.
It is the frequency difference between the half-power
points.
Two types:
1. Related to radiation pattern.
2. Related to input impedance.

14. Bandwidth, Beam width and Polarization(ctn’ed…)
(ii)Beam width:
Beam width is defined as the angular separation
between the two half power points on the power density
radiation pattern.Beam width is expressed in degrees.
Fig. 6.3: Beamwidth
Fig. 6.3 shows the example, where beam angle is
30, which is sum of the two angles created at the points
where the field strength drops to 0.707 field strength in
(V/m) of the maximum voltage at the center of the lobe.
(These points are called the half-power points).

15. Bandwidth, Beam width and Polarization(ctn’ed…)
(iii) Polarization:
Polarization is defined as the direction of the electric
vector in the electromagnetic wave radiated by the transmitting
antenna. (Fig. 6.4).
Fig. 6.4: Polarization of the antenna showing E and M fields
Low frequency antennas are usually, vertically polarized because
of ground effect (reflected waves) etc. and physical construction.
High frequency antennas are generally horizontally polarized.
Horizontal polarization is the more desired of the two because of
its rejection to noise made by people.

16. Types of Antenna
Antennas are mainly classified into two types.
Antennas
Resonant Antennas Non-Resonant Antennas

17. Resonant Antennas
Resonant antennas are opened out transmission line
i.e. they are open circuited at one end .
They have resonant lengths i.e. multiple of half-wave
length.
The lengths of the antennas are L = /2, L = , L =
3/2 and so on.
A resonant antenna corresponds to resonant
transmission line.
Radiated patterns of resonant dipoles are shown in
Fig. 6.5.

18. Resonant Antennas(Continued…)
Fig. 6.5 : Radiation Pattern of Various Resonant
Dipoles

19. Non-Resonant Antennas
Non-resonant antennas are the antennas in which the
source is matched to the load (i.e. they don’t have open
circuit).
A non-resonant antenna is like a properly terminated
transmission line, produces no standing waves.
They are suppressed by the use of a correct termination
resistor and no power is reflected, ensuring that only
forward travelling waves will present.
In a correctly matched transmission line, all the
transmitted power is dissipated in the terminating
resistance.
When an antenna is terminated as shown in Fig. 6.6 (a)
about two-third of the forward power is radiated and
remaining is dissipated in the antenna.

20. Non-Resonant Antennas (continued…)
(a) Layout and Current Distribution (b) Radiation Pattern
Fig. 6.6: Non-Resonant Antenna
As shown in Fig. 6.6 (b), the radiation pattern of the resonant antenna
and a non-resonant antenna are same except one major difference i.e.
the
non-resonant antenna is unidirectional.
Demerits of Non-Resonant Antennas:
(i) Low gain.
(ii) Low efficiency.
(iii) Occupy more space.

21. Comparison between Resonant and Non-resonant Antennas

22. Dipole Antenna
•An antenna is some form of electrical conductor.
•It may be a length of wire, a metal rod, or a piece of
tubing.
•Many different sizes and shapes are used.
•The length of the conductor is dependent upon the
frequency of transmission.
•Antennas radiate most effectively when their length
is directly related to the wavelength of the
transmitted signal.
•Most of the antennas have a length that is some
fraction of a wavelength.
•The most common lengths are one-half and one-
quarter wavelengths.

23. Half Wave Dipole Antenna
Half wave dipole is a resonant antenna. A resonant antenna
corresponds to resonant transmission line.
One of the most widely used antenna types is the half-wave
dipole shown in Fig. 6.7 also called a doublet.
Fig. 6.7: A Half-wave Dipole
Half-wave dipole antenna corresponds to a resonant
transmission line.
i.e. exact half-wave length (/2) long and open-circuited at
one end.
The dipole antennas have lengths of /2, , 3/2 etc. which
are all multiple of /2. Hence, the dipole antennas are
resonant antennas

24. Half Wave Dipole Antenna(continued…)
Radiation Pattern:
• If you look down on the top of the dipole, the radiation pattern
appears as figure eight shown in Fig. 6.8.
Fig. 6.8: Horizontal Radiation Pattern of Half-wave Dipole
• In resonant antennas there exist forward (incident waves) and
backward (reflected waves) i.e. standing waves exist and hence radiation pattern
is bi-directional.
(a) Radiation Pattern Due to Forward Wave
(b) Radiation Pattern Due to Reverse Wave
(c) Combined Pattern
Fig. 6.9

25. Folded Dipole Antenna
Fig. 6.10: (a) Folded Dipole
Radiation Pattern:
The folded dipole has the same direction pattern as the
ordinary dipole.
Fig. 6.10: (b) Radiation Pattern of Folded Dipole

26. Loop Antenna
(a) (b)
Fig. 6.11: (a) Loop Antenna
Fig. 6.11: (b) Directional Radiation Pattern of Loop
Antenna

27. Loop Antenna(Continued…)
Advantages
Loop antenna has the following advantages:
(i) Highly directive.
(ii) Small size.
Disadvantage
Loop antenna has very low radiation efficiency.
Applications
Loop antenna has following applications.
1. For direction finding.
2. In portable receivers.
3. In navigation.
For more detail contact us