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CH.6
Antennas
( 16 Marks)
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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
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 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.
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.
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