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Notes of Electromagnetics [EX 503]



Introduction to Antenna


- Antenna is a structure made of metallic object like wires, used to convert high frequency current into electromagnetic wave and vice versa.
- It can acts as both transmitter and receiver.
- It radiates electromagnetic energy into space and also accumulates electromagnetic energy from space.
- A good antenna design improves performance of communication system.
- The basic principle of antenna are:
a) The radiation is produced by accelerated or decelerated charge.
b) I L = Q v

Parameters or Properties of Antenna

1. Antenna Pattern:
- It is the 3 Dimensional plot of its radiation.
- If amplitude of component of E vector is plotted, field or voltage pattern is obtained.
- The square of amplitude of E vector when plotted, gives power pattern.

2. Radiation Intensity:
- It indicates the strength of electromagnetic wave radiation.
- It is given as:
U = r^2 * P(ave)
where, P(ave) = total average power radiated.

3. Directive Gain [Gd(θ, ϕ)]
- It is the measure of concentration of radiated power in particular direction.
- It is the ability to direct radiated power in given direction.
- The general equation is:
(Max directive gain / Directivity(D)) = (Max. radiation intensity / Average radiation intensity)

4. Power Gain [Gp(θ, ϕ)]
- It is the ratio of radiation intensity to the total input power to antenna multiplied by 4π.
- The general equation is:
Gp = 4 π * { U(θ, ϕ) / P(in) }
P(in) = P(i) + P(rad) ; P(i) is ohmic power loss and P(rad) is power radiated by antenna.
So, P(in) = 1/2 * [ I(m) ^ 2 * { R(i) + R(rad) } ]

5. Radiation Efficiency
- The radiation efficiency is defined as the ratio of power gain to directive gain.
- It is given as:
Efficiency = Gp / Gd = P(rad) / P(in) = R(rad) / { R(rad) + R(i) }

Antenna Types and Properties

Types of Antenna

1. Wire Antenna:
- It is the most commonly used antenna type.
- It may be straight wire (dipole) antenna, loop antenna or helix antenna.

2. Aperture Antenna:
- It is used in aircraft and spacecraft.
- It can be covered with dielectric for environmental protection.
- It can be flush mounted on skin of aircraft.

3. Microstrip Antenna:
- It is used for governmental and commercial applications.
- It has metallic patch on grounded substrate.
- It has low profile.
- It is comfortable, simple and inexpensive.

4. Array Antenna:
- It is used to obtain radiation characteristics by different arrangement of radiating elements.

5. Reflector Antenna:
- It is used for very long distance communication.

6. Lens Antenna:
- It is used to collimate incident divergent energy so as to prevent it from spreading in undesired direction.

Radiation From Dipole Antenna

The two conductors are very close, the fields produced by individual conductor cancel each other. So, there will be no radiation from the line. But, if portion of line is slowly bent outward, the cancellation of field gradually decrease. When it is bent as shown in given figure, no cancellation occurs. So, it radiates electromagnetic waves out into the surrounding medium.




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