Types of Antenna

On the basis of their Structure and applications Antennas can be divided into five major parts. They are following...

1. Wire Antennas
2. Microstrip Antennas
3. Reflector Antennas
4. Travelling Wave Antennas
5. Aperture Antennas

Now Wire Antennas are also of  five different types. They are...

1. Short dipole
2. Dipole
3. Monopole  
4. Small loop
5. Half wave dipole

1) Short Dipole: It is the simplest of all antennas. It can be made of a simple wire, with a center-fed driven element. They are used as antennas, notably in traditional "rabbit ears" television antennas. Word "short" or "small" in antenna always imply "relative to a wavelength".

2) Dipole Antenna: The dipole antenna is similar to the short dipole except it is not required to be small compared to the wavelength at which antenna is operating. 

   Dipole Antenna

3) Monopole Antenna:  A monopole antenna is one half of a dipole antenna, always mounted above some ground plane. 

Monopole Antenna

One side of antenna feed line is attached to the lower end of the monopole, and the other side is attached to the ground plane, generally to the ground.

4) Loop Antenna: A small loop antenna is a loop of wire or tubing with its end connected to a transmission line. Due to radiation resistance and high reactance they are often used as receiver antennas. They have poor efficiency. 

Loop Antenna

5) Half Wave dipole: Half wave dipole is a special type of dipole antenna but is enough important to have a different type. The half wave term means that the length of this dipole antenna is half wavelength of operating frequency. 

Continued...

Antenna- Working Principle

Antenna is mostly used for Radio waves due to which it is often referred as 'Radio Antenna'. Radio waves are electromagnetic waves which carry signals through the air (or through space) at the speed of light with almost no transmission loss. The first antennas were built in 1888 by German physicist Heinrich Hertz in his pioneering experiments to prove the existence of electromagnetic waves predicted by the theory of James Clerk Maxwell.

An antenna is an Impedance matching and directing device. It is an electrical device which converts the electrical currents into radio waves or radio waves into electrical currents. Since they are directing devices they are used with transmitters and receivers. Antennas are arrangements of electrical conductors. These conductors are electrically connected to transmitters or receivers. The antenna at the receiver intercepts the small amount of EM wave and develops tiny voltage which forms small current this current is amplified at the receiver.

When a two wire transmission line is open circuited at the ends it will have a standing wave pattern with voltage maximum at the end of the line. From quarter wave from the end it will have a minimum. The transmission line looses very little amount of energy because field of each wire cancels each other. By folding that wire we form a dipole antenna. Now both fields are added.The name dipole is given due to it has to electrical poles not the physical poles.

Maximum power will be emitted from the Antenna if it follows the maximum power transfer theorem. Which is matching the impedance of an antenna system to the complex conjugate of impedance of transmitter or receiver system. ANY piece of conducting material will work as an antenna on any frequency. The  reason for building sophisticated antennas is to allow us to Control The Radiation Pattern for maximum efficiency and effectiveness.

 Antennas are basically two types according to technology: Omnidirectional and Directional (in one or two direction).

Although according to their structure and application antennas are of various types...