5 Antennas
Suchit Purohit
Learning Objective
- Introduction
- Overview of Antenna
- Characteristics
- Classification
- Additional Antenna
- Application
- Summary
Introduction
This module is talking about an antenna its definition, characteristics, types and different parameters that affects the efficiency of an antenna. Based on different factors, it has been classified in different groups. At end of the module its application area are listed.
Overview of Antenna
An antenna is a metallic device for radiating or receiving radio waves. It is the transitional structure between free space and guiding device. The guiding device may take the form of coaxial line, which transport electromagnetic energy from the transmitting source to the antenna or from antenna to receiver.
Figure 1: Antennas
Antennas Characteristics
Antennas size and shapes
It depends on following characteristics
- Frequency for transmitting and receiving
- Direction of radiated electromagnetic wave
The size of antenna is inversely proportional to the wavelength signal to which it is designed to transmit and receive. Lower signal require large antenna.
Radiation Pattern
The radiation pattern is the mathematical function which describes graphically radiation property with respect to space coordinates.
Radiation Power Density
Power has been associated with the waves. Electromagnetic waves are used to transport information from one point to another. The power intensity decide the distance covered by the EM wave.
Radiation Intensity
It is defined as power radiated from an antenna per unit solid angle .It is multiplication of radiation density to square of distance.
Beam width
It is the angular distance between two identical points on the opposite side.
Directivity
It is the ratio of radiation intensity in a direction to the overall radiation intensity in all directions.
Efficiency
The efficiency of antenna is defined as the ration of power radiated to the total input power supplied to the antenna.
Gain
Gain is the ratio of maximum radiation intensity in a given direction to the maximum radiation intensity from a reference antenna produced in the same direction with same power input.
Polarization
The polarization of an antenna defines the orientation of the E and H waves transmitted or received by the antenna
Classification of Antenna
There are numbers of basis on which the antennas are classified. Mainly the bases are Radiation, Frequency, Aperture and Polarization. Figure 2 is describing categories of some of antennas.
Figure 2: Classification of Antennas
- Radiation: It is differentiated on the basis of Radiation Patte Radiation pattern is a mathematical function which represents the radiation properties of antenna. It includes density, intensity, field strength, directivity, and phase.
Following are the types of antenna on the base of radiation pattern:
| Isotropic Antenna
An isotropic antenna is a fabricated antenna. It radiates uniformly in all directions. An isotropic antenna is a hypothetical lossless antenna. An isotropic antenna is used as reference antenna with which practical antennas are compared. |
Figure 3:Isotropic Antenna |
Figure 4:Omnidirectional Antenna |
Omnidirectional Antenna
They are used to send and transmit equal intensity signals in all the directions. These antennas will cover equally in azimuth direction and having angle in elevation direction. Mostly the wire antennas are having Omnidirectional radiation pattern. Figure 4 shows Omnidirectional antenna. |
| Directional Antenna
They are used when signals are required to propagate in one directional only.
|
 Figure 5(a):Directional Antenna |
Figure 5(b):Yagi Antenna |
Figure 5(c): Dish Antenna |
2) Frequency
The antennas are working with some specific frequency. The frequency range is defined as under:
- Very Low Frequency (VLF) & Low frequency (LF) antenna
- Medium Frequency (MF) antennas
- High Frequency (HF) antennas
- Very High Frequency (VHF) & Ultra High Frequency (UHF) antennas
- Super High Frequency (SHF) & Extremely High Frequency (EHF) antennas
3) Aperture based antennas:
These are sophisticated antennas which utilizes higher frequencies. They are mainly used in aircraft and spacecraft.
| Wire Antennas
These are common antennas virtually seen everywhere – aircraft, building, car etc. There are different shapes of these antennas dipole, loop and helix. |
(a) (b) (c)
Figure 6(a-c): (a) Wire dipole (b) Monopole (c) Loop |
Figure 7 (a) E-Plane (b)Conical |
Horn Antenna
It is most widely used microwave antenna. It is having different forms. It is basically a hollow pipe of different cross sections, which has been pointed towards a larger opening. The type, direction and amount of opening will affect the overall performance. Typically there are four forms of Horn antennas such as E-plane, H-plane, Pyramidal and Conical. |
| Parabolic Antenna
A parabola is a two dimensional plane curve. It is having three dimensional curved surfaces. It is formed by rotating a parabola about its axis. The surface is known as paraboloid or microwave dish or parabolic reflector.
The antenna consists of a primary antenna (dipole or horn) situated at the focal point of a paraboloid reflector. |
Figure 8: Parabolic Antenna |
Figure 9: Microstrip Antenna |
Microstrip Antenna
They became popular in 1970 for space borne applications. They are low profile antennas, appropriate for plane and non plane surfaces.
Its main application is in spacecraft or aircraft where size, weight, cost, performance, ease of installation all different constraints exist. |
4) Polarization
Every sine wave has some orientation while exiting from antenna. The orientation of a wave leaving the antenna is called polarization.
| Vertical Polarization Antenna
If antenna is transmitting or receiving
Vertical E field vector, then antenna is said to be vertically polarized antenna. |
Figure 10:Vertical Polarized Antenna |
 Figure 11:Horizontal Polarized Antenna |
Horizontal Polarization Antenna
If antenna is transmitting/receiving
horizontal E field vector, then antenna is said to be horizontally polarized antenna. |
|
Circular polarization antenna
If the antenna is able to transmit or receive
E field vectors of any orientation, then antenna is said to be Circularly polarized antenna. |
Figure 12:Circular Polarized Antenna |
Some Additional Antennas
Array Antennas
Some application require aggregation of radiation, there requirement does not be met by one radiator. So the arrangement of array has been made to add up all the radiation and get maximum radiation in a specific direction.
Figure 13: Array Antennas
Reflector
When the communication is required in great distance, sophisticated antennas are used. These antennas are made of large diameters. The signals can travel miles of distance.
Figure 14: Corner Reflector Antenna
Smart Antenna
It will increase the capacity and useful received signal level by lowering the interferences level. It will increase the range because they are more directional as compared to Omnidirectional or sectorized antennas. They are more secure because it is difficult to tap a connection in smart antenna. They can be used for location based services.
Antenna Applications
- Defense Services
- Astronomy
- Satellite communication
- Geographical information system
- Wireless communication
Summary
- The field of wireless communication is growing fast and antennas plays a vital role in it.
- There are many categories of antenna.
- Radiation pattern plays an important role in the performance of an antenna.
- Satellite communication is done only with the help of antennas.
- The signal transmission across the globe is done with the help of antennas.
| you can view video on Antennas | |
Suggested Reading:
- Mobile Communication 2nd edition by Jochen Schiller, Pearson education
- Mobile Computing by Asoke Talukder, Roopa Yavagal (Tata McGraw Hill)
- “Wireless communication and networking” by William Stallings
- Mobile Cellular Telecommunications — W.C.Y. Lee, Mc Graw Hill
- Wireless Communications – Theodore. S. Rapport, Pearson Education
- Reza B’Far (Ed), “Mobile Computing Principles”, Cambridge University Press.