Table of Contents
Antenna is a metallic conductor system capable of capturing and radiating electromagnetic energy means it can radiate and receive electromagnetic wave from one position to another. It converts the electromagnet energy in to electrical energy and electrical energy into electromagnetic energy. Antenna and aerial couple or match the receiver and transmitter, electromagnetic wave is radiate in free space. The working of the antenna is similar like transmission line but there is slightly difference in it. The transmission line is bounded medium like copper wire, optical fiber, wave guide in which signal is transmitted. While the antenna transmits the signal in free space and does not require material medium. The antenna is a system in which transmitter and receiver are couple and match. The transmitter and receiver are couple in free space. The electromagnetic waves are transmitted from the transmitter and these waves will be received through the receiver. When the electromagnetic waves radiates and receives this will required antenna.
The higher the frequency, the shorter the wavelength and the smaller the antenna. For example the wavelength for 433.92 MHz is 0.69 m and the wavelength for 916 MHz is 0.33M.
The transmitters are connected with the transmitted lines and will produce electromagnetic waves. These waves will travel in free state and these wave at the speed of velocity of light which is equal to 3 x 108 m/s.
The receiving antenna will be connected with radio receiver. It will receive the waves which were transmitted by the transmitting antenna.
The transition is the region between the free space and wave guide. The transmitter will transmit the signal through the transmission lines or wave guide. The transmission line is device which transmits the radio frequency and transmits the energy from one point to another point. It will transmit the wave in such way that the attenuation will be minimum. So that the radiation loss may be less when the energy will be transfer from one point to another point through transmission line. The energy that will be transmitted will be one dimensional between it will have a region which will be antenna and transition line. After it will have free space where the wave will travel as the wave is one dimension so it will not spread in the free space and if we have infinite losses in the transmission. In this condition the source which is connected with the transmitter generator the incident wave from the source will travel in the transmission line and will show maximum energy. So in this way maximum energy is transferred from the source to the load. In infinite losses we do not have particular load. While in the finite loss we will have particular load there will be characteristics impedance of the line which is denoted by usually it is between the line. The characteristics impedance will be equal to the load at far end. In this case the transmission line will terminate from the load and having characteristics impedance. If the characteristics impedance are not equal to the load it means that whether it is going maximum or it is coming minimum due to this mismatch will be produce in the signal. Due to this mismatch backward wave will be generated. This backward load will be from load to the source and this will be reflected which is generated in the line and will moves in backward direction from load to source. Due to which interference will be produce between the forward and backward wave. Due to this interference standing wave will be generated which will also consist of energy losses.
Loss less infinite line:
In this line as the name suggest that it have no losses in the transmission now in this case. The transmission line will be terminated at the characteristics impedance and all energy of the system will move in forward direction means the incident wave will be completely absorbed at the load. When it’s completely absorbed at the load no backward wave or reflected wave will be generated. The energy will be transmitted from the source to the load without any loss.
Two wire transmission line:
Now consider we have two wire transmission line which is completely connected with the transmitter and these line will be matched with the forward wave or incident wave and will travel in its direction the region between the wave guide and free space is antenna and transition line.
How Antennas Work:
The antenna will converts the electrical signal in to electromagnetic signal and will transmit it in the free space. While on other side we have receiving antenna which receive the electromagnetic wave and will convert this wave in the electrical signal.
There is difference between the normal wave and electromagnetic wave. The electromagnetic waves do not require any medium for its propagation it can travel in vacuum as well as in free state. The electromagnet waves consists of two components electrical and magnetic components. Both waves will be perpendicular 90° to each other and travel with speed of light.
Generation of electromagnetic waves from antenna:
In the generation of electromagnetic wave there are two rules of physics working according to the first rule when the charge is static it will produce only electric field and according to the second rule when the charge flow in the conductor it will produce magnetic field as well as electric field. The direction of the magnetic field can be find from right hand rule when the current flow in upward direction the thumb will show the direction of the electric field and curl finger will show the direction of the magnetic field.
In antenna input we give high frequency signal as it is the ac signal so it has positive and negative peaks the charge flow will vary its direction due to which electrical and magnetic field will be produce and in this way changing electric field along the length of the antenna electromagnetic waves will be generated from the antenna.
When the antenna is in vertical position then the electrical field generated will be vertical and the magnetic field will be horizontal. But when we place the antenna horizontally then the electrical field will be generated horizontally and electric field will be generated vertically. So the position of the antenna tells us that if the antenna position is vertical the electrical field will be vertical and if the antenna position is horizontal the electric field will be horizontal. When we obtain the electrical field horizontal then we can say that antenna is horizontal polarized and when we obtain the electrical field vertical then we can say that antenna is vertical polarized.
One point to be noted that if the transmitting antenna is vertical polarized than the receiving antenna must also be vertical polarized. When both antennas will match maximum power will be transmitted. When one antenna is horizontal polarized and other is vertical polarized then there will be power loss.
Radiation pattern can be defined as the pictorial representation of the power distribution from the antenna or received by the antenna. At the end of the antenna the power will be minimum while at the middle the power radiated will be maximum. Every antenna has different radiation pattern. It depends upon on the construction of the antenna.
The antenna resolution gives us resolution power for example we have antenna on which different types of waves are falling these wave has different nature and frequencies. For example we are using FM on our phone so at that time our mobile is connected with different type of FM station. This means that the resolution of our phone is high. If the mobile is connected with the several stations then it means that the resolution power of the mobile is low. In case of antenna if it is receiving different types of waves and if this antenna treat these waves differently then its resolution power is maximum. The resolution of the antenna is equal to the half of the first null beam width:
Resolution = FNBW/2
Directivity of the antenna:
The directivity of the antenna is the energy focus in a particular direction when to receiving or transmitting a signal.
Gain of the antenna:
Gain the parameter that measures the degree of the directivity of radial pattern. An antenna with high a higher gain is more effective in it radiation pattern.
Efficiency of the antenna:
Efficiency is a measurement of how much of the energy put in to the antenna actually gets radiated in to free space rather than los as heat on the antenna structure or reflected back in to the source.
Types of Antenna:
Isotropic radiator Antenna:
The isotropic radiator antenna receives and radiates electromagnetic waves connecting wired system with satellite for radio transmission. The isotopic means equal so the isotropic radiator antenna transmit equal radiations in all direction. For example if we see it in three dimensions the radiation transmission in X-axis will be equal the radiation transmission along Y-axis and so on. This type of antenna is idealistic in nature. The principal of this antenna is that it radiates power in all direction. The radiation is in form of circle.
Simple dipole antenna:
Isotropic radiator antenna is idealistic in nature means it is not possible for antenna to transmit equal amount of radiation in all directions. The real antenna will not be idealistic and is called simple dipole antenna whose length will be equal to the wave length.
- Hertzian dipole:
This is infinitesimally small dipole have length
l < λ/50
It is minimum use with marginal application and has less efficiency and has higher loss. It has larger region of reactive field. It is difficult to fabricate.
- Small dipole:
It is use less because the radiation efficiency is less. The length of this antenna is:
λ/50 < l < λ/10
It has higher losses and has larger region of reactive field but smaller then hertzian dipole. If we want to send signal to few meter we use this type of antenna.
- Dipole antenna:
It has maximum radiation efficiency in dipole antenna category. It has length:
l < λ/2
Directional antenna is the antenna which is specified to receive or transmit the signal in particular direction. The directional antenna has high directivity and gain. The working of directional antenna is like a laser which transmits light in one direction. These antennas are used in point to point link and dish antenna is its example which is in 2 feet to 8 feet in size.
Omnidirectional directional antenna:
Omnidirectional antenna radiates in 360° pattern and is typically have lower gain. The omnidirectional antenna radiates uniformly in horizontal direction.
In this type of antenna the antenna is divided in to various sector to receive or transmit the particular signal from different direction.
The grouping of two or more antenna is known as diversity antenna. It increases the strength of the receiving signal. Receiver can choose the largest output. It can combine the output power of antenna to produce power gain.
Yagi Uda antennas:
It is directional antenna and has operating frequency above 10 MHz. it can be used for 40 Km to 80 Km distance and has two types of elements.
- Active elements (driven elements)
- Parasitic elements (reflector , director)
Structure of Yagi Uda antennas:
The driven element will be connected with the supply and other elements are not connected with the supply which is known as directors which is also parasitic. The reflector is not connected with the supply that it is parasitic.
Advantages of Yagi Uda antennas:
- It has high gain about 9 db
- It has front to back ratio
- It is cheap and light weight
Disadvantages of Yagi Uda antennas:
- The antenna becomes large for high gain level
- Gain limitation is about 20 dB
Application of Yagi Uda antennas:
- It is used in HF ( 3 – 30 MHz )
- VHF ( 30 – 300 MHz )
- UHF ( 300 – 3000 MHz )
- It is used in home TV receiver
- It is used for point to point communication
Log periodic antennas:
Log periodic antenna is a very important antenna because it is independent of the frequency it means that if we give different frequencies to it its properties will be not changed like directivity, bandwidth, impedance, beam area. For this antenna, the bandwidth ratio is larger than 10:1. The characteristics depend upon angle rather than frequency. The Array of dipole antenna will follow the periodic log structure.
Application of log periodic antennas:
- This type of antenna is mostly used in the TV signal reception
- It is used ultra high-frequency communication ( UHF )
- It is used for monitoring purposes