# Antenna

We often define antennas and antenna terminology in terms of a transmitting antenna, but all the definitions apply to receiving antennas as well. In fact, an antenna's properties are the same in either operating mode. So, whether it is stated or not, all the definitions and descriptions describe antennas that are either part of a transmitter or a receiver.

An antenna is a transducer between a guided wave and a radiated wave, or vice versa. The structure that "guides" the energy to the antenna is most evident as a coaxial cable attached to the antenna. The radiated energy is characterized by the antenna's radiation pattern.

1. ## Power Density

When the Electromagnetic waves are propagated from the antenna, its field strength decreases as it go away from its source. This variation in the power of the electromagnetic waves per unit length is called power density. It’s abbreviated as ρ.

2. ## Inverse Square Law

Inverse square law defines that the power density of the electromagnetic waves propagated from the source is inversely proportional to the square of the distance from the source and directly proportional to the total power radiated by the source.
ρ = PT/4πr2
Here PT stands for total power dissipated by the source.
r stands for the distance from the source
ρ stands for power density

3. ## Zo in Free Space

Characteristic impedance is the square root of the ratio between permeability and permittivity.

Where μ stands for permeability
ε stands for permittivity
4. ## Maximum Usable Frequency (MUF)

As the frequency of the microwave transmission is very high and with the increase of frequency the wave length of the electromagnetic waves keeps on decreasing.
As a result we get a stage that beyond certain value of high frequency we are unable to get the response of the reflected waves. The stage at which the response or reflection of electromagnetic waves is not present, that is called the maximum usable frequency.

1. muf = critical frequency/ Cosθ
2. muf = fc Secθ
5. ## Interference of E.M Waves

When the electromagnetic wave of the same frequency is propagated from the source (antenna) and it is received by the load through two or more than two different paths, it is called the interference of the electromagnetic waves, due to this reason the Electromagnetic wave receive by the load from different paths will in phase or out of phase with each other. This phase difference will cause to increase or decrease the signal strength of the load. This variation in the signal is known as electromagnetic waves interference.

6. ## Skip Distance

It is the distance from the source to the point where the first sky wave touches the ground as shown in the given diagram. The B ray is considered as the first sky wave which touches the ground now the distance from the source to the B is known as skip distance. In the skip distance if we want to receive some signal, the reception will not be possible until we reach the end of the distance.

7. ## Critical Frequency

It is the frequency above which the reception of the signal is not possible. When the electromagnetic waves are transmitted from the source, it is received back at the load after reflecting from the sky. If the angle of transmission is small, the angle of reflection will also be small. As the frequency keep on increasing the distance between the source and load keeps on increasing. Ultimately we reach the frequency of transmitter above which the reception of the signal is not possible. This frequency is known as critical frequency.

8. ## Radiation Resistance

Radiation resistance is the ratio between the power dissipated by the antenna to the square of the current which produces this radiated power

9. ## Directivity

It is the ability of the antenna to transmit the maximum power in the desired direction in case of transmission and to receive the maximum energy in from the desired direction in case of reception.

10. ## Power Gain

Power gain is the ratio between the powers of the isotropic antenna to the power of the actual antenna at some reference point.

.

A (dB) = 10log10  P2/P1

Here P2 stands for power of isotropic antenna and P1 for power of actual antenna.

11. ## Beam Width

Beam width is the angular separation between two half power points at the radiation plane of the antenna.

12. ## Front to Back Ratio

It is the ratio between the power radiations by the antenna in the desire direction to the power radiation by the antenna in the opposite direction.

13. ## Physical Length & Electrical Length of the Antenna

The electrical length of the antenna is determined by the formula

λ = c/f

Where ‘C’ is the velocity of the electromagnetic wave

This wave length is taken in free space but in case of antenna where the first half cycle changes to the negative cycle the direction of flow of electromagnetic wave also changed as the physical change to the antenna. This change in the direction is not as quick as in case of change in direction of electrical waveform, therefore the physical length of the antenna is kept smaller with respect to its electrical length.

This decrease in size of the physical length of antenna is 50% less than its electrical length.

Formula for electrical length of antenna

ϱ = 150 / f

Here ϱ stands for the electrical length of antenna.

f stands for the frequency in MHz

Formula for Physical length of antenna

ϱ = 142 / f

Here ϱ stands for physical length of antenna

f stands for the frequency in MHz

14. ## Lobe

The graphical presentation of radiation pattern of the antenna is known as Lobe. There are two types of lobes.

1. ### Main Lobe or Major Lobe

The graphical presentation of the radiation of antenna is direction of maximum radiation power is called the main lobe or major lobe.

2. ### Side Lobe or Minor Lobe

Beside the maximum radiated power from the antenna there is a small amount of energy which radiated from the other directions of the antenna. As the radiated energy is less with respect to the maximum radiated energy, therefore the lobe which forms due to this energy is known as minor or side lobe.