An antenna is simply called a transducer, Which converts radio frequency (RF) electrical current into an electromagnetic (EM) wave at the same frequency and vice versa. It can works as a transmitter of signals as well as receivers them.

An antenna is also known as Ariel. It can be understood in the following different ways

- It may be a piece of a connecting material in the form of wire, rod or any other shape which can be excited.
- This is a transmitter or radiator of RF electromagnetic waves.
- It is an impedance matching device which matches the impedance of one medium to another medium.
- It acts as a coupler, which couple a generator/transmission line and space or vice versa.

## Properties of antenna

- It has identical impedance when used for transmitting and receiving purpose. This property is called equality of impedance.
- It has identical directional characteristics/patterns when it is used for transmitting & receiving purpose. This property is called equality of directional pattern.
- It has the same effective length when it is used for transmitting & receiving purpose. This property is called equality of effective length.

## Types of Antennas

- Yagi Uda Antenna
- Log Periodic Antenna
- Horn Antenna
- Helical Antenna
- Microstrip Antenna

## Antenna Parameters

### Impedance

It is defined as the ratio of input voltage to input current or

Here,

is a complex quantity & it is written as

Here, the reactive part

results from fields surrounding the antenna. The resistive part,

is given by

= Losses

= Radiation Resistance

### Radiation Resistance ( )

It is defined as fictitious or hypothetical resistance that would dissipate an amount of power equal to the radiated power.

### Radiation pattern

An antenna radiation pattern is a three-dimensional variation field. It is a pattern drawn as a function of θ and Ø. The pattern consists of one main lobe & a number of minor/side lobes.

### Radiation Intensity

It is defined as the power radiated in a specific direction per unit solid angle.

= Intrinsic Impedance of the medium (Ω) R = radius of sphere, (m) P = Power radiated instantaneous E = Electric field strength (V/m)

is a function of θ and Ø.

### Directional Gain ()

It is defined as the ratio of radiation intensity in a specified direction to the average radiation intensity.

### Directivity

It is defined as the ratio of the maximum radiation intensity to the average radiation intensity. It is also called as the maximum directive gain of an antenna. It is denoted by D.

### Power Gain

Power Gain is defined as the ratio of the radiation intensity in a given direction to the average total input power. It is denoted by

It is also defined as the ratio of 4 π times of radiation intensity to the total input power.

= ohmic losses in the antenna

### Efficiency

It is defined as the ratio of the radiated power to input power. It is denoted by η.

### Effective Area

It is defined as

Or

Where,

= received power (Watt)
P = Power flow per square meter (Watt/m^{2})