Question

What is an antenna?What are the different types? What are its parameters(Explain it thoroughly)

Answer 1

An antenna is a specialized transducer that converts radio-frequency (RF) fields into alternating current (AC) or vice-versa. There are two basic types: the receiving antenna, which intercepts RF energy and delivers AC to electronic equipment, and the transmitting antenna, which is fed with AC from electronic equipment and generates an RF field.

TYPES

Wire Antennas

One of the most commonly used antennas are wire antennas. They can be found in vehicles (automobiles), ships, aircrafts, buildings etc. Wire Antennas come in different shapes and sizes like straight wire (Dipole), Loop and Helix.

Short Dipole Antenna

Perhaps the simplest of all antennas is the Short Dipole Antenna. It is a special case of the Dipole antenna.

In its simplest form, it is basically an open circuit wire with the signal being fed at the centre. The term “short” in short dipole antenna doesn’t directly refer to its size but rather to the size of the wire relative to the wavelength of the signal.

For a typical Short Dipole Antenna, the length of the wire les less than the tenth of the wavelength of the frequency of operation.

Dipole Antenna

A Dipole Antenna is made up two conductors in the same axis and the length of the wire need to be small compared to the wavelength.

Loop Antenna

A Loop antenna is formed by a single or multiple turn of wire forming a loop. The radiation produced by loop antenna is comparable to a short dipole antenna.

Monopole Antenna

A special case of Dipole antenna is the monopole antenna i.e. it is half of the dipole antenna.

Aperture Antennas

A class of directional antennas, Aperture Antennas have an opening in the surface. Usually, aperture antenna consists of Dipole or Loop Antenna in a guiding structure with an opening to emit radio waves.

Slot Antenna

A type of aperture antenna which contains one or more slots cut on the surface of the waveguide. They are usually used in microwave frequencies and have an omnidirectional radiation pattern.

Horn Antenna

One of the most popular antennas is the Horn Antenna, which effects the transition between transmission line and wave propagating in free space. It acts as a natural extension to a waveguide.

Fundamental Parameters of Antennas (Characteristics)

These parameters are sometimes also called as Properties of Antenna or Characteristics of Antenna. Certain basic characteristics of antenna are listed below:

• Antenna Radiation Pattern
• Radiation Intensity
• Directivity and Gain
• Radiation Efficiency and Power Gain
• Input Impedance
• Effective Length
• Bandwidth
• Effective Aperture
• Antenna Polarization

Let us understand these properties of antennas one-by-one.

Radiation Pattern

A practical antenna cannot radiate energy in all directions with equal strength. Radiation from an antenna is usually found to be maximum in one direction whereas it is minimum or almost zero in other directions.

Field Strength is the quantity that is used to represent the radiation pattern of an antenna. It is usually measured at a point located at a particular distance from the antenna.

You can measure field strength by calculating voltage at two different points on an electric line and dividing the result with the distance between the two points. Hence, the units of field strength are volts per meter.

Graphically speaking, the field of strength is plotted as a three-dimensional graph as it is the measure of the electromagnetic field strength at equidistant points from the antenna.

If the graph of radiation of antenna is just a function of direction, then it is simply called as Radiation Pattern. But if it is expressed in terms of the electric field strength in V/m, then it is called Field Strength (or Radiation) Pattern.

Radiation Intensity

The radiation intensity of an antenna is the power per unit solid angle. It is represented by U and is independent to distance from the antenna. Units of radiation intensity are Watts per steradian (W/Sr).

Directivity and Gain

Ideally, an antenna which radiates energy in all directions equally i.e. an omnidirectional antenna is called as Isotropic Antenna. This is just a hypothetical situation and an isotropic antenna doesn’t exist in practice.

But if we consider an isotropic antenna, then its power density will be same at all points on the sphere of radiation. Hence, the average power of an antenna as a function of radiated power is

P_avg = P_rad / 4πr² W/m²

The ratio of power density to the average power radiated is known as Directive Gain.

Directivity of an Antenna is a measure of concentration of radiation in the direction of maximum radiation or the ratio of maximum power density to average power radiated.

Radiation Efficiency and Power Gain

All practical antennas will have Ohmic losses as they are made up of conducting materials with finite conductivity. Radiation Efficiency is ratio of radiated power to the input power.

η_r = P_rad / P_in

Power Gain of an Antenna is the ratio of the power radiated in a direction to the total input power.

Input Impedance

We should take care to match the input impedance of the antenna to that of the input transmission line. If the input impedance doesn’t match, then the system degrades over time due to the reflected power.

Effective Length

The length of an imaginary linear antenna with uniformly distributed current is defined as the Effective Length of an antenna such that both these antennas have same far field in π/2 plane.

Bandwidth

The bandwidth of antenna is defined as the range of frequencies over which the characteristics of the antenna are maintained to the specified value. This is because, the requirements for the characteristics of an antenna like the gain, impedance, standing wave ratio etc. may change during operation.

Effective Aperture

Generally, the term effective aperture or effective area is associated with the receiving antenna. Effective Aperture or Area of an antenna is the measure of the ability of an antenna to extract energy from the electromagnetic wave.

Effective Aperture of an antenna is the ratio of the Power Received at the load to the average power density produced by the antenna.

Antenna Polarization

The Antenna Polarization refers to the physical orientation of the electromagnetic wave radiated in a given direction. Polarization of an EM Wave is a time varying direction and relative magnitude of the electric field vector.