What is an antenna?

The antenna is an important concept in the domain of electronics and communication engineering. An antenna forms an integral part of many commonly used devices such as wireless modems, routers, game controllers, mobile phones, bluetooth devices, and so on. In simple terms, any wireless communication makes use of antennas to serve the purpose. An antenna, also known by the term 'arial', is an electrical device that converts electrical signals into electromagnetic waves or electromagnetic waves into electrical signals. An antenna contains a transmission line usually made up of co-axial cable, it then receives the signals and converts them into electromagnetic waves, which are then transmitted via space into their required destination. A communication system essentially makes use of two antennas, a transmission antenna, which converts electrical signals into electromagnetic waves, and a receiving antenna, which receives the electromagnetic waves and converts them back into electrical signals.

Antennas generally make use of radio waves, as can be found in the electromagnetic spectrum. After a radio wave has been generated, usually known as the RF (radio frequency) signal, a means needs to be set up so as to transmit this RF signal through the free space, the device that does this task is the antenna. Upon receiving these RF signals by the conductor of the receiving antennas, a voltage is induced in it, which is processed by the receiver of the antenna to extract the required information. Hence, in a nutshell, an antenna can be compared with a transducer, which converts RF energy into electrical energy and vice-versa. There are various ways of using antennas, one of the major use is for long-distance communication, where using a cable is not feasible, for instance, an aero-plane cannot communicate with the air traffic control (ATC) unit via cable connection.

Types of antennas

There are various types of antennas used in wireless communication. Some of the main types of antennas are discussed below.

Wire antennas

A wire antenna is a radio wave antenna. The wire antennas have a long wire suspended above the ground. In such antennas, the other end of the receiver or transmitter is always grounded. These antennas are widely used for long, short, and medium wavebands. There are various kinds of wire antennas, they are:

• Short dipole antenna: Short dipole antennas are the special case of a dipole antenna whose radiation pattern is similar to the dipole antennas. A dipole antenna has two conductors, and its wire is shorter than its wavelength. The term 'short' in the short dipole antenna refers to the wire length, which is smaller than the wavelength. These antennas are an open circuit, where the signals are fed at the center.
• Loop antenna: Loop antennas are formed by multiple turns of conductive wires that form a loop. The radiation pattern formed by the loop antenna is similar to that of short dipole antennas.
• Monopole antenna: Monopole antennas are referred to as the special case of a dipole antenna. They are always mounted over a ground plane. They are half of the dipole antenna, with their length twice the length of the dipole antenna.
• Aperture antenna: The aperture antennas consist of dipole and loop antenna with an opening structure. They are a class of directional antennas. Directional antennas are beam antennas, they are characterized by greater power radiation patterns in a specific direction which provides greater performances and reduced interferences.

Isotropic antenna

An isotropic antenna is also known as an omnidirectional antenna. This antenna radiates waves equally in all directions. These are generally hypothetical antennas, whose radiation intensity is equal in all directions. The power density or the power gain of an isotropic antenna is equal to 1.0.

Parameters of antenna characteristics

Radiation pattern

A practical antenna, unlike a hypothetical antenna, cannot radiate energy equally in all directions. The intensity of radiation emitted by an antenna varies when the directions covered by an antenna are concerned. Some parts may have maximum radiation, while some parts may have minimum radiation, while some parts may have almost zero radiation. Field strength is the quantity, which represents the radiation intensity at a point emitted by an antenna. The field strength can be measured by measuring the voltage at two different points in an electric field, and the results are divided by the distance between the two points.

Directivity and antenna gain

The average power of an antenna is a function of radiated power of the antenna. The ratio of power density to the average power of an antenna is known as the directive gain of the antenna. Directivity is the ratio between the maximum power radiated by an antenna and the average power radiated.

The ability of a practical antenna is compared with the hypothetical antenna, that is, an isotropic antenna. Hence, antenna gain is the ability of an antenna to radiate as compared with the hypothetical antenna.

Effective area

The effective area of an antenna is also known as the effective aperture of an antenna. This term is usually related to the receiving antenna. Effective area is the ratio between the power received at the load and the average power density emitted by the antenna. This measure simply represents the ability of a receiving antenna to extract energy from an electromagnetic wave, that is, the radio signal.

Radiated power

The radiated power represents the amount of power that is emitted by the antenna when the antenna is connected to the transmitter. The radiated power of an antenna is estimated by considering the product of the input power of the antenna and the gain of the antenna.

Antenna transmission line

The transmission lines of an antenna are the wires of the antenna, specially co-axial cables. These lines carry the power to and fro from the antennas. The transmission lines are connected to the circuits, which are used for transmitting and processing information. The transmission lines essentially guide the RF from one point to another in a contained manner. Ordinary cables are not meant for carrying currents in the RF range which are above 30 kHz, as the energy moves out of the cables in the form of radio waves and causes power losses.

Context and Applications

This topic is taught in many undergraduate and postgraduate degree courses like:

• Bachelors of Technology (Electronics and Communications Engineering)
• Bachelors of Technology (Electronics Engineering)
• Bachelors of Technology (Electrical Engineering)
• Masters of Technology (Electronics and Communication Engineering)
• Masters of Technology (Antenna Engineering)

Practice Problems

Q1. Which of the following is true for an electromagnetic wave?

a. It only consists of an electric field.

b. It only consists of magnetic field.

c. It consists of both electric and magnetic fields.

d. None of these

Answer: Option c

Explanation: An electromagnetic consists of both electric and magnetic fields. The electric and magnetic fields are mutually perpendicular to each other.

Q2. What is the effective area of an antenna?

a. It is the distance at which a receiver antenna can transmit RF.

b. It is the ability of an antenna to radiate energy in all directions.

c. It is known as the effective aperture of an antenna.

d. It is referred to as the power gain of an antenna.

Answer: Option c

Explanation: The effective area is also known as the effective aperture of an antenna. It denotes the ability of a receiving antenna to extract energy from an electromagnetic wave.

Q3. Why ordinary electric cable cannot be used for antenna transmission?

a. Energy moves out of the cable as radio waves

b. The cable gets heated

c. Both a and b

d. None of these

Answer: Option a

Explanation: Ordinary electric cables cannot be used for radio transmissions above 30 KHz as the energy moves out of the cable in the form of radio waves and causes power loss. Radio waves also get reflected from the presence of discontinuities present in the cables.

Q4. Which of the following is a hypothetical antenna?

a. Dipole antenna

b. Short dipole antenna

c. Isotropic antenna

d. Directional antenna

Answer: Option c

Explanation: The isotropic antennas are known as hypothetical antennas. These are the antennas, whose radiation of waves is equal in all the directions with the same intensities.

Q5. Which antennas are referred to as the special case of dipole antennas?

a. Short dipole antennas

b. Monopole antennas

c. Aperture antennas

d. Loop antennas

Answer: Option b

Explanation: The monopole antennas are referred to as the special case of dipole antennas. They are half of the dipole antennas and their length is twice that of a dipole antenna.

Related concepts

• Radio frequency antenna types
• Electromagnetism
• RF connector