Transmission and Receiving Antennas

 

   

 

An antenna is a device that serves as interface between the electric circuit and space, and it is designed to transmit and receive electromagnetic waves. 

Each antenna is designed to transmit and/or to receive signals within a certain frequency range according to its size and form.

 

   

 

Antennas transmit and receive electromagnetic waves.  Generally composed of metals (mainly copper or aluminum), antennas can convert an electric current into electromagnetic radiation and vice versa.  Every wireless communication device contains at least one antenna.

 

Transmission antennas produce radiofrequency radiation that propagate in space

 

Receiving antennas perform the reverse process: they receive radiofrequency radiation and convert them into the required signals (e,g. sound, picture) in the receiving device (e.g., radio, television, mobile phone).

 

The simplest type of antenna consists of two metal rods, and is known as a dipole.  One of the commonest types of antennas is the monopole antenna, consisting of a rod situated vertical to a large metal board that serves as a ground plane.  The antenna mounted on vehicles is usually a monopole, with the metal roof of the vehicle serving as the ground plane.

 

The shape and size of the antenna determines its operative frequency and its other radiation characteristics.  One of the important attributes of an antenna is its directionality.  In communication between two fixed targets, as in communication between two fixed transmission stations, or in radar applications, a directional antenna is required, in order to direct the transmission energy to the receiver exclusively.  Conversely, when the transmitter or receiver is not stationary, as in cellular communication, a non-directional system is required.  In such cases an omnidirectional antenna is required, that transmits (and receives) all frequencies uniformly in all directions of the horizontal plane, while in the vertical plane, radiation is not uniform and very low.

 

 

Transmission antenna – the basic source of radiofrequency (RF) radiation

A transmission antenna is the basic element of radio technology. This type of antenna is composed of a conductor that carries an electric current whose intensity fluctuates over time and converts it into radiofrequency radiation that propagates in space. 

 

TV transmission antenna-Berlin

TV transmission antenna-Berlin

Radio transmission antenna

Radio transmission antenna

Receiving antenna - a device for reception of radiofrequency (RF)

A receiving antenna performs the reverse of the process performed by the transmission antenna.  It receives radiofrequency radiation and converts it into electric currents in an electric circuit connected to the antenna.  

                         Household TV receiving antenna

Household TV receiving antenna

 

TV and radio broadcasting stations, use transmission antennas to transmit specific types of signals that propagate through the air. These signals are detected by receiving antennas which convert them into signals, and are received by the appropriate device (e.g., TV, radio, mobile phone).

 

 

 

Radio and TV receiving antennas are designed exclusively for receiving radiofrequency radiation, and they do not produce radiofrequency radiation

 

 

Cellular communication installations (e.g. base stations, repeaters) and mobile phones are equipped with designated transmission and receiving antennas that emit radiofrequency radiation and serve the cellular communication networks, in accordance with communication network technologies. 

 

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Main characteristics of antennas

Directionality – an antenna may be non-directional (isotropic), transmitting uniformly in all directions, or directional –transmitting preferentially in one or more directions at a higher power.

 

Radiation curve of an antenna – a graphic three dimensional representation of the radiation intensity of the antenna, representing the distribution of the electric field and/or the radiation energy in the space surrounding it.

 

Enhancement (amplification) of the antenna – the ratio between the radiation power of an antenna in a certain direction, and the radiation power of an isotropic (omnidirectional) antenna under identical conditions of transmission (distance and input power of the antenna).

 

Polarization – direction of the electric field radiating from the antenna in areas sufficiently distant from the antenna (a distance of a number of wavelengths).  A radio wave may be non-polarized or polarized (linear or circular/elliptical polarization).

 

Efficiency – The ratio between the total power radiated by the antenna and the net electric power fed to the antenna.

 

Band width – The width of a range of frequencies at which the antenna transmits its maximal radiation and intensity.

 

Antenna array – A systematic deployment of antennas that operate together.  The individual antennas in an array are usually of the same type, and are situated in close proximity and at a fixed distance from one another. An antenna array enables increasing the directionality and control of the main radiation beams and the lateral beams. 

 

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Basic types of antennas

Dipole antenna - consists of two wires (or rods), usually metallic, placed in a straight line, so that an alternating current flows through the center of the antenna.

 

Monopole antenna (Marconi) – a single straight wire or rod (metallic), mounted perpendicularly over an infinite conductive surface (called a ground plane). In practice, a smaller plane is sufficient.

 

Yagi antenna – consists of an active antenna and a number of passive antennas, serving as directors or reflectors.  This type of antenna is used mainly for transmission and reception of FM radio and TV broadcasting.  In the past this type of antenna was also used in radar systems.

 

Yagi antenna for TV reception

Yagi antenna for TV reception

 

 

Horn antenna – a funnel-shaped wave-guiding antenna.  This type of antenna is used mainly for frequencies greater than 1 GHz.

 

Printed antenna – an antenna that is composed of a conductive layer etched on an insulating substrate.  The back of the substrate is covered by a conductor and serves as the ground plane. This method enables creating a large number of antennas on a single substrate, thus achieving great enhancement.  The printed antenna has come into use during the last decades for example in mobile phones

 

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References

  • Joel R. Hallas, Basic Antennas: Understanding Practical Antennas and Design, February 28, 2009, 1st edition, ISBN-13: 978-0872599994.

 

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