Date Posted: 2/22/2017 2:01:44 AM
Posted By: Abced Membership Level: Gold Total Points: 1085
An antenna is an electromagnetic radiator that converts electromagnetic wave into electrical signals and vice versa. Antennas are means of coupling the transmitter to the medium(free space).The origin of antennas dates back in the mid 1800s.Since then much evolution has occurred . A major advancement was propagated by Guglielmo Marconi who set up a wireless call that traveled to boats across the Atlantic Ocean. Previous to this radio transitions had a very limited range such that houses very close could talk to each other, but communication over great distances was unlikely to occur. The next breakthrough was around 1920s when operators at Radio Arlington were able to transmit the sound of a human voice up and down the Atlantic coast. With this advancement in antenna communication,it was considered a major breakthrough and the beginning of Amplitude Modulation radio. Radio waves are similar to light waves but vary in some aspects. Whereas light waves always follow the inverse square law ,the radio waves do not. There are many external conditions that affect a radio wave such as atmospheric conditions. Discovery of radio signals as electromagnetic waves is attributed to Heinrich Hertz. In radio applications there is a radiated field that leaves the conductor and travels through space. Antennas create a series of oscillation waves with specified frequencies and wavelengths. The electromagnetic wave travels away from the antenna up to a distance where the energy is completely damped by the environment. Antennas exhibit the following properties: 1.Gain2.polarization3. direction and 4.aperture.1.Gain is a measure of increase in power.An antenna with higher gain is more effective in its radiation pattern.Antennas are designed such that power raises in wanted direction and decreases in unwanted direction. As the gain of antenna increases,the angle of radiation reduces hence increasing coverage distance.This is why omnidirectional antennas are considered low-gain antennas whereas directional antennas are considered high-gain antennas.2.Direction is the shape of the transmission pattern.The directivity of an antenna is considered as the capability of an antenna to radiate signal power to a given direction.3.Polarization.An electromagnetic Wave may be polarized vertically or horizontally. Combination of both vertical and horizontal polarization results to circular polarization.4.Aperture can be said to be a collective area of an antenna that actively takes part in transmission and reception of electromagnetic waves.Antennas can be classified in three broad categories:Omni-directional,directional and semi directional.Omni-directional antennas propagate wave signals in all directions i.e at an angle of 360°.They are considered low-gain antennas(LGAs) with high bandwidth, therefore can propagate in all directions regardless of terrain.Their application is mainly in spacecraft where they are used as a backup to high gain antennas(HGAs).Omnidirectional antennas include; dipole antenna.Directional antennas refer to the high-gain antennas that radiate or receive high radio signal power in a speficific direction,hence improving performance and reducing interference.They have a narrow beam that make their propagation highly directional.Directional antennas include;Parabolic/dish antenna,helical antenna,quad antenna,billboard antenna,log periodic antenna and yagi antenna.They offer enhanced performance in some directions at the expense of other directions.Semi directional antennas propagate at a specific angle.Semi-directional antennas include; Patch,panel and yagi antenna.After having looked at the three broad categories of antennas,let's now look at the various types of antennas.I have already named them under the categories above.They include;1.Parabolic antennas.Also known as dish reflector.A parabolic reflector has a high degree of directivity,hence has a high ability to focus radio frequency waves in to a narrow beam.Beamwidth is <25°.It converts diverging spherical wavefront into parallel wavefront that produces a narrow beam of the antenna.They are mainly used for long distance communication links over large geographical areas.2. Helical/helix antenna.A simple structure made up of wire(s) wound to form a helix.The most common design is a single wire backed by the ground and fed with coaxial line.3.Yagi antenna.It's actually Yagi-Udah Array,named after its founders,Shintaro Uda and Hidetsugu Yagi.Can be constructed with one or more reflector and director elements. It uses several dipoles(reflector and director elements) to form a directional array.They have a beam-width of between 30° and 80°.A dipole propagates radio frequency (RF) energy,then the immediate dipoles reradiate the RF energy in phase and out of phase respectively.4.Patch antenna.A semi directional antenna that uses a flat metal strip mounted above a ground plane.Forward directionality is enhanced since radiation from the back is cut off by the ground plane.It can have a beamwidth of between 30° and 180°.5.Log periodic antenna.It is made of a series of dipoles placed along the antenna axis at different space intervals of time followed by a logarithmic function of frequency .Mainly used in wide range of applications where variable bandwidth is required alongside antenna gain and directivity. Some of its versions are bow-tie and logperiodic dipole array.Bowtie antenna,also known as butterfly antenna, resembles a bowtie.Logperiodic dipole is mainly used in wireless communication technology.6.Dipole antennas.Consists of two thin metal rods with a sinusoidal voltage difference between them.They are very simple to construct and use.They are the most common omnidirectional antenna used.They propagate a radio frequency energy of 360° in the horizontal plane.Consists of two metallic rods through which current and frequency flow.Its other versions include;short dipole, monopole and loop antenna.7.Grid antennas.To prevent dish reflectors from moving out of alignment or deforming due to high wind conditions,the parabolic antenna can be perforated to form grid antenna with square meshes or simply grids.It's gain and beamwidth are similar to that of dish/parabolic antenna.
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