Rhombic and Helical antenna

Information about Rhombic and Helical antenna

Published on December 18, 2009

Author: gkraman_16

Source: authorstream.com


Slide 1: Rhombic and Helical Antenna Kothanda Raman G Agenda : Agenda Antenna Antenna parameters Types of antenna Rhombic antenna Helical antenna Why Antenna? : Why Antenna? The need for antenna arisen when two person wanted to communicate between them when they are separated by some distance and wired communication not possible. Antenna : Antenna An antenna or aerial is a metallic conductor system capable of radiating and receiving electromagnetic waves. Transmission radiates electromagnetic energy into space Reception collects electromagnetic energy from space Where antenna used? : Where antenna used? Antennas are used in systems such as radio and television broadcasting, point-to-point radio communication, wirelessLAN, radar, and space exploration. Antennas are most commonly employed in air or outer space. But can also be operated under water or even through soil and rock at certain frequencies for short distances. Antenna reciprocity : Antenna reciprocity A transmitting antenna can also be used for reception and vice versa. In two-way communication, the same antenna can be used for transmission and reception. This property of transmit/receive is known as antenna reciprocity. Antenna parameters : Antenna parameters Beam width Directivity Gain Efficiency Radiation pattern Polarization The radiation intensity, directivity and gain are measures of the ability of an antenna to concentrate power in a particular direction. Beamwidth : Beamwidth Half-power beamwidth (HPBW) is the angle between two vectors from the pattern’s origin to the points of the major lobe where the radiation intensity is half its maximum. First-null beamwidth (FNBW) is the angle between two vectors, originating at the pattern’s origin and tangent to the main beam at its base. Often FNBW ≈ 2*HPBW Example : Example Source: NK Nikolova Slide 10: Directivity Directivity relates to the power radiated by antenna in particular direction. Gain Gain refers to the efficiency of antenna with respect to given normal. It relates to the power delivered to antenna. Slide 11: Antenna efficiency Antenna Efficiency) is defined as ratio of power actually radiated to the power input to antenna terminals. η= Pr/(Pr+ Pd) where Pr = power radiated by antenna Pd = power dissipated in antenna Alternatively η = Rr/( Rr+ Re ) where Rr = radiation resistance Re = effective antenna resistance Antenna Polarization : Antenna Polarization Polarization is referred to the orientation of the electric field radiated from an antenna. Vertically/Horizontally Polarized Antenna the antenna radiates a vertically/horizontally polarized electromagnetic wave Linearly polarized Antenna the antenna lies either in horizontal or vertical plane Slide 13: Elliptical Polarization the radiated field rotates in elliptical manner. Circular Polarization the radiated field rotates in circular manner. Bandwidth It refers to the range of frequencies over which antenna is effective. Radiation pattern The radiation pattern of an antenna is a polar diagram or graph representing field strengths or power density at various angular positions relative to an antenna. Induction & Radiation Fields : Induction & Radiation Fields Induction Field( Near Field ) is the field pattern close to the antenna energy is returned to antenna in the second half cycle of excitation similar to inductor that stores & releases energy Radiation Field( Far Fields ) the field pattern at great distance power radiated outward and never returned to antenna Antenna types : Antenna types Isotropic antenna Dipole antenna Yagi-Uda antenna Rhombic antenna Helical antenna Parabolic antenna Horn antenna Patch antenna Isotropic antenna : Isotropic antenna Isotropic antenna radiates equally in all directions. Its radiation pattern is represented by a sphere whose center coincides with the location of the isotropic radiator. Isotropic antenna or isotropic radiator is not physically realizable, used as a useful reference to describe real antennas. Rhombic Antenna : Rhombic Antenna Rhombic Antenna : Rhombic Antenna Structure and construction 4 wires are connected in rhombic shape and terminated by a resistor. Mounted horizontally and placed > λ/2 above ground Directional property The fields in the X right direction is additive Fields in other directions are self cancelled out. Rhombic antenna : Rhombic antenna The highest development of the long-wire antenna is the RHOMBIC ANTENNA It consists of four conductors joined to form a rhombus, or diamond shape. The antenna is placed end to end and terminated by a noninductive resistor to produce a uni-directional pattern. A rhombic antenna can be made of two obtuse-angle V antennas that are placed side by side, erected in a horizontal plane, and terminated so the antenna is nonresonant and unidirectional. Radiation pattern : Radiation pattern Features of Rhombic Antenna : Features of Rhombic Antenna Advantages Easier to construct Its input impedance & radiation pattern are relatively constant over range of frequencies. Maximum efficiency High gain can be achieved Disadvantages large site area & relatively large side lobes Applications : Applications The rhombic antenna is widely used for long-distance, high-frequency transmission and reception. In point-to-point communications. Radio communications. Short wave radio broadcasting. For long distance communications. Helical antenna : Helical antenna Helical antenna : Helical antenna The most popular helical antenna (often called a 'helix') is a travelling wave antenna in the shape of a corkscrew that produces radiation along the axis of the helix. These helixes are referred to as axial-mode helical antennas. Slide 26: B: Central Support,C: Coaxial Cable,E: Spacers/Supports for the Helix,R: Reflector/Base,S: Helical Aerial Element Slide 28: Helical antennas can operate in one of two principal modes: Normal (broadside) mode Axial (end-fire) mode. The normal mode, which yields radiation broadside to the helix axis, occurs when the helix diameter is small with respect to a wavelength. The axial mode, the most commonly used mode, provides maximum radiation along the helix axis, which occurs when the helix circumference is of the order of one wavelength. Modes Slide 30: Radiation pattern Advantages : Advantages Overall length/height of antenna is reduced Can be easily constructed It produce circularly polarized fields. Disadvantages : Disadvantages Poor reception and transmission properties. Bandwidth too narrow for cellular communication. Applications : Applications Global Positioning Systems(GPS) Wireless LAN Satellite communication Animal tracking Dual ISM(Industry Scientific& Medical) bands

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