Laserfiche WebLink
Jo <br />Oeam <br />earn <br />ine), <br />am t <br />same conditicxis ^ <br />The radiation angle of the toi^est lobe tor a hcv <br />zomai antenna above the ground may be determined <br />mathematically <br />e 0 25 <br />where <br />^ • the wave angle or radiation angle <br />h - the antenna height above ground m <br />waveengfhs (see Tade 2i <br />In Short, higher the horizontal antenna the lower <br />1$ the lowest tooe ot the pattern Therefore the higher <br />an hF .menn. can £* placed me -annec „ .,11 prpv.de <br />effective communications because ot the resu iinc w <br />radiation angle This .s true tor any honzcntai antenna <br />Electrical Characteristics ot MF Antennas <br />As mentioned previously, a oipoie antenna w,,en <br />Viewed from ore end cl me conductor, raoiates <br />amount of powe'm all directions The plotted ^ad.at.on <br />pattern is a perfect crde If the dicoie is vertical, m.s <br />circle represents the azimuth coverage, with an equal <br />amount of power radiated m every compass <br />For point-to-point communications. novi.ever. it is be <br />ficial to concentrate the radiated energy mto a beam <br />which IS aimed toward a single distant point. An analogy <br />can be made by comparing the light emanating from a <br />bare electric lamp to that from an automcPile ^^^d ig t <br />For Illuminating a distant point, the headlight is far more <br />0ff0CtlV6 <br />Antennas designed to concentrate the radiated <br />energy mto a beam are called beam antennas For a <br />fixed amount of transmitter power fed to the tratismiumg <br />antenna, beam antennas provide increased signa <br />strength at a o-stant receiver In radio communications, <br />the use of a beam antenna is also beneficial during <br />reception, because the antenna pattern for transmission <br />IS the same as for reception A beam antenna helps to <br />reiect signals from unwanted directions, and m effect <br />boosts the strength of signals received from the desired <br />direction ^ ^ <br />The increase in signal or field strength which a beam <br />antenna offers is frequently referenced to a dipole <br />antenna by a term called gam Gam is common y <br />expressed m oecibels One type of beam antenna is a <br />Yagi named after one of its Japanese inventors. <br />Ditterenf varieties of Yaoi antennas exist, each having <br />somewhat diHerent characteristics. Many antennas <br />erected for television reception are a form of multi­ <br />element Yagi beam <br />Subsequent discussions m this paper refer to a <br />3-element Yag* array, one having a driven element, a <br />parasitic director and a parantic reflector The parasitic <br />elements are not led any power directly; instead they are <br />excited through mutual coupling to the driven element, <br />and in turn also radiate power Each parasitic element <br />IS spaced 0.2 wavelength from the driven element, and <br />/ X / ‘ <br />/ > <br />\ <br />\ <br />2'Ct U-M- <br />•vTu>. y.\ vv <br />(<V <br />_ <br />L I ! <br />X <br />/ <br />4 <br />-•I N-\. <br />'v.N' <br />\ <br />t <br />w lN.V \V/ <br />-x>' <br />K-X so <br />i I <br />-90 <br />Fig 7-Computer-calculated elevation-plane or vertical- <br />profile pattern of a horizontal Yagi <br />for a comparison dipole, broken line, <br />ere those seen from the ends of the elements with the <br />antennas in free space <br />the theoretical gam of this array, determined through <br />computer analysis, is 7 2 dB over a dipole. This rneans <br />that fcr the same transmitter power, the nrmerthat <br />DOwer of the Yagi m its favored direction is 5.2 times <br />S?the diDOle in the same direction. The theoretical front^ <br />to-back ratio of the Yag. antenna is ’2 dB. <br />power radiated in its forward direction is 157 <‘mes lha <br />fad.ated m the opposite direction Values for a P^^t'cal <br />antenna of this type are quite dose to ^a'' <br />Compute'calculated radiation % <br />horizontal Yag. array are presented in Figs 6 and 7^ <br />Fig 6 shows the azimuth plane, the response in various