Introduction to Antenna principles

Transcription

1 What is an antenna? Intoduction to ntenna pinciples n antenna is a passive stuctue that seves as tansition between a tansmission line and ai used to tansmit and/o eceive electomagnetic waves. D. Sanda Cuz-ol INEL 5305 Electical and Compute Engineeing Univesity of ueto ico at Mayaguez Souce T eceive Cicuit ntenna Types of antennas Can be divided into two goups Wie antennas: dipoles, loops, Yagi-Uda petue antennas: paabolic, hons, micostip antennas Ulaby, Wie antennas Wie antennas Log peiodic Yagi Log peiodic Yagi Yagi-Uda with eflecto

2 petue antennas eflecto and yamidal hon antennas Dipole with paabolic and cone eflecto Spheical (main eflecto) with Gegoian feed elated paametes Solid angle, Α and adiation intensity, U adiation patten, n, sidelobes, HBW Fa field zone, ff Diectivity, D o Gain, G ntenna iation impedance, Effective ea, e Spheical coodinates z (zenith) θ θ0 ll of these paametes ae epessed in tems of a tansmission antenna, but ae identically applicable to a eceiving antenna. We ll also study: Fiis Tansmission Equation ada Equation φ azimuth θ elevation θ90 φ0 φ θ90 φ90 y Solid ngle adiation Intensity Is the powe density pe solid angle: s dθ s sin θ dø s θ aco d s s d sin θ dø dθ d θ ángulo plano d elemento de ángulo sólido El aco total en un cículo: El áea total en una esfea: π ngulo total: π [ianes] ngulo sólido total: [ ] [s] steian (s) ( ian) U whee ½ e{e [W/s] H*}ˆ [W/m is the powe density also known as oynting vecto. ]

3 Total iated powe by antenna Can be calculated as; o U d[w] ds [W] adiation patten is the 3D plot of the gain, but usually the two dimensional hoizontal and vetical coss sections of the iation patten ae consideed. efes to the vaiation of the elative amplitude of the iation as a function of diection. adiation atten Field patten: E( θ, E n E owe patten: ( θ, U F n U ma ma ma Whee U is the iation intensity to be defined late. Total Solid ngle of an antenna Isotopic antenna d F n [s] z ž It s an hypothetic antenna, i.e., it does not eist in eal life, yet it s used as a measuing ba fo eal antenna chaacteistics. Is as if you changed the iation patten beam of an antenna into a pencil beam shape and find out what s the equivalent solid angle occupied by this patten. atón n y It s a point souce that occupies a negligible space. Has no diectional pefeence. Its patten is simply a sphee so it has isotopic [steians]. isotopic π π θ 0 φ 0 () d () sinθ dθ dφ adiation atten atten pola plot Wheneve we speak of iation pattens, we nomally mean we ae at a distance fa enough fom the antenna known as the fa field. En _ db -3dB -0dB ø HBW HBW atón de Campo (Escala lineal) atón nomalizado atón de campo o de potencia (Escala logaítmica) COODENDS ECTNGULES Note that when plotted in decibels, the powe and field pattens look eactly the same. HBW NNBW n.5 Lóbulo pincipal ("Mainlobe") } Lóbulos menoes TON TIICO (Coodenadas polaes esféicas, dimensiones) 3

4 Dipole antenna patten Sidelobes ntennas sometimes show side lobes in the iation patten. Side lobes ae peaks in gain othe than the main lobe (the "beam"). Note the iation patten is donut shaped. Side lobes have bad impact to the antenna quality wheneve the system is being used to detemine the diection of a signal, fo eample in D systems. Sidelobes of dipole aays ntenna atten with sidelobes sidelobe Many applications equie sidelobe levels (SLL) to be below -0dB. Gain o Diectivity Diectivity and Gain ll pactical antennas iate moe than the isotopic antenna in some diections and less in othes. Gain is inheently diectional; the gain of an antenna is usually measued in the diection which it iates best. D D U ma ma / ave ma /U ave n isotopic antenna and a pactical antenna fed with the same powe. Thei pattes would compae as in the figue on the ight. If lossless antenna, GD 4

5 Gain o Diectivity Gain is measued by compaing an antenna to a model antenna, typically the isotopic antenna which iates equally in all diections. D( θ, F n d p D o isotopic / Diectivity Fo an antenna with a single main lobe pointing in the z-diection, can be appoimated to the poduct of the HBW then β z β D / yz The Diectivity: β β z yz e λ Fa field The distance at which the fields tansmitted by an antenna (spheical) can be appoimated to plane waves. It s defined as ff D / λ D is the lagest physical dimension of the antenna λ wavelength of opeation ff distance fom the antenna to the obsevation point Beamwidth, HBW Is the distance in ians o degees between the diection of the iation patten whee the iated powe is half of the maimum. 0 log log HBM 70-3 db o λ D -3 db fo "pencil beam" shape; ntenna Impedance n antenna is seen" by the geneato as a load with impedance Z, connected to the line. ( + L ) jx Z + The eal pat is the iation esistance plus the ohmic esistance. Minimizing impedance diffeences at each inteface will educe SW and maimize powe tansfe though each pat of the antenna system. Comple impedance, Z, of an antenna is elated to the electical length of the antenna at the wavelength in use. The impedance of an antenna can be matched to the feed line and io by adjusting the impedance of the feed line, using the feed line as an impedance tansfome. Moe commonly, the impedance is adjusted at the load (see below) with an antenna tune, a balun, a matching tansfome, matching netwoks composed of inductos and capacitos, o matching sections such as the gamma match. Z ntenna efficiency, η Efficiency is the atio of powe put into the antenna teminals to the powe actually iated adiation in an antenna is caused by iation esistance which can only be measued as pat of total esistance including loss esistance. η in G η D 5

6 adiation esistance The antenna is connected to a T.L., and it sees it as an impedance. The powe iated is Io The loss powe is η + loss loss Io + L loss What is a a? ada equation eceived powe by a a fom a single taget is G t e ( ) o λo τ σ 3 4 Whee σ is the backscatteing coefficient of the taget [m ] ntenna polaization The polaization of an antenna is the polaization of the signals it emits. The ionosphee changes the polaization of signals unpedictably, so fo signals which will be eflected by the ionosphee, polaization is not cucial. Howeve, fo line-of-sight communications, it can make a temendous diffeence in signal quality to have the tansmitte and eceive using the same polaization. olaizations commonly consideed ae vetical, hoizontal, and cicula. ntenna Bandwidth The bandwidth of an antenna is the ange of fequencies ove which it is effective, usually centeed aound the opeating o esonant fequency. The bandwidth of an antenna may be inceased by seveal techniques, including using thicke wies, eplacing wies with cages to simulate a thicke wie, tapeing antenna components (like in a feed hon), and combining multiple antennas into a single assembly and allowing the natual impedance to select the coect antenna. Effective ea How a antenna etacts enegy fom incident wave and delives it to a load? ec λ D e inc bove is valid fo any antenna unde matched-load conditions Eample Detemine the diection of maimum iation, patten solid angle, diectivity and HBW in the y-z plane fo an antenna with nomalized iation intensity given by cos θ F( θ, 0 π fo 0 θ elsewhee π nswes (: 0,0),, 6, 90 3 and 0 φ π o 6

7 Fiis Tansmission Eq. In any communication link, thee is a tansmitting antenna and a eceive with a eceive antenna. TX isot t Gtisot t G tt λ t t ec t ec λ G tg t λ ( ) t t X ntenna ays Uses many antennas synchonized with each othe to incease atten multiplication, F atón Facto E() antena del individual eglo Nψ sin F N ψ N sin lana ays Unifom illumination Tschebyscheff Illumination T () F n M N sin ψ sin ψ y M ψ N ψ y sin sin 7