Dr. S. X-ol What is an antenna? Introduction to ntennas n antenna is a passive structurethat serves as transition between a transmission lineand airused to transmit and/or receive electromagnetic waves. Dr. Sandra Cruz-ol Electrical and Computer Engineering University of uerto ico at Mayaguez Source Tx eceiver Circuit x ntenna Types of antennas Can be divided into two groups Wireantennas: dipoles, loops, Yagi-Uda pertureantennas: parabolic, horns, microstrip antennas http://www.kyes.com/antenna/antennatypes/antennatypes.html Ulaby, 1999 http://en.wikipedia.org/wiki/ntenna_(electronics)#overview Wire antennas Wire antennas Log periodic Yagi Log periodic Yagi Yagi-Uda with reflector Intro to ntennas 1
Dr. S. X-ol perture antennas eflector and yramidal horn antennas Dipole with parabolic and corner reflector Spherical (main reflector) with Gregorian feed Outline ntenna parameters Solid angle, Ω Α and adiation intensity, U adiation pattern, n, sidelobes, HBW Far field zone, r ff Directivity, D or Gain, G ntenna iation impedance, Effective rea, e ll of these parameters are expressed in terms of a transmission antenna, but are identically applicable to a receivingantenna. We ll also study: Friis Transmission Equation adar Equation Spherical coordinates φ azimuth θ elevation x θ90 φ0 z (zenith) φ θ θ0 θ90 φ90 y Solid ngle adiation Intensity Is the power density per solid angle: s 1 r dθ s r sin θ dø s θr arco d s 1 s d r sin θ dø dθ r dω θ ángulo plano dω elemento de ángulo sólido El arco total en un círculo: El área total en una esfera: πr r ngulo total: π [ianes] ngulo sólido total: [ ] [sr] 1 steian (sr) (1 ian) U where r [W/sr] r r ½ e{e H*}rˆ [W/m ] is the power density also known as oynting vector. Intro to ntennas
Dr. S. X-ol Total iated power by antenna Can be calculated as; or U dω [W] ds [W] r adiation pattern is the 3D plot of the gain, but usually the D horizontal and verticalcross sections of the iation pattern are considered. efers to the variation of the relative amplitude of the iation as a function of direction. adiation attern Field pattern: E( θ, E n ( θ, E ( θ, ower pattern: ( θ, U ( θ, F n ( θ, ( θ, U ( θ, Where U is the iation intensity to be defined later. Total Solid ngle of an antenna Isotropic antenna Ω ( θ, dω F n [sr] z Ωž It s an hypothetic antenna, i.e., it does not exist in real life, yet it s used as a measuring bar for real antenna characteristics. Is as if you changed the iation pattern beam of an antenna into a pencil beam shape and find out what s the equivalent solid angle occupied by this pattern. atrón n x y It s a point source that occupies a negligible space. Has no directional preference. Its pattern is simply a sphereso it has Ω Ω isotropic [steians]. Ωisotropic (1) dω π π (1)sin θ dθ dφ θ 0 φ 0 adiation attern attern polar plot Whenever we speak of iation patterns, we normally mean we are at a distance far enough from the antenna known as the far field. En _ 1 -.7 -.5 atrón normalizado - 0 db -3dB -10dB ø HBW HBW atrón de Campo (Escala lineal) atrón de campo o de potencia (Escala logarítmica) COODENDS ECTNGULES Note that when plotted in decibels, the power and field patterns look exactly the same. n 1 Lóbulo principal HBW ("Mainlobe").5 NNBW } Lóbulos menores TON TIICO (Coordenadas polares esféricas, dimensiones) Intro to ntennas 3
Dr. S. X-ol Dipole antenna pattern Sidelobes ntennas sometimes show side lobesin the iation pattern. Side lobes are peaks in gain other than the main lobe (the "beam"). Side lobes have bad impact to the antenna quality whenever the system is being used to determine the directionof a signal, for example in Dsystems. Note the iation pattern is donut shaped. Sidelobes of dipole arrays ntenna attern with sidelobes sidelobe Many applications require sidelobe levels (SLL) to be below -0dB. Gain or Directivity Directivity and Gain ll practical antennas iate more than the isotropic antenna in some directions and less in others. Gain is inherently directional; the gain of an antenna is usually measured in the direction which it iates best. n isotropic antenna and a practical antenna fed with the same power. Their patterns would compare as in the figure on the right. D D ( θ, U / If lossless antenna, GD ave /U ave Intro to ntennas 4
Dr. S. X-ol Gain or Directivity Gain is measured by comparing an antenna to a model antenna, typically the isotropic antennawhich iates equally in all directions. ( θ, r ( θ, D( θ, / VE 1 d U Do / Ω Ω isotropic / Ω Directivity For an antenna with a single main lobe pointing in the z- direction, Ω can be approximated to the product of the HBW Ω β β then xz yz The Directivity: π D 4 / Ω β β xz yz Far field The distance at which the fields transmitted by an antenna (spherical) can be approximated to plane waves. It s defined as r ff D / λ D is the largest physical dimension of the antenna λ wavelength of operation r ff distance from the antenna to the observation point Beamwidth, HBW Is the distance in ians o degrees between the direction of the iation pattern where the iated power is half of the imum. Can be found by solving F n (θ,.5 10 log 0. 5 0 log 0. 707 for "pencil beam" shape; HBM 70-3 db o λ -3 db D ntenna Impedance n antenna is seen" by the generator as a load with impedance Z, connected to the line. Z Z ( + L ) + jx The real part is the iation resistance plus the ohmic resistance. Minimizing impedance differences at each interface will reduce SWand imize power transfer through each part of the antenna system. Compleximpedance, Z,of an antenna is related to the electrical 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 transformer. More commonly, the impedance is adjusted at the load (see below) with an antenna tuner, a balun, a matching transformer, matching networks composed of inductorsand capacitors, or matching sections such as the gamma match. ntenna efficiency, η Efficiencyis the ratio of power put into the antenna terminals to the power actually iated adiation in an antenna is caused by iation resistancewhich can only be measured as part of total resistanceincluding loss resistance. η in G η D Intro to ntennas 5
Dr. S. X-ol adiation esistance The antenna is connected to a T.L., and it sees it as an impedance. The power iated is The loss power is loss 1 1 I o I o L adar equation What is a ar? eceived power by a ar is ( ) o λo τ σ 3 4 G t r e Where σis the backscattering coefficient of the target [m ] η + loss + loss LICTIONS pplication to several research projects: CS, NS-F, NS-TCESS Show results from undergs working in NS and NSF projects elation to G students ntenna polarization The polarization of an antenna is the polarization of the signals it emits. The ionosphere changes the polarization of signals unpredictably, so for signals which will be reflected by the ionosphere, polarization is not crucial. However, for line-of-sight communications, it can make a tremendous difference in signal quality to have the transmitter and receiver using the same polarization. olarizations commonly considered are vertical, horizontal, and circular. ntenna Bandwidth The bandwidth of an antenna is the range of frequencies over which it is effective, usually centered around the operating or resonant frequency. The bandwidth of an antenna may be increased by several techniques, including using thicker wires, replacing wires with cages to simulate a thicker wire, tapering antenna components (like in a feed horn), and combining multiple antennas into a single assembly and allowing the natural impedance to select the correct antenna. Effective rea How a x antenna extracts energy from incident wave and delivers it to a load? rec D e inc bove is valid for any antenna under matchedload conditions λ Intro to ntennas 6
Dr. S. X-ol Friis Transmission Eq. Example In any communication link, there is a transmitting antenna and a receiver with a receiver antenna. adar and Friis TX isotr t tx Gt isotr G tt λ t t rec rt rec λ t r t G tg rt λ ( ) X ntenna rrays Uses many antennas synchronized with each other to increase attern multiplication Example Determine the direction of imum iation, pattern solid angle, directivity and HBW in the y-zplane for an antenna with normalized iation intensity given by cos θ F( θ, 0 π for 0 θ elsewhere and 0 φ π Intro to ntennas 7