VIEWS: 7 PAGES: 58 POSTED ON: 10/14/2011
IEEE EMC Symposium, 2010 Introduction to Antennas Presented by Vince Rodriguez, Ph.D. 1 IEEE EMC Symposium, 2010 Outline I • What is an antenna • How do they radiate • Radiation pattern – What is it – E and H plane – Far and near field – Omnidirectional/Directional 2 IEEE EMC Symposium, 2010 Outline II • Radiation pattern (cont’) – Isotropic – Main, side and back lobes. – Half power and 3dB beamwidth – Side lobe level – Directivity and gain 3 IEEE EMC Symposium, 2010 Outline III • Antenna Factor • Input impedance • S parameters and VSWR • Polarization 4 IEEE EMC Symposium, 2010 What is an antenna? “An antenna is a device that provides a means for radiating and receiving radio waves” Book definition 5 IEEE EMC Symposium, 2010 What is an antenna? Radiated energy Guided energy 6 IEEE EMC Symposium, 2010 How do antennas radiate? Electromagnetic energy does not like to go from the guided medium to the free space medium 7 IEEE EMC Symposium, 2010 How do antennas radiate? Guided waves in the cable Free Space waves 8 IEEE EMC Symposium, 2010 How do antennas radiate? A good antenna will ease all the guided energy that it receives in to a free space wave ANTENNA Guided waves in the cable Free Space waves 9 IEEE EMC Symposium, 2010 How do antennas radiate? 10 IEEE EMC Symposium, 2010 How do antennas radiate? So, one way of accomplishing radiation is to smoothly change from the transmission line (cable) environment to an environment. open space environment The antennas will receive in the same way that they radiate. 11 IEEE EMC Symposium, 2010 How do antennas radiate? As the currents in the antenna change direction the wave propagates outward as is the case when we shake a rope 12 IEEE EMC Symposium, 2010 Radiation Pattern “a 3D plot that displays the strength of the radiated fields or power density as a function of Book definition direction” 13 IEEE EMC Symposium, 2010 Radiation Pattern The radiation is then a representation of how much Electromagnetic energy is concentrated in each direction around the antenna 14 IEEE EMC Symposium, 2010 Radiation Pattern Because of the difficulty of plotting a 3D plot usually the patterns are shown as E and H planes 90 0 120 60 -10 150 30 -20 -30 30 -40 180 0 -30 -20 210 330 -10 240 300 0 Eplane18GHz 270 Hplane18GHz 15 IEEE EMC Symposium, 2010 Radiation Pattern: E and H Plane The E plane is the plane that is parallel to the Electric field The l is the l Th H plane i th plane th t that is parallel to the Magnetic field The Electric and Magnetic fields are perpendicular to each other 16 IEEE EMC Symposium, 2010 Radiation Pattern: Near and Far Field To put it on simple terms, the near field has spherical waves and standing wave behaviors The far field the sphere is large that it resembles a plane and the wave is traveling 17 IEEE EMC Symposium, 2010 Radiation Pattern: Omnidirectional and Directional OMNI = Latin for Every or All So, Omnidirectional radiates in “every” direction? 18 IEEE EMC Symposium, 2010 Radiation Pattern: Omnidirectional and Directional No, Omnidirectional radiates in every direction on one of the principal planes A dipole antenna 19 IEEE EMC Symposium, 2010 Radiation Pattern: Omnidirectional and Directional Omni directional on the H plane. It radiates equally on all directions on this plane But not on this plane 20 IEEE EMC Symposium, 2010 Radiation Pattern: Omnidirectional and Directional Can you radiate equally on all directions Isotropic, Greek, isos meaning same, tropos meaning direction, ΑΡΗΣΤΟΤΕΛΗΣ 21 IEEE EMC Symposium, 2010 Radiation Pattern: Omnidirectional and Directional There is no such thing in real life. An isotropic radiator is a mathematical concept, an idea that belongs in the world of ideas like my professor, Plato, would say. p It is however use as a comparison to determine the Directive Gain of a real antenna ΑΡΗΣΤΟΤΕΛΗΣ 22 IEEE EMC Symposium, 2010 Radiation Pattern: Omnidirectional and Directional Directional?, well that is plain English, The antenna radiates mainly in one direction. Lets look again at the beautiful EMCO 3117 90 0 120 60 -10 150 30 -20 -30 Radiation is -40 180 0 mainly in this direction -30 -20 210 330 -10 240 300 0 Eplane18GHz 270 Hplane18GHz 23 IEEE EMC Symposium, 2010 Radiation Pattern: Main, Side and Back 90 0 Side 120 60 Lobes: -10 smaller than the -20 150 30 main lobe -30 Is a side Main lobe that Lobe: -40 180 happens 0 Is the to be on strongest -30 the in level. opposite -20 direction 210 330 than the -10 main lobe 240 300 0 270 24 IEEE EMC Symposium, 2010 Radiation Pattern: Half Power Beamwidth 90 Half power 0 120 60 1 Half = = 0.5 -10 2 150 30 In decibels -20 ⎛1⎞ 10 × log10 ⎜ ⎟ = −3.02 ≈ −3dB 30 -30 ⎝2⎠ -40 180 0 -3dB= half power -30 -20 210 330 -10 240 300 0 270 25 IEEE EMC Symposium, 2010 Radiation Pattern: Half Power Beamwidth Let’s represent the pattern in Cartesian coordinates, for clarity C omput e d pat t er n 18 GH z 3 1 17 0 90 -3 0 120 60 -6 -10 -9 150 30 -12 -20 -15 -30 -18 -21 -40 180 0 -24 -30 -27 -30 -20 210 330 -33 -10 -36 240 300 -39 18G Hz 3117 H plane 0 270 -42 -180 -150 -120 -90 -60 -30 0 30 60 90 120 150 180 P h i (d e g r e e s) 26 IEEE EMC Symposium, 2010 Radiation Pattern: Half Power Beamwidth C omput e d pa t t e r n 1 8 GH z 3 1 1 7 -3dB 0 Half -3 -6 power -9 -12 -15 -18 21 -21 -24 -27 -30 -33 -36 -39 1 1 8G Hz 31 7 H plane -42 -180 -150 -120 -90 -60 -30 0 30 60 90 120 150 180 P h i (d e g r e e s) About 25 degrees 27 IEEE EMC Symposium, 2010 Radiation Pattern: Sidelobe Level Comput ed pa t t er n 1 8GHz 3 11 7 0 The power -3 level at the -6 -9 strongest -12 side-lobe in -15 relation to the -18 -21 level of the -24 main lobe -27 -30 -33 In this case -36 15dB -39 18GHz 3117 H plane -42 -180 -150 -120 -90 -60 -30 0 30 60 90 120 150 180 P h i (d e g re e s) 28 IEEE EMC Symposium, 2010 Radiation Pattern: Front to Back Ratio Similar to the Comput ed pa t t er n 1 8GHz 3 11 7 side lobe level, but it 0 -3 applies to -6 those especial -9 side lobes, the -12 -15 back lobes b k l b or -18 to the -21 radiation level -24 -27 opposite the -30 main lobe -33 -36 -39 18GHz 3117 H plane -42 -180 -150 -120 -90 -60 -30 0 30 60 90 120 150 180 P h i (d e g re e s) 29 IEEE EMC Symposium, 2010 Radiation Pattern: Front to Back Ratio 90 It may be 0 120 60 easier to see -10 in polar coordinate 150 30 -20 representation -30 -40 180 0 -30 -20 210 330 -10 240 300 0 270 30 IEEE EMC Symposium, 2010 Radiation Pattern: Directivity and Gain General Gain definition Power in Power out Powerout Gain = Powerin 31 IEEE EMC Symposium, 2010 Gain(IEEE) Power out= Total radiated Power in power Pr Gain = Pin 32 IEEE EMC Symposium, 2010 Traditional Gain(or directive gain or directivity) WHAT?? Fmax D= average over sphere of F (θ , ϕ ) maximum radiated power D= total radiated power surface of a sphere 33 IEEE EMC Symposium, 2010 Traditional Gain(or directive gain or directivity) 90 0 120 60 -10 Oh! OK. 150 30 -20 -30 -40 180 0 -30 Maximum radiation -20 210 330 density Total radiated power add the -10 radiation on every direction 240 0 and divided it by300 the 270 π spherical surface 4π radians 34 IEEE EMC Symposium, 2010 Traditional Gain vs. IEEE Gain IEEE gain takes in to account the losses in the antenna since it looks at how much power is radiated versus how much power goes in. Traditional gain looks at how Directive the antenna is how much power we receive (or transmit) in one direction versus the total radiated power. It is a measure of how the antenna concentrates the radiation. 35 IEEE EMC Symposium, 2010 Antenna Factor Antenna factor – an antenna sensor lib ti hi h calibration which permitsit measuring an unknown electric field strength. Antenna factors are commonly expressed in terms of dB (1/m). Book definition 36 IEEE EMC Symposium, 2010 Antenna Factor The receiver, (spectrum analyzer, The antenna is RF voltmeter, vector receiving an Electric voltmeter, etc) field unbounded measures signal wave that is in V/m, level in volts, it (Volts per meter) measures potential 37 IEEE EMC Symposium, 2010 Antenna Factor • The AF is the electric field at a given distance divided by the voltage measured at the antenna input. It is a value that relates the voltage at the antenna input to the field seen by the antenna r E 38 IEEE EMC Symposium, 2010 Antenna Factor Recall that the Electric and magnetic field can be related to each other by the impedance of free space ηo = 120π Ω ≈ 377 Ω 39 IEEE EMC Symposium, 2010 Antenna Factor • Antenna factor – for magnetic antennas the magnetic antenna factor (MAF) is measured. It relates the magnetic field to the voltage at the antenna input. • Because we can relate the E and the H, we can relate the AF to the MAF r H 40 IEEE EMC Symposium, 2010 Input Parameters At the input port of the antenna we can Measured a set of parameters that are related to How good the antenna is. These are measured without looking at the radiated power 41 IEEE EMC Symposium, 2010 Input Parameters 1. S11 parameter 2. The VSWR 3. The input impedance 42 IEEE EMC Symposium, 2010 VSWR and S11 When entering a different medium the waves bounces back Pref S11 = Piin V peak VSWR = Vvalley 43 IEEE EMC Symposium, 2010 VSWR and S11 VSWR and S11 These two are related to the amount of energy that does not go in to the antenna but it is reflected back to the generator 100% of power in to antenna 0% of power in to antenna 64% of power in to antenna 44 IEEE EMC Symposium, 2010 VSWR and S11 VSWR and S11 These two are related to the amount of energy that does not go in to the antenna but it is reflected back to the generator S11=<-50 S11=0dB S11=-4.6 VSWR=1:1 VSWR=>30 VSWR=4:1 45 IEEE EMC Symposium, 2010 Input Impedance The reason that the energy bounces back at the input to the antenna is that the impedance of the antenna is not the same as the one of the cable the slight mismatch causes part of the wave to bounce and part to travel 46 IEEE EMC Symposium, 2010 Input Impedance Connector 50 ohm impedance Input impedance is the impedance that can be measured at the input of the antenna. Since most equipment uses 50 ohm cables and 50 ohm receivers we want our antenna input impedance to be as close as possible to 50 ohms NOTE: the antenna connector may be 50 ohms, but what is behind the connector is what matters when talking about the input impedance ridges ??ohm impedance 47 IEEE EMC Symposium, 2010 Polarization When a propagating wave ill i l l oscillates on a single plane then it is called a linear polarized wave 48 IEEE EMC Symposium, 2010 Polarization A linearly polarized wave will radiate linearly polarized waves 49 IEEE EMC Symposium, 2010 Polarization An antenna that can generate two linearly polarized waves simultaneously at the same time is called a dual polarized antenna 50 IEEE EMC Symposium, 2010 Polarization When a linearly polarized antenna is set in a horizontal position it produces a horizontal polarized wave 51 IEEE EMC Symposium, 2010 Polarization When a linearly polarized antenna is set in a vertical position it produces a vertical polarized wave 52 IEEE EMC Symposium, 2010 Polarization A circular or elliptical polarized wave the plane of oscillation rotates as the wave propagates 53 IEEE EMC Symposium, 2010 Polarization An antenna that radiates a circular polarized wave is called a circularly polarized antenna This is one of our conical log spiral antennas. They can be manufactured to be left or right hand circularly polarized. 54 IEEE EMC Symposium, 2010 Polarization Left Hand Circular Polarization 55 IEEE EMC Symposium, 2010 Polarization Right hand circular polarization 56 IEEE EMC Symposium, 2010 Polarization Transmits and receives 100% of linear polarization that is co-polarized Receives ½ of circular polarized waves Transmits and receives 100% of linear polarization that is co-polarized Receives ½ of circular polarized waves Transmits and receives 100% of linear polarization and 100% of co-polarized circular polarized wave Receives 0% of cross polarized circular polarized waves 57 IEEE EMC Symposium, 2010 Questions? 58