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Basic antenas
 

Basic antenas

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    Basic antenas Basic antenas Presentation Transcript

    • Key Points 1. Principals of EM Radiation 2. Introduction to Propagation & Antennas 3. Antenna Characterization 1
    • 1. Principals of Radiated electromagentic (EM) fields two laws (from Maxwell Equation) 1. A Moving Electric Field Creates a Magnetic (H) field 2. A Moving Magnetic Field Creates an Electric (E) field 2
    • An AC current i(t), flowing in a wire produces an EM field Assume i(t) applied at A with length l = λ/2 • EM wave will travel along the wire until it reaches the B • B is a point of high impedence  wave reflects toward A and is reflected back again • resistance gradually dissipates the energy of the wave • wave is reinforced at A  results in continuous oscillations of energy along the wire and a high voltage at the A end of the wire. A l = λ/2 B c ≈ 3 ×108m/s l = λ/2: wave will complete one cycle from A to B and back to A λ = distance a wave travels during 1 cycle f = c/λ = c/2l 3
    • Dipole antenna: 2 wires each with length l = λ/4 • attach ends to terminals of a high frequency AC generator • at time t, the generator’s right side = ‘+’ and the left side = ‘−’ − + B i(t) l = λ/4 current distribution at time t A − ------------------------------------------------------------------------ A + ++++ +++++++ +++++++++++ +++++++++++++++ +++++++++++++++ B voltage distribution at time t • electrons flow away from the ‘−’ terminal and towards the ‘+’ terminal • most current flows in the center and none flows at the ends • i(t) at any point will vary directly with v(t) ¼ cycle after electrons have begun to flow  max number of electrons will be at A and min number at B vmax(t) is developed i(t) = 0 4
    • Standing Wave • center of the antenna is at a low impedance: v(t) ≈ 0, imax(t) • ends of antenna are at high impedence: i(t) ≈ 0, vmax(t) • maximum movement of electrons is in the center of the antenna at all times  Resonance condition in the antenna • waves travel back and forth reinforcin • maximum EM waves are transmitted into at maximum radiation EM patterns on Dipole Antenna: • sinusoidal distribution of charge exists on the antenna that reverses polarity every ½ cycle • sinusoidal variation in charge magnitude lags the sinusoidal variation in current by ¼ cycle. • Electic field E and magnetic field H 90° out of phase with each other • fields add and produce a single EM field • total energy in the radiated wave is constant, except for some absorption • as the wave advances, the energy density decreases 5
    • POLARIZATION • EM field is composed of electric & magnetic lines of force that are orthogonal to each other • E determines the direction of polarization of the wave vertical polarization: electric force lines lie in a vertical direction horizontal polarization : electric force lines lie in a horizontal direction circular polarization: electric force lines rotate 360° every cycle An antenna extracts maximumenergy from a passing EM wave when it is oriented in the same direction as E • use vertical antenna for the efficient reception of vertically polarized waves • use horizontal antenna for the reception of horizontally polarized waves if E rotates as the wave travels through space  wave has. horizontal and vertical components 6
    • Ground wave transmissions missions at lower frequencies use vertical polarization • horizontal polarization E force lines are parallel to and touch the earth. earth acts as a fairly good conductor at low frequencies  shorts out • vertical electric lines of force are bothered very little by the earth. 7
    • 2. Introduction to Antennas and Propagation Types of antennas • simple antennas: dipole, long wire • complex antennas: additional components to shape radiated field provide high gain for long distances or weak signal reception size ≈ frequency of operation • combinations of identical antennas phased arrays electrically shape and steer antenna transmit antenna: radiate maximum energy into surroundings receive antenna: capture maximum energy from surrounding • radiating transmission line is technically an antenna • good transmission line = poor antenna 8
    • Transition from guided wave to free space wave 9
    • Major Difference Between Antennas And Transmission Lines • transmission line uses conductor to carry voltage & current • radio signal travels through air (insulator) • antennas are transducers - convert voltage & current into electric & magnetic field - bridges transmission line & air - similar to speaker/microphone with acoustic energy Transmission Line • voltage & current variations  produce EM field around conductor • EM field expands & contracts at same frequency as variations • EM field contractions return energy to the source (conductor) • Nearly all the energy in the transmission line remains in the system 10
    • Antenna • Designed to Prevent most of the Energy from returning to Conductor • Specific Dimensions & EM wavelengths cause field to radiate several λ before the Cycle Reversal - Cycle Reversal - Field Collapses  Energy returns to Conductor - Produces 3-Dimensional EM field - Electric Field ⊥ Magnetic Field - Wave Energy Propagation ⊥ Electric Field & Magnetic Field 11
    • Antenna Performance depends heavily on • Channel Characteristics: obstacles, distances temperature,… • Signal Frequency • Antenna Dimensions transmit & receive antennas theoretically are the same (e.g. radiation fields, antenna gain) practical implementation issue: transmit antenna handles high power signal (W-MW) - large conductors & high power connectors, receive antenna handles low power signal (mW-uW) 12
    • Propagation Modes – five types (1) Ground or Surface wave: follow earths contour • affected by natural and man-made terrain • salt water forms low loss path • several hundred mile range • 2-3 MHz signal htx (2) Space Wave • Line of Sight (LOS) wave • Ground Diffraction allows for greater distance • Approximate Maximum Distance, D in miles is hrx D = 2htx + 2hrx (antenna height in ft) • No Strict Signal Frequency Limitations 13
    • (3) Sky Waves • reflected off ionosphere (20-250 miles high) • large ranges possible with single hop or multi-hop • transmit angle affects distance, coverage, refracted energy refracted wave ionosphere reflected transmitted wave wave skip distance 14
    • Ionosphere • is a layer of partially ionized gasses below troposphere - ionization caused by ultra-violet radiation from the sun - affected by: available sunlight, season, weather, terrain - free ions & electrons reflect radiated energy • consists of several ionized layers with varying ion density - each layer has a central region of dense ionization Layer D E F1 F2 altitude (miles) Frequency Availability Range 20-25 several MHz day only 55-90 20MHz day, partially at night 90-140 30MHz 24 hours 200-250 30MHz 24 hours F1 & F2 separate during daylight, merge at night 15
    • Usable Frequency and Angles Critical Frequency: frequency that won’t reflect vertical transmission - critical frequency is relative to each layer of ionosphere - as frequency increases  eventually signal will not reflect Maximum Usable Frequency (MUF): highest frequency useful for reflected transmissions - absorption by ionosphere decreases at higher frequencies - absorption of signal energy = signal loss - best results when MUF is used Frequency Trade-Off • high frequency signals eventually will not reflect back to ground • lower frequency signals are attenuated more in the ionosphere 16
    • Critical Angle angle of radiation: transmitted energy relative to surface tangent - smaller angle requires less ionospheric refraction to return to earth - too large an angle results in no reflection - 3o-60o are common angles critical angle: maximum angle of radiation that will reflect energy to earth Determination of minimum skip distance: - critical angle - small critical angle  long skip distance - height of ionosphere - higher layers give longer skip distances for a fixed angle multipath: signal takes different paths to the destination ionosphere angle of radiation 17
    • (4) Satellite Waves Designed to pass through ionosphere into space • uplink (ground to space) • down link (space to ground) • LOS link Frequencies >> critical frequency • penetrates ionosphere without reflection • high frequencies provide bandwidth Geosynchronous orbit ≈ 23k miles (synchronized with earth’s orbit) • long distances  result in high path loss • EM energy disperses over distances • intensely focused beam improves efficiency 18
    • Free Space Path Loss equation used to determine signal levels over distance 2 Pt  4πfd  =  Pr  c   4πfd  20 log10    c  (dB) G = antenna gain: projection of energy in specific direction • can magnify transmit power • increase effective signal level at receiver total loss = Gt + Gr – path loss (dB) 19
    • (5) radar: requires • high gain antenna • sensitive low noise receiver • requires reflected signal from object – distances are doubled • only small fraction of transmitted signal reflects back 20
    • 3. Antenna Characterization antennas generate EM field pattern • not always possible to model mathematically • difficult to account for obstacles • antennas are studied in EM isolated rooms to extract key performance characteristics antenna design & relative signal intensity determines relative field pattern absolute value of signal intensity varies for given antenna design - at the transmitter this is related to power applied at transmitter - at the receiver this is related to power in surrounding space 21
    • Polar Plot of relative signal strength of radiated field • shows how field strength is shaped • generally 0o aligned with major physical axis of antenna • most plots are relative scale (dB) - maximum signal strength location is 0 dB reference - closer to center represents weaker signals 90 o forward gain = 10dB backward gain = 7dB 0o 180o 270o +10dB +7dB + 4dB 22
    • radiated field shaping ≈ lens & visible light • application determines required direction & focus of signal • antenna characteristics (i) radiation field pattern (ii) gain (iii) lobes, beamwidth, nulls (iv) directivity (i) antenna field pattern = general shape of signal intensity in far-field far-field measurements measured many wavelengths away from antenna near-field measurement involves complex interactions of decaying electrical and magnetic fields - many details of antenna construction 23
    • Measuring Antenna Field Pattern field strength meter used to measure field pattern • indicates amplitude of received signal • calibrated to receiving antenna • relationship between meter and receive antenna known measured strength in uV/meter received power is in uW/meter • directly indicates EM field strength 24
    • Determination of overall Antenna Field Pattern form Radiation Polar Plot Pattern • use nominal field strength value (e.g. 100uV/m) • measure points for 360o around antenna • record distance & angle from antenna • connect points of equal field strength practically • distance between meter & antenna kept constant 90o • antenna is rotated • plot of field strength versus angle is made 0o 180o 270o 100 uV/m 25
    • Why Shape the Antenna Field Pattern ? • transmit antennas: produce higher effective power in direction of intended receiver • receive antennas: concentrate energy collecting ability in direction of transmitter - reduced noise levels - receiver only picks up intended signal • avoid unwanted receivers (multiple access interference = MAI): - security - multi-access systems • locate target direction & distance – e.g. radar not always necessary to shape field pattern, standard broadcast is often omnidirectional - 360o 26
    • (ii) Antenna Gain Gain is Measured Specific to a Reference Antenna • isotropic antenna often used - gain over isotropic - isotropic antenna – radiates power ideally in all directions - gain measured in dBi - test antenna’s field strength relative to reference isotropic antenna - at same power, distance, and angle - isotropic antenna cannot be practically realized • ½ wave dipole often used as reference antenna - easy to build - simple field pattern 27
    • Antenna Gain ≠ Amplifier Gain • antenna power output = power input – transmission line loss • antenna shapes radiated field pattern • power measured at a point is greater/less than that using reference antenna • total power output doesn’t increase • power output in given direction increases/decreases relative to reference antenna e.g. a lamp is similar to an isotropic antenna a lens is similar to a directional antenna - provides a gain/loss of visible light in a specific direction - doesn’t change actual power radiated by lamp 28
    • • transmit antenna with 6dB gain in specific direction over isotropic antenna  4× transmit power in that direction • receive antenna with 3dB gain is some direction receives 2× as much power than reference antenna Antenna Gain often a cost effective means to (i) increase effective transmit power (ii) effectively improve receiver sensitivity may be only technically viable means • more power may not be available (batteries) • front end noise determines maximum receiver sensitivity Rotational Antennas can vary direction of antenna gain Directional Antennas focus antenna gain in primary direction 29
    • (iii) Beamwidth, Lobes & Nulls Lobe: area of high signal strength - main lobe - secondary lobes Nulls: area of very low signal strength Beamwidth: total angle where relative signal power is 3dB below peak value of main lobe - can range from 1o to 360o Beamwidth & Lobes indicate sharpness of pattern focus 90o beam 0 width 180 o o null 270o 30
    • Center Frequency = optimum operating frequency Antenna Bandwidth ≡ -3dB points of antenna performance Bandwidth Ratio: Bandwidth/Center Frequency e.g. fc = 100MHz with 10MHz bandwidth - radiated power at 95MHz & 105MHz = ½ radiated power at fc - bandwidth ratio = 10/100 = 10% 31
    • Antenna Design Basics Main Trade-offs for Antenna Design • directivity & beam width • acceptable lobes • maximum gain • bandwidth • radiation angle Bandwidth Issues High Bandwidth Antennas tend to have less gain than narrowband antennas Narrowband Receive Antenna reduces interference from adjacent signals & reduce received noise power Antenna Dimensions • operating frequencies determine physical size of antenna elements • design often uses λ as a variable (e.g. 1.5 λ length, 0.25 λ spacing) 32
    • Testing & Adjusting Transmitter  use antenna’s electrical load • Testing required for - proper modulation - amplifier operation - frequency accuracy • using actual antenna may cause significant interference • dummy antenna used for transmitter design (not antenna design) - same impedance & electrical characteristics - dissipates energy vs radiate energy - isolates antenna from problem of testing transmitter 33
    • Testing Receiver • test & adjust receiver and transmission line without antenna • use single known signal from RF generator • follow on test with several signals present • verify receiver operation first  then connect antenna to verify antenna operation Polarization • EM field has specific orientation of E-field & M field • Polarization Direction determined by antenna & physical orientation • Classification of E-field polarization - horizontal polarization : E-field parallel to horizon - vertical polarization: E-field vertical to horizon - circular polarization: constantly rotating 34
    • Transmit & Receive Antenna must have same Polarization for maximum signal energy induction • if polarizations aren’t same  E-field of radiated signal will try to induce E-field into wire ⊥ to correct orientation - theoretically no induced voltage - practically – small amount of induced voltage Circular Polarization • compatible with any polarization field from horizontal to vertical • maximum gain is 3dB less than correctly oriented horizontal or vertically polarized antenna 35
    • Antenna Fundamentals Dipole Antennas (Hertz): simple, old, widely used - root of many advance antennas • consists of 2 spread conductors of 2 wire transmission lines ½λ • each conductor is ¼ λ in length • total span = ½ λ + small center gap ¼λ ¼λ Distinct voltage & current patterns driven by transmission line at midpoint • i = 0 at end, maximum at midpoint • v = 0 at midpoint, ±vmax at ends +v • purely resistive impedance = 73Ω • easily matched to many transmission lines Transmission Line gap i -v High Impedance 2k-3kΩ Low Impedance 73Ω 36
    • E-field (E) & M-field (B) used to determine radiation pattern • E goes through antenna ends & spreads out in increasing loops • B is a series of concentric circles centered at midpoint gap E B 37
    • 3-dimensional field pattern is donut shaped antenna is shaft through donut center radiation pattern determined by taking slice of donut - if antenna is horizontal  slice reveals figure 8 - maximum radiation is broadside to antenna’s arms Azimuth Pattern Polar Radiation Pattern Elevation Pattern 38
    • ½λ dipole performance – isotropic reference antenna • in free space  beamwidth = 78o • maximum gain = 2.1dB • dipole often used as reference antenna - feed same signal power through ½ λ dipole & test antenna - compare field strength in all directions Actual Construction (i) propagation velocity in wire < propagation velocity in air (ii) fields have ‘fringe effects’ at end of antenna arms - affected by capacitance of antenna elements 1st estimate: make real length 5% less than ideal - otherwise introduce reactive parameter Useful Bandwidth: 5%-15% of fc • major factor for determining bandwidth is diameter of conductor • smaller diameter  narrow bandwidth 39
    • Multi-Band Dipole Antennas use 1 antenna  support several widely separated frequency bands e.g. HAM Radio - 3.75MHz-29MHz Traps: L,C elements inserted into dipole arms • arms appear to have different lengths at different frequencies • traps must be suitable for outdoor use • 2ndry affects of trap impact effective dipole arm length-adjustable • not useful over 30MHz λ2/4 L λ2/4 L C λ1/4 λ1/4 C Transmission Line 40
    • Transmit Receive Switches • allows use of single antenna for transmit & receive • alternately connects antenna to transmitter & receiver • high transmit power must be isolated from high gain receiver • isolation measured in dB e.g. 100dB isolation 10W transmit signal ≈ 10nW receive signal 41
    • Elementary Antennas low cost – flexible solutions Long Wire Antenna • effective wideband antenna • length l = several wavelengths - used for signals with 0.1l < λ < 0.5l - frequency span = 5:1 Transmission Line n en nt A a R=Z0 earth ground • drawback for band limited systems - unavoidable interference • near end driven by ungrounded transmitter output • far end terminated by resistor - typically several hundred Ω - impedance matched to antenna Z0 • transmitter electrical circuit ground connected to earth 42
    • practically - long wire is a lossy transmission line - terminating resistor prevent standing waves Polar radiation pattern • 2 main lobes - on either side of antenna - pointed towards antenna termination • smaller lobes on each side of antenna – pointing forward & back • radiation angle 45o (depending on height)  useful for sky waves feed horizon polar ration pattern angular radiation pattern 43
    • poor efficiency: transmit power - 50% of transmit power radiated - 50% dissapated in termination resistor receive power - 50% captured EM energy converted to signal for reciever - 50% absorbed by terminating resistor 44
    • Folded Dipole Antenna - basic ½λ dipole folded to form complete circuit - core to many advanced antennas λ/2 - mechanically more rugged than dipole - 10% more bandwidth than dipole - input impedance ≈ 292 Ω - close match to std 300Ω twin lead wire transmission line - use of different diameter upper & lower arms  allows variable impedance 45
    • Loop & Patch Antenna – wire bent into loops Patch Antenna: rectangular conducting area with || ground plane V = k(2πf)BAN V = maximum voltage induced in receiver by EM field B = magnetic field strength flux of EM field N-turns A = area of loop N = number of turns f = signal frequency k = physical proportionality factor Area A Antenna Plane 46
    • Radiation Pattern • maximum ⊥ to center axis through loop • very low broadside to the loop • useful for direction finding - rotate loop until signal null (minimum) observed - transmitter is on either side of loop - intersection with 2nd reading pinpoints transmitter • Loop & Patch Antennas are easy to embed in a product (e.g. pager) • Broadband antenna - 500k-1600k Hz bandwidth • Not as efficient as larger antennas 47
    • Name Isotropic Shape Gain (over Beamwidth isotropic) -3 dB 0 dB 360 2.14 dB 55 Turnstile -0.86 dB 50 Full Wave Loop 3.14 dB 200 Yagi 7.14 dB 25 Helical 10.1 dB 30 Parabolic Dipole 14.7 dB 20 Horn 15 dB 15 Biconical Horn 14 dB Radiation Pattern 360x200 Dipole 48