3 basic antenas1

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3 basic antenas1

  1. 1. 1 Key Points 1. Principals of EM Radiation 2. Introduction to Propagation & Antennas 3. Antenna Characterization
  2. 2. 2 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
  3. 3. 3 c ≈ 3 ×108 m/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 l = λ/2 A B 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. An AC current i(t), flowing in a wire produces an EM field
  4. 4. 4 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 = ‘−’ • 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) current distribution at time t − + i(t) l = λ/4 A B − + ++++ +++++++ +++++++++++ +++++++++++++++ +++++++++++++++ ----- ----------- ---------------- -------------------- ------------------------ voltage distribution at time t A B 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
  5. 5. 5 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 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
  6. 6. 6 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
  7. 7. 7 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.
  8. 8. 8 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 2. Introduction to Antennas and Propagation 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
  9. 9. 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. 10. 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. 11. 11 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) Antenna Performance depends heavily on • Channel Characteristics: obstacles, distances temperature,… • Signal Frequency • Antenna Dimensions
  12. 12. 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 (2) Space Wave • Line of Sight (LOS) wave • Ground Diffraction allows for greater distance • Approximate Maximum Distance, D in miles is (antenna height in ft) • No Strict Signal Frequency Limitations rxtx hh 22 +D = hrx htx
  13. 13. 13 (3) Sky Waves ionosphere transmitted wave reflected wave refracted wave skip distance • reflected off ionosphere (20-250 miles high) • large ranges possible with single hop or multi-hop • transmit angle affects distance, coverage, refracted energy
  14. 14. 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 altitude (miles) Frequency Range Availability D 20-25 several MHz day only E 55-90 20MHz day, partially at night F1 90-140 30MHz 24 hours F2 200-250 30MHz 24 hours F1 & F2 separate during daylight, merge at night
  15. 15. 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. 16. 16 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 angle of radiation ionosphere Critical Angle
  17. 17. 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. 18. 18 total loss = Gt + Gr – path loss (dB) Free Space Path Loss equation used to determine signal levels over distance G = antenna gain: projection of energy in specific direction • can magnify transmit power • increase effective signal level at receiver 2 4       = c fd P P r t π       c fdπ4 log20 10 (dB)
  19. 19. 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. 20. 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 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 antenna design & relative signal intensity determines relative field pattern
  21. 21. 21 forward gain = 10dB backward gain = 7dB +10dB +7dB + 4dB 0o 270o 180o 90o 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
  22. 22. 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 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 (i) antenna field pattern = general shape of signal intensity in far-field
  23. 23. 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. 24. 24 0o 270o 180o 90o 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 100 uV/m practically • distance between meter & antenna kept constant • antenna is rotated • plot of field strength versus angle is made
  25. 25. 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. 26. 26 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 (ii) Antenna Gain
  27. 27. 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. 28. 28 Rotational Antennas can vary direction of antenna gain Directional Antennas focus antenna gain in primary direction • 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
  29. 29. 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 0o 270o 180o 90o beam width null
  30. 30. 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. 31. 31 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 Design Basics 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. 32. 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. 33. 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. 34. 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. 35. 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 • each conductor is ¼ λ in length • total span = ½ λ + small center gap Distinct voltage & current patterns driven by transmission line at midpoint • i = 0 at end, maximum at midpoint • v = 0 at midpoint, ±vmax at ends • purely resistive impedance = 73Ω • easily matched to many transmission lines gap ¼ λ¼ λ ½ λ Transmission Line +v -v i High Impedance 2k-3kΩ Low Impedance 73Ω
  36. 36. 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. 37. 37 Azimuth Pattern Elevation Pattern Polar Radiation Pattern 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
  38. 38. 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. 39. 39 Multi-Band Dipole Antennas Transmission Line λ1/4C L C L λ1/4 λ2/4λ2/4 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
  40. 40. 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. 41. 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 • 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 Antenna Transmission Line earth ground R=Z0
  42. 42. 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 angular radiation pattern horizon feed polar ration pattern
  43. 43. 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. 44. 44 Folded Dipole Antenna - basic ½λ dipole folded to form complete circuit - core to many advanced antennas - 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 λ/2
  45. 45. 45 Loop & Patch Antenna – wire bent into loops Patch Antenna: rectangular conducting area with || ground plane Area A N-turns V = maximum voltage induced in receiver by EM field B = magnetic field strength flux of EM field A = area of loop N = number of turns f = signal frequency k = physical proportionality factor V = k(2πf)BAN Antenna Plane
  46. 46. 46 • 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 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
  47. 47. 47 552.14 dB Dipole 3600 dBIsotropic Beamwidth -3 dB Gain (over isotropic) ShapeName Radiation Pattern 20 30 50 200 25 14.7 dB 10.1 dB -0.86 dB 3.14 dB 7.14 dB Parabolic Dipole Helical Turnstile Full Wave Loop Yagi Biconical Horn 1515 dBHorn 360x20014 dB

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