Antenna

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FUNDAMENTAL OF ANTENNA SYSTEM

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Antenna

  1. 1. ElectroMagnetic wave theoryyAntenna-Radiation pattern etc. SKGOCHHAYAT SDERTTC BHUBANESWAR
  2. 2. Antenna - How it WorksThe antenna converts radio frequency electrical energy fed to it (via q y gy (the transmission line) to an electromagnetic wave propagated intospace.The h i l iTh physical size of the radiating element is proportional to the f th di ti l ti ti l t thwavelength. The higher the frequency, the smaller the antenna size.Assuming that the operating frequency in both cases is the same,the antenna will perform identically in Transmit or Receive mode.
  3. 3. The type of system you are installing will help determine thetype of antenna used. Generally speaking, there are two ‘types’ yp y p g, ypof antennae:1. Directional1 Di ti l - this type of antenna has a narrow beamwidth; with the power being more directional, greater distances are usually directional achieved but area coverage is sacrificed - Yagi Panel, Sector and Parabolic antennae Yagi, Panel - an EUM, NCL Station/Master will use this type of antenna p in both Point to Point and Point to Multipoint
  4. 4. 2. Omni-Directional - this type of antenna has a wide beamwidth and radiates 3600; with the power being more spread out, shorter distances are achieved but greater coverage attained - Omni antenna - a CCU or an NCL Master will use this type of antenna
  5. 5. Yagi- b tt suited for shorter links better it d f h t li k- lower dBi gain; usually between 7 and 15 dBi
  6. 6. Typical Radiation Pattern for a Yagi
  7. 7. Parabolic- used in medium to long links g- gains of 18 to 28 dBi- most common
  8. 8. Typical Radiation Pattern for a Parabolic
  9. 9. Sectoral- directional in nature, but can be adjusted anywhere from 450 to nat re b t adj sted an here1800- typical gains vary from 10 to 19 dBi
  10. 10. 0 0 -15 -15 -20 -20 -30 30 -30 30270 0 -3 -6 -10 dB 90 270 0 -3 -6 -10 dB 90 180 180 Typical Radiation Pattern for a Sector
  11. 11. Omni- used at the CCU or Master NCL for wide coverage- typical gains of 3 to 10 dBi
  12. 12. Typical Radiation Pattern for an Omni
  13. 13. Antenna Radiation PatternsCommon parameters – main lobe (boresight) – half power beamwidth (HPBW) half-power – front-back ratio (F/B) – pattern nulls pTypically measured in two planes: • Vector electric field referred to E-field • Vector magnetic field referred to H-field
  14. 14. PolarizationAn antennas polarization is relative to the E-field of antenna. E field antenna– If the E-field is horizontal, than the antenna is HorizontallyPolarized.Polarized– If the E-field is vertical, than the antenna is Vertically Polarized.No matter what polarity you choose, all antennas in the same RFnetwork must be polarized identically regardless of the antennatype.
  15. 15. Polarization may deliberately be used to: – Increase isolation from unwanted signal sources (Cross g ( Polarization Discrimination (x-pol) typically 25 dB) – Reduce interference – Help define a specific coverage area l d fi ifi Horizontal Vertical
  16. 16. PARABOLIC ANTENNA• The parabolic antenna is used almost universally i d l t i ll in point-to-point systems. The parabolic antenna utilizes a reflector consisting of a paraboloid of revolution and primary radiator at the focal point .The reflector converts the spherical wave radiating from the focus to the planar wave across the face of the paraboloid t concentrate b l id to t t the energy in a beam much like a searchlight beam as discussed below below.
  17. 17. Reflector Optics: Limitations• Prime focus – Over-illumination (spillover) can increase system temperature due to ground pick-up – Number of receivers, and access to them, is limited• Subreflector systems – Can limit low frequency capability. Feed horn too large. – Over-illumination by feed horn can exceed gain of reflector’s Over illumination reflector s diffraction limited sidelobes • Strong sources a few degrees away may limit image dynamic range• Offset optics – Support structure of offset feed is complex and expensive
  18. 18. Reflector OpticsPrime focus Cassegrain focusOffset Cassegrain NaysmithBeam Waveguide Dual Offset
  19. 19. Reflector Optics: ExamplesPrime focus Cassegrain focus (GMRT) (AT)Offset Cassegrain Naysmith (VLA) (OVRO)Beam Waveguide Dual Offset (NRO) (GBT)
  20. 20. directrix Q’QP P’ b F FP+Fp’=FQ+FQ’
  21. 21. Cassegrain• If a 30cm diameter reflector is placed at the centre of a 3-m dish, simple arithmetic p shows that the area obstructed is only 1 percent of the total. Similar f th t t l Si il reasoning is applied to the horn primary, which primary obstructs an equally small proportion of the total area. area
  22. 22. Feed designs• There are different types of feed designs for various frequency bands and different system applications Feeds for the 890 to applications. 2,300 MHz bands are generally coaxial dipoles, dipoles slot excited circular wave guide reverse horns or printed circuit arrays.
  23. 23. The image cannot be display ed. Your computer may not hav e enough memory to open the image, or the image may hav e been corrupted. Restart y our computer, and then open the file again. If the red x still appears, y ou may hav e to delete the image and then insert it again. Feed Systems GBT VLA EVLA
  24. 24. Aperture EfficiencyOn axis response: A0 = ηAEfficiency: η = ηsf × ηbl × ηs × ηt × ηmiscηsff = Reflector surface efficiency Due to imperfections in reflector surface rms error σ ηsf = exp(−(4πσ/λ)2) e.g., σ = λ/16 , ηsf = 0.5ηbl = Blockage efficiency Caused by subreflector and its support structureηs = Feed spillover efficiency Fraction of power radiated by feed intercepted by subreflectorηt = Feed illumination efficiency Outer parts of reflector illuminated at lower level than inner partηmisc= Reflector diffraction feed position phase errors feed match and diffraction, errors, loss
  25. 25. Aperture EfficiencyPrimary Beam πDll=sin(θ), D = antenna diameter in contours:−3,−6,−10,−15,−20,−25, wavelengths −30,−35,−40 dBdB = 10log(power ratio) = 20log(voltage ratio)VLA: θ3dB = 1.02/D, First null = 1.22/D Voltage radiation pattern, |F(l,m)|
  26. 26. Antenna Pointing: Practical Considerations SubreflectorReflector structure mount Quadrupod El encoderAlidade structure Rail flatness Foundation Az encoder
  27. 27. Cassegrain feed
  28. 28. Cassegrain feed• The Cassegrain feed is used when it is desired to place the primary antenna at a convenient position and to shorten the length of the transmission line or wave guide connecting the receiver (or transmitter) to the primary. This requirement in the line or waveguid may not be tolerated, specially over lengths which may exceed 30 m in large antennas. Another solution to the problem is to place the active part of the transmitter or receiver at the l h i f h i i h focus. With transmitters this can almost never be done because of their size, and it may also be difficult to place the RF amplifier of the receiver there This is either there. because of its size or because of the need for cooling apparatus for very low-noise applications in which case the RF amplifier may be small enough, but the ancillary equipment is not. In any case, such placement of the RF amplifier causes servicing and replacement difficulties, and the Cassegrain feed is often the best solution.
  29. 29. ELECTRICAL CHARACTERISTICS• Antenna Gain• Beam width• Voltage to Standing Wave Ratio (VSWR)• Radiation Patterns• Front-to-back ratio
  30. 30. Antenna Gain
  31. 31. Conventional feeds provide an illumination ofapproximately -10 dB at the edge of the parabolafrom that at the centre which results in an antenna centre,efficiency of 58 to 63 percent for productionantennas. Taking other factors into account, mostmanufacturers guarantee antenna efficiencies of 55 f t t t ffi i i fpercent.
  32. 32. TYPICAL ANTENNA GAINS AND BEAMWIDTH FOR VARIOUS SIZES AND FREQUENCIESAntenna 2 GHA t GHz 6 GH GHz 11 GH GHzDiameter Gain Beamwidth Gain Beamwidth Gain Beamwidth DBi degrees dBi degrees dBi degrees1.2m1 2m 25.4 25 4 8.8 88 35.0 35 0 2.8 28 40.3 40 3 1.6 161.8m 29.0 5.7 38.8 1.9 43.8 1.12.5 31.5 4.3 41.2 1.4 46.2 0.83.030 33.4 33 4 3.5 35 43.0 43 0 1.2 12 48.1 48 1 0.6 063.7 35.0 2.9 44.8 1.0 49.6 0.54.6 36.9 2.3 46.2 0.8 ----- ----
  33. 33. Beam widthThe value of θ is approximately 1.1 degree at 6 GHz and 3.4degree at 2 GHz for a 3.0 m diameter antenna. The mainlobe drops off to a null at 1.1 degree beam width off axis.This may mean that signal could drop as much as 40 dB if a3.0 m antenna at 6 GHz is moved 1.1 degree off axis. Onecan appreciate the need for sturdy mounts and careful towerdesign .
  34. 34. Antenna ImpedanceA proper Impedance Match is essential for maximum powertransfer. The antenna must also function as a matching load forthe Transmitter ( 50 ohms).Voltage Standing Wave Ratio (VSWR), is an indicator of howwell an antenna matches the transmission line that feeds it. Theantenna VSWR is the ratio of the amplitude of the voltage standing wave at themaximum to the amplitude at the minimum. It is the ratio of the forward voltageto the reflected voltage. The better the match, the Lower the VSWR. VSWR is always equalto or greater than 1.0. A 1.000 VSWR indicates that an antenna is perfectlymatched to a transmission line. A value of 1.5:1 over the frequencyband of interest is a practical maximum limit. limit
  35. 35. VOLTAGE STANDING WAVE RATIO (VSWR)• The antenna VSWR is the ratio of the amplitude of the voltage standing wave at the maximum to the amplitude at the minimum. VSWR is always equal to or greater than 1 0 1.0.• A 1.000 VSWR indicates that an antenna is perfectly matched to a transmission line line.
  36. 36. Voltage Standing Wave Ratio• Since the feedhorn, located at the focus, has , , some physical size, it will catch some reflected energy from the parabola causing a mismatch. mismatch This is termed dish effect .• The VSWR contribution may be 1.02 or more, depending upon size and frequency. Placing a raised circular plate, of the proper thickness, called a vertex plate, at the centre of the reflector can cancel out these dish reflections. fl t l t th di h fl ti Vertex plates are usually installed along with the feed when t e a te a is asse b ed eed e the antenna s assembled.
  37. 37. Voltage Standing Wave Ratio• The VSWR over the operating frequency bands for standard microwave antennas will be approximately 1 10 1.10. Through extreme care in manufacture and additional tuning and matching, low VSWR matching antennas can achieve a VSWR of 1.04 to 1.06. 1 06
  38. 38. Voltage Standing Wave Ratio• Low VSWR antennas are necessary to ensure minimum echo di t ti i l h distortion in long h l microwave systems. A hi h haul i t high antenna VSWR will cause some of the transmitted signal to be reflected back down the transmission line. This reflected signal can be again reflected at the RF equipment and sent towards the antenna. This delayed signal causes unwanted noise which can be compounded in a long microwave system. A low VSWR antenna minimizes the amount-of reflected signal.• Standard Parabolic Antennas- make a reliable economic choice for the majority of thin route systems. These j y y antennas have a VSWR of about 1.10:1 which is satisfactory for low to medium channel densities and moderate length systems.
  39. 39. Return LossReturn Loss is related to VSWR, and is a measure of thesignal power reflected by the antenna relative to the i l fl db h l i hforward power delivered to the antenna.The higher the value (usually expressed in dB), thebetter. A figure of 13.9dB is equivalent to a VSWR of1.5:1. Return L15 1 AR t Loss of 20dB i considered quite good, f is id d it dand is equivalent to a VSWR of 1.2:1.Return Loss [in db] = 20 log VSWR +1 VSWR –1
  40. 40. Return Loss & Transmission lossVSWR Return Loss Transmission Loss1.0:1 ∞ 0.0 dB1.2:1 20.83 dB 0.036 dB1.5:1151 13.98 13 98 dB 0.177 0 177 dB5.5:1551 3.19 3 19 dB 2.834 2 834 dB
  41. 41. Return LossReturn Loss [in db] = 20 log VSWR +1 VSWR –1 VSWR R. L. R L VSWR R. L. R L VSWR R. L. R L 40.1 1.07 29.4 1.15 23.0 1.02 1.03 36.6 1.08 28.3 1.20 20.8 1.04 34.1 1.09 27.3 1.25 19.0 1.05 32.2 1.10 26.4 1.30 17.8 1.06 30.7 1.12 24.9 1.40 15.4
  42. 42. RADIATION PATTERNS• The radiation patterns of antennas have become more important with the increase in microwave p congestion and the need for careful coordination to prevent interference between systems.• In planning a route a system engineer will route, evaluate the potential interference from microwave systems operating on the same frequencies up to 160 to 320 km away km. away.• If the carrier to interference signal ratio, C/l, is 70 dB or more, interfering noise is negligible. g g g However if the C/l ratio is low, then preventive measures must be taken such as using a high p performance or ultra high pg performance antenna.
  43. 43. RADIATION PATTERNS
  44. 44. RADIATION PATTERNS
  45. 45. RADIATION PATTERNS
  46. 46. 0102030405060708090
  47. 47. Front to back Front-to-back ratio• The front to back ratio of an antenna is defined front-to-back as the ratio of the power received from [or transmitted to] the main beam of the antenna to the power received from [or transmitted to] the back side.• In order to operate satisfactorily on a two frequency plan, using the same transmitting frequencies in two directions at a repeater it is repeater, necessary to have high front- to back ratios.
  48. 48. Horn Reflector Antennas
  49. 49. Horn Reflector Antennas• The horn reflector [cornucopia] antenna has a [ p ] section of a very large parabola mounted at such an angle that the energy from the feed horn is simultaneously focused and reflected at y right angles. A horn antenna having the equivalent gain of a 3 meter parabolic antenna is over 6 meters in height and causes a much greater load from wind on the tower,• However, it has a much higher front- to- back ratio than the standard parabolic antenna but antenna, has about the same front -to-back ratio as higher performance antenna of the same gain .
  50. 50. Horn Reflector Antennas• This type of antenna has good VSWR characteristics and with suitable coupling networks [which are quite complex and [ q p very expensive], can be used for multi- band operation on both polarizations. However, th H there are moding problems, di bl particularly at the higher frequencies which, uncorrected, which if uncorrected can cause severe distortions, Correcting of these moding Problems is a very difficult task task.
  51. 51. Horn Reflector Antennas• Disadvantages are that this antenna is very big, heavy and complex to mount. The cost of one antenna with suitable coupling networks to provide dual polarization at 4 GHz and 6 GHz band far exceeds the cost of two separate parabolic high performance antennas providing antennas, equivalent or better electrical performance.
  52. 52. Horn Reflector Antennas
  53. 53. Horn Reflector Antennas• Disadvantages are that this antenna is very big, heavy and complex to mount. The cost of one antenna with suitable coupling networks to provide dual polarization at 4 GHz and 6 GHz band far exceeds the cost of two separate parabolic high performance antennas providing antennas, equivalent or better electrical performance.
  54. 54. WAVEGUIDES AND TRANSMISSION LINES• Wave guide and transmission line is important, not only for its loss characteristics, characteristics which enter into the path loss calculation, but also for the degree of impedance matching attainable attainable, because of the effect on echo distortion noise The later becomes noise. important with high-density systems having long waveguide runsruns.
  55. 55. Coaxial Transmission Lines• In bands up to 2 GHz coaxial cable is GHz, usually used, and except for very short runs, runs it is usually of the air dielectric type type. Typical sizes are: 2.2 cm. diameter. Andrew type HJ 5-50 with attenuation of 5 50, about 6dB per 100 meters at 2 GHz, and 4.1 cm. diameter, 4 1 cm diameter Andrew type HJ 7-50 7 50 with an attenuation of about 3 dB per 100 meters. meters
  56. 56. Wave guides• Bands higher than 2 GHz require the use of GH req ire se waveguides almost exclusively and one of three basic types may be used rigid rectangular rigid circular and rectangular, circular, flexible elliptical. The latter is of continuous construction, having the advantages of minimizing the g g g number of flange connection usually of two.
  57. 57. Rectangular Guide oxygen-free,highhi h conductivity copper (OFHC) d i i
  58. 58. Rectangular Guide
  59. 59. Circular Guide• Circular waveguide has the lowest loss of all, and in g addition, it can support two orthogonal polarizations within the single guide. It is also capable of carrying more than one frequency band in the same g q y guide. For example, WC 281 circular, guide is normally used with horn reflector antennas to provide two polarizations at 6 GHz. But circular guide has certain disadvantages. It is g g practical only for straight runs, requires rather complicated and extremely critical networks to make the transitions from rectangular to circular and can have g significant molding problems, when the guide is large enough to support more than one mode for the frequency range in use. q y g
  60. 60. Elliptical Guide• Semi-flexible elliptical waveguide is available in sizes p g comparable to most of the standard rectangular guides, with attenuations differing very little from the rectangular equivalents. The distinctive features of elliptical g q p guide is that it can be provided and installed as a single continuous run, with no intermediate flanges. When carefully transported and g y p installed it can provide good VSWR performance but relatively small deformations can introduce enough impedance mismatch to p p produce severe echo distortion noise. However, usually the effect of small deformations can be tuned out.
  61. 61. Elliptical Guide4 GHz band : EW - 37 Approximately 2.8 dB per 100 meters6 GHz band : EW – 56 Approximately 5.7 dB per 100 meters7-8 GHz band : EW – 71 Approximately 8.2 dB per 100 meters11 GHz band : EW – 107 Approximately 12.1 dB per 100 meters12-13 Approximately GHz band : EW – 122 14.7 dB per 100 meters All attenuation figures given at mid band

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