1. Summer 2013 © Tim Pratt 2013 1 REU June 2013 Antennas Tim Pratt, Instructor tipratt@vt.edu

2. Topics Antenna types Antenna gain and beamwidth Antenna patterns Yagi, horns and helix Reflector antennas Phased array principles Fixed beam phased array antennas Electronically steered phased arrays Summer 2013 © Tim Pratt 2013 2

3. Summer 2013 © Tim Pratt 2013 3 Antennas All antennas serve several purposes Generate controlled beam Provide gain Interface from waveguide or feed line to medium (air) Antennas are often the limiting factor in a radio communication system Definition of antenna gain: Increase in power radiated in one direction relative to an antenna that radiates equally in all directions

4. Fig 10.1 Antenna Gain Summer 2013 © Tim Pratt 2013 4 Isotropic or omni-directional antenna G = 1 or 0 dB Directional antenna Axis or boresight G >> 1 Dish

5. Summer 2013 © Tim Pratt 2013 4 Fig 10.2 Low Gain, Wide Beam Antennas Dipole Monopole Helix Open ended waveguide Small hornGain 0 to 3 dB Patch /4 /2 Ground plane

6. Cellular Handset Antennas Antenna must be nearly omni-directional Simplest antenna is a monopole Length is one quarter wavelength At 2000 MHz, = 0.15 m = 15 cm = 6 inches Quarter wavelength = /4 = 3.75 cm or about 1.5 ins Older cell phones had antenna on top Newer phones bury antenna inside case Ground plane is user’s hand Wider bandwidth requires a more sophisticated antenna GPS signals are at 1575 MHz – a folded F may be used Summer 2013 © Tim Pratt 2013 6

7. Summer 2013 © Tim Pratt 2013 7 Transmission Lines Antennas must be fed by transmission lines Open wire pair - rarely used now, low frequencies only Coaxial line - available as rigid or flexible Impedance is set by d 1, d 2, dielectric constant  Impedances typically 50 ohms or 75 ohms Waveguide – hollow rectangular or circular tube Electric field in waveguide controls radiation 

8. Summer 2013 © Tim Pratt 2013 8 Fig 10.3 Rectangular Waveguide TE 10 is the dominant mode in rectangular waveguide Waveguide has dimensions a = 0.6 to 0.9 b < 0.5 E-filed is zero parallel to a conducting surface TE 10 E-field distribution b a

9. Summer 2013 © Tim Pratt 2013 9 Fig 10.4 Circular Waveguide TE 11 is the dominant mode in circular waveguide Waveguide has dimensions D ~ 1 E field must terminate normal to a conducting surface TE 11 E-field distribution

10. Summer 2013 © Tim Pratt 2013 10 Fig 10.5 Planar Transmission Lines Stripline and microstrip w t Dielectric,  Impedance is set by w, t,  Thickness of dielectric must be controlled carefully at high microwave frequencies Microstrip Substrate

11. Summer 2013 © Tim Pratt 2013 11 Medium Gain Antennas Higher gain can be obtained with horns and traveling wave antennas Horns are aperture antennas Flare out waveguide to make aperture Field in aperture is same as in waveguide Must be a plane wave for maximum gain Traveling wave antennas are long – helix and Yagi

12. Summer 2013 © Tim Pratt 2013 12 Waveguide horn with large aperture Maximum gain ~ 25 dB Phase front in horn aperture is curved Phase center E field In waveguide Fig 10.6 Waveguide Horn

13. Summer 2013 © Tim Pratt 2013 13 Traveling Wave Antennas Yagi Helix These antennas have small aperture, medium gain Gain of Yagi antenna is approximately equal to the number of elements (not in dB) E.g. 20 elements, G ~ 13 dB Elements are spaced about / 4 Reminder: dB value = 10 log (P2 / P1) Never 20 log [ ]

14. Summer 2013 © Tim Pratt 2013 14 Reflector Dipole Directors (passive) Feed line End fire array Gain is roughly equal to the number of directors (not in dB) Fig 10.7 Yagi (-Uda) Ante nna

15. Summer 2013 © Tim Pratt 2013 15 High Gain Antennas In order of popularity: Reflector with feed Phased array Lens with feed (rare) Phased arrays are generally much more expensive than reflector antennas for a given gain Phased arrays allow electronic steering of beam – used mainly in radars

16. Summer 2013 © Tim Pratt 2013 16 Reflector (Dish) Waveguide horn feed Edge illumination - 10 to –13 dB Feed pattern Fig 10.8 Front Fed Reflector Antenna Parallel rays

17. Unit 2 Analysis of Antennas Gain and beamwidth Antenna patterns Examples Beam squint Summer 2013 © Tim Pratt 2013 17

18. Summer 2013 © Tim Pratt 2013 18 Antenna Gain Accurate formula for Gain (not in dB) G =  A 4  A / 2 = 4  A e / 2 A = physical area of aperture A e = effective area of aperture = wavelength  A = aperture efficiency Aperture efficiency is typically 65% in well designed antenna For an antenna with a circular aperture diameter D G =  A (  D / ) 2

19. Summer 2013 © Tim Pratt 2013 19 Approximation for Antenna Gain Approximate formula for G [empirical] G = 33,000 / (  a   b ) [not dB] where  a and  b are antenna beamwidths in degrees in two orthogonal planes E.g. Azimuth and Elevation Equal beamwidths: G = 33,000 /  2 This formula is empirical and not particularly accurate It is useful for finding gain when beamwidths are known Example: Satellite transmit antenna creates 3 o x 4 o beam Gain is approximately 33,000 / 12 = 2750 or 34.4 dB

20. Summer 2013 © Tim Pratt 2013 20 Plane wave aperture Main beam sidelobes Aperture is derived from concept in optics Same effect when a plane wave is created by an antenna Fig 10.9 Creation of Antenna Pattern with an Aperture

21. Antenna Pattern Antenna pattern shows the distribution of transmitted power with angle for axis of beam (where gain is maximum) - see Fig 10.10 Usually plotted in Cartesian coordinates with relative gain on vertical axis and maximum gain set to 0 dB Beamwidth is defined between half power (-3 dB) angles Pattern has main beam and sidelobes Gain is highest and sidelobes largest with uniform illumination – constant field across aperture Tapered illumination has lower field at aperture edge Gain is lower, beam is broader and sidelobes are lower Summer 2013 © Tim Pratt 2013 21

22. Summer 2013 © Tim Pratt 2013 22 Relative Gain dB 0o0o Uniform illumination Tapered illumination Fig 10.10 Antenna Patterns 0 dB -13.2 dB axis -3 dB 3 dB beamwidth angle First sidelobeMain beam First null

23. Reciprocity in Antennas Gain and Pattern of any antenna is always same for transmit and receive We usually think of antenna pattern of transmitting antenna Same pattern applies to antenna when receiving Principle is called reciprocity Summer 2013 © Tim Pratt 2013 23

24. Summer 2013 © Tim Pratt 2013 24 Rayleigh Range Antenna requires uniform phase across aperture Field radiates from antenna, diffracts in far field (long distance from antenna) Forms far field pattern at R > D 2 / Pattern is constant beyond Rayleigh Range R = 2 D 2 / Beamwidth in far field is set by aperture dimension D and illumination Typical 3 dB beamwidth for reflector antenna is 75 / D Example: Antenna with 5 m aperture diameter at 12 GHz = 0.025 m, 3 dB B/W = 75 x 0.025 / 5 = 0.375 o

25. Summer 2013 © Tim Pratt 2013 25 Antenna Beamwidths and Sidelobes Beamwidth depends on aperture dimension D and illumination of aperture Examples: Uniform illumination  = 51 D / degrees Tapered illumination  > 57 D / degrees Depends on taper - heavier taper, wider beam First sidelobe peak: Uniform illumination - 13.2 dB linear, -17.6 dB circular Tapered illumination << -13.2 dB

26. Summer 2013 © Tim Pratt 2013 26 Antenna Example Large antenna at 6 GHz with 30 m aperture, 65 % aperture efficiency G = 10 log ( 0.65 x (  D / ) 2 ) At 6 GHz, = 0.05 m G = 10 log ( 0.65 x (  x 30 / 0.05) 2 ) = 10 log ( 0.65 x (1885) 2 ) = 63.6 dB This is the upper limit for gain with a reflector antenna Holding surface accuracy of paraboloidal dish for diameters > 30 m is difficult

27. Summer 2013 © Tim Pratt 2013 27 Antenna Example 3 dB Beamwidth Typical value is 75 / D degrees For D = 30 m, = 0.05 m 3 dB Beamwidth = 0.125 degrees Antenna will have to be steered to follow any signal source that moves more than 0.03 degrees, such as a GEO satellite Typical cost: $ 5 M

28. Summer 2013 © Tim Pratt 2013 28 Antenna Imperfections Blockage of aperture causes null filling, higher sidelobes Phase errors caused by surface errors also cause null filling Asymmetric phase errors cause coma distortion Sidelobes on one side of beam are higher than on other Seen when feed is displaced transverse from focus of a reflector antenna Displacing feed gives beam squint

29. Summer 2013 © Tim Pratt 2013 29 ReflectorOff-axis feed Distorted wavefront phase Fig 10.11 Beam Squint with Off-axis Feed Focus Center ray

30. Summer 2013 © Tim Pratt 2013 30 Gain, dB 0 angle Coma distortion Fig 10.12 Antenna Pattern with Off-axis Feed 0 Nulls filled Low sidelobes High sidelobes

31. Summer 2013 © Tim Pratt 2013 31 Reflector Antenna Configurations Front feed: Feed blocks aperture Offset front feed: Feed below aperture, avoids blockage Cassegrain : Dual reflector - sub reflector inside focus of main reflector Gregorian: Dual reflector - sub reflector outside focus of main reflector

32. Summer 2013 © Tim Pratt 2013 32 Reflector (Dish) Waveguide horn feed Edge illumination - 10 to –13 dB Feed pattern Fig 10.13 Front Fed Reflector Antenna Parallel rays

33. Summer 2013 © Tim Pratt 2013 33 Offset Reflector Feed Feed pattern Fig 10.14 Offset Front Fed Reflector Parallel rays This configuration is used by Directv and Dish network for direct to home satellite TV reception. The feed and LNB are below the antenna beam, avoiding blocking.

34. © Tim Pratt 2013 34 January 2013 Fig 10.15 DBS-TV receive antennas Offset front fed dish Summer 2013

35. © Tim Pratt 2013 35 Main Reflector Paraboloid Feed Fig 10.16 Cassegrain Antenna Secondary Reflector (sub-reflector) Hyperboloid Parallel rays Cassegrain antennas are used mainly for large earth stations that need high gain antennas Main reflector focus

36. Summer 2013 © Tim Pratt 2013 36 Fig 10.17 Large Cassegrain antenna - fully steerable

37. Summer 201337 ECE 4644Telecomms - II Copyright Tim Pratt 2013 Fig 10.18 Echostar transmit / receive station © Tim Pratt 2013

38. Summer 2013 © Tim Pratt 2013 38 Main Reflector Paraboloid Feed Fig 10.19 Gregorian Antenna Sub-reflector Paraboloid Gregorain antennas are used for mid size transportable earth stations on satellite trucks Main reflector focus

39. Summer 2013 © Tim Pratt 2013 39 Fig 10.20 Mobile earth station with Gregorian antenna

40. Summer 2013 © Tim Pratt 2013 40 Reflector Antennas Much of the development of antennas came from radar requirements Surveillance radar needs high gain antenna Beam scans horizon to warn of approaching aircraft, ships, missiles Scan rate is typically 6 to 10 rpm Tracking antenna: radar locks to target Needs smaller antenna with high slew rate, tracking beams

41. Summer 2013 © Tim Pratt 2013 41 Phased Arrays Uses same aperture illumination principles as reflector antenna Can be a fixed array or an electronically steered array Electronic steering is used mainly in radar antennas Beam is scanned electronically with variable phase shifters Multiple beams are possible Random scan possible (defeat ECM) Nulling of jammers is possible Complex and expensive

42. Summer 2013 © Tim Pratt 2013 42 Radar Antennas The ideal radar antenna is a phased array Advantages: Great flexibility Multiple functions Disadvantages: Cost Complexity

43. Summer 2013 © Tim Pratt 2013 43 Fig 10.21 Planar Phased Array with Square Aperture a a Aperture area = a 2 Element spacing = d N elements, with N = ( a/d ) 2 d Element

44. Summer 2013 © Tim Pratt 2013 44 Phased Array Antennas Phased array is made up of many active elements Element is a dipole, helix, open ended waveguide – any small antenna with gain ~ 0 dB Element spacing is typically d = 0.6 wavelengths Look at number of elements needed: For gain of G = 36 dB = 4000 =  A 4  A / 2 Assume  A = 0.6 A / 2 = 531 and a =  531 2 = 23 For d = 0.6 N = 38 elements per side Total number of elements in array is N 2 = 38 2 = 1444

45. Summer 2013 © Tim Pratt 2013 45 Phased Array Antennas If beamwidth = 1.4 o Length of side increases to 46 Area increases to 4 x 531 2 = 2124 2 Gain increases to 36 + 6 = 42 dB N = 76 elements per side Total number of elements in the array is N 2 76 x 76 = 5776 Making and driving 5776 microwave devices is expensive

46. Summer 2013 © Tim Pratt 2013 46 Phased Array Antennas Phased array design and cost are dominated by requirement for thousands of active elements Useful number of elements is in range 1000 to 10,000 for array scanned in two planes Example: 5,000 elements at $1000 each gives phased array cost of $ 5M A reflector antenna will cost much less Electronic scanning in one plane only reduces cost

47. Summer 2013 © Tim Pratt 2013 47 Phased Array Antennas High cost of an electronically steered phased array antenna can be justified when: Antenna performs multiple functions Antenna must have good ECCM capability Random beam pointing is required Rapid beam steering is required Variable dwell time is needed Radar has been the primary user of phased arrays

48. Summer 2013 © Tim Pratt 2013 48 Fig 10.22 Pave Paws phased array antenna Long range VHF radar for detection of ICBMs

49. Summer 2013 © Tim Pratt 2013 49 Fig 10.23 A/N SPY-1 phased array radar antenna

50. Summer 2013 © Tim Pratt 2013 50 Fig 10.24 A/N SPY-1 phased array radar antenna

51. Summer 2013 © Tim Pratt 2013 51 Fig 10.25 Phased array using slotted elements Flat panel is scanned mechanically

52. Summer 2013 © Tim Pratt 2013 52 One Dimensional Phased Array Simplest phased array is a linear (1-D) array Elements are in a straight line, spaced d wavelengths apart Element spacing d must be in range 0.5 to 1.0 wavelengths Used to create omni directional antennas with a narrow beam in the vertical plane

53. Summer 2013 © Tim Pratt 2013 53 Broadside beam Dipoles with uniform phase Fig 10.26 Omni directional dipole array Transmitted RF wave is vertically polarized

54. Summer 2013 © Tim Pratt 2013 54Summer 2013 © Tim Pratt 2013 54 Beam is depressed to improve coverage of ground Dipoles with progressive phase shift Fig 10.27 Omni directional dipole array Transmitted RF wave is vertically polarized 10 o 0o0o 20 o 30 o 40 o Phase

55. Summer 2013 © Tim Pratt 2013 55 Equal time delays TT TT TT TT dipoles wavefront wavelets Fig 10.28 Series Fed Linear Array TT Beam direction load

56. Cellular Phone Base Station Antennas Cell phone base stations use linear array antennas like the one shown in Fig 10.27. The linear array makes a narrow beam in the horizontal plane that is tilted down a little for best coverage over the earth’s surface Directional elements can be used to make the beam cover a sector Cell phone towers with a large number of antennas are covering several frequency bands and providing sector coverage to increase the number of users Summer 2013 © Tim Pratt 2013 56

57. Summer 2013 © Tim Pratt 2013 57 Fig 10.29 Cell phone tower with sector and omni antennas. Small dishes link tower to cell phone HQ and other towers.

58. Summer 2013 © Tim Pratt 2013 58 Fig 10.30 Cell phone tower with sector antennas

59. Summer 2013 © Tim Pratt 2013 59 Fig 10.31 Cell phone tower disguised as a tree

60. Summer 2013 © Tim Pratt 2013 60 Progressive time delays TT dipoles wavefront wavelets Fig 10.32 Parallel Fed Linear Array TT TT TT Splitters TT Beam direction

61. Summer 2013 © Tim Pratt 2013 61 Beam Steering Beam is steered by changing relative phase between elements Fixed time (phase) delays give fixed beam Cell phone tower antenna has fixed beams Electronically controlled phase shifter gives movable beam Used mainly in military radars because of high cost Elements must be excited with correct amplitude distribution to control sidelobes

62. Summer 2013 © Tim Pratt 2013 62 Beam Steering Time delays are difficult to achieve Phase delay is used instead Relative phasing between elements determines beam direction Analyze for transmit case … apply reciprocity for receive Consider case of two adjacent elements One element has phase shift 0 o Adjacent element has phase shift  Beam direction is  to the array normal where  = (2  / ) d sin 

63. Summer 2013 © Tim Pratt 2013 63 0o0o  d  d x  x = d sin   = (2  / ) d sin  Fig 10.33 Setting Beam Angle  Broadside Beam angle

64. Summer 2013 © Tim Pratt 2013 64 To form beam at angle  we need progressive phase delay along array of 0 o, , 2 , 3 , 4 , 5  … Eventually N  > 360 o Reset phase to N  - 360 o Beam is correctly pointed only in steady state Beam transient occurs as wavelets emerge from elements Beam Steering

65. Summer 2013 © Tim Pratt 2013 65 Beam Steering Example #1: Steer beam to 30 o from normal (broadside) Element spacing d = 0.6  = (2  / ) d sin  = 2  x 0.6 x sin 30 o = 1.2  x ½ = 0.6  = 108 o We must insert 108 o phase shift between elements

66. Summer 2013 © Tim Pratt 2013 66 0 Phase Shifts in degrees wavefronts Fig 10.34 Parallel fed linear array scanned 30 o 108 Splitters 108 o Beam direction 30 degrees 216 324

67. Summer 2013 © Tim Pratt 2013 67 Beam Steering Example #2: Steer beam to 45 o from normal Element spacing d = 0.6  = (2  / ) d sin  = 2  x 0.6 x sin 45 o = 1.2  x ½ = 0.848  = 153 o We must insert 153 o phase shift between elements

68. Summer 2013 © Tim Pratt 2013 68 0 Phase shifts in degrees wavefronts Fig 10.35 Linear array scanned 45 o 153 Splitters Beam direction 45 degrees 306 99 153 o

69. Conclusion All radio transmitters and receivers need antennas Simplest antenna is a monopole or dipole Omni-directional radiation pattern, low gain Used in mobile and portable radios – e.g. cell phone Reflector and phased array antennas provide high gain and narrow beam Used mainly for microwave links, satellite comms and radars Summer 2013 © Tim Pratt 2013 69