2. Antenna Measurements: Dihedrals, ground targets and antenna beam patterns AMS Radar Calibration Workshop Albuquerque, New Mexico 13-14 January 2001 Ronald E. Rinehart University of North Dakota Grand Forks, ND 58202-9006 Voice: 701-777-2183; fax: 701-777-5032 email: rinehart@aero.und.edu or radarwx@aol.com 1/19/1 1

3. Other speakers in this session:  Ken Tapping  John Lutz  Dave Brunkow & John Hubbe  Dick Doviak 1/19/1 2

4. Speakers in this session: 1/19/1 3

5. Beware of the hazard associated with this talk: 1/19/1 4

6. Growing a crop of antennas at EEC, Enterprise, AL 1/19/1 5

7. More antennas growing in New Mexico 1/19/1 6

8. Why we need to know antenna parameters:  Point target radar equation:  Meteorological target radar equation: 1/19/1 7

9. receiver transmitter modulator master clock display signal processor/ computer r 1/19/1 8

10. receiver transmitter modulator master clock display signal processor/ computer r 1/19/1 9

11. Here’s what we think happens: Here’s what actually happens: And it gets even worse! A radar’s view of a storm: 1/19/1 10

12. Antenna characteristics than need to be measured:  gain  mainlobe  sidelobes  complete pattern  beamwidth 1/19/1 11

13. How can we measure beamwidth, gain and antenna beam pattern?  Antenna range  Signal generator/horn  Standard target  Secondary-standard target  Sun 1/19/1 12

14. Use of antenna range:  Requires moving the antenna to the antenna range.  Expensive  Time-consuming  Excellent results 1/19/1 13

15. Signal Generator & Horn  Aim antenna at S/G and horn  Scan antenna in azimuth & elevation  S/G needs to be in far field (?):  Far field distance = 2D 2 /   D = antenna diameter, = wavelength  Examples of two antennas:  C-band, 3.66 m (12 ft) --> 495 m ~0.5 km  S-band, 8.53 m (28 ft) --> 1360 m ~1.5 km  Excellent results 1/19/1 14

16. Standard Targets  Sphere  Tethered  Lots of work, good results  Dihedral  Surveyed position  Gives gain, azimuth & range  Can also give beam pattern  Quite convenient; good results 1/19/1 15

17. Gain using standard target - sphere  Use sphere on tethered balloon at some location 3- 15 km from radar.  Location must be free of ground clutter.  Scan target in range and azimuth and use peak value recorded.  Use point radar equation to calculate gain.  Backscattering cross-sectional area of sphere is either geometric or resonant region.  if resonant, use Fig. 4.2, pg. 72, Radar for Meteorologists, Fig 4.2, pg. 37, Battan, 1973: Radar Observation of the Atmosphere; 1/19/1 16

18. …a pet peeve: 1/19/1 17

19. 1/19/1 18

20. Antenna gain using dihedral target  Mount dihedral target 5-15 km from radar  Avoid nearby ground clutter.  Using motorized nodding mechanism, allow dihedral to nod up and down through a position normal to beam.  Aim antenna in azimuth and elevation for peak signal.  Record signal amplitude and use strongest found.  Calculate gain using radar equation for point targets. 1/19/1 19

21. Nodding Dihedral Top view Side view Perspective view Nodding action 1/19/1 20

22. Side view of dihedral target Pivot point Eccentric cam Motor 1/19/1 21

23. 1/19/1 22

24. Working on the dihedral Bill Bradley (on pole) Greg Muir observing. Looking west; radar located NW 1/19/1 23

25. Signal from dihedral while nodding (+ calibration signal) 1/19/1 24

26. Advantages of dihedral  Excellent way to get antenna gain  Good check on range and azimuth of radar  Inexpensive to operate (once installed)  Not labor intensive  Quick: can get G within 10 min or so  Can use it without nodding once set  then it’s even faster 1/19/1 25

27. Secondary Standard Targets  Strong, isolated radio towers, water towers, or buildings  Beware of changes  Useful for quickly monitoring overall system “health”  Check of receiver, transmitter, azimuth and range 1/19/1 1

28. Sun  Useful for measuring antenna gain  Too weak to get a full beam pattern  Also, not quite a point target, so more difficult to use. http://134.153.112.105/t-se-anim.gif 1/19/1 2

29. Antenna  the transducer that converts the electrical signal into an electromagnetic signal  the interface between the hardware and the medium carrying the EM signal  consists of actual antenna and a reflector 1/19/1 3

30. Reflector  parabolic in cross-section focus reflector 1/19/1 4

31. Reflector rays from focus are reflected parallel into space 1/19/1 5

32. Reflector rays from space are reflected back to the focal point 1/19/1 6

33. Antenna  Actual antenna is either a horn or a dipole: half-wavelength dipole antenna sub-reflector Feed horn 1/19/1 7

34. Feedhorn  Need to connect feedhorn to the rest of the system somehow. 1/19/1 8

35. Alternate arrangements Off-set Parabolic 1/19/1 9

36. Feedhorn and waveguide; tabs for supports 1/19/1 10

37. 1/19/1 11

38. NCAR CP-2 dual- polarization antenna 1/19/1 12

39. Dual- polarization feedhorn and antenna ( CSU- CHILL) http://radarmet.atmos. colostate.edu/CHILL/Pix.html 1/19/1 13

40. Dual-polarization feed on EEC radar 1/19/1 14

41. Reflector cross-section (viewed from front or back) Circular “orange peel” vertical (height-finding) horizontal (azimuth finding) 1/19/1 15

42. Reflector  Directs signal into space, i.e., focuses it in the desired direction  Generally parabolic in shape  Larger antennas give smaller beamwidths (for the same wavelength signal)  Higher frequencies require smaller antennas for the same beamwidth  aircraft usually use X or C band  ground-based radars usually use S or C band 1/19/1 16

43. Isotropic antenna  An isotropic antenna radiates equally in all directions  Examples:  the sun and other stars  a candle (except downward)  fireworks or explosions  Real antennas are never truly isotropic 1/19/1 17

44. The advantage of using a reflector  Reflectors focus energy into a particular direction.  Reflectors make the energy at some point stronger than it would have been otherwise.  Reflectors allow us to determine direction to a target. 1/19/1 18

45. Intensity at target without reflector 1/19/1 19

46. Intensity at target with reflector Reflector 1/19/1 20

47. Antenna gain  The gain of antenna is the ratio of the power at a point when an antenna is used to that from an isotropic antenna at the same point. 1/19/1 21

48. Gain of real antennas  isotropic1.0  simple dipole1.5  small circular parabolic4000  UND (12-ft diameter, C-band)23700  WSR-88D (28-ft dia., S-band)31600 1/19/1 22

49. Logarithmic units  Because some parameters vary over several orders of magnitude, it is sometimes convenient to convert to a logarithmic scale: logarithmic power ratio [dB] = 10log 10 (p 1 /p 2 ) where the logarithmic units are decibels. 1/19/1 23

50. Logarithmic gain where p 1 is the (linear) power with the antenna, p 2 is the (linear) power of an isotropic antenna, g is the linear gain (unitless number) and G is the logarithmic gain of the antenna measured in decibels. p 1 and p 2 need to be measured or converted to the same units; milliwatts are frequently used. 1/19/1 24

51. Gain of real antennas (logarithmically)  isotropic 0 dB  simple dipole1.8 dB  small circular parabolic 36 dB  UND 12 ft antenna43.75 dB  WSR-88D 28-ft antenna45 dB 1/19/1 25

52. Antenna beamwidth  The angular width of an antenna pattern  The angular width where the power density is 1/2 that on the axis of the beam.  half-power point or 3-dB point 1/19/1 1

53. Antenna beamwidth Antenna beam axis Double the angle to get the half-power point antenna beamwidth.  Measure power on beam axis Measure angle from axis to half-power point (at the same range). 1/19/1 2

54. Gain vs. Beamwidth Gain and beamwidth are related by equation (Battan, 1973): where g is the linear gain of the antenna, k 2 depends upon the shape of the antenna. k 2 = 1 for circular reflectors.  and  are the horizontal and vertical beamwidths, respectively. Beamwidths must be measured in radians. 1/19/1 3

55. Antenna sidelobes  There are no perfect antennas!  All antennas have antenna patterns which include  main lobe  side lobes  back lobes 1/19/1 4

56. Top-hat beam pattern  Simplest assumption - no power at all until in the beam pattern, then uniform power. Power Relative angle 1/19/1 5

57. Top-hat pattern (in polar coordinates) Antenna beam axis 1/19/1 6

58. Gaussian beam pattern Relative angle Power 1/19/1 7

59. Gaussian beam pattern (in polar coordinates) Antenna beam axis 1/19/1 8

60. Simple sidelobes - CPS-9 0 1 2 3 4 5 Relative angle (deg) Relative gain (dB) 0 -10 -20 -30 -40 1/19/1 9

61. CPS-9 Gain in Polar Coordinates 1/19/1 10

62. Examples of real antennas  NCAR CP-2 S- and X-band dual- wavelength radar  Lincoln Lab FL2 S-band radar  UND C-band radar 1/19/1 11

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64. Notice (20 years too late?)  Slight (~0.3°) offset in S- and X- band mainlobe pointing directions!  Different sidelobes  Different mainlobe widths 1/19/1 13

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69. Using ground targets to measure beam patterns  Pick strong target fairly close by.  If needed, use very strong target for sidelobes and somewhat weaker target for mainlobe  If target saturates receiver, top of mainlobe will be lost.  If target is too weak, sidelobes are lost.  Scan with resolution about a third to fifth of the beamwidth in azimuth & elevation. 1/19/1 18

70. UND radar beam pattern  Used 2000-ft radio tower located 67 km toward SSW.  Scanned with 0.2° average interval in azimuth and 0.2° elevation steps  Note: beamwidth is 0.97° 1/19/1 19

71. UND radar beam pattern  Adjusted raw azimuths to the nearest 0.2°  Found CW vs. CCW azimuth offset  Adjusted one of these to agree with the other.  Kept CW data the same, adjusted CCW 0.2° CCW to reduce offset to zero).  It would be better to use calculated azimuth to target as standard.  Get from GPS positions. 1/19/1 20

72. Procedure for getting pattern Scan target & record data Edit data to common angles & remove spurious targets Smooth data, Correct for hysteresis Plot pattern 1/19/1 21

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76. Conclusions:  Knowledge of antenna characteristics will make you a better meteorologist (and better person).  Adopt a pet target or two and use them occasionally to check on the health of your system  Be aware that what you see on a radar is biased by the antenna beam pattern. 1/19/1 25

77. References added after RADCAL Workshop:  Rinehart, R. E., P. J. Eccles, 1976: Use of a nodding dihedral target for antenna gain measurements. 17 th Conf. on Radar Meteorology, Seattle, WA, pp 66-71.  Rinehart, R. E., and Charles L. Frush, 1983: Comparison of antenna beam patterns obtained from near-field test measurements and ground target scans. 21 st Radar Meteorology Conf., Edmonton, Canada, pp 291- 295.  Rinehart, R. E., and John D. Tuttle, 1981: A technique for determining antenna beam patterns using a ground target. 20 th Conf. on Radar Meteorology. Boston, MA, pp 672-675. 1/19/1 26