Ronald E. Rinehart University of North Dakota Grand Forks, ND 58202-9006

1. Antenna Measurements:Dihedrals, ground targets and antenna beam patternsAMS Radar Calibration WorkshopAlbuquerque, New Mexico13-14 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 1/19/1

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

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

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

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

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

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

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

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

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

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

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

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

14. 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 = 2D2/l, • D = antenna diameter, l = 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 14 1/19/1

15. 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 15 1/19/1

16. 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; 16 1/19/1

17. …a pet peeve: 17 1/19/1

18. 18 1/19/1

19. 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. 19 1/19/1

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

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

22. 22 1/19/1

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

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

25. 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 25 1/19/1

26. 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 1/19/1

27. 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. 2 http://134.153.112.105/t-se-anim.gif 1/19/1

28. 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 3 1/19/1

29. Reflector • parabolic in cross-section reflector focus 4 1/19/1

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

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

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

33. Feedhorn • Need to connect feedhorn to the rest of the system somehow. 8 1/19/1

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

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

36. 11 1/19/1

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

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

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

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

41. 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 16 1/19/1

42. 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 17 1/19/1

43. 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. 18 1/19/1

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

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

46. 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. 21 1/19/1

47. Gain of real antennas • isotropic 1.0 • simple dipole 1.5 • small circular parabolic 4000 • UND (12-ft diameter, C-band) 23700 • WSR-88D (28-ft dia., S-band) 31600 22 1/19/1

48. 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] = 10•log10(p1/p2) where the logarithmic units are decibels. 23 1/19/1

49. Logarithmic gain where p1 is the (linear) power with the antenna, p2 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. p1 and p2 need to be measured or converted to the same units; milliwatts are frequently used. 24 1/19/1

50. Gain of real antennas(logarithmically) • isotropic 0 dB • simple dipole 1.8 dB • small circular parabolic 36 dB • UND 12 ft antenna 43.75 dB • WSR-88D 28-ft antenna 45 dB 25 1/19/1