Antenna Measurements:Antenna Measurements:Dihedrals, ground targets and antenna Dihedrals, ground targets and antenna

beam patternsbeam patterns

AMS Radar Calibration WorkshopAMS Radar Calibration WorkshopAlbuquerque, New MexicoAlbuquerque, New Mexico

13-14 January 200113-14 January 2001

Ronald E. RinehartRonald E. Rinehart

University of North DakotaUniversity of North Dakota

Grand Forks, ND 58202-9006Grand Forks, ND 58202-9006Voice: 701-777-2183; fax: 701-777-5032Voice: 701-777-2183; fax: 701-777-5032

email: rinehart@aero.und.edu or email: rinehart@aero.und.edu or radarwx@aol.comradarwx@aol.com

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Other speakers in this Other speakers in this session:session:

Ken TappingKen Tapping John LutzJohn Lutz Dave Brunkow & John HubbeDave Brunkow & John Hubbe Dick DoviakDick Doviak

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Speakers in this session:Speakers in this session:

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Beware of the hazard Beware of the hazard associated with this talk:associated with this talk:

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Growing a crop Growing a crop of antennas atof antennas at

EEC, Enterprise, ALEEC, Enterprise, AL

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More antennas growing More antennas growing in New Mexicoin New Mexico

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Why we need to know Why we need to know antenna parameters:antenna parameters:

Point target radar equation:Point target radar equation:

Meteorological target radar equation:Meteorological target radar equation:

43

22

64 r

Agpp t

r

pp g c K zl

rrt

5 2 2

2 21024 2ln

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receivertransmitter

modulatormasterclock

displaysignal processor/computer

43

22

64 r

gpp t

r

r

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receivertransmitter

modulatormasterclock

displaysignal processor/computer

r

pp g c K zl

rrt

5 2 2

2 21024 2ln

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Here’s what we think happens:Here’s what

actually happens:And it gets even worse!

A radar’s view of a storm:A radar’s view of a storm:

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Antenna characteristics Antenna characteristics than need to be than need to be

measured:measured: gaingain

mainlobemainlobe sidelobessidelobes complete pattern complete pattern

beamwidthbeamwidth

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How can we measure How can we measure beamwidth, gain and beamwidth, gain and

antenna beam pattern?antenna beam pattern? Antenna rangeAntenna range Signal generator/hornSignal generator/horn Standard targetStandard target Secondary-standard targetSecondary-standard target SunSun

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Use of antenna range:Use of antenna range:

Requires moving the antenna to the Requires moving the antenna to the antenna range.antenna range. ExpensiveExpensive Time-consumingTime-consuming Excellent resultsExcellent results

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Signal Generator & HornSignal Generator & Horn Aim antenna at S/G and hornAim antenna at S/G and horn

Scan antenna in azimuth & elevationScan antenna in azimuth & elevation S/G needs to be in far field (?):S/G needs to be in far field (?):

Far field distance = Far field distance = 2D2D22// DD = antenna diameter, = antenna diameter, = wavelength = wavelength Examples of two antennas:Examples of two antennas:

C-band, 3.66 m (12 ft) --> 495 m ~0.5 kmC-band, 3.66 m (12 ft) --> 495 m ~0.5 km S-band, 8.53 m (28 ft) --> 1360 m ~1.5 kmS-band, 8.53 m (28 ft) --> 1360 m ~1.5 km

Excellent resultsExcellent results

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Standard TargetsStandard Targets

SphereSphere TetheredTethered Lots of work, good resultsLots of work, good results

DihedralDihedral Surveyed positionSurveyed position

Gives gain, azimuth & rangeGives gain, azimuth & range Can also give beam patternCan also give beam pattern

Quite convenient; good resultsQuite convenient; good results

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Gain using standard target - sphereGain using standard target - sphere Use sphere on tethered balloon at some location 3-15 km Use sphere on tethered balloon at some location 3-15 km

from radar.from radar. Location must be free of ground clutter.Location must be free of ground clutter.

Scan target in range and azimuth and use peak value Scan target in range and azimuth and use peak value recorded.recorded.

Use point radar equation to calculate gain.Use point radar equation to calculate gain. Backscattering cross-sectional area of sphere is either Backscattering cross-sectional area of sphere is either

geometric or resonant region.geometric or resonant region. if resonant, use Fig. 4.2, pg. 72, if resonant, use Fig. 4.2, pg. 72, Radar for MeteorologistsRadar for Meteorologists , Fig 4.2, , Fig 4.2,

pg. 37, Battan, 1973: pg. 37, Battan, 1973: Radar Observation of the AtmosphereRadar Observation of the Atmosphere; ;

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……a pet peeve:a pet peeve:

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Antenna gain Antenna gain usingusing dihedral target dihedral target

Mount dihedral target 5-15 km from radar Mount dihedral target 5-15 km from radar Avoid nearby ground clutter.Avoid nearby ground clutter.

Using motorized nodding mechanism, allow dihedral Using motorized nodding mechanism, allow dihedral to nod up and down through a position normal to to nod up and down through a position normal to beam.beam.

Aim antenna in azimuth and elevation for peak signal.Aim antenna in azimuth and elevation for peak signal. Record signal amplitude and use strongest found.Record signal amplitude and use strongest found. Calculate gain using radar equation for point targets.Calculate gain using radar equation for point targets.

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Nodding DihedralNodding Dihedral

Top view Side view Perspectiveview

Noddingaction

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Side view of dihedral Side view of dihedral targettarget

Pivot point

Eccentric cam

Motor

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Working on Working on the dihedralthe dihedral

Bill Bradley (on pole)Bill Bradley (on pole)Greg Muir observing.Greg Muir observing.

Looking west;Looking west;radar located NWradar located NW

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Signal from dihedral while noddingSignal from dihedral while nodding (+ calibration signal) (+ calibration signal)

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Advantages of dihedralAdvantages of dihedral Excellent way to get antenna gainExcellent way to get antenna gain Good check on range and azimuth of radarGood check on range and azimuth of radar Inexpensive to operate (once installed)Inexpensive to operate (once installed)

Not labor intensiveNot labor intensive

Quick: can get G within 10 min or soQuick: can get G within 10 min or so Can use it without nodding once setCan use it without nodding once set

then it’s even fasterthen it’s even faster

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Secondary Standard Secondary Standard TargetsTargets

Strong, isolated radio towers, water Strong, isolated radio towers, water towers, or buildingstowers, or buildings

Beware of changesBeware of changes Useful for quickly monitoring overall Useful for quickly monitoring overall

system “health”system “health” Check of receiver, transmitter, azimuth and Check of receiver, transmitter, azimuth and

rangerange

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SunSun

Useful for measuring antenna Useful for measuring antenna gaingain

Too weak to get a full beam Too weak to get a full beam patternpattern

Also, not quite a point target, so Also, not quite a point target, so more difficult to use.more difficult to use.

http://134.153.112.105/t-se-anim.gif1/19/1

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AntennaAntenna

the the transducertransducer that converts the that converts the electrical signal into an electromagnetic electrical signal into an electromagnetic signalsignal

the the interfaceinterface between the hardware and between the hardware and the medium carrying the EM signalthe medium carrying the EM signal

consists of actual consists of actual antennaantenna and a and a reflectorreflector

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ReflectorReflector parabolic in cross-sectionparabolic in cross-section

focus

reflector

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ReflectorReflector

rays from focus are reflectedparallel into space

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ReflectorReflector

rays from space are reflectedback to the focal point

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AntennaAntenna Actual antenna is either a horn or a Actual antenna is either a horn or a

dipole:dipole:half-wavelengthdipole antenna

sub-reflector

Feed horn

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FeedhornFeedhorn Need to connect feedhorn to the Need to connect feedhorn to the

rest of the system somehow.rest of the system somehow.

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Alternate arrangementsAlternate arrangements

Off-set Parabolic

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Feedhorn andFeedhorn andwaveguide;waveguide;

tabs for supportstabs for supports

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NCAR CP-2 NCAR CP-2 dual-dual-

polarization polarization antennaantenna

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Dual-Dual-polarization polarization feedhorn and feedhorn and antenna (antenna (CSU-CSU-

CHILL)CHILL)

http://radarmet.atmos.http://radarmet.atmos.colostate.edu/CHILL/Pix.htmlcolostate.edu/CHILL/Pix.html

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Dual-polarization feed on Dual-polarization feed on EEC radarEEC radar

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Reflector cross-sectionReflector cross-section(viewed from front or back)(viewed from front or back)

Circular“orange peel”

vertical(height-finding)

horizontal(azimuth finding)

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ReflectorReflector Directs signal into space, i.e., focuses it in the Directs signal into space, i.e., focuses it in the

desired directiondesired direction Generally parabolic in shapeGenerally parabolic in shape Larger antennas give smaller beamwidths (for the Larger antennas give smaller beamwidths (for the

same wavelength signal)same wavelength signal) Higher frequencies require smaller antennas for Higher frequencies require smaller antennas for

the same beamwidththe same beamwidth aircraft usually use X or C bandaircraft usually use X or C band ground-based radars usually use S or C bandground-based radars usually use S or C band

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Isotropic antennaIsotropic antenna

An An isotropic antenna isotropic antenna radiates equally in radiates equally in all directionsall directions

Examples:Examples: the sun and other starsthe sun and other stars a candle (except downward)a candle (except downward) fireworks or explosionsfireworks or explosions

Real antennas Real antennas are never truly isotropicare never truly isotropic

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The advantage of using a The advantage of using a reflectorreflector

Reflectors focus energy into a Reflectors focus energy into a particular direction.particular direction.

Reflectors make the energy at Reflectors make the energy at some point stronger than it some point stronger than it would have been otherwise.would have been otherwise.

Reflectors allow us to determine Reflectors allow us to determine direction to a target.direction to a target.

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Intensity at target Intensity at target withoutwithout reflector reflector

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Intensity at target Intensity at target withwith reflector reflectorReflectorReflector

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Antenna gainAntenna gain

The gain of antenna is the ratio of the The gain of antenna is the ratio of the power at a point when an antenna is used power at a point when an antenna is used to that from an isotropic antenna at the to that from an isotropic antenna at the same point.same point.

gp

pwith antenna

isotropic antenna _

_

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Gain of real antennasGain of real antennas isotropicisotropic 1.01.0 simple dipolesimple dipole 1.51.5 small circular parabolicsmall circular parabolic 40004000 UND (12-ft diameter, C-band)UND (12-ft diameter, C-band) 2370023700 WSR-88D (28-ft dia., S-band)WSR-88D (28-ft dia., S-band) 3160031600

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Logarithmic unitsLogarithmic unitsBecause some parameters vary over Because some parameters vary over several orders of magnitude, it is sometimes several orders of magnitude, it is sometimes convenient to convert to a logarithmic convenient to convert to a logarithmic scale:scale:

logarithmic power ratio [dB] logarithmic power ratio [dB] = 10•= 10•loglog1010((pp11/p/p22))

where the logarithmic units are decibels.where the logarithmic units are decibels.

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Logarithmic gainLogarithmic gain

gp

pG 10

2

110 log10log10

where where pp11 is the (linear) power with the is the (linear) power with the antenna, antenna, pp22 is the (linear) power of an is the (linear) power of an isotropic antenna, isotropic antenna, gg is the linear gain is the linear gain (unitless number) and (unitless number) and GG is the is the logarithmic gain of the antenna logarithmic gain of the antenna measured in decibels. measured in decibels. pp11 and and pp22 need to need to be measured or converted to the same be measured or converted to the same units; milliwatts are frequently used.units; milliwatts are frequently used.

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Gain of real antennasGain of real antennas(logarithmically)(logarithmically)

isotropicisotropic 0 dB 0 dB simple dipolesimple dipole 1.8 dB1.8 dB small circular parabolicsmall circular parabolic 36 dB 36 dB UND 12 ft antennaUND 12 ft antenna 43.75 dB43.75 dB WSR-88D 28-ft antennaWSR-88D 28-ft antenna 45 dB45 dB

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Antenna beamwidthAntenna beamwidth

The angular width of an antenna patternThe angular width of an antenna pattern The angular width where the power The angular width where the power

density is 1/2 that on the axis of the density is 1/2 that on the axis of the beam.beam. half-power point or 3-dB pointhalf-power point or 3-dB point

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Antenna beamwidthAntenna beamwidth

AntennaAntenna

beam axisbeam axis

Double the angle to getDouble the angle to getthe the half-power pointhalf-power pointantenna beamwidth.antenna beamwidth.

Measure Measure power on power on beam axisbeam axis

Measure angle from axisMeasure angle from axisto half-power point to half-power point (at the same range).(at the same range).

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Gain Gain vs. vs. BeamwidthBeamwidthGain and beamwidth are related by equation (Battan,Gain and beamwidth are related by equation (Battan,1973):1973):

22k

g

where where gg is the linear gain of the antenna, is the linear gain of the antenna, kk22 depends dependsupon the shape of the antenna. upon the shape of the antenna. kk22 = 1 for circular = 1 for circularreflectors. reflectors. and and are the horizontal and verticalare the horizontal and verticalbeamwidths, respectively. Beamwidths must be beamwidths, respectively. Beamwidths must be measured in measured in radiansradians..

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Antenna sidelobesAntenna sidelobes

There are no perfect antennas!There are no perfect antennas! All antennas have antenna patterns All antennas have antenna patterns

which includewhich include main lobemain lobe side lobesside lobes back lobesback lobes

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Top-hat beam patternTop-hat beam pattern Simplest assumption - no power at all until Simplest assumption - no power at all until

in the beam pattern, then uniform power.in the beam pattern, then uniform power.

PowerPower

Relative angleRelative angle

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Top-hat patternTop-hat pattern(in polar coordinates)(in polar coordinates)

AntennaAntenna

beam axisbeam axis

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Gaussian beam patternGaussian beam pattern

Relative angleRelative angle

PowerPower

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Gaussian beam patternGaussian beam pattern(in polar coordinates)(in polar coordinates)

AntennaAntenna

beam axisbeam axis

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Simple sidelobes - CPS-9Simple sidelobes - CPS-9

-80

-60

-40

-20

0

0 1 2 3 4 5Angular distance from mainlobe (deg)

Tw

o-w

ay g

ain

(dB

)

0 1 2 3 4 5

Relative angle (deg)

Rela

tive g

ain

(d

B)

0

-10

-20

-30

-40

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CPS-9 Gain in Polar CoordinatesCPS-9 Gain in Polar Coordinates

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Examples of real antennasExamples of real antennas

NCAR CP-2 S- and X-band dual-NCAR CP-2 S- and X-band dual-wavelength radarwavelength radar

Lincoln Lab FL2 S-band radarLincoln Lab FL2 S-band radar UND C-band radarUND C-band radar

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CP-2 S-Band and X-Band Antenna Gain

Based on 6 J uly 1981 data, radio tower 2.18 km away.

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191

Azimuth (deg)

Rel

ativ

e Gai

n (d

B)

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Notice (20 years too late?)Notice (20 years too late?)

Slight (~0.3°) offset in S- and X-Slight (~0.3°) offset in S- and X-band mainlobe pointing band mainlobe pointing directions!directions!

Different sidelobesDifferent sidelobes Different mainlobe widthsDifferent mainlobe widths

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FL2 Antenna Beam Pattern(smoothed, so nearby sidelobes do not show)

-70

-60

-50

-40

-30

-20

-10

0

-120 -60 0 60 120 180

Relative Angle (°)

Rel

ativ

e O

ne-

Way

Gai

n (

dB

)

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FL2 Antenna Beam Pattern (smoothed)

Relative

Two-Way

Gain (dB) - 20

- 40

- 60

0

- 10

- 30

- 50

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Using ground targets to Using ground targets to measure beam patternsmeasure beam patterns

Pick strong target fairly close by.Pick strong target fairly close by. If needed, use very strong target for sidelobes and If needed, use very strong target for sidelobes and

somewhat weaker target for mainlobesomewhat weaker target for mainlobe If target saturates receiver, top of mainlobe will be lost.If target saturates receiver, top of mainlobe will be lost. If target is too weak, sidelobes are lost.If target is too weak, sidelobes are lost.

Scan with resolution about a third to fifth of Scan with resolution about a third to fifth of the beamwidth in azimuth & elevation.the beamwidth in azimuth & elevation.

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UND radar beam patternUND radar beam pattern

Used 2000-ft radio tower located 67 km Used 2000-ft radio tower located 67 km toward SSW.toward SSW.

Scanned with 0.2° average interval in Scanned with 0.2° average interval in azimuth and 0.2° elevation stepsazimuth and 0.2° elevation steps Note: beamwidth is 0.97°Note: beamwidth is 0.97°

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UND radar beam patternUND radar beam pattern Adjusted raw azimuths to the nearest 0.2° Adjusted raw azimuths to the nearest 0.2° Found CW vs. CCW azimuth offsetFound CW vs. CCW azimuth offset Adjusted one of these to agree with the other.Adjusted one of these to agree with the other.

Kept CW data the same, adjusted CCW 0.2° CCW Kept CW data the same, adjusted CCW 0.2° CCW to reduce offset to zero).to reduce offset to zero).

It would be better to use calculated azimuth to It would be better to use calculated azimuth to target as standard.target as standard. Get from GPS positions.Get from GPS positions.

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Procedure for getting patternProcedure for getting pattern

Scan target& record data

Edit data to common angles

& remove spurious targets

Smooth data, Correct for hysteresis

Plot pattern

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Height of UND Radar Beam Looking Toward Radio TowerTower at 66.25 km range, 193.4° azimuth

0

5

10

15

20

25

30

0 20 40 60 80 100Range (km)

Hei

ght

(kft

)

4 3.8 3.6 3.4 3.2 3 2.8 2.6 2.4 2.2 2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 GroundTower

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-66-60-54-48-42-36-30-24-18-12-60

Rel

ativ

e T

wo-

way

Gai

n

(dB)

UND Radar Antenna Beam Pattern5 December 2000

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Conclusions:Conclusions: Knowledge of antenna characteristics will Knowledge of antenna characteristics will

make you a better meteorologist (and better make you a better meteorologist (and better person).person).

Adopt a pet target or two and use them Adopt a pet target or two and use them occasionally to check on the health of your occasionally to check on the health of your systemsystem

Be aware that what you see on a radar is Be aware that what you see on a radar is biased by the antenna beam pattern.biased by the antenna beam pattern.

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References added after References added after RADCAL Workshop:RADCAL Workshop:

Rinehart, R. E., P. J. Eccles, 1976: Use of a nodding dihedral target for Rinehart, R. E., P. J. Eccles, 1976: Use of a nodding dihedral target for antenna gain measurements. 17antenna gain measurements. 17thth Conf. on Radar Meteorology, Seattle, Conf. on Radar Meteorology, Seattle, WA, pp 66-71.WA, pp 66-71.

Rinehart, R. E., and Charles L. Frush, 1983: Comparison of antenna Rinehart, R. E., and Charles L. Frush, 1983: Comparison of antenna beam patterns obtained from near-field test measurements and ground beam patterns obtained from near-field test measurements and ground target scans. 21target scans. 21stst Radar Meteorology Conf., Edmonton, Canada, pp 291- Radar Meteorology Conf., Edmonton, Canada, pp 291-295.295.

Rinehart, R. E., and John D. Tuttle, 1981: A technique for determining Rinehart, R. E., and John D. Tuttle, 1981: A technique for determining antenna beam patterns using a ground target. 20antenna beam patterns using a ground target. 20 thth Conf. on Radar Conf. on Radar Meteorology. Boston, MA, pp 672-675.Meteorology. Boston, MA, pp 672-675.

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