parabolic dish focus, zoom and tilt - ntms · dish pattern simulation • ansoft hfss • more...
TRANSCRIPT
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Parabolic DishFocus, Zoom
and TiltPaul Wade
W1GHZ
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EME Prevention
• TREES – no window
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Solution – new QTH
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Clear View
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Dish Pattern Simulation
• Ansoft HFSS• More detail than
measurement• No reflections• See affect of
focus, zoom, andtilt
15Dish,
Coffee-can feed
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BetterFeed
• More gain• Smaller
sidelobes• Same
beamwidth
15Dish,
Super-VE4MA feed
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Dish Focus
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0.25Closer to Reflector
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0.5Closer to Reflector
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0.75Closer to Reflector
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1Closer to Reflector
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1.25Closer to Reflector
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1.5Closer to Reflector
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1.75Closer to Reflector
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2Closer to Reflector
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2.5Closer to Reflector
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3Closer to Reflector
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Plot the gain vs focus shiftDish with Axial Feed Displacement
-5
0
5
10
15
20
25
30
35
-3 -2 -1 0 1 2 3
Feed Aperture Position in Wavelengths from Dish Focal Point
Gai
n(d
Bi)
Super VE4MA
Coffee-can
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Axial Defocusing Loss
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Focus Summary
• As Feed moves toward reflector, patterngets wider, gain decreases (Conservationof Energy)
• Other direction similar• Focus is critical for deep dish
– ¼ error = ~1dB– For any diameter dish
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How to focus accurately
• Range > 50x Rayleigh Distance (Imbriale)– Not practical for EME dish
• Sun Noise• Celestial Sources (Larger dishes)
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Practical Focus Error
• Normal antenna range ~= Rayleigh Distance• Focus at infinity = measured gain ~1 dB low• Focus on range = real gain ~ 1 dB low
– Feed will be too far from reflector
> Focus at 5x Rayleigh Distance = tiny error
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Focus for BIG Dish
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Offset Dish
18 inch DSS Dish,
W2IMU dual-mode feed
15.7 wavelengths
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BetterFeed
• More gain• Smaller
sidelobes• Same
beamwidth18 inch DSS Dish,Skobelev optimizeddual-mode feed
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Feed 0.83closer to reflector
uptilt
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Feed 1.66closer to reflector
uptilt
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Feed 0.83away from reflector
downtilt
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Offset Dish Focus Shift
• Axial feed shift tilts pattern– Closer tilts up– Away tilts down
• Maximum gain – only small decrease• Axial gain – decrease due to pattern tilt
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Lateral Feed Shift
• Beam Scanning• Multiple feeds – several bands
• Lateral Feed shift causes beam shift inopposite direction
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Lateral Feed Shift - Focus
• Shifted Focus follows Petzval Surface– Parabola with ½ radius of curvature– Tangent to focal plane at focus
• Shifted Focus farther away from reflector• Adjust for Maximum Gain
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Shift - Maximum Gain Contour
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Scanning Limit (1 dB Loss)
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Feed-scanning Loss
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Feed Pointing
StraightAheadBest
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Zoom Control
• Move feed in or out to broaden pattern• Easier to find signals
• BUT• Maximum beamwidth increase ~3 times• Gain ~10 dB lower at 3X beamwidth• Gain never higher anywhere than focused• So zoom only finds strong signals• Small shift can fill in null around main beam
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Alternative to Zoom
• Align bearing accurately– Beacon– Sun– Compass
• Know your pattern• Search by ½ beamwidth
– For 4 degree beam, move in 2 degree steps
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Tilt
• A few degrees uptilt can enhanceterrestrial signals
• More for rain scatter• Offset zoom = tilt
– Bad idea – no intuitive reference for level
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RV tilt gauge good for dishes
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Summary• Accurate focus is important, especially for
deep dishes• Sun noise or celestial source needed for
accurate focus• Lateral shifted feed adjusted independently• Zoom has little benefit• Tilt is useful, best done mechanically• Location of dish is important• www.w1ghz.org
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W1GHZ --> Vermont
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