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Phased-Array Feed Measurements at Green Bank
Karl Warnick, Brian JeffsJonathan Landon, Mike Elmer
Roger Norrod, Rick Fisher
R&D Challenges
• Technical– Mutual coupling– Receiver/element noise– Calibration– Total bandwidth
• Cost Drivers– Receiver system complexity– Signal processing
20-Meter Experiments
• October 2007– Tsys ~150K
• July-August 2008– LNA Ta = 30K, Tsys ~65K– Sky noise measurements– Direct Control of 20-m
• ~1600 MHz, 0.5 MHz BW• 19 A/D samplers to disk• Software beam-forming
Mutual Coupling Effects
• Changes element impedances– Beam-dependent
• Couples amplifier noise• Changes element patterns• All frequency dependent
Matching network
0
Noise Budget
Measured Model
LNA Tmin 33 K 33 K
Noise coupling 20 K 23 K
Spillover 5 K 5 K
Sky 3 K 3 K
Loss 4 K --
Tsys: 65 K 64 K
Noise matching efficiency 60%Noise Matching Efficiency = 60%
---------- ----------
Calibration
• Optimize Ta/Tsys at each beam position in the FOV– Use strong radio source
– R = cross-correlation matrix of all elements on and off source– w = beam-forming complex weights– λmax = highest eigenvalue corresponding to the dominant eigenvector
of Roffw
• Experience thus far is that calibration is stable for hours and possibly days in the experimental setup.
• Calibration method does not distinguish between various noise and efficiency factors.
Cygnus-X Region Mosaic
Cross Elevation (Degrees)
Ele
vatio
n (
Deg
ree
s)
-4 -2 0 2 4-4
-2
0
2
4
0
50
100
150
Canadian Galactic Plane SurveyConvolved to 20-m Beam
RFI Cancellation with Array Processing
Steps Toward a Cooled PFA
May 2009 9
Cool the Element Antennas?
May 2009 10
Polystyrene cover – low loss, low k, and strong enough, but outgassed heavily.
Machined PTFE cover – low loss, moderate k, strong.
Cool the Antenna Elements?
May 2009 11
So, PTFE vacuum cover is probably workable but concerns remain: Radiation loading, scattering, loss, robustness, interface with ground plane.
Too thin cover after ~1mo under vacuum. PTFE creep.
Stress analysis
Noise Budget ~20cm
Item Noise Comment
MW Background 3 K
Atm 1
LNA 4 Balanced @ 15K
Spillover/Scatter 3-6 Antenna dependent
Subtotal 11-14 K We’re stuck with this!
Array Noise 3-6 ? Mutual coupling effects
Input Losses 3-6 ? Ohmic to LNA input
Total 17-26 K
May 2009 12
Warm Antenna Elements?
May 2009 13
Item Loss Temp Noise
Dipole Post 0.02 dB 300 K 1.4 K
Hermetic Bead 0.07 300 4.9 !!
SS Coax 0.09 150 3.1
SMA Conn 0.02 15 0.1
Subtotal 9.5 K
Test Facility
May 2009 14
New absorber in retractable roof, surveyed local obstacles and ground shield EM models for spillover estimation.
Summer ‘08