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%

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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