1. ngVLA Receiver/Feed Options: Overview
Wes Grammer
U.S. Radio/Millimeter/Submillimeter Science Futures II
August

2. Introduction and Purpose
Map out possible receiver configurations for ngVLA to:
Compare relative sensitivity, to a first order
Obtain feedback on bandwidths, band boundaries, science impact
Briefly assess current technical risks and tradeoffs of configurations
Provide inputs to overall cost model (# of bands, dewars)

3. Assumptions For All Configurations
Near-continuous frequency coverage from 1.2 – 116 GHz
Large gap from GHz, due to atmospheric absorption
W-band ( GHz) receiver identical in all configurations
LNAs in all bands are cryogenically cooled to ~20K.
Linearly-polarized outputs
VLA site typical weather, 45 degree elevation angle
Antenna type assumed:
Offset Gregorian optics, f/D ~ 0.55
‘Feed-low’ configuration
Unshaped optics (for this comparison; antenna will likely be shaped)
Subreflector ground shield for reduced spillover.

4. ‘Baseline’ Configuration
6 receiver bands, 3 dewars (full frequency range)
4 receiver bands, 1 dewar ( GHz)
Waveguide-bandwidth (~1.66:1) feeds and receivers > 11 GHz
Band configuration proposed by S. Srikanth, NRAO CDL
Feed horns cooled to ~20K on highest two bands (Q, W)
Packaged in a single, moderately-sized dewar
Performance slightly better than comparable EVLA receivers
Wideband (3:1) feeds and receivers < 11 GHz
Based on CSIRO ATCA feed, OMT, LNA designs
Performance possibly degraded relative to EVLA; TBC

5. Baseline Configuration Performance Estimates
Band #
Dewar #
fL GHz
fM GHz
fH GHz
fH: fL
BW GHZ
Feed
OMT
Pol. Out
Aperture Eff., ηA
Spillover, K
@ fL
@ fM
@ fH 1 A
1.2
2.1
3.6
3.00
2.4
Warm QR
Lin.
0.75 10 3 2 B
6.2
10.8
7.2
0.74 C 11
14.1 18
1.64 7 WG
0.73 4
23.2 30
1.67 12
0.72
0.71 5
38.7 50 20
Cold
0.68
0.64 6 70 90
116
1.66 46
0.52
0.43
0.31
TLNA, K
TRX, K
TSKY, K
TSYS, K
(TSYS/ηA), K
4.4
4.5
4.6 24 19 33 25 26 13 14
4.8
5.5 21 23 27 28
5.6
6.5
10.6 35 8
38.2
14.9 55 36 76 51 22
20.1
68.1 45 99 66
154 40 60 84 43
138
137 81
201
262
187
643

6. Wideband Receiver Configuration
4 receiver bands, 1 dewar (full frequency range)
Proposed by S. Weinreb, Caltech
All feeds are cooled (Band 1 to 80K, others to 20K)
Frequency range 1.2 – 50 GHz covered in 3 bands:
Band 1: 1.2 – 4.2 GHz (3.50:1 bw)
Band 2: 4.2 – 15 GHz (3.57:1 bw)
Band 3: 15 – 50 GHz (3.33:1 bw)
Wideband feeds are optimized quad-ridge type w/coax outputs (Akgiray / Soliman, Caltech)
Performance currently worse than baseline & UWB configs
UWB shown with CSIRO dielectric-loaded feed, a big difference
Shaping the antenna to raise illumination efficiency?

7. Wideband Configuration Performance Estimates
Dewar #
fL GHz
fM1 GHz
fM2 GHz
fH GHz
fH: fL
BW GHZ
Feed
OMT
Pol. Out
Aperture Eff., ηA
@ fL
@ fM1
@ fM2
@ fH 1 A
1.2
2.2
3.4
4.2
3.50 3
Cold QR
Lin.
0.63 2 8 10 15
3.57
10.8
0.62 23 35 50
3.33
0.61
0.59
0.54 4 70 90
100
116
1.66 46 WG
0.52
0.43
0.38
0.31
Spillover, K
TLNA, K
TRX, K 11 12 6 7 13 20 18 19 29 40 25 30 45 60
TSKY, K
TSYS, K
(TSYS/ηA), K
4.4
4.5
4.6
4.7 31 5
5.3 32 37
41.3
16.6
68.1 28 62 39
102 66
185 84 43 48
138
137 86 96
201
262
199
249
643

8. Ultra-Wideband (UWB) Receiver Configuration
3 receiver bands, 1 dewar (full frequency range)
Technology for 8-48 GHz receiver under development at NASA JPL (Lazio et. al.), in collaboration with Caltech.
All feeds are cooled (Band 1 to 80K, others to 20K)
Frequency range 1.2 – 48 GHz covered in 2 bands:
Band 1: 1.2 – 8 GHz (6.67:1 bw)
Band 2: 8 – 48 GHz (6.0:1 bw)
Wideband feed design based on Akgiray quad-ridge type:
May use dielectric loading for improved full-band aperture efficiency (D. Hoppe, JPL; A. Dunning, CSIRO). CSIRO data is shown.
New wideband LNAs under development at JPL (J. Velazco)
Performance good at lower frequencies, worse > 8 GHz

9. UWB Configuration Performance Estimates
Band #
Dewar #
fL GHz
fM1 GHz
fM2 GHz
fM3 GHz
fH GHz
fH: fL
BW GHZ
Feed
OMT
Pol. Out
Aperture Eff., ηA
@ fL
@ fM1
@ fM2
@ fM3
@ fH 1 A
1.2 3 5 6 8
6.67
6.8
Cold QR
Lin.
0.76
0.80
0.77
0.72 2 15 23 32 48
6.00 40
0.73
0.65 70 80 90
100
116
1.66 46 WG
0.52
0.48
0.43
0.38
0.31
Spillover, K
TLNA, K
TRX, K 10 4 11 12 13 20 19 22 24 25 34 30 50 35 45 60
TSKY, K
TSYS, K
(TSYS/ηA) , K
4.4
4.6
4.7
4.8 21 27 28 7
41.3
15.1
48.1 29 87 38 93 61
133 84 44 43
138
137 82 91 96
201
262
172
210
249
643

10. Precipitable Water Vapor (PWV) = 13mm
Elevation Angle = 45
Surface Accuracy (um RMS) = 185

11. Precipitable Water Vapor (PWV) = 13mm
Elevation Angle = 45
Surface Accuracy (um RMS) = 185

12. Quick Comparison Baseline configuration: Wideband configuration:
Has best overall sensitivity, especially above 10 GHz (at present)
Proven, mature technology => Low technical risk
Operating cost very high, if low frequency bands included
Wideband configuration:
Single dewar for GHz => Much lower operating cost
Sensitivity appears degraded relative to others (at present)
Moderate technical risk: development needed for feed, dewar pkg.
Ultra-wideband configuration:
Largest instantaneous bandwidth (40+ GHz/pol), fewest receivers
Sensitivity appears degraded relative to baseline (at present)
New technology => Higher technical risk

13. Summary Single dewar solution for high-frequency coverage > 10 GHz
Operating costs comparable to VLA, for N ~ 300
Optimized cryogenic receiver and system design may allow > N
Too early to settle on a receiver configuration
Need to settle on frequency limits for ngVLA
Most numbers shown are fairly rough estimates, and need to be refined by measurement of prototypes.
Wideband configurations have numerous advantages, but need to be more fully developed and tested.

14. Acknowledgments
Drs. Sivasankaran Srikanth and Marian Pospieszalski at the NRAO CDL, for review of noise and feed/antenna efficiency estimates in the baseline configuration.
Dr. Peter Napier, for his review of the estimates, and many helpful suggestions and comments, especially regarding antenna optics.
Dr. Larry D’Addario of NASA/JPL, for reviewing the estimates and for his many helpful and constructive suggestions.