1. EM COMPONENTS UPDATE & PERFORMANCE RESULTS
S. Srikanth
Critical Design Review
GBT K-Band Focal Plane Array
January 30-31, 2009

2. KFPA Feed Frequency 18-26.5 GHz Feed taper -13 dB
Compact/Profile taper horn
Ring-loaded corrugations
Aperture ID = 2.93” (5.5λ22)
OD = 3.4”
Length = 6.658”
Pitch = 0.082” (λ18/8)
Vane = 0.026”
Depth = 0.13” (0.28λ26.5)
69 corrugations
Θi = 8º ; Θmax = 17º

3. Layout of Elements (61 feeds)
Element spacing 3.45”

4. 22 GHz; Telescope beams in Azimuth (asym.) plane HPBW = 34”X”
Throw
(arcsec)
(HPBW)
3.45 94
2.7
6.9
189
5.5
10.4
284
8.2
13.8
378
11.0
17.3
473
13.7
Beam throw 27.4”/inch; 0.43HPBW/λ

5. 22 GHz; Telescope beams in Elevation (symm.) plane HPBW = 34”
Throw
(arcsec)
(HPBW) 6
164
4.8 12
331
9.6 18
498
14.5
Beam throw 27.4”/inch; 0.43HPBW/λ

6. Feed Offset & Aperture Efficiency
Circle #1 (3.4”rad): 7 feeds 62.9% min.
Circle#2 (6.9”rad): 19 feeds 62.9% min.
Circle#3 (10.4”rad): 37 feeds, 61.7% min.
Circle#4 (13.8”rad): 61 feeds, 58.9% (az); 61.6% (el)
Circle 18.0” : 91 feeds, 54.7% (az); 60.2% (el)

7. Optimum orientation of Elements

8. Beam width & Beam spacing
Freq.
(GHz)
HPBW
Beam Spacing
(HPBW) 18
40”
2.3 22
34”
2.7 24
31”
3.0
26.5
29”
3.2
Element spacing 3.45”; Beam spacing 94arcsec
Beam throw 27.4 arcsec/”

9. Measured Patterns & Taper
Freq.
(GHz)
H-pln
(dB)
E-pln 18
-11.7
-11.2 19
-12.6
-12.3 20
-13.2
-13.6 21
-13.7
-14.2 22 23
-13.5
-14.0 24 25
-13.0
-13.4 26
-14.3
26.5
-13.9
-14.7
Avg.

10. Measured Cross-polarization in 45 deg. plane

11. KFPA (Profile Horn) Feed Vs. 20 GHz (Linear Taper) Feed
Measured feed patterns

12. KFPA (Profile Horn) Feed Vs. 20 GHz (Linear Taper) Feed
Telescope beam patterns, Efficiency
Freq.
(GHz)
Linear taper
Profile
ήapl- ήapp
ήapl
(%)
Edgetaper (dB)
Edge taper (dB)
ήapp 18
-12.64
0.6995
-11.45
0.6965
0.43 20
-12.23
0.7284
-13.41
0.6560
9.94 22
-14.07
0.6888
-13.95
0.6189
10.15

13. KFPA (Profile Horn) Feed Vs. 24 GHz (Linear Taper) Feed
Measured feed patterns

14. KFPA (Profile Horn) Feed Vs. 24 GHz (Linear Taper) Feed
Telescope beam patterns, Efficiency
Freq.
(GHz)
Linear taper
Profile
ήapl- ήapp
ήapl
(%)
Edgetaper (dB)
Edge taper (dB)
ήapp 22
-12.10
0.6770
-13.95
0.6189
8.58 24
-12.58
0.6669
-13.48
0.6050
9.28
26.5
-12.46
0.6456
-14.32
0.5864
9.17

15. 18-26.5 GHz Phase Shifter 0.474 x 0.452; Length 4.84”
S. Srikanth, “A Wide-Band Corrugated Rectangular Waveguide Phase Shifter for Cryogenically Cooled
Receivers”, IEEE Microwave and Guided Wave Letters, Volume 7, pp , June 1997.

16. MEASURED RETURN LOSS

17. MEASURED INSERTION LOSS

18. Measured Phase Difference

19. ORTHO MODE JUNCTION 6061T6 Aluminum 353 Leaded brass
WR42 waveguide; 4.5” x 2.8”
Septum 0.010” (Beryl. Cu)
Pin 0.040” dia. (Beryl. Cu)
E. Wollack, “A Full Waveguide Band Orthomode Junction”, EDIR No. 303, NRAO, May 16, 1996

20. 6061T6 ALUMINUM Vs. 353 LEADED BRASS

21. Measured of S-parameters- Brass OMJ

22. Measured of S-parameters- Al. OMJ
Side arm
Main arm