1. Upgrade of the VLBA C-Band Feed & Receiver Design Review Presented by S durand Designed by Bob Hayward and: Sivasankaran Srikanth (Feed Design) Hollis Dinwiddie (Mechanical Design) Everett Callan (Master Receiver Builder) Gordon Coutts & Craig Hennies (OMT Testing) Marian Pospieszalski & the CDL Amplifier Group Pat Madigan & the VLA Machine Shop Michael Hedrick & the Green Bank Machine Shop 21 April 2011

2. Astronomers were eager to: Access the Methanol Maser at 6,668 MHz GHz Access to OH at 6016-6049 MHz Carry out continuum observations with two IF’s separated by several GHz to allow the effects of the atmosphere to be subtracted out Example : IF-AC=4.0-4.5 and ID-BD=7.5-8.0 GHz Limitations of the existing C-Band system: Mediocre sensitivity due to old GAsFET LNA’s Narrow bandwidth due to: –Septum Polarizer (4.5-5.2 GHz) –LNA’s (4.5-5.2 GHz) –Warm IF (4.5-5.2 GHz) –T105 Conversion (4.5-5.2 GHz) Why Upgrade the VLBA C-band ?

3. T(Rx) for Old VLBA vs. New EVLA Rx’s Average of 8 Dual-Channel Receivers

4. Required Specifications To modify the VLBA Receiver to give us the RF performance that is as close to that achieved by an EVLA Receiver as possible.

5. Sri’s VLBA 4-8 GHz Feed Design Profiled Corrugated Horn Four machined sections Ring-Loaded Mode Converter Plugs into old feed location Total Length= 37.5” Outer Diameter= 18.5” Weight= 55 lbs

6. Installation of the New Feed at VLBA Pie Town Done with Feed S/N 01 And Unmodified C-Band Rx Uses a Transition Plate so an old Receiver can be attached to a new Feed and thus allow installation of lateral support brackets and turnbuckles. Luckily feed is small enough to fit through the hatch (i.e., a crane is not required for hoisting it over the dish).

7. Existing VLBA C-Band Receiver Septum Pol RCP LCP LNA Pamtech CTB1107 Pamtech CTB1107 Omni-Spectra 2089-6203-00 T -30 dB Narda 4014C-30 Atlantic Microwave AMC 0935 GAsFET 4.5-5.2 GHz GAsFET 4.5-5.2 GHz T Noise Diode +20 -10 dB High-Cal ( Not implemented on VLBA) T Noise Diode -6 dB Omni-Spectra 2020-6617-10 Omni-Spectra 2020-6616-06 Pulse Cal T Cal PA Reactel 6B1-4850-700S12 4.5-5.2 GHz Miteq AMF-2B-4552 4.5-5.2 GHz Ditom D3I4080-1 4-8 GHz Ditom D3I4080-1 4-8 GHz PA Reactel 6B1-4850-700S12 4.5-5.2 GHz Miteq AMF-2B-4552 4.5-5.2 GHz Ditom D3I4080-1 4-8 GHz Ditom D3I4080-1 4-8 GHz T Narda 4014C-30 -30 dB T Cal & P Cal T Cal & P Cal The Cryogenic Pamtech Isolators were a surprise. There was no mention of them in the VLBA Technical Report #3. There must have been added after the 1 st prototype Receiver was built. Keep New N/A Toss Key: Cryo

8. Upgraded VLBA C-Band Receiver OMT T -30 dB RCP LCP T Cal & P Cal LNA Hybrid Dorado 3ICC60-1 Dorado 3ICC60-1 Mactech CA7205U In HEMT 4-8 GHz InP HEMT 4-8 GHz -30 dB T Narda 4014C-30 Narda 4014C-30 Omni-Spectra 2089-6203-00 T Noise Diode -10 dB Noise/Com NC3205-G 10 dB Omni-Spectra 2020-6617-10 Pulse Cal T Cal & P Cal RCP 4-8 GHz Out PA TTE K5221-4/8G 4-8 GHz Ciao CA48-281 G=26dB Ditom D3I4080-1 4-8 GHz Ditom D3I4080-1 4-8 GHz PA TTE K5221-4/8G 4-8 GHz Ciao CA48-281 G=26dB LCP 4-8 GHz Out Ditom D3I4080-1 4-8 GHz Ditom D3I4080-1 4-8 GHz Note that we leave out the High-Cal feature, which has never been used on the VLBA Keep New N/A Toss Key: Cryo

9. Modifications & Problems Making the modifications to the RF path of the receiver were relatively straight forward. However, cooling the beast was a problem. –The old VLBA receiver had its Septum Polarizer tied to the 50°K Stage. –We wanted the new OMT to be tied to the 15°K Stage since it has more resistive losses. Hollis spent quite a while fighting to minimize the final temperature that the 15°K Stage reached. –It was hard to get it much below 30°K and the temperature was very dependent on how many other receivers were on the same Helium line. –Required reducing the weight of the OMT as well as adding Space Blankets around the 50°K Radiation Shield. It was also decided that the Thermal Gap assembly used in the EVLA design would need to be improved.

10. EVLA-Style OMT & Thermal Gap Includes: “Old” Thermal Gap + Heavy Blocks + Absorber Strip AN72 Absorber wrapped around Thermal Gap to prevent Cavity Resonances.

11. VLBA-Style OMT & Thermal Gap Includes: New Circular Thermal Gap with Choke Ring and long standoffs + “Swiss Cheese” Blocks to reduce thermal mass + Mount for Hybrid makes for an integrated Circular Polarizer

12. Recent Cool-down Results Thanks to Hollis’ new Thermal Gap Assembly, the Model 22 is now cooling the RF Tree in the Prototype Receiver down to acceptable temperature levels. The 15°K Cold Stage temperature is also less susceptible to variations from other fridges being connected to the Helium lines: – ~12-14°K when alone – ~14-16°K with one Model 350 on the loop – ~16-18°K with two Model 350 on the loop The 2 x Model 350 scenario is what occurs on a VLBA Antenna. VLBA C-Bands typically 15-25°K

13. VLBA C-Band Prototype (VC#11)

14. Early RF Sensitivity Tests on VC#11 Old EVLA vs. New VLBA Thermal Gap Using the new T-Gap assembly with its proper Choke Ring yields a vast improvement over that achieved on the EVLA with the old T-Gap design.

15. VLBA C-Band Receiver Old vs. Interim vs. New Configuration C RCP LCP RCP LCP Old T105 4.5-5.2 GHz Converter Module 4.5-5.2 GHz L104 #2 IF Output A & C 500-1000 MHz 4.5-5.2 GHz C RCP LCP RCP LCP New T405 4 - 8 GHz Converter Module 4.0-8.0 GHz IF Output A, B, C & D 500-1000 MHz 4.0-8.0 GHz L104 #2 L104 #3 Old C-Band Rx with Old T105 Modified C-Band Rx with New T405 C RCP LCP RCP LCP Old T105 4.5-5.2 GHz Converter Module 4.5-5.2 GHz 4.0-8.0 GHz IF Output A & C 500-1000 MHz 4.0-8.0 GHz (From old Rx) L104 #2 4.5-5.2 GHz Modified C-Band Rx with Old T105 SMA

16. Downconverter Design 3900-5900 and 5600-7900MHz

17. Conclusions Thanks to the new Thermal Gap design, the upgraded VLBA C-Band is now superior to the EVLA receiver. Expect T(Rx) < 10°K from 4.2-8.0 GHz It is now probably the most sensitive wideband system in the world at this frequency.

18. Questions ?