Preloaded Parabolic Dish Antennas for SKA

1. Preloaded Parabolic Dish Antennas for SKA By G. Swarup (NCRA-TIFR) & N. UdayaShankar (RRI)

2. Plan of the Talk • Summary • Concept of the Pre-loaded Parabolic Dish Antennas • Description and Fabrication details of 12-m PPD Antenna • Cost Estimates • Strawman's Design For SKA • Conclusion

3. Summary • Preloaded Parabolic Dish Antennas can provide a large collecting area over a wide frequency range for SKA at an affordable cost. • A set of high tensile tubular members - • are clamped to a central hub, • bent elastically to lie close to a parabolic curve. • finally clamped to a similar no. of tubes placed circumferentially

4. Important to Note • The novel design of preloaded parabolic dish stores sufficient initial strain energy in the back up structure for resisting front and back winds, • Also minimizes gravitational distortions of the Parabolic Dish due to change in its elevation. Patent applications by G. Swarup & S.C. Tapde on 1st Aug, 2000, no. 721/MUM/2000 and on 30 July 2001 no. PCT/In 01/00137.

5. Basic Design Concepts of PPD • The back up structure of a parabolic dish must be designed to withstand survival wind velocity. • Desirable to minimize labour cost. • The preloaded concept is based on the principle that if a structure has an initial stored energy it can offer a large stiffness to external loads. • As an example consider an elastically bent member connected to an anchor (Fig.1) • When the preload is removed strain energy is stored in the anchor as well in the elastically bent member.

7. Details of the 12-m PPD Antenna • 12-m PPD consists of • a central hub of diameter 4 m • 24 elastically bent stainless steel tubes with 8 mm wall thickness and 40 mm O.D. • an outer circumferential ring to hold the elastically bent radial tubes of 40 mm O.D. • an intermediate ring of 40 mm O.D.

11. 12-m Dish • Next picture shows a view of the assembled dish at RRI. It is seen that preloading simplifies the back-up structure. • The reflecting panels are made of welded wire mesh of size 6 mm x 6 mm. The diameter of the stainless steel wires is 0.55 mm. The mesh is attached by resistive arc welding by a s.s. frame of thin tubes. • Measured rms error of the assembled reflecting surface is about 2 mm which should allow operation of the 12-m dish from nearly 150 MHz to 10 GHz.

15. Fabrication and Assembly Procedure(Labour content is low) • Fabricate 4m diameter HUB made of 8 mm mild-steel plates. • Anchor the 24 radial tubes of 40 mm OD, 8 mm thickness • Adjust the clamps at the Hub to ensure that the ends of the tubes are in one plane using a theodolite. The take-off angle is important. • Bend the radial tubes elastically using a central ring and 12 diagonally placed ropes and turn-buckles (spider). • Clamp the radial tubes with circumferential tubes along the RIM and at intermediate locations. • Remove the spider and the ropes. • Attach the wire mesh panels using adjustable bolts using a theodolite. • Note: Possible to automate a major part of the fabrication and assembly procedure, except panel mounting.

16. Finite Element (FEM) Analysis of 12-m PPD (Prof. Ashok Joshi, IIT, Mumbai) • Detailed finite element stress and dynamic analysis have been done for the operational and survival winds. It was decided to use S.S. tubes of yield strength of 60 kg/ mm2 Important to Note • For wind towards the front of the dish, circumferential tubes go in tension and take up the survival wind load. Tension in the radial tubes decreases only marginally. • For wind in the back direction, stress in the circumferential tubes reduces, but FEM analysis shows a 20% margin before the tubes go in compression. • Dish distortion is only a few mm.

17. Mechanical Drive System • A contract has been placed with a design firm at Bangalore for designing a suitable mechanical drive system and a structural mount for the 12-m PPD. • Two designs have been examined : • Elevation: (Al) : Screw Drive (A2): Sector Gear • Azimuth: (B1): Slew Ring Bearing (B2): Wheel & Track • Preliminary cost estimates for the two alternatives for El and Az are nearly the same. • For the present, we have selected Sector Gear for Elevation and Slew Ring Bearing for Azimuth. • Designs to be finalized by end 2002. Drive system to be fabricated by MID-2003.

18. Table 1: Parameters of the Preloaded Parabolic Dishes * Values for 25m dish are tentative, although a preliminary FEM analysis has been done.

19. Strawman's Design For SKA • In order to meet SKA design goals, it is proposed to use • About 8343 nos. of 12 m diameter dishes, say 103 stations of 9 x 9 dishes. OR • 5047 nos. of 15 m diameter dishes, say 103 stations of 7 x 7 dishes. • Table 2 gives performance parameter of a 12 m dish • Table 3 a comparison with SKA goals.

20. Table 2: Approx. parameters for a set of 8343 dishes of 12m dia * Assuming (a) Antenna aperture efficiency due to illumination of 0.5 at 150 MHz and 0.65 at higher frequencies. (b) Rms error of the reflecting surface = 2mm (c) Reflectivity leakage of the wire mesh of size 6mm x 6mm x 0.55 mm size as a function of frequency, based on calculations. ** Assuming galactic background temperature of 50 x (f(MHz)/300)-2.6K, ATA type receiver but with a primary feed of 4 log-periodic antennas and ground leakage for the wire mesh of size 6mm x 6mm x 0.55mm.

21. Table 3: Design Compliance Matrix

22. … Table 3: …

23. … Table 3:

24. A proposal to cover 30 MHz 20 GHz using a parabolic dish of about 12-m diameter for SKA • Use Al panels or a formed dish of 6 m diameter for the Central part for operation upto 20 GHz • Use wire mesh (welded) of size 6 mm x 6 mm x 0.55 mm for operation from 150 MHz to 10 GHz. • Place active antennas on the rim to cover 30 to 150 MHz. (details not worked out)

26. Antenna Elements for SKA • In order to make a relative comparison of various concepts being developed by the SKA partners, it is suggested that we may consider a "Figure of merit", F, for each concept. • An additional parameter, F2, will be the no. of instantaneous beams, which will increase the cost of SKA. Therefore, the total cost of the antenna and receiver system will have to be considered, along with F1 & F2, and some other factors such as achievable dynamic range, RFI susceptibility, maintenance aspects, etc.

27. Conclusion • Parabolic dishes offer an attractive option for SKA. • Estimated cost of the antenna system of SKA for achieving 2 x 104 m2 k-1 at 1.4 GHz will be about $ 500 x 106 using about 8300 Pre-loaded Parabolic Dishes of 12 m diameter, with frequency coverage of at least 150 MHz to 10 GHz. (Dish cost including mechanical and servo system ~$ 500 /m2). The cost would be only ~$250/m2 if the upper frequency is only 2 GHz. • Better estimates would be available by mid-2003.

29. Advantages of Parabolic Dishes • Parabolic dishes have several advantages : • Steerability over all the sky. • Wide frequency coverage ( < 100 MHz to > 20 GHz) • Wide bandwidths are possible which will allow dynamic channel assignments for minimizing RFI

30. … Advantages … • Side-lobes of each 12 m antenna are likely to be about • -25 dB and further a null can be produced in specified directions by suitable phasing of the array of 9 x 9 antennas at each station and of few thousand antennas in the central array.

31. … Advantages • Good polarization characteristics. • Less complex electronics than phased arrays which may allow : • Lower system temperature. • Better phase and gain stability resulting in better calibration capabilities, which is extremely important for achieving the required 106 dynamic range.