Phased Array Feed Scattering Analysis. Stuart Hay, Raj Mittra, Neng-Tien Huang and Wenhua Wu

Transcription

1 Phased Array Feed Scattering Analysis Stuart Hay, Raj Mittra, Neng-Tien Huang and Wenhua Wu 3 May 200

2 Outline of talk Motivation Phased array feed (PAF) scattering analysis Two approaches Characteristic Basis Function Method Finite Difference Time Domain Preliminary results Conclusions

3 Motivation Parkes Testbed Better estimates of various contributions to noise Hence LNA properties and arrary/lna matching

4 Motivation Ditto for ASKAP Prototype 5x4 ASKAP ~ 9x0 Variation with size? Finer detail on expected ASKAP performance

5 Motivation - SKA Test against measurements on Parkes Testbed and ASKAP Accurate and significant? Capability applicable to SKA design Both front- and offset-fed systems (perhaps with multiple feeds) will involve scattering requiring design May be important to achieving required dynamic range

6 Initial thoughts Aperture efficiency decreased by blockage Proportional to area at high frequency However PAF not electrically large (~6λ ASKAP) PAFs probably too small to efficiently null pattern in blocked area Possibly area squared impact on signal power Scattering may increase ground noise pickup Complex struts Order wavelength in cross section LNA noise coupling via reflector Variation in array/lna matching requirement Long-range coupling Is residual impact with equal optimum LNAs still small Frequency ripples Dependence on array size and supporting structure

7 Approaches CBFM Extends array analysis by CBFM to including coupling via reflector Some simplifying assumptions Efficient Initial indication of effects and magnitude Parallel FDTD In principle accurate and handles all detail However very large computational effort required Any performance issues on this type of problem Any significant high order effects (eg guided waves on complex struts)

8 CBFM Green s function for currents in presence of reflector (approximate by GO or PO) Use currents from array Characteristic Basis Function Method (CBFM) Existing CBFM for connected patch array Extension CBF coupling via reflector Additional CBFs plane waves from reflector directions modest increase in unknowns Solve for CBF coefficients as usual LU factorization

9 Connected patch CBFM Verified against FDTD and measurements

10 Connected patch CBFM Core element Primary CBF Secondary CBF Tertiary CBF

11 CBF coupling via reflector s J s E J 2 i E CBF ds J CBF 2 Mutual impedance 2, = dv i ( E E s ) J 2 = direct 2, via reflector 2, via reflector 2, 2 ds ( E i 2 H i ) (PO) dv E s,go J 2 (GO)

12 System representation R n V w I Y c G n -Y c Y LNA Y t 2 x 2 Y A M x M R n V M w M M I M Y c G n -Y c Y LNA Y t 2 x 2 Σ Equivalent LNA noise sources M Vbeam = wivi = i= Equivalent currents for external sources (signal and noise) t w V Array admittance matrix

13 Sensitivity formulation Sensitivit y = 8 S T 0 ( I φ signal t G t tot φ 2 ) φ The antenna part of the matrix G tot is given by ( G ant ) i, j t = ds T Ei E j ds z T 0 0 farfield sphere antenna surface R s J i J t j where E i and J i are the electric field and current produced by volt at port i while all other ports are short circuited T and R s are the brightness temperature distribution and antenna surface resistance z 0 and T 0 are the impedance of free space and physical antenna temperature Φ is equivalent beamformer weight vector at LNA input The receiver part of G tot is given by G rec = ( F min ) G A N( Y A Y opt U ) G opt ( Y A Y opt U ) where F min, N and Y opt are a complete set of two-port LNA noise parameters Y A =G A jb A

14 Sensitivity matching F min and N are Lange noise constants (invariant in lossless transformation). Sensitivity Y opt = 0 Y φ φ φ B φ φ 2 t A A 2 Y = ( ) opt 2 2 t φ B φ A j 2 φ 2. Sensitivity φ = 0 φ = G tot I signal Iteration of and 2 is rapidly convergent

15 Loading and beamforming configurations Patch Patch Groundplane V i C V i - C Patch Patch D Groundplane V o =A (V i -V i- ) Beamformer V i V i - Differential (D) SE SE A V i A V - i Beamformer Patch Patch Groundplane V i V i - Single-ended (SE) SE SE ie C = SE / 2 D = SE x 2 A (V i -V i- ) Beamformer Differential single-ended (DSE)

16 Maximum sensitivity -5x4 array No blockage (left) and with scattering analysis (right)

17 Max sensitivity impedance

18 Extended groundplane J J 2 J 2 Array Extended groundplane reflector via 2, reflector via 2,2 direct 2,2 2 reflector via 2, reflector via 2,2 direct 2,2 reflector via,2 reflector via, direct, 2 reflector via 2,2 direct 2,2 reflector via 2, reflector via,2 reflector via, direct, 2 reflector via 2,2 direct 2,2 reflector via 2, direct 2, reflector via,2 direct,2 reflector via, direct, ) ( ] ) ( [ 0 0 J V J V J J V J J = = = = Extended groundplane

19 Sensitivity with extended groundplane Without (left) and with (right) extended groundplane

20 Larger array 8x7 8x7 array and conjugate match pattern

21 Sensitivity - 5x4 vs 8x7 Both without any extended groundplane

22 Parallel FDTD Method Single Computer Computer Cluster Domain Decomposition moderate jobs easy jobs Parallel FDTD Technique Hard jobs hard jobs 3 2 hard jobs easy jobs

23 Reflector Antenna Antenna size: 200mm x 200 mm x 663mm Fine structure

24 Antenna With Feed Diameter of reflector is 2 meters Patch array includes 32 elements Fed by 32 probes Antenna Feed and support

25 Simulation Technique Excitation PMC Images PMC PEC - - PEC = PMC PEC - - PEC - - PMC = Not independent Simulate quarter of the problem 4 times to get the solution of the original problem =

26 One Quarter of Antenna

27 Field Distribution With Strut

28 Return Loss S- (db) -5-6 Patch Array only Patch with reflector Frequency (ns) Red curve: Return loss for the patch array only. Blue curve: Return loss for the patch array when the reflector is present in the simulation domain.

29 Simulation Summary Hardware platform: 0 Intel Q GHz CPUs EM software: GEMS (parallel version) Problem size: 060 Mcells ( 993 x 989 x 007) Memory usage: 40GB Number of time steps: 8722 Simulation time: 9 hours 40 minutes Output parameters: S-parameters and far field patterns

30 Conclusions Rigorous analysis by FDTD appears feasible CBFM results suggest Blockage and scattering may increase Tsys/ηap by 20-25% in 5x4 array in Parkes Testbed Scattering structures around PAFs should be minimized Ripples produced by checkerboard decrease as its size increases Should be small for ASKAP-sized array Further work Complete FDTD analysis Testbed and ASKAP Compare analysis results against consistent measurements

31 CSIRO ICT Centre Stuart Hay Research Team Leader Electromagnetics Phone: Stuart.Hay@csiro.au Web: Thank you Contact Us Phone: or enquiries@csiro.au Web: