Departement Elektriese en Elektroniese Ingenieurswese  Department of Electrical and Electronic Engineering Ultra Wideband Reflector Antenna Feeds Dirk de Villiers Post Doctoral Fellow: University of Stellenbosch 5 December 2009

2 Agenda Overview of Wideband Feeds Log-Periodic Dipole Array Above a Ground Plane Direct Radiating Log-Periodic Dipole Arrays Log-Periodic “Thunderbolt” Antenna Conclusions

Overview of Wideband Feeds What is required of a wideband feed good enough for meerKAT? Illuminate the entire dish properly:  Amplitude  Phase  Polarisation Have no spillover Have a constant impedance Excellent polarisation purity Low (no…) noise All this over a 4:1 band, maybe even 20:1 for SKA 3

Overview of Wideband Feeds 4

From Bradley and Gawande, URSI Meeting, Boulder, Jan

Log-Periodic Dipole Array Above a Ground Plane Major problem with wideband feeds is the moving phase centre Ground plane will stabilise the phase centre Simple dipole model: 6

Log-Periodic Dipole Array Above a Ground Plane Elements driven independently to build a circuit model Gives the expected results 7

Log-Periodic Dipole Array Above a Ground Plane Test a circuit model with different feeding networks Simple directly connected LP type feed: 8

Log-Periodic Dipole Array Above a Ground Plane Results similar to other previously tested LP arrays over ground planes Also similar to the Eleven feed reflection results (Suspect) Reflections from ground back into the antenna killing us here… 9

Log-Periodic Dipole Array Above a Ground Plane Attempt to include a more complicated feeding network that can still be realistically constructed… 10

Log-Periodic Dipole Array Above a Ground Plane Without going into too much detail: We found a nice way to colour in a Smith chart! 11

Direct Radiating Log-Periodic Arrays Moving phase centre is a “problem”, but how big is this problem? meerKAT only requires a 4:1 bandwidth Maybe the effect of the moving phase centre is not as bad after all? Direct radiating log periodic type arrays can easily achieve a 10:1 pattern and impedance bandwidth Check the effect of the moving phase centre by designing some antennas and analysing the performance over the required meerKAT bandwidth 12

Direct Radiating Log-Periodic Dipole Arrays 13

Direct Radiating Log-Periodic Dipole Arrays 14

Direct Radiating Log-Periodic Dipole Arrays 15

Direct Radiating Log-Periodic Dipole Arrays 16

Log-Periodic “Thunderbolt” Antenna 17

Why “Thunderbolt”? 18

Log-Periodic “Thunderbolt” Antenna 19

Log-Periodic “Thunderbolt” Antenna 20

Log-Periodic “Thunderbolt” Antenna Feed at tip of antenna Need to bring cooling as close as possible to the tip Solution similar to the ATA feed Does not have a significant effect on performance Lowers impedance somewhat 21

Log-Periodic “Thunderbolt” Antenna Results with f/d = 0.5 Z in = 230 Ω differential Differential feeds and baluns are being investigated and will not be reported on yet… 22

Log-Periodic “Thunderbolt” Antenna Properly sampled mid-band results: 23

Log-Periodic “Thunderbolt” Antenna Efficiency due to copper losses (conservative estimate of conductivity taken as 3e07 S/m): 24

Log-Periodic “Thunderbolt” Antenna Need to calculate the A/T results Use simple noise model  Main beam sees the sky temperature  Spillover sees the ground temperature  Include antenna noise  Add 20 K for feed and cryogenically cooled amps Results shown for different f/d’s, thus different illuminations of the dish: 25 f/d = 0.5 (10 dB taper)f/d = 0.4 (15 dB taper)

Log-Periodic “Thunderbolt” Antenna Problem with small f/d: Phase efficiency drops significantly! 26

Log-Periodic “Thunderbolt” Antenna Must try to maintain large f/d Direct spillover energy at the sky Well known issue…  Skirts around main dish?  Offset dual reflectors best here! meerKAT spec: A/T > 2.63 per dish 27

Conclusions Wideband feeds that make meerKAT and SKA specs are very difficult to design (Don’t exist yet…) Promising topologies are in development around the world, but none are good enough yet A promising new topology has been presented Initial simulations show that the specs might be achieved if the spillover issue is properly addressed and low noise feeds can be implemented Expect the first prototype in the first half of next year 28