Aperture blockage in mechanically scanned multi-beam lens antenna for satellite communications in Ku band. John Thornton Department of Electronics, University of York, YO10 5DD. United Kingdom. jt21@ohm.york.ac.uk 1
Background scanning lens antenna developed for 28-31 GHz (Framework 6 CAPANINA projects on High Altitude Platforms, ~ 2004-2006.) GEO satellite Current project: Multi-beam Scanning Antenna for Satellite Communications ( MULTISCAN, funded by the European Space Agency.) GEO satellite for satcomms to trains at Ku band (...and Ka band if possible.) terminal 1 2 train
Existing lens antenna developed at University of York f r 2 Scale model : 35 dbi @ 28 GHz, 236 mm diameter, using two dielectric layers. ε r 1 ε r 2 r 1 Bibliography: J. Thornton, "Scanning Ka-band Vehicular Lens Antennas for Satellite and High Altitude Platform Communications" 11th European Wireless Conference, Nicosia, Cyprus, 10-13 April 2005 J. Thornton, Wide-scanning Multi-layer Hemisphere Lens Antenna for Ka band. IEE Proceedings Microwaves, Antennas & Propagation, Volume 153, Issue 6, December 2006. pp. 573-578. J. Thornton, T.C.Tozer, Lens antennas for multi-satellite and multi-frequency band communications on trains. IET Seminar: "Broadband on Trains", London, 20 February 2007. 3
Mechanical design (i). This version keeps the upper ground plane clear, but places components beneath the ground plane. This adds to the height. primary feed on elevation trolley centre bearing direction of beam azimuth carriage elevation motor azimuth motor coax or optic fibre lens pedestal circumfrential azimuth rail this region not blocked ground plane (has to be terminated here) 4
Mechanical design (ii). This version puts no components beneath the ground plane. This potentially blocks the aperture....but is our preferred solution. direction of beam coax or optic fibre (reel) lens circumfrential azimuth rail...aperture blockage azimuth carriage ground plane (extends as far as space allows) 5
effect of elevation angle high elevation feed rail virtual lens rail low elevation 6
Elevation mechanism required elevation angles derived from range of latitudes plus train maximum roll angle C L R 340.3 mm R 305.0 mm 62.00 13.00 C 7
Azimuth carriage & rail (i) 141 mm 84:1 gearbox L gearbox motor elevation rail cable trunking position sensor azimuth rail 8
Azimuth carriage & rail (ii) L gearbox 84:1 gearbox motor elevation rail 141 mm magnetic sensor rack and pinion cable trunking IF coaxial cables cable termination area 457.5mm radius (1.5 x 305mm) azimuth rail 9
Antenna under construction, March 2008 lens diameter 610 mm 10
Aperture blockage by azimuth rail? We expect the azimuth rail to block the aperture. We expect this to be worse at low elevation angles. But how big an effect should we expect? Difficult to quantify from theory (although I m trying!)...but convenient to revert to the 28 GHz scale model. full size scale model lens diameter (mm) 610 236 rail height (mm) 27 or 36 10.4 or 13.9 rail radius* (mm) 480 186 * from lens centre 11
Scale model for the azimuth rail feed lens copper wall 12
Radiation patterns and loss due to rail 14 mm rail, H-plane, 30 elevation degrees -5 5 10 15 20-5 without rail -10 with rail -15 loss due to rail ~ 0.3 db -20-25 -30-35 -40 db 30 elevation 24 elevation Eh Ev Eh Ev 0.4 0.3 0.9 1.5 13
Measurement results: low elevation 18 elevation: first ground plane too small (120 mm) extend to 240 mm (=lens diameter), gain recovers: ~ 0.8 db angle ( ) -30-20 -10 10 20 30 ground plane 1-20 -10 ground plane 2-30 -40-50 db 14
Extended ground plane for low elevation 18 elevation. 240 mm 15
Azimuth rail...again... L gearbox 84:1 gearbox motor elevation rail magnetic sensor rack and pinion cable trunking IF coaxial cables cable termination area 457.5mm radius (1.5 x 305mm) azimuth rail 16
rack and pinion this space reserved for magnetic strip (to be fitted) 17
Reduced height rail if we use optical fibre instead of coax, and re-orient magnetic strip, we can considerably reduce the rail height: magnetic sensor optic fibre trunking pinion prototype rail (coax) 36 mm (14 mm scaled) alternative rails (fibre) 9 1.5? 25 mm, or less... 13 mm minimum? (scales to 5 mm) 18
Measurement results: medium elevation -60-40 -20 20 40 60 10 mm rail H-plane, 28 GHz -10-20 -30-40 28 GHz 31 GHz (10.8 12.0 GHz, scaled) 45 to 30 elevation: rail has no measurable effect db -50 45 elevation no rail with rail -30-20 -10 10 20 30 H-plane, 28 GHz -20-30 19 30 elevation db -40-50
effect of 10 mm rail at low elevation (24 ) degrees -30-20 -10 10 20 30 degrees -30-20 -10 10 20 30 H-plane 31 GHz 24 elevation -10 E-plane 28 GHz 24 elevation -10-20 -20-30 -30-40 -40 db db without rail with rail 20
Summary of measured results all measurements at 28 GHz loss due to rail (db) model rail height (mm) scale height (mm) 30 elevation 24 elevation 18 elevation Eh Ev Eh Ev Eh Ev 14 36* 0.4 0.2 0.9 1.5 1.5 1.1 10 26 0 0 0.7 0.9 1.3 0.9 5 13 0 0 0.2 0.4 0.2 0.7 * was we ve made for first mechanical prototype with coax cables 21
frequency effects loss due to 10 mm rail model, 24 elevation 1 db 0.8 0.6 0.4 0.2 E-field horizontal E-field vertical 28 29 30 31 f (GHz) 22
Conclusions First version of azimuth rail designed to route a pair of coaxial cables, stacked vertically. also vertical magnetic strip, to allow curvature. Rail height of 36 mm scales to 14 mm for scale model measurements. Significant scan loss (~ 1.5 db) at 18 & 24 elevation is a consequence of a conservative mechanical design. Can reduce scan loss by reducing height of rail. Optical fibre would help here consumer satellite LNBs with optical feed close to market. 13 mm (full) rail height typical scan loss 0.2 db 0.4 db. Might improve further by using plastic rail not metal. 23
Acknowledgement Thanks to the European Space Agency for funding this work under their Innovation Triangle Initiative Contract No. 20836/07/NL/CB 24