Steerable antennas Meeting July 2005 John Thornton WP3.2 Steerable antennas Meeting July 2005

WP3.2 Summary Steerable antennas For the train: array antenna. CSEM Single beam antenna to be implemented. Mechanically steered. For the HAP: multi-beam lens antenna. UOY Hemisphere lens with ground plane. Variant of Luneburg lens, but using only 2 layers. Electromagnetic demonstrator successful. ...could also be used for train antenna. the last bit has recently moved from WP3.3

Space constraints The antenna may be constrained by available space Hemisphere with ground plane reflector antenna plane wave dish feed radome feed reflective plane virtual lens maximum height of reflector effective height 1 beam per antenna N beams per antenna

2004 results 28 GHz prototype has demonstrated concept single layer hemisphere has sub-optimum efficiency lens diameter is 160 mm this test used a pyramid waveguide horn. ~ 30 % aperture efficiency. a better feed (scalar feed horn) was later used scans to +/- 75° mixer horn hemisphere lens ground plane

2004 results: radiation patterns reference antenna 90° elevation 45° elevation -100 -50 50 100 150 degrees from zenith 15° elevation 65° elevation -10 -20 -30 -40 -50 dB

effect of primary feed scalar waveguide feed improves gain by 2 dB scalar feed + lens 30 dBi (40 % aperture efficiency) can also circular polarisation

theory and measurement now using scalar feed angle (degrees) -75 -50 -25 25 50 75 measurement -10 theory uses modal expansion for spherical wave, then construct hemisphere case from real and virtual components due to ground plane -20 -30 -40 relative gain (dB)

Multi-layer Structures Luneburg and quasi-Luneburg type lenses. dielectric constant r varies R r concentric shells as approximation e.g. Luneburg: all power at aperture is focussed. Uniform sphere: not all power is focussed (but can be quite good). imperfect focus (but can be quite good...) best (perfect focus) difficult to make !

Ray Tracing r = 2.28 r = 2.53 r = 2.53 illustrates properties not a rigorous analysis two layers single layer r = 2.28 r = 2.53 incident plane wave 0.2 0.35 -1.5 -1.5 -1 1 -1 1 r = 2.53

Numerical techniques Modal analysis Like a waveguide or a cavity, free space has modes ...these are the basis functions from which any radiation pattern can be synthesised. m and n are vector spherical wave functions function of spatial co-ordinates r,,  amn and bmn are the coefficients (or weights) for each mode this is all in the literature, e.g. J. A. Stratton, Electromagnetic Theory, 1942

Scattering source an, bn (outward) The multi-shell scattering analysis was published by John Sanford in IEEE Trans.Ant.Prop. in 1994 z source (primary feed) source an, bn (inward) 2 2 2 r scattered n, n (outward)  1 1 scattered an, bn (inward) r1 y r2  The total field in the exterior is the sum of the source and scattered outward travelling waves. scattering of a sphere by a plane wave is a well-established analysis. Sanford extended the analysis to a source near the sphere. Multiple-shell structures also analysed. x boundary condition: E and H are continuous

Two-shell prototype core: Rexolite (cross-linked polystyrene) r = 2.53 outer layer: polyethylene r = 2.28 good (low loss) materials er 1 er 2 r1 r2 f r1 = 5.3  r2 = 11  diameter = 236 mm r_feed = 11.5  have to work with practical materials. Directivity = 36 dBi (76 % aperture efficiency) from theory Published at 11th European Wireless Conference,Cyprus, May 2005

Fabricated 2-layer lens components machined at University of York polyethylene outer layer Rexolite core

Measurement results Measured gain approximately 35.1 dBi at 28 GHz Aperture efficiency of about 68 % is comparable with a dish (a) Theoretical E- and H-plane far field patterns (b) H-plane patterns at 2.7 m measurement distance

Other work in progress Have developed a new theory for the effect of the outer layers (radome) which cover the lens and feed. Steering mechanism is under consideration. Possibly develop a Ku-band system.

Conclusions, July 2005 A two-layer lens antenna has excellent electromagnetic performance Equivalent to dish but with lower profile and offering multiple beams. 35 dBi at 28 GHz from 236 mm aperture (118 mm height)