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

2. 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

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

4. 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

5. 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

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

7. 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)

8. 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 !

9. 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

10. 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

11. 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

12. 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

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

14. 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

15. 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.

16. 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)