1 John Thornton WP3.2 Steerable antennas Meeting July 2005.
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Transcript of 1 John Thornton WP3.2 Steerable antennas Meeting July 2005.
1
John ThorntonWP3.2
Steerable antennas
Meeting July 2005
2
WP3.2 Summary
Steerable antennas
For the train: array antenna. CSEMSingle beam antenna to be implemented.Mechanically steered.
For the HAP: multi-beam lens antenna. UOYHemisphere lens with ground plane.Variant of Luneburg lens, but using only 2 layers.Electromagnetic demonstrator successful....could also be used for train antenna.
3
Space constraints
The antenna may be constrained by available space
maximum height of reflector
N beams per antenna1 beam per antenna
Hemisphere with ground plane reflector antenna
feed
radome dishfeed
virtual lensreflective plane
plane wave
effective height
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°
horn
mixer
hemisphere lens
ground plane
5
2004 results: radiation patterns
-100 -50 50 100 150
degrees from zenith
-50
-40
-30
-20
-10
dB
15° elevation
45° elevation
65° elevation
90° elevation reference antenna
6
effect of primary feed
scalar waveguide feed improves gain by 2 dB
scalar feed + lens 30 dBi (40 % aperture efficiency)
can also circular polarisation
7
theory and measurement
now using scalar feed
-75 -50 -25 25 50 75
-40
-30
-20
-10measurement
theory
angle (degrees)
relative gain (dB)
uses modal expansion for spherical wave, then construct hemisphere case from real and virtual components due to ground plane
8
Multi-layer Structures
Luneburg and quasi-Luneburg type lenses.
dielectric constant r varies
2
2
Rr
rconcentric shells asapproximation
e.g.
rR
best (perfect focus)difficult to make !
imperfect focus(but can be quite good...)
9
Ray Tracing
illustrates properties not a rigorous analysis
-1.5 -1 1
0.35
-1.5 -1 1
0.2
single layer two layers
0 0
incident plane wave
r = 2.53
r = 2.53
r = 2.28
10
Numerical techniques
)nm(,, )(
0
)(
10
imnemn
m
imnomn
n
n
jbajErE
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
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 functionsfunction of spatial co-ordinates r,,
11
Scattering
y
z
x
r
source an, bn (outward)
scattered n, n (outward)
scattered an, bn (inward)
source
The total field in the exterior is the sum of the source and scattered outward travelling waves.
boundary condition: E and H are continuous
1 1
source an, bn (inward)2 2
2 2
r1r2
The multi-shell scattering analysis was published by John Sanford in IEEE Trans.Ant.Prop. in 1994
(primary feed)
12
Two-shell prototype
core: Rexolite (cross-linked polystyrene) r = 2.53
outer layer: polyethylene r = 2.28
good (low loss) materials
r 1
r 2
r1
r2
fr1 = 5.3
r2 = 11 diameter = 236 mm
r_feed = 11.5
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
Rexolite core
polyethylene outer layer
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)