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John ThorntonWP3.2

Steerable antennas

Meeting July 2005

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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.

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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

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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

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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

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effect of primary feed

scalar waveguide feed improves gain by 2 dB

scalar feed + lens 30 dBi (40 % aperture efficiency)

can also circular polarisation

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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

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Multi-layer Structures

Luneburg and quasi-Luneburg type lenses.

dielectric constant r varies

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2

Rr

rconcentric shells asapproximation

e.g.

rR

best (perfect focus)difficult to make !

imperfect focus(but can be quite good...)

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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

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Numerical techniques

)nm(,, )(

0

)(

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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,,

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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)

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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

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Fabricated 2-layer lens

components machined at University of York

Rexolite core

polyethylene outer layer

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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

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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.

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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)