Microwave response of tessellated
metal surfaces and their
constituent elements
Matthew Paul Biginton
School of Physics
University of Exeter
A thesis submitted for the degree of
Doctor of Philosophy in Physics
1st June 2012
Microwave response of tessellated metal surfaces and their
constituent elements
Submitted by Matthew Paul Biginton to the University of Exeter as a thesis for
the degree of Doctor of Philosophy in Physics
1st June 2012
This thesis is available for Library use on the understanding that it is copyright
material and that no quotation from the thesis may be published without proper acknowledgement.
I certify that all material in this thesis which is not my own work has been identified
and that no material has previously been submitted and approved for the award of a
degree by this or any other University.
Matthew Paul Biginton
1st June 2012
Abstract
Over the last century the electromagnetic (EM) spectrum has become ever
more accessible with advances in technology. As a result, EM filters (or
Frequency Selective Surfaces (FSSs)) have been developed for many applications. Such filters have been used on satellites and radomes. In this thesis,
novel single layer and dual layer FSS have been studied and characterised,
experimentally and using Finite Element Method (FEM) modelling, showing very good agreement between the data and models. The interesting
transmission properties of these structurally complicated FSS are explained
and the physics of the resonant modes that mediate transmission is explored.
Enhanced transmission through an array of sub-wavelength apertures close
to the diffraction limit has been a popular area of physics for many years.
In addition enhanced reflection from metal patch arrays has been of great
interest. This thesis studies original extensions of conventional FSS. The
work is split into two main sections: single layer FSS and dual layer FSS.
In the first experimental chapter (chapter 5) two new single layered FSS
comprising complementary elements tessellated into composite arrays are
explored (a connected array and a disconnected array). The behaviour of
these arrays is compared with that of arrays of the constituent elements
that either exhibit enhanced transmission or enhanced reflection phenomena. The behaviour of the connected composite array can be inferred from
the behaviour of arrays of the constituent elements. Interestingly for the
disconnected composite array, the behaviour can not be inferred from the
constituent elements as without one or the other of the elements in situ, the
modes supported on the composite array are not supported for the arrays
of constituent elements.
The second and third experimental chapters (Chapters 6 and 7) explore the
transmission through dual layer arrays composed of either capped holes or
capped annuli. Despite the holes being capped with a metal disc, the array
exhibits a remarkably high transmission, mediated by the annular cavity
formed between the caps and apertured metal sheet. In Chapter 7 concentrically nested annular patches above annular slots are used to achieve
multiple transmission pass bands.
For many applications it is often desirable to miniaturise resonant elements.
Developing this concept further, chapter 8 explores the resonant frequency
of a structured capped aperture. The internal structure of metal inclusions,
give control over the resonant frequency of the cavity, reducing it’s resonant
frequency significantly and miniaturising the size of the cavity compared to
the incident wavelength.
Contents
Contents
iv
1 Introduction
1
2 Microwave transmission through metallic arrays
5
2.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5
2.2
Frequency selective surfaces . . . . . . . . . . . . . . . . . . . . . . . . .
7
2.2.1
Response of electrons driven by an incident plane wave
. . . . .
7
2.2.2
Electromagnetic filters
. . . . . . . . . . . . . . . . . . . . . . .
7
Capacitive and Inductive filters . . . . . . . . . . . . . .
7
Mesh filters and frequency selective surfaces . . . . . . . . . . . .
8
2.2.2.1
2.2.3
2.3
2.4
Surface waves on planar metal
. . . . . . . . . . . . . . . . . . . . . . .
8
2.3.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
2.3.2
Overview of surface waves . . . . . . . . . . . . . . . . . . . . . .
9
2.3.3
Surface wave derivation . . . . . . . . . . . . . . . . . . . . . . .
11
2.3.4
Spatial extent of surface waves . . . . . . . . . . . . . . . . . . .
14
2.3.4.1
Permittivity and conductivity at microwave frequencies
14
2.3.4.2
Propagation length . . . . . . . . . . . . . . . . . . . .
15
2.3.4.3
Penetration depth and skin depth . . . . . . . . . . . .
17
2.3.5
Surface wave dispersion curve . . . . . . . . . . . . . . . . . . . .
20
2.3.6
Surface impedance . . . . . . . . . . . . . . . . . . . . . . . . . .
22
Surface waves on structured metal surfaces . . . . . . . . . . . . . . . .
23
2.4.1
Equivalent circuit model of periodic arrays . . . . . . . . . . . .
23
2.4.1.1
Parallel circuit . . . . . . . . . . . . . . . . . . . . . . .
23
2.4.1.2
Series circuit . . . . . . . . . . . . . . . . . . . . . . . .
24
2.4.1.3
Reflection and transmission from an FSS layer . . . . .
25
2.4.1.4
TM surface waves on a planar interface . . . . . . . . .
26
2.4.1.5
TE surface waves on a planar interface . . . . . . . . .
26
2.4.1.6
Surface waves on a high impedance surface . . . . . . .
28
iv
CONTENTS
2.4.2
Diffraction gratings . . . . . . . . . . . . . . . . . . . . . . . . . .
28
2.4.2.1
Light lines and light cones . . . . . . . . . . . . . . . .
30
2.4.2.2
Dispersion of surface modes on a grating . . . . . . . .
33
2.4.2.3
Energy gaps in the dispersion of a surface wave . . . . .
34
2.5
The phenomena of enhanced transmission through periodic arrays . . .
35
2.6
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
36
3 Circular and annular waveguide theory
3.1
Introduction
3.2
Fields at a perfectly conducting interface
3.3
Helmholtz wave equation
3.4
Bessel functions
3.5
3.6
3.7
37
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
37
. . . . . . . . . . . . . . . . .
38
. . . . . . . . . . . . . . . . . . . . . . . . . .
39
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
39
Waveguides with cylindrical symmetry . . . . . . . . . . . . . . . . . . .
41
3.5.1
TE solutions for cylindrical symmetry . . . . . . . . . . . . . . .
43
3.5.2
TM solutions for cylindrical symmetry
. . . . . . . . . . . . . .
45
3.5.3
TEM solutions for cylindrical symmetry
. . . . . . . . . . . . .
46
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
47
3.6.1
TE modes of the circular waveguide . . . . . . . . . . . . . . . .
48
3.6.2
TM modes of the circular waveguide . . . . . . . . . . . . . . . .
52
Circular Waveguide
Annular waveguide
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
56
3.7.1
TEM modes of the annular waveguide . . . . . . . . . . . . . . .
57
3.7.2
TE modes of the annular waveguide . . . . . . . . . . . . . . . .
59
3.7.3
TM modes of the annular waveguide
. . . . . . . . . . . . . . .
64
3.7.4
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
67
4 Experimental and modelling methods
68
4.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
68
4.2
Measurements of transmission in the far field . . . . . . . . . . . . . . .
68
4.2.1
Apparatus for microwave scalar measurements . . . . . . . . . .
68
4.2.2
Experimental arrangement
. . . . . . . . . . . . . . . . . . . . .
70
4.2.3
Microwave beam alignment for transmission measurements . . .
73
4.2.4
Measuring transmission . . . . . . . . . . . . . . . . . . . . . . .
74
Finite element method modelling . . . . . . . . . . . . . . . . . . . . . .
75
4.3.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
75
4.3.2
Finite element method . . . . . . . . . . . . . . . . . . . . . . . .
75
4.3.3
Drawing a model . . . . . . . . . . . . . . . . . . . . . . . . . . .
78
4.3.4
Material assignment . . . . . . . . . . . . . . . . . . . . . . . . .
78
4.3.5
Electromagnetic boundaries and sources . . . . . . . . . . . . . .
79
4.3
v
CONTENTS
4.3.6
4.4
4.3.5.1
Electromagnetic boundaries
. . . . . . . . . . . . . . .
79
4.3.5.2
Radiation sources . . . . . . . . . . . . . . . . . . . . .
80
Thesis specific modelling of FSS . . . . . . . . . . . . . . . . . .
81
4.3.6.1
Plane wave modelling of FSS . . . . . . . . . . . . . . .
81
4.3.6.2
General model design . . . . . . . . . . . . . . . . . . .
82
4.3.6.3
Composite array of complementary elements . . . . . .
82
4.3.6.4
The capped aperture array . . . . . . . . . . . . . . . .
84
4.3.6.5
Capped slot array and extensions . . . . . . . . . . . .
85
4.3.6.6
Modelling single capped apertures with sub-wavelength
structure . . . . . . . . . . . . . . . . . . . . . . . . . .
85
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
86
5 Composite array of complementary elements
88
5.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
88
5.2
Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
89
5.2.1
Babinet’s principle . . . . . . . . . . . . . . . . . . . . . . . . . .
89
5.2.1.1
. . . . . . . . . . . .
90
Square waveguide with a central cylindrical post . . . . . . . . .
92
5.2.2
Derivation of Babinet’s principle
5.3
Experimental samples and measurement techniques
. . . . . . . . . . .
98
5.4
Experimental data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
5.4.1
Connected composite array . . . . . . . . . . . . . . . . . . . . . 100
5.4.2
Disconnected composite array . . . . . . . . . . . . . . . . . . . . 106
5.4.3
Babinet’s comparison of the connected (A) and disconnected (B)
composite arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
5.4.4
Discussion of results for the composite arrays and comparisons
to the component arrays . . . . . . . . . . . . . . . . . . . . . . . 110
5.4.5
5.5
Connected array on different thickness dielectrics . . . . . . . . . 111
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
6 Capped circular holes in a metal sheet
118
6.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
6.2
Experimental sample and measurement techniques . . . . . . . . . . . . 119
6.3
Single annular cavity theory
6.4
Normal incidence response
6.5
Oblique incidence response . . . . . . . . . . . . . . . . . . . . . . . . . 125
6.6
Dispersion
6.7
Comparisons with a single capped aperture . . . . . . . . . . . . . . . . 132
6.8
Comparison with a uncapped aperture array
. . . . . . . . . . . . . . . . . . . . . . . . 120
. . . . . . . . . . . . . . . . . . . . . . . . . 123
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
vi
. . . . . . . . . . . . . . . 135
CONTENTS
6.9
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
6.10 Future work and extensions . . . . . . . . . . . . . . . . . . . . . . . . . 137
7 Capped annular slots in a metal sheet
138
7.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
7.2
Experimental samples and measurement techniques . . . . . . . . . . . . 141
7.3
Modes supported by the capped annulus array (B) . . . . . . . . . . . . 143
7.4
Numerically modelled arrays (A), (B), (C) and (D) that have multiple
annular cavities
7.5
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
8 Sub-wavelength structuring of a capped aperture
153
8.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
8.2
Analytic models for a structured capped aperture
. . . . . . . . . . . . 154
8.2.1
Parallel plate capacitor driven by an alternating current source . 154
8.2.2
Circuit theory for a single capped aperture . . . . . . . . . . . . 157
8.2.3
Circuit theory for the stepped capped aperture . . . . . . . . . . 159
8.2.4
Circuit theory for a connected capped aperture
. . . . . . . . . 161
8.3
Experimental technique . . . . . . . . . . . . . . . . . . . . . . . . . . . 164
8.4
Stepped capped aperture
8.5
. . . . . . . . . . . . . . . . . . . . . . . . . . 166
8.4.1
Experimental samples and measurement techniques . . . . . . . . 168
8.4.2
Analytic results and comparisons to FEM modelling . . . . . . . 169
8.4.3
Experimental results for the stepped capped aperture . . . . . . 170
Connected capped aperture . . . . . . . . . . . . . . . . . . . . . . . . . 173
8.5.1
Experimental samples for the connected capped aperture . . . . 174
8.5.2
Results for the connected capped aperture . . . . . . . . . . . . . 175
8.6
Stepped and connected capped aperture . . . . . . . . . . . . . . . . . . 177
8.7
Concentric capped apertures
8.8
. . . . . . . . . . . . . . . . . . . . . . . . 180
8.7.1
Experimental samples . . . . . . . . . . . . . . . . . . . . . . . . 180
8.7.2
Results for the concentric capped apertures . . . . . . . . . . . . 182
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184
9 Conclusions and Future work
185
9.1
Summary of Thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185
9.2
Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187
9.3
List of publications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191
9.4
List of presentations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191
References
192
vii