15-01-2014, 04:35 PM
Review of Electromagnetic Bandgap Technology and Applications
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ABSTRACT
This paper reviews the primary application areas of Electromagnetic BandGap (EBG) technology at
microwave and (sub)millimetre wave frequencies. Examples of EBG configurations in the microwave
region are shown and include array antennas, high precision GPS, mobile telephony, wearable antennas
and diplexing antennas. In the submillimetre wave region a 500 GHz dipole configuration and a novel
heterodyne mixer is shown. Some emphasis is also placed on EBG waveguides, high impedance planes
(AMC’s), resonators and filters.
As most fundamental components would be available in EBG technology, a fully integrated receiver
could be developed in order to take full advantage of this technology. True integration of passive and
active components can now begin to materialise using EBG technology.
INTRODUCTION
Microwave engineers are familiar with the concept of electromagnetic waves interacting with periodic
structures. Periodic structures in both closed metallic or open waveguides have been used for many years,
for example, in filters and travelling-wave tubes. Planar versions of these can be found in the form of
frequency selective surfaces (FSS) and phased array antennas.
In the late 1980’s a fully 3-D periodic structure, working at microwave frequencies, was realised by
Yablonovitch [1] and his co-workers by mechanically drilling holes into a block of dielectric material.
This so-called material, “Yablonovite”, prevents the propagation of microwave radiation in any three-
dimensional spatial direction whereas the material is transparent in its solid form at these wavelengths.
These artificially engineered materials are generically known as photonic bandgap (PBG) materials or
photonic crystals. Although “photonic” refers to light, the principle of “bandgap” applies to
electromagnetic waves of all wavelengths. Consequently, there is a controversy in the microwave
community about the use of the term “Photonic”, [2] and the name Electromagnetic bandgap (EBG)
material or Electromagnetic crystal is being proposed.
NUMERICAL MODELING OF ELECTROMAGNETIC BANDGAP CRYSTALS
As in any novel technological field, on occasion a cut and try method was applied initially because of the
lack of reliable prediction methods. The theoretical description of electromagnetic waves in
electromagnetic band gap crystals involves the exact solution of Maxwell's equations in a periodic
medium. Over the past few years, several techniques have emerged which allow us to predict the
performance of EBG’s providing useful prefabrication data.
The first models were based on scalar theory, but, it was soon discovered that this did not provide the
accuracy required; consequent models are based on the full vectorial Maxwell’s equations. While other
techniques exist (Order-N [3]) the following methods have been used very frequently:
1) Plane wave expansion or Spherical wave expansion.
This method starts with the Maxwell’s equations in a generalized eigenvalue form. The plane wave
expansion allows this set of equations to be solved by converting them into a Hermitian eigenvalue
problem [4] and many commercial packages exist to aid in their solution. Because the plane wave method
is easy to understand and is computationally very straightforward to implement, it became the first
method to find widespread use. As many plane waves are usually required in order to obtain good
convergence this can limit the use of the method for treatment of more complicated crystals. Several
means have been proposed to improve the convergence of the plane-wave expansion [5]. Spherical waves
may be used instead of plane waves as a basis set if the electromagnetic crystal is composed of spherical
or cylindrical parts. This method is called the spherical-wave expansion method of the vector KRR
(Koringa-Kohn-Rostker) method [6].
APPLICATIONS OF MICROWAVE ELECTROMAGNETIC BANDGAP ANTENNAS
A multitude of basic EBG applications exist especially within the microwave and low millimetre-wave
region, for example in electronically scanned phased arrays, high precision GPS, Bluetooth, mobile
telephony, wearable antennas, etc.
Electronically scanned phased arrays find their use in many applications. For example constellations of
Low Earth orbit satellites can be used for high data-rate transmission for multi-media applications. These
applications require scanned multi-beam antennas with relatively wide bandwidth. Each beam is usually
working in dual circular polarization. Most of these constellations will work at frequencies up to 30 GHz.
The use of active phased array made in microstrip technology is then an attractive solution. However, the
need for bandwidth and scanning increases the undesirable effects caused by surface waves. A very
promising way to eradicate the problems created by surface waves, e.g. scan blindness, while at the same
time improving performance, is to substitute standard dielectric substrates by electromagnetic bandgap
crystals [14], [15].
SUB) MILLIMETRE WAVE ELECTROMAGNETIC BANDGAP ANTENNAS
A new generation of scientific space borne instruments, included in both Earth observation and scientific
missions, is under consideration at millimetre and sub-millimetre wavelengths. As the frequency
increases, a planar structure that integrates the antenna, mixer, local oscillator and all peripheral circuitry
onto one single substrate becomes an attractive option. While conceptually simple, in practice it is
challenging to develop and test an integrated planar antenna on a semiconductor substrate that has good
radiation efficiency and can be easily integrated with the active circuit. One of the problems encountered,
is that planar antennas on high dielectric constant substrates couple a significant fraction of the input
power into substrate modes. Since these do not contribute to the primary radiation pattern, substrate mode
coupling is generally considered as a loss mechanism. By removing the possible existence of substrate
modes by using a EBG substrate the problem can be overcome, exemplifying the application of EBG
materials. The radiation pattern of an integrated antenna system at 500 GHz [23] is shown in figure 10.
The EBG crystal used is the so-called layer-by-layer or woodpile structure [55].
CONCLUSION
Currently, there is a need for wide, multi-band device functionality, and multifunctional devices. As long
as the market generates a technology pull, the development of novel components and subsystems will
always be in demand. Ideally, these components and sub-systems would be required to be dynamic, re-
configurable and multifunctional.
The technological potential of electromagnetic crystals for developing such novel components and
subsystems offers a very promising alternative that could potentially overcome the limitations of current
technology. EBG technology is a breakthrough, mainly due to their ability to guide and control efficiently
electromagnetic waves.
In order to drive this technology towards the market place we will need to identify component feature(s)
of electromagnetic bandgap structures that give added value over and above current approaches. This
paper has presented several applications where EBG technology could play a major role. Several antenna,
waveguide and filter configurations are discussed. It is also shown that tunable EBG devices can be made.
As long as primary EBG components emerge with functional efficiency, the realisation of a complete
system would become a distinct possibility.