The Fundamental Physics of Directive Beaming at
Microwave and Optical Frequencies in Terms of
Leaky Waves
Saman Kabiri, Master’s Student
Dept. of Electrical and Computer Engineering
Texas A&M University
Course: Laser Spectroscopy
Instructor: Dr. Hans Schuessler
Monday, November 26th, 2012
Outline
- Introduction and Motivation
- Definition of Antenna’s Parameters
- Antennas Based on a PRS
- Directive Beaming at Optical Frequencies
2
Outline
- Introduction and Motivation
- Definition of Antenna’s Parameters
- Antennas Based on a PRS
- Directive Beaming at Optical Frequencies
3
Introduction
•
Directive beaming occurs in antenna design where a narrow beam is desirable by
using fairly simple planar structure excited by a single source.
•
This topic is fairly rich, and extending from the 1950s until the present time.
•
Several applications:
1- Construction of novel highly directive antenna
2- Optical effect such as narrow beaming of light from a subwavelength aperture
3- Enhanced transmission of light from a subwavelength aperture
•
The purpose of this talk is to give a common explanation of the directive-beaming
phenomenon in terms of leaky wave.
•
Leaky-wave theory is used to present simple design formulas.
4
Introduction
•
Three main structures are used as highly directive antenna.
Fabry-Perot
cavity
Three main
structures
Metamaterial
structures
Directive beaming at
optical frequencies
5
Introduction
•
Three main structures are used as highly directive antenna.
Fabry-Perot
cavity
Three main
structures
Metamaterial
structures
Directive beaming at
optical frequencies
6
Outline
- Introduction and Motivation
- Definition of Antenna’s Parameters
- Antennas Based on a PRS
- Directive Beaming at Optical Frequencies
7
Definition of Antenna’s Parameters
•
A radiation pattern is a graphical representation of the far-field properties of an
antenna.
•
Principle plane:
– E-plane: Contains the electric vector
– H-plane: Perpendicular to the E-plane containing
the magnetic field
Field components and the pattern
measurement scheme for an ideal
dipole
E-plane radiation
pattern
H-plane radiation
pattern
8
Definition of Antenna’s Parameters
•
Directivity is the ratio of the maximum power radiated from an antenna in one
direction in respect to the power radiated from an isotropic antenna.
•
Therefore, the directivity of an isotropic antenna is 1.
•
The directivity of an actual antenna can vary from 1.76 dB for a short dipole, to as
much as 50 dB for a large dish antenna.
Directivity of an actual antenna comparing to an isotropic antenna
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Outline
- Introduction and Motivation
- Definition of Antenna’s Parameters
- Antennas Based on a PRS
- Directive Beaming at Optical Frequencies
10
Antennas Based on a PRS
A leaky-wave antenna made from a PRS
over a grounded substrate layer
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Antennas Based on a PRS
A symmetric pencil
beam at broadside
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Antennas Based on a PRS
•
Various types of PRS surfaces have been developed so far:
A)
B)
C)
D)
Multiple dieletric-superstrate PRS
Periodic metal patch PRS
Periodic slot PRS
Periodic wire or metal strip grating PRS
Different PRS surfaces
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Antennas Based on a PRS
(1)
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Antennas Based on a PRS
•
The two modes have different wavenumbers:
From the figure
and Snell’s law
•
•
The PRS structure showing
the leaky parallel-plate
modes emanating from the
dipole source.
(1)
Equation (1) is an approximate result for the optimum substrate thickness,
which assumes an ideal parallel-plate waveguide.
By considering the loading effect of the PRS on the waveguide cavity,
equation (1) can be modified as
where
The transmission line
model of the PRS
15
Antennas Based on a PRS
• The normalized admittance is either the TM or
the TE value, depending on whether the beam is
being optimized in the E-plane or the H-plane.
The admittances in these planes are different for
conical beam, so that the beam cannot be
optimized simultaneously in both planes.
• An analysis based on the transverse equivalent network (TEN) shows that the
TMz and TEz leaky modes have nearly the same wavenumber, and furthermore,
the phase and attenuations constants are nearly the same, so that
• The far field from the TEN model is given by
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Antennas Based on a PRS
Design Restrictions
• For an air substrate, the scan angle is limited to 60º.
• To allow for a single conical beam that can scan down to endfire, the substrate
must have a refractive index sufficiently large, satisfying
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Antennas Based on a PRS
Radiation Characterizations
• Directivity is approximately related to the E-plane and H-plane half-power
beamwidth (angle in radians between the -3-dB points)
• The pattern bandwidth is defined as
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Antennas Based on a PRS
Table 1. Expression for Peak Field value
19
Antennas Based on a PRS
Table 2. Expression for Beamwidth
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Antennas Based on a PRS
Table 3. Expression for Bandwidth
•
The product of directivity and pattern bandwidth for a broadside can be calculated. The
result for this figure of merit (FoM) is
21
Antennas Based on a PRS
Results
Far field pattern radiation for the slot PRS
structure (a) E-plane pattern for a broadside
design, (b) H-plane pattern for a broadside
design, (c) E-plane pattern for a 45º scan angle,
and (d) H-plane pattern for a 45º scan angle
22
Antennas Based on a PRS
Normalized susceptance of the slot PRS
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Antennas Based on a PRS
•
In this figure, patterns for a metal
patch PRS structure for varying
substrate thicknesses is shown using
an air substrate at a frequency of 12
GHz.
•
The substrate thickness is varied so
that the beam scans from broadside to
45º.
•
For a scan angle beyond 60º an
undesirable secondary beam forms.
•
This especially pronounced for the 75º
scan, where a secondary beam
(pointing at about 43º) is larger than
the primary beam at 75º. Another
secondary beam at about 12º is also
observed in this case.
Various substrate thicknesses
are used to obtain different
scan angles
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Outline
- Introduction and Motivation
- Definition of Antenna’s Parameters
- Antennas Based on a PRS
- Directive Beaming at Optical Frequencies
25
Directive Beaming at Optical Frequencies
26
Directive Beaming at Optical Frequencies
•
•
•
On the exit face, the aperture acts as a source which
is fairly well approximated as a magnetic line source.
The source radiates into space, producing a direct
“space- wave” radiation.
In addition, the source launches a plasmon surface
wave that propagates away from the source in both
directions.
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Directive Beaming at Optical Frequencies
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References
1. D. R. Jackson, P. Burghignoli, G. Lovat, F. Capolino, C. Ji, D. R. Wilton, and A.
A. Oliner, “The Fundamental Phyiscs of Directive Beaming at Microwave and
Optical Frequencies and the Role of Leaky Waves,” Proceeding of the IEEE, vol.
99, pp. 1780-1805, 2011
2. T. Zhao, D. R. Jackson, and J. T. Williams, “General formulas for 2D leaky wave
antennas,” IEEE Trans. Antenna Propag., vol. 53, no. 11, pp. 3525-3533, Nov.
2005
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Thank You
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