An Adaptive Rectangular Microstrip Patch Antenna
Array Element Using Photonic Controls
Randy L. Haupt
Applied Research Laboratory
Penn State University
PO Box 40
State College, PA 16804
814-865-7299 x210
hauptWieee.org
Abstract This paper shows how certain photoconductive
materials can be used to design an adaptive array element
with a center frequency of 2 GHz. The resonant frequency
of the patch gradually shifts to a lower frequency as the
conductivity off its gap filled with photonic material
increases. The resulting increase in the reflection coefficient
at the center frequency and decrease in gain acts as a
continuous amplitude weight. This approach offers
continuous variation of the conductive portions of the patch
rather than an "on" or "off' approach offered by the
switches. Varying the amplitude of the elements allows
dynamic control over the array sidelobe levels.
power of the array. It requires constraints on the hardware
phase shifters, amplitude weights, or number of adaptive
elements, so the desired signal is not nulled in the main
beam. This approach has been implemented in a phase-only
algorithm [4] and amplitude and phase adaptive nulling
using a genetic algorithm [5]. The goal is to minimize the
total output power while at the same time minimizing the
perturbations to the main beam. Limiting the amount of
controls by using a subset of all the elements or least
significant bits of the weights prevents the algorithm from
placing a null in the main beam and reducing the desired
signal strength.
TABLE OF CONTENTS
This paper explores the use of photoconductive materials in
the design of microstrip patch antennas for adaptive arrays.
Making part of a patch antenna from photoconductive
material allows the tuning of that element via an optical
signal and hence controlling the reception of that signal at
the element. Normally, the adaptive weights are digital
amplitude and/or phase weights or the weights are applied
in software if a digital beamformer is used. A patch design
is optimized for control at 2 GHz. The patch consists of a
rectangular perfect electric conductor (PEC) separated from
a thin rectangular PEC section by a thin rectangular piece of
silicon (see Figure 1). Five of these patches are placed in a
linear array to demonstrate sidelobe control. Sidelobes are
reduced through altering the conductivity of the small strip
of silicon placed in the patch.
1. INTRODUCTION......................1
2. PATCH DESIGN......................1
ARRAY DESIGN...................... 3
ANTENNA
3.
4. CONCLUSIONS......................5
REFERENCES .....................5
BIOGRAPHY .....................6
1. INTRODUCTION
Adaptive antennas have found use in many wireless
applications. An adaptive antenna has the ability to change
its antenna pattern in order to enhance reception of a desired
signal while minimizing undesired signals. The antenna
patterns are manipulated by controlling the amplitude and/or
phase of the signals received at each element in the array.
Various antenna configurations and adaptive algorithms are
outlined in the literature [1].
2. PATCH DESIGN
A simple pin-fed rectangular microstrip patch serves as the
Hardware requirements for an adaptive antenna can be quite
expensive. The most elegant signal processing algorithms
require a receiver at each element with an associated
calibration scheme. These approaches are derivatives of the
Applebaum adaptive loop [2] and the least mean square
(LMS) algorithm [3]. Signals from the elements are used to
form a covariance matrix from which the adaptive weights
are derived. Another approach minimizes the total output
starting point for this adaptive element. The majority of the
patch is made from a PEC with a small portion made from
photonic material with variable conductivity. There are
several different types of materials whose conductivity can
be changed using an electrical signal. Conductive
electroactive polymers, such as polypyrroles and
polyanilines, have a conductivity that is proportional to an
1
1-4244-1488-1/08/$25.00 C 2008 IEEE.
IEEEAC paper paper# 1 184, Version 3, Updated 15 Nov 2007
1
coordinate system is centered on the patch. The pin-feed is
located at x = 7.6 mm.
applied electric potential. Conducting polymeric materials
have controllable conductivity at microwave frequencies
[6]. A small dc potential applied across a poly(aniline)silver-polymer electrolyte composite changes its
conductivity. These materials have been incorporated into a
Salisbury screen to alter the radar cross section of large
surfaces [7]. Organic photoconductors have been used in
photocopying for many years [8]. They were not used in
circuit design due to the low conductivity. Recent advances
in increasing the conductivity of organic photoconductors
have become popular for use in plastic circuits [9]. Sheets
of single wall carbon nanotubes have been designed to
exhibit photoconductivity. They are not yet practical to use
in circuits though [10]. Probably the most common type of
variable conductive material is silicon. Silicon has a relative
permittivity of £r = 11.7 and an electrical conductivity that
varies from an insulator to a good conductor, depending
upon the intensity of the optical source or the biasing
current. Silicon is used in the manufacture of many different
types of electronic devices, as well as photo voltaic cells. It
has been used to reconfigure dipole antennas [11] as well as
electromagnetic bandgap surfaces [12]. Other applications
include a reconfigurable Fresnel-zone plate antenna [13]
and a reconfigurable reflectarray [14].
A graph of the amplitude of the return loss is shown in
Figure 2 for the following conductivities: 0, 1, 2, 5, 10, 20,
30, 50, 75, 100, 200, 500, and 1000 S/m. At 2 GHz, there is
a distinct resonance when the silicon has no conductivity.
As the conductivity increases, the resonance at 2 GHz
transitions to a new resonance near 1.78 GHz. As a result,
the s1l at 2 GHz increases from zero to 0.9. The amount of
power delivered to the patch at 2 GHz reduces as the
conductivity increases. Consequently, the photoconductive
silicon acts as an amplitude control to that element. This
amplitude control could be useful in receive array
applications. Figure 3 shows the return loss as a function of
conductivity at 2 GHz. Since the conductivity is directly
related to the intensity of the optical source, then increasing
the optical intensity decreases the signal at the patch. In
effect, this configuration is an optically controlled
attenuator built into the patch element.
CST Microwave Studio [15] performs the simulations in
this paper. It uses the finite integration technique (FIT) with
the perfect boundary approximation and a multilevel
subgridding scheme. The time domain approach covered a
frequency band from 1.7 to 2.3 GHz.
The microstrip patch model is shown in Figure 1. It consists
of a main rectangular patch made from a PEC that is
58.7 x 39.4 mm. The substrate is a slab of optically
transparent fused quartz with Er = 3.78 backed by a PEC
groundplane. The substrate is 88.7x69.4mm and is 3 mm
thick. To the right of the patch is a thin strip of silicon (G)
58.7x2mm with Er = 11.7 . To the right of the silicon is
another thin strip of PEC (F) 58.7x 4.2 mm. An optical
source illuminates the bottom of the small rectangular sliver
of silicon from below. A laser or LED beneath the
groundplane can illuminate the silicon through small holes
in the groundplane or by making the groundplane from a
transparent conductor, such as indium tin oxide [16].
Increasing the optical source intensity increases the silicon
conductivity, thus promoting the flow of current from the
main patch to the small rectangle. Enlarging the patch
lowers the resonant frequency.
0
in
7.6 mm
I
Figure 1. Diagram of the adaptive patch.
1(}
0,41I- 000)SAIIn
.
The dimensions for the patch and the location of the feed
point were found using the numerical optimization
algorithm in CST Microwave Studio. The optimization was
initially done assuming the silicon has zero conductivity.
The patch was designed to be resonant at 2 GHz. The length
of the main patch and the location of the feed point along
the x-axis were optimized to minimize
at 2 GHz. The
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fteqtwncy (GHz)
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Figure 2. Plots of the magnitude of s1l for silicon
conductivities of 0, 1, 2, 5, 10, 20, 30, 50, 75, 100, 200,
500, and 1000 S/m.
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z
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600
conldLiclvlitv (SAin)
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Figure 3. Plot of the magnitude of s11 versus conductivity at
2 GHz.
A three dimensional view of the antenna pattern is shown in
Figure 4. The peak gain is 5.22 or 7.22 dB. Orthogonal cuts
of the element pattern are shown in Figure 5. The gain and
pattern are typical of a rectangular microstrip patch.
Figure 4 Plot of the linear antenna gain at 2 GHz when
v= 0 . The peak gain is 5.22.
l 0 1'""""""""""""""""'"""""""""""""""""""""""'
The first attempt at an adaptive patch did not have the small
PEC to the right of the silicon strip. This configuration did
not allow a large change in s 1 with a small change in
conductivity, because the resonance did not move far from 2
GHz.
+
The signal strength received by an element is a function of
the element gain and the s1l. Consequently the received
signal is proportional to the gain times the power not
reflected by the element mismatch. Figure 6 is a plot of the
gain times the portion of power not reflected (1-_s2).
Significant attenuation is possible with modest increases in
silicon conductivity.
0
-80
,40
60
SO
Figure 5. Graphs of the element gain patterns (dB) for
0=0 and 0=90'.
3. ANTENNA ARRAY DESIGN
The next step places these adaptive elements in a small
array. A five element linear array of the photoconductive
patches is shown in Figure 7. The spacing between
elements is 75 mm or 0.51Z. If the silicon insets all have a
conductivity of zero, then the array is uniform with a far
field pattern shown in Figure 8. This quiescent pattern has a
gain of 12.81 dB and a relative peak sidelobe level of 13.84
dB.
...
The element patterns of the uniform array are shown in
Figure 9. The average gain of these patterns at boresight is
6.14 dB. This average gain is over 1 dB less than the
isolated element pattern. Mutual coupling also flattens the
gain patterns and adds a ripple due to the finite length of the
2
20
40
60
conductivity (S/ii)
80)
lot
Figure 6. Plot of the magnitude of s1l times patch gain
versus conductivity at 2 GHz.
array.
3
Illuminating the silicon at each element with a different
optical intensity produces a conductivity taper across the
array. Increasing the conductivity of the silicon in a patch
decreases the product of the patch gain times the power
delivered to the patch. Thus, the conductivity taper induces
an amplitude taper. An array pattern with equal sidelobes
results when the conductivity has values of [16 5 0 5 16]
S/m. The corresponding antenna pattern is shown in Figure
10. It has a gain of 10.4 dB and a peak relative sidelobe
level 23.6 dB below the main beam. The element patterns
have different gain patterns (Figure 11) than those of the
uniform array. The gain of the elements with the silicon
illuminated goes down as predicted. The general element
pattern shape also changes slightly due to the change in
coupling.
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qiall LSflt
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Figure 7. A five element linear array of photoconductive
patches.
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40
20
0
20
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40
60
80
Figure 10. The quiescent pattern is the dashed line and has
all the conductivities set to 0. The adapted pattern is the
solid line and has the silicon conductivities set to [16 5 0 5
16] S/m.
-W
-20
I
10
-80
-60
-40
-20
0
20
4(
60
SO
Figure 8. Far field pattern of a five element uniform array.
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40
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Figure 9. The element patterns of the quiescent
numbers correspond to the elements in Figure 7.
40
20
0
20
4
Figure 11. The element patterns of the array that has the
silicon conductivities set to [16 5 0 5 16] S/m. The numbers
correspond to the elements in Figure 7.
A linear conductivity taper of [16 8 0 8 16] S/m reduces the
inner sidelobes even farther. The resulting antenna pattern
in Figure 12 has a gain of 10.14 dB and a first sidelobe
level of 24 dB below the peak of the main beam. The
10
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60
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so
80
array. The
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corresponding element patterns appear in Figure 13. The
increased conductivity at elements 2 and 4 reduced their
gains. Altering the conductivity at certain elements changes
the array pattern. It would be possible to switch the pattern
between uniform with high gain and low sidelobe with
lower gain depending upon the interference present. It
would also be possible to use an adaptive algorithm to
adjust the conductivity of the patches to reject interference
entering certain sidelobes. Figure 14 demonstrates the
effects on the array pattern of varying the conductivity at
element 2 from 0 to 500 S/m while keeping the conductivity
at the other elements at 0 S/m. Sidelobes and nulls are
changed, but the gain remains relatively unchanged. The
minimum of s1l of element 2 moves to below 1.8 GHz when
its conductivity is 500 S/m. The minimum of s1l of elements
1, 3, and 4 occur at about 1.98 GHz, while The minimum of
s1l of elements 5 stays at 2.0 GHz.
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This work was sponsored by Army CECOM under contract
N00024-02-D-6604
DO-295.
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REFERENCES
[1] R.A. Monzingo and T.W. Miller, Introduction to
Adaptive Arrays, Raleigh, NC: SciTech Publishing,
Inc., 2003.
1
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\.\..
-20
0
20
0 (d1eeL)
40
.........................................
0 rr....................
XY"
-40
20
much. Also, optimization would produce a lower maximum
sidelobe level. Adding more elements to the array would
reduce the effects of errors and allow lower sidelobe levels.
Extensions to planar arrays are also possible.
X
A\
'd X
., j,.'S.\
-60
0
Optimizing the conductivity taper would make it more
efficient, so the main beam gain would not be reduced so
h, M \
1so
=20
This paper shows how to control the signal strength
received by a patch antenna by altering the conductivity of
part of the patch. Photoconductive elements can be used to
place an amplitude taper on a linear array to lower the
sidelobe level. This paper demonstrated the
photoconductive element concept through computer
modeling. The relative peak relative sidelobe level of a five
element array was lowered by about ten dB through a linear
conductivity taper.
S->b-b \
i;
40
......
4. CONCLUSIONS
0
20
0 (:degJeg)
.>^s^xt.--ss,
4-0
S6i0
.............
Figure 14. The array patterns associated with varying the
conductivity at element 2 from 0 to 500 S/m.
- t.1
01H
MI
O
Figure 12. The quiescent pattern is the dashed line and has
all the conductivities set to 0. The adapted pattern is the
solid line and has the silicon conductivities set to [16 8 0 8
16] S/m.
1Ol
Xfm
60
[2] S.P. Applebaum, "Adaptive arrays," Syracuse
University Research Corporation Report SPL TR 66-1,
Aug 1966.
80
Figure 13. The element patterns of the array that has the
silicon conductivities set to [16 8 0 8 16] S/m. The numbers
correspond to the elements in Figure 7.
[3] B. Widrow, et al., "Adaptive antenna systems," IEEE
Proc., Vol. 55, No. 12, Dec 1967, pp. 2143-2159.
5
,,,,,,,,
[4] C. A. Baird and G. G. Rassweiler, "Adaptive sidelobe
nulling using digitally controlled phase-shifters," IEEE
AP Trans., Vol 24, No. 5, pp. 638-649, Sep 76.
BIOGRAPHY
Randy
Haupt
is an IEEE Fellow and
Head
Department
of
Computational
Electromagnetics and Senior Scientist at
the Penn State Applied Rwsearch
Laboratory. He has a Ph.D. in Electrical
Engineering from the University of
Michigan, MS in Electrical Engineering
from Northeastern University, MS in
Engineering Management from Western
New England College, and BS in Electrical Engineering
from the USAF Academy. He was Professor and
Department Head of Electrical and Computer Engineering
at Utah State University from 1999-2003. He was a
Professor of Electrical Engineering at the USAF Academy
and Professor and Chair of Electrical Engineering at the
University of Nevada Reno. In 1997, he retired as a Lt. Col.
in the USAF. Dr. Haupt was a project engineer for the
OTH-B radar and a research antenna engineer for Rome
Air Development Center. He was the Federal Engineer of
the Year in 1993 and is a member of Tau Beta Pi, Eta
Kappa Nu, URSI Commission B, and Electromagnetics
Academy. He served on the board of directors for the
Applied Computational Electromagnetics Society and is on
the IEEE Antenna and Propagation Society Administrative
Committee. He has many journal articles, conference
publications, and book chapters on antennas, radar cross
section and numerical methods and is co-author of the book
Practical Genetic Algorithms, 2 ed., John Wiley & Sons,
2004 and Genetic Algorithms in Electromagnetics, John
Wiley & Sons, 2007. He has eight patents in antenna
technology.
[5] R.L. Haupt and H.L. Southall, "Experimental adaptive
nulling with a genetic algorithm," Microwave Journal,
vol. 42, no. 1, Jan 99, pp. 78-89.
[6] P.V. Wright, et.al., "Progress in smart microwave
materials and structures," Smart Mater. Struct. Jun
2000, 9, (3), pp. 273-279.
[7] B. Chambers, "Surfaces with adaptive radar reflection
coefficients," Smart Mater. Struct., Oct 1997, 6, (5), pp.
521-529.
[8] S. Forrest, P. Burrows, and M. Thompson, "The dawn
of organic electronics," IEEE Spectrum, Vol. 37, No. 8,
Aug 2000, pp. 29-34.
[9] S.K. Moore, "Just one word plastics," IEEE
Spectrum, Vol. 39, No. 9, Sep 2002, pp. 55-59.
[10] S. Lu and B. Panchapakesan, "Photoconductivity in
single wall carbon nanotube sheets," Nanotechnology,
Vol. 17, 2006, pp. 1843-1850.
[11] A.E. Fathy, et al., "Silicon-based reconfigurable
antennas concepts, analysis, implementation, and
feasibility," IEEE MTT Trans., Vol. 51, No. 6, Jun 03,
pp. 1650-1661.
[12]V.J. Logeewaran, et al., "Switching between positive
and negative permeability by photoconductive coupling
for modulation of electromagnetic radiation," Appl.
Phys. A 87, 2007, pp. 209-216.
[13]M. Hajian, G.A. de Vree, and L.P. Ligthart,
"Electromagnetic analysis of beam-scanning antenna at
millimeter-wave band based on photoconductivity
using Fresnel-zone-plate technique," IEEE AP Mag.,
Vol. 45, No. 5, Oct 2003, pp. 13-25.
[14] M.R. Chaharmir, J. Shaker, M. Cuhaci, and A.R.
Sebak, "Novel photonically-controlled reflectarray
antenna," IEEE AP Trans., Vol. 54, No. 4, Apr 2006,
pp. 1134-1141.
[15]CST Microwave Studio, Version 2006.05, April 19,
2006.
[16] R.G. Gordon, "Criteria for choosing transparent
conductors," MRS Bulletin, Aug 2000, pp. 52-57.
6
L.