Adaptive Reflector Antenna with Smart Materials
*Randy Haupt and Joseph Flemish
The Pennsylvania State University, Applied Research Laboratory
State College, PA 16804-0030
Abstract
This paper describes a method for placing nulls in a reflector antenna pattern by
incorporating elements which have a reflectivity that can be varied over a wide
range using electrical or optical control.
Introduction
Adaptive nulling in an antenna is usually associated with expensive phased array
systems, although several approaches exist to place nulls in reflector antennas.
For example, some novel ideas on placing movable metallic elements on the
reflector surface to cancel interference have been reported [1], [2]. Although these
approaches are functional, the mechanical movement of the plates is undesirable.
A method that accomplishes the same goals without moving the plates would
make this approach more attractive.
New developments in polymers and electronic materials now make it possible to
replace the moving plates on the adaptive reflector with materials that have
properties that can be altered by electrical or optical means. It has been shown
that conducting polymeric materials have controllable resistance at microwave
frequencies [3]. For example, applying a small dc potential across a poly(aniline)silver-polymer electrolyte composite quickly changes its reflectivity in a process
that is reversible. Experiments have shown a reflectivity change of greater than 20
dB can be obtained and that these materials can be incorporated into a Salisbury
screen to dynamically alter the radar cross section of large surfaces [4]. Other
approaches to obtaining adaptive Salisbury screens involve using moving parts.
This paper demonstrates the use of adaptive Salisbury screens on the surface of a
cylindrical reflector antenna. The results of the simulations suggest that this
approach would be a viable way to place nulls in the sidelobes of reflector
antennas.
The Reflector
We used the two-dimensional perfectly conducting parabolic reflector antenna
model shown in Figure 1. The feed is a line source and the reflector has a focal
length (f) to diameter (D) ratio of 0.5 with the reflector havingD=102. This
reflector was modeled using the method of moments. Its radiation pattern in space
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at a specified time is shown in Figure 2. The smart materials are Salisbury screens
made from variable resistive sheets in front of the reflector surface extending
from to the edges.
/
relector
D
eed
F
S
j a!daptive
|reflector
elements
conducting
reflector
x\
\
/
~~~~feed
scattering
Figure 1. The right reflector replaces the movable scattering element in the
left reflector with smart material.
Results
A genetic algorithm (GA) varies the resistivity of the smart materials until the
output power of the reflector antenna is minimized. Since the scattered field is too
small to cancel the field at the main beam, but large enough to cancel a sidelobe,
the desire signal entering the main beam is minimally affected, while the
interference signal entering the sidelobe is eliminated. Figure 2 shows a plot of
the adapted pattern (solid line) superimposed on the quiescent pattern (dashed
line). The main beam gain goes down a couple of dB as a result of the deep null
placed at 26.5 degrees. In this case, the GA placed the null very quickly as shown
in Figure 3. Figure 4 shows the placement and values for the normalized (to free
space) resistivity of the Salisbury screens.
384
E- -II ,It
-20-
-30
0
50
+ in degrees
Figure 2. Resulting null in the far field pattern due to the adaptable element.
-50
-5
-10
-15
8-20populaton average
-25
0
10
20
40
30
generation
50
60
Figure 3. The GA found the null very quickly as shown by this convergence
plot.
385
/0;=0.224
4
2
.s O1
-2
\\=.982
4
-6
4
0
2
6
x in .
Figure 4. Diagram of the adaptive reflector.
-4
-2
Conclusions
Electrically changing the reflectivity of parts of a reflector antenna surface can
place nulls in the far field pattern sidelobes with some main beam degradation.
References
[1]
[2]
[3]
[4]
D. Jacavanco, "Reflector antenna having sidelobe suppression elements,"
U.S. Patent 4,631,547, Dec 23, 1986.
J. L. Poirier, "Reflector antenna having sidelobe nulling assembly with
metallic gratings," U.S. Patent 4,725,847, Feb 16, 1988.
P.V. Wright, et.al., "Progress in smart microwave materials and
structures," Smart Mater. Struct. vol. 9, no. 3, Jun 2000, pp. 273-279.
B. Chambers, "Surfaces with adaptive radar reflection coefficients," Smart
Mater. Struct. vol. 6, no. 5, Oct 1997, pp. 521-529.
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