Near Field Nulling with a Cylindrical Reflector and Moving Plates
Nabeel Malik'
214 Zachry Engineering Center
Department of Electrical Engineering
Texas A & M University
College Station, TX 77843
nmalik@ee.tamu.edu
Randy Haupt
The Pennsylvania State University
Applied Research Laboratory
P. 0. Box 30
State College, PA 16804-0030
hauot@ieee.org
abstract. A cylindrical reflector with a single element feed and movable disks on
the surface is modeled using finite elements. The disks are mechanically moved in
and out of the surface of the reflector in order to place a null in the near field.
I. Introduction
One idca for placing a null in the sidelobe of a reflector antenna is to use another
object to scatter sufficient field strength to cause cancellation at the sidelobe.
Jacavanco experimentally demonstrated this concept with moving disks on a
parabolic reflector. Scattering from the disk added deshuctively with a sidelohe in
order to create a null. This paper revives this concept through modeling the
problem using finite elements.
11. The Reflector
We experimented with the two-dimensional perfectly conducting parabolic
reflector antenna shown in Figure 1. The feed is a line source and the reflector has
ratio of 0.5 with the reflector having D = 101.
a focal length v) to diameter (0)
This reflector was modeled using the FEM. Its radiation panem in space at a
specified time is shown in Figure 2. Dimensions are in meters. The frequency is at
3 GHz.
111. Placing Nulls in the Reflector Pattern
We decided to take Jacavanco's approach and try placing nulls in the radiation
pattem of a reflector antenna by moving metal plates back and forth from the
reflector surface until the scattered field from the plates cancels the sidelobe
where the interference is present. Since the plates are mechanically moved, the
interference must be stationw or slowly moving to be cancelled.
Figure 3 is a diagram of the reflector with a single nulling plate. The plate is small
enough to have only a small impact on the main beam but large enough to cause
destmctive interference in the sidelobe region. A genetic algorithm finds the
optimal placement of the plate in order to minimize the total output power from
the reflector. Since the plate is not large enough to cancel the main beam, the
desired signal entering the main bean remains relatively unaffected while the
nulling process takes place.
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f = Frequency (3GHz)
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Figure 1 . Geometry o f the parabolic reflector.
0.''
2.5
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1 0.5
.o
. -0.5
-2.5
Figure 2. E-field panern at 3 GAz from a point source (U10 in size) at the focal
point ofthe reflector (Xand Y in meters and Elechic Field Intensity in V h ) .
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Flat Metal Plate
nt of the Reflector
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Figure 3. Schematic diagram of flat metal plate of size h moving at y = 0.2m.
1
0.8
. .
0.6
0.4
0.2
y
o
-0.2
-0.4
i0.6
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Figure 4. Electric field pattems in presence of a flat metal plate of size h with null
at ( I , 0.7) (X and Y in meters and Electric Field Intensity in V h ) .
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TABLE 1 COMPARISON BETWEEN TWO DIFFERENT VERTICAL
POSITIONS OF THE FLAT METAL PLATE (dB REDUCTION)
IV. Results
We tried three different sized metal plates each at two different vertical positions.
A genetic algorithm (CA) was used to move the plate to find the greatest amount
of sidelohe reduction. Table 1 shows the dB reduction for each size and vertical
position. The amount of reduction was found to be directly proportional to the
size of the metal plate regardless of its vertical position. This conforms to the fact
that as the surface area of the plates is increased, we get more control over the
scattered field and thus get more reduction at the desired location where the
interference is present. In general, the plate closest to the axis provides the most
sidelobe level reduction. It also causes the greatest amount of main beam
distortion.
V. Conclusions
A plate can he mechanically moved on the reflector surface to create a null in a
sidelobe. The size of the plate is chosen such that the scattered field is large
enough to cancel the sidelohe but small enough to cause minimal distortion to the
main beam.
Bibliography
[I] D. Jacavanco, "Reflector antenna having sidelohe suppression elements," US.
Patent4,631,547, Dec 23, 1986.
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