APS-2 1-7
EXPERIMENTAL P-ARTIALLY ADAPTIVE NULLING
IN A LOW SIDELOBE
PHASED ARRAY
Randy L. Haupt
Robert A. Shore
Rome Air Development Center
Hanscom AFB, YA 01731
1.
Introduction
Recent interest in adaptive phase-only control of array
element weights reflects the growth
in importance of both phased
arrays and adaptive processing. In a phased array, control of
the element phases is already available
as part of a beam steering system, and s o it is natural to attempt adaptive cancellation
with phase control alone. Although a number of papers have considered phase-only adaptive nulling and null synthesis, references containing experimental results are extremely scarce, and
considerably more work is needed
to test the practicality of
phase-only adaptive nullingin phased array antennas.
2.
Antenna and Adaptive Algorithm
The antenna usedi n this investigation is a low sidelobe
linear array of 80 H-sectoral horn radiators. Figure 1 is the
quiescent far-field pattern of the array. A Taylor amplitude
t a p e r produces the low sidelobes.
Phase concrol of each elemenr
is accomplished withan 8-bit phase shifter. Although the antenna was not designed to
be "adaptive," we were able to use it
adaptively by linking the phase controls to the HP-2lMX computer
in the SA-2020 computer controlled antenna measurement system
and programming the computer to perform adaptive suppression.
Adaptive cancellation was performed
by minimizing the total
array output power using the gradient search algorithm
PHNEW~= PHOLD~-
AP'%
(1)
A PH
Here PHh'EWi and PHOLDi are the new and old phase shifter settings respectively for theith element at each iteration. APWRi
is the changein total out ut power resulting from a change of
APH in the phase of theit{ elememt keeping all other phases
fixed at their old values, and
u , the gradient stepsize, is
823
where
The algorithm is iteratively repeated starting
with a given APH
until no further reductionin total output power is obtained;
then APH is reduced by one bit and the procedure
repeated, and
s o on until APR equals one least significant
bit (LSB).
3.
ExperimentalResults
The adaptive performanceof the antenna and gradient algorithm were tested with a variety
of different partially adaptive
configurations. For all cases a singleCW noise signal was
located at 22" from broadside,and the initial phase shiftfor
determining the gradient vector was taken to5 be
LSB or 7".
The results are summarizedin Table 1. All but one of the configurations were effective
in placing a pattern nullof better
at the location
than -20 dB below the quiescent pattern value
22".
The one exceptionwas a configurationwith all adaptive
elements placed on one side of the array center. Figures 2 and
3 show representative adapted patterns.
4.
SimulationResults
was modeled on a TekThe 80 element experimental array
tronix 4052 computer using measured amplitude weights from the
experimental array,8 bit phase shifters, and cosine element
patterns. Many adaptive element and jammer configurations
were
tried with successful results. Figure 4 is an example of the
adapted far field pattern superimposed
on the quiescent far
field pattern. For this case, therewere 10 adaptive elements
(elements 20-24 and 57-61)and three jammersat 5', B o , and
47.6".
Adaptation lowered the sidelobe levels at the three
jammer locations-21.7 dB, -31.4 dB, and -17.0 respectively.
8 24
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F i g u r e 2. Measured p a t t e r n of
t h e a r r a y w i t h 10 a d a p t i v e e l e m e n t s
(20-24 a n d 57-61)and
jammer a t 22".
F i g u r e 1. M e a s u r e dq u i e s c e n t
p a t t e r n of t h ee x p e r i m e n t a l
array.
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F i g u r e 4 . S i m u l a t e dq u i e s c e n p
t att e r n (---) a n da d a p t e dp a t t e r n
(-)
for the array with
10 a d a p t i v e elements(20-23and57-61)and
jammers
a t 5",38",and47".
of
F i g u r e 3. M e a s u r e dp a t t e r n
t h e a r r a y w i t h 4 a d a p t i v e elements(20,23,58,61)and
jammer
a t 22'.
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