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Analysis and Design
of
a Microstrip Reflectarray Using Patch es of Variab le Size
S.D .
Tagonski* and D.M. Pozar
Department of Electrical and Computer Engineering
University
of
Massachusetts
Amherst. MA 01003
Introduction
In a great number of m icrowave applications a highly directive antenna
with a main beam scanned to a certain angle is required. To ach ieve this a certain
aperture illumination with progressive phasing is used. The two primary ways to
do this are reflectors and arrays. The reflector antenna uses its geometry to create
the desired phase across the aperture, while the array employs distinct elements fed
with progressiv e phasing. Reflecto r antenna s are advantage ous in the fact that
they typically exhibit large bandwidth and low
loss
The main disadvantage
of
the
reflector is the geometrical constrain t it imposes on the design. The most p opular
reflector, the parabolic reflector, also exhibits inherently high cross polarization
levels. Micros trip patch arrays are lightweight, low-profile antennas that are
capable
of
low
cross
polarization levels but typically have small bandwidth and
fairly large loss at microwave frequencies
Obviously, it would be beneficial to combine some of the more attractive
features of reflectors and arrays. This is accomplished by the reflecta rray
11
In
the reflectarray, a primary source illuminates a reflecting surface, producing a
specularly reflected field, as in the case of a reflector antenna. However, the
reflecting surface is covered with an array
of
radiating elements that produce a
scattered field These elements can be tuned to produ ce the required phasing over
the aperture, thereby eliminating the geometrical constraint of ordinary reflectors
In [2], the author designed a rricrostrip reflectarray using patches with an
attached stu b for phasing as radiating elemen ts. In [3], the authors show ed that
the same type of phase control could be achieved by using patches of variable
resonan t length. This technique eliminates the need for a triangula r grid spac ing,
and produces a larger bandwidth since the bandwidth of the stubs is no longer a
factor.
In this paper the steps taken in the design
of
a microstrip reflectarray using
patches of variable size are outlined. Measured and theoretical results are shown
for the finished design, and several important performance criteria are compared
with the microstrip reflectarray of [2]
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where Qiis the reflection co efficient phase,
R,
s the distance from the phase
center of the feed to each element,
r
is the vector from the center of the
reflectarray to each element, and
re
s the unit vector in the main beam direction.
The length
of
each element is then adjusted to produce the desired phase.
An
interesting effect occurs if the specularly reflected field is included in
the analysis of
[3].
At the design frequency of the reflectarray, the scattered field
from the patches is of nearly equal amplitude and has an approximate phase
difference of
180
degrees as compared to the specularly reflected field.
This
effectively eliminates the specular reflection, and there is
no loss
of efficiency due
to the specular reflection.
Results for the MicrostripReflectarray
Using the design procedures described above, a microstrip reflectarray was
designed and fabricated with the Same parameters of [2] but using patches of
variable sue. Figure
2
shows the measured and theoretical E-plane pattems for the
reflectarray taken at 5 305GHz The main beam is scanned to 25 degrees with a
beamwidth
of
7.5 degrees. The peak sidelobe level was measured to be 20 5dB
down from the main beam amplitude. The results are in good agreem ent with
theoretical calculations. The measured and computed H-plane patterns are shown
in Figure 3 The agreement between computed and measured patterns is excellent,
and is better than in the H-plane case since aperture blockage effects are reduced.
4
10
16
m
-26
30
-36
4
6
-10040 do
4 20
0 20
40 w 8
10
10
-10040
do
4 0 0 20
40
w w
100
Theta (degrees) Theta (degrees)
Figure
2. Measured
and
theoretical
E-plane
patternsat 5.305 GHz
Figure 3.Measured
and theoretical
H-plane
pattems at
5.305GHz
Table 1shows a comparison between the reflectarray described in this
paper and the reflectarray of
[2]
Several important performance criteria of the
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two different designs are compared. The two designs exhibit similar
performance
in most areas, however, the reflectarmy using variable size patches
has
a slightly
larger bandwidth, which is
expected
because of the absence of t uni ngstubs. The
incre se n bandwidth may also result from
the
f ct that the r e f l m y
consists
of
many Merent sized patches, each operating at a
slightly
different resonant
frequency. The bandwidth is de he d in
this
c se as the gain bandwidth I down
from the
peak
gain and th reflectm ay of [2] exhibits a bandwidth of 3.7% while
the reflectarray using variable size patches h s a 4.6 bandwidth.
Table 1.
Micmstrip
nfled rr y
perfomawe
comparison.
In conclusion, a microstrip reflectarray using variable
size
patches was
designed and tested. It exhibited S i a r performance to the microstrip reflectarray
using patches w ith tuning stubs attached, with the additional advantage o f having
less restriction on the array grid spacing. It lso exhibited a slightly larger
bandwidth.
References:
11
R.G. alech, The Reflectarray Antenna System , 12th Annual
Antenna
Symposium,
USAF Antenna Research and Development
Program 1, University of
IuinOis,
1962.
[2]
T.A. etzler, Design and nalw'sof a Microstrio
Reflectarray
h.D.
dissertation, University of Massachusetts, September 1992.
D.M.
Pozar
nd
T.A.
Metzler, Analysis of a R eflectarray Antenna Using
Microstrip Patches of V ariable
Size ,
Electronics
Letters
vol. 29, pp.
657
658 April 1993.
[3]
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