analusis and design of a microtrip reflectarray using patches of variable size

8/10/2019 Analusis and Design of a Microtrip Reflectarray Using Patches of Variable Size

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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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analusis and design of a microtrip reflectarray using patches of variable size

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