A new approach for designing low profile antennas for MSAT applications is presented. It is based on stacking two microstrip antennas that operate at two adjacent modes. The beam scanning is achieved by introducing a phase shift between the stacked elements and consequently a low cost self scan array element is developed. The concept is used to investigate two different antenna types: (1) a single element unit, with its phase shifter, that provides a moderate gain of about 7 dBic; and (2) a seven element array with a peak gain of about 14 dBic. Computed and measured data for each design are presented and discussed

Shafai, L.

English

NASA Technical Reports Server

N92-24156
MSAT Vehicular Antennas with
Self Scanning Array Elements
L. Shafai
Dept. of Electrical and Computer Engineering
University of Manitoba
Winnipeg, Manitoba, Canada R3T 2N2
Tel. (204) 474-9615 FAX (204) 261-4639
ABSTRACT
This paper presents a new approach for
designing low profile antennas for MSAT
applications. It is based on stacking two
microstrip antennas that operate at two adjacent
modes. The beam scanning is achieved by
introducing a phase shift between the stacked
elements and consequently a low cost self scan
array element is developed. The concept is used
to investigate two different antenna types: 1) a
single element unit, with its phase shifter, that
provides a moderate gain of about 7 dBic, and 2)
a seven element array with a peak gain of about
14 dBic. Computed and measured data for each
design are presented and discussed
BEAM SCANNING CONCEPT
Assume N-stacked circular patches, where
each operates at one of TMnl modes. If the
radiation is circularly polarized, the components
of the far field pattern can be written as
N
EO.¢ = E fn(O) ein¢
n=l (1)
where
= _ grikr [ J_+] (k0a Sin0)- J_-i (k0a Sin0)l (2)
and
=-yak0 j"e-i_ cos0 [Jo+, (koaSin0)- Jn-, (k0a Sin0)]
2 r (3)
when the excitation of each TMnl mode (i.e.
each patch) has a relative phase difference of 5n,
equation (1) modifies to
N
n=l (4)
and selecting 5n = nS1 provides
N
EO,¢= Z fn(O)@(¢-h)
n=l (5)
For equation (1) the beam peak is located in
the ¢ = 0 plane, and moves to _ = _51 plane for
the pattern of equation (5). Now, if the phase
differences of nS1 are generated by external
phase shifters, the array beam can be scanned by
varying the relative phase of each stacked
element. In particular, increasing the number of
stacked elements reduces the beamwidth and
consequently increases the array gain.
The above array formation by stacking the
TMnl patches also alters the radiation patterns in
the 0- direction. Only the TM11 mode radiates
axially in the 0 = 0 direction. All other TMnl
modes radiate conical beams, with a null at 0 = 0
direction. Consequently, the pattern peak of the
stacked arrays occurs, somewhere between 0 = 0
and 90 °, depending on the number of stacked
elements and the substrate dialectric constant.
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SINGLE STACKED ELEMENT
In the present study, since only two stacked
microstrip patches are utilized, the operating
modes are the TMll and TM21 modes. Fig. 1
shows the geometry of the structure. Their
individual radiation patterns, when each patch is
over an infinite ground plane, are shown in Figs.
2 and 3. The substrate dielectric constant is _ r =
2.52. Consequently, the principal plane patterns
are not identical, but can be made to be similar by
selecting an appropriate ground plane size. For
the stacked configuration the computed circularly
polarized patterns, in the 0 = 0, 180 plane, are
shown in Figs. 4a and 4b. The peak gain is 7.9
dBic, which occurs at about 0 = 32 °, Fig. 4a,
and the beamwidth in the 0-direction is about 30
degrees. The generated sidelobe is small, about -
20 dB, and occurs at an angle of 70 degrees
behind the main beam, 0 = 180° plane. The
horizontal copolar pattern, in a plane containing
the beam peak, i.e. 0 = 32 °, is shown in Fig. 4b.
The beamwidth is about 180 degrees, and the
pattern has no sidelobes.
An experimental unit was fabricated and
tested. Fig. 5 shows the measured copolar
patterns at two adjacent frequencies. The centre
frequency of the patches was selected to be 3.165
GHz, to reduce the element size. The antenna
ground plane was small, 4 in x 4 in, but the
measured patterns still agree well with
computations. The achieved peak gain of 7.56
dBic is also close to computed value of 7.9 dBic.
SEVEN ELEMENT ARRAY
Using the above stacked TM11, TM21 mode
element, a 7-element array was formed, that is
shown in Fig. 6, and its performance, with and
without mutual coupling effects was investigated.
For the symmetric planes, i.e. q_= 0 and 30 °, the
computed results are shown in Figs. 7 and 8.
Fig. 7a shows the copolar pattern with and
without mutual coupling in the (_ = 0 plane. For
the selected dimensions the effect of mutual
coupling is small and does not alter the gain or
the pattern shape significantly. The beam peak is
at 0 = 47 ° (elevation of 43°), where the array gain
is about 13.8 dBic and reduces to 11.0 dBic at
the elevation angle of 22 °. Since the ground
plane is selected to be an infinite one, the co- and
cross-polar fields become equal at the horizontal
plane. The computed patterns in the 0 = 47 °, are
shown in Fig. 7b, and for the selected beam the
sidelobe levels are reasonably high.
Figs 8a and 8b show the corresponding
results when the beam is scanned to _0 = = 30 °,
00 -- 60 ° direction. For this beam the grating
lobe of the array falls outside the real space and
the array gain and sidelobes improve. The beam
peak occurs at _ = 30 °, 0 = 48 ° (elevation of 42°),
where the array gain is increased to 14.3 dBic.
The copolar patterns in the 0 = 48 °, are shown in
Fib. 8b and again have reduced sidelobe levels.
The 7-element stacked array was fabricated
and tested for performance at both beams.
Sample of measured results are shown in Figs. 9
and 10, respectively for (q_0 = 0, 00 = 60 °) and
(_b0 = 30 °, 00 = 60 °) beams. The measured peak
gain of these beams, excluding circuit losses,
were at 12.34 dBic and 13.80 dBic, respectively,
which are somewhat below the computed values.
The discrepancies are partly due to the mismatch
and polarization losses, and partly due to the
phase errors and finite ground plane size. The
effect of phase error is evident from the beam
peak, where the pattern is nearly flat between 48 °
and 60 ° range.
To scan the array beam one needs two
different sets of phase shifters. Array elements
require seven corse phase shifters ( 2 or 3 bits),
since their azimuthal pattern is broad and has a 3
dB beamwidth of about 180 °. For interelement
phasing, six finer (4 bit) phase shifters are
necessary to scan the array beam. That is in total
one requires 13 phase shifters, which is five less
than the number needed for a 15-element TMll
mode array. Both arrays have similar gains for
the 20 ° to 60 ° elevation range.
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International Mobile Satellite Conference, Ottawa, 1990
Geometry of Twn Stacked Micr(}_trip An{enn,_s
Fig. 1.
i
'-90.00
Fig. 3.
- ]0. O0 O. O0 30. O0
T_ETR IN OE_REE
E and H plane patterns for
the TM21 mode patch,
E-plane, -- - H-plane
DO. O0 -60. O0 - ]0. O0 O. O0 30 O0 iO. O0 _10. O0
_ETA _N OEO_E
Fig. 2. E and H plane patterns of
the TM 11 mode patch over an
infinite ground plane,
E r = 2.52
E-plane
.... H-plane
-]0.00 0.00 ]0.30 60aO 90.00
THET_ IN DEZREF
Co- and cross-polar (circular)
patterns of stacked TM 1 1 +
T/I_21 modes.
= 0.01180, E r = 2.52
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525
-- , , I _ i _ ' _ _ i . , _ I i _ I : E i _ t ,
] t_ ,,
i r
i i i ' ' : ' I
L
_-I.00. OCt -_2'0.O@ -_0.00 0'.0'0 50.00 ' _0.00 L@O. O0
_HI N 0EG_E_
Fig. 4b. Co- and cross-polar
(circular)patterns of
stacked TM]I +TM2]
modes.
I , +lllllll
li lllllP
Fig. 5. Experimental patterns of
stacked TM]] + TM?] modes,
I_ r = 2.52, h = 1.g mm,
gain = 7.56 dBic
Y AXIS
I
X AXIS
Fig.6.
Triangular A_ray of Seven Dual Mode Microstrip Elements
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International Mobile Satellite Conference, Ottawa, 1990
' I /_' '
, / ;
J
_, I I, -'¢-'_, L ,/ ' "
i
i
] / : ,, !
o_Q.30 -;JO, O0 --_Q. _0 _.00 ]0.30 _0,00 90,30
:_ET_ IN OE3al[-_"
Fig. 7.
i
Computed co-polar and cross-polar patterns of the 7-element array,
ft.0degree beam (_o = 0.0, % = 60.0)
without mutual coupling, with mutual coupling
_r = 2.52,
o
z
_=°
=: _-_, i I t I_ ,J
_': '_ "' _ I_ "
, i
-_O. 0{3 -_O. 30 -30.O0 3.30 _0.00 _0.00 30 10
TH_T_ IN _,E._REE
.... _'°:'°....' (a) .,... ,o0.o ....,
-i I i i , , , I ,
9 1 ' 1 _ ' _ ....
"-! ........ /i .....i _ : : d i
i .... ,L !
J
Fig. 8. Computed co-polar and cross-polar patterns of the 7-element array, _r = 2.52,
30-degree beam (@^ = 30.0, 0_ = 60.)
without mutual couphng, ..... with mutual coupling
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o.0
-4.o
-e.Ol
-t2,a
-|s.O
'0
-;_0.0
C
m
-24,0
-al.o
-3_.0
-3el. o
_d_l _ I I I f t! /
l,l,, , I-Ifr'
-L;I.O 12.0 3e.o 80.0 14.o soe.o 132.0 151.0 Io0.o
AZ Imuth (deg)
Fig.9. (to = 0.0,Oo=60.),gain: 12.34dBic
-_o ...........
-80 ....
-12.o
-le.o
-20.0
-_.m. o
-32, o
- 38.0
-180.O-|BI.O-131,O-lOQ.O -B4.0 -60,0 -36.0 -I1.0 ll.O 38.0 60.0 84.0 tOB.g 13;LO IDS.0 |DO 0
AZ imuth (deg)
Fig. I0. (_o = 30.0,0o = 60.0),gain = 13.80dBic
Fig.9 and 10. Measured patterns of the 7-element array,Er
f = 3.16 GH z
= 2.52,
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International Mobile Satellite Conference, Ottawa, 1990


MSAT vehicular antennas with self scanning array elements

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