DUAL-POLARIZED SLOTTED SQUARE
G. S. Binoy,' C. K. Aanandan,' P. Mohanan,' K. Vasudevan,'
and K. G. Nair'
'Department of Electronics
Cochin University of Science & Technology
Cochin-22, Kerala, India
Received 5 January 2000
ABSTRACT: A novel dual frequency dual-polarized square microstrip
patch antenna embedded with a slot is presented. The proposed antenna
offers tunability of the frequency ratio between the two frequencies by
adjusting the slot dimensions. This configuration also provides a size
reduction up to -51 and 35% for the two modes as compared to a
square microstrip patch antenna. ® 2000 John Wiley & Sons, Inc.
Microwave Opt Technol Lett 25: 395-397, 2000.
Key words: microstrip antennas; dual frequency; dual polarization; slot
Dual-frequency operating microstrip antennas find wide ap-
plications in GPS, regional mobile satellite arrays, large feed
arrays for offset reflectors, and other common transmit and
receive antennas [1, 21. A compact dual-band dual-polariza-
tion microstrip patch antenna capable of generating- two
distinct frequencies with different polarizations recently has
MICROWAVE AND OPT
been reported . In this letter, we present a novel design of
a square-shaped microstrip antenna embedded with a square
slot, with an arm extended giving dual-frequency operation
with the polarization planes perpendicular to each other. This
new design with a placard-shaped slot exhibits a reduction in
antenna size for dual-frequency operation compared to con-
ventional microstrip antennas. By changing the dimensions of
the arm of the slot, the ratio of the two operating frequencies
can be adjusted. Details of the antenna design, impedance,
and radiation characteristics at the two resonant frequencies
are described, and experimental results are presented.
ANTENNA DESIGN AND EXPERIMENTAL DETAILS
The geometry of the proposed slotted microstrip antenna for
dual-frequency operation is depicted in Figure 1. A square
microstrip antenna with side dimensions L, is fabricated on
a substrate of thickness It and relative permittivity e, . The
placard-shaped slot is centered in the square microstrip patch
antenna. It consists of a slot of equal side dimensions Ls =
Ws, with an arm having length L,, and width W. (L0 >- W,)
extending toward one of the edges of the square microstrip
patch, as shown in Figure 1.
When the protruding slot is not present (i.e., L. = 0), it is
seen that the fundamental resonance is at 1.611 GHz. How-
ever, it is observed that, when L, L., and W. are properly
chosen, the excited patch surface current densities of the
TMI0 and TM01 modes are perturbed such that these two
modes are excited for dual-frequency operation. Coaxial
feeding is used, which is located along the diagonal of the
patch, very close to the corner of the square slot, as shown in
The proposed antenna with various slot dimensions was
constructed and investigated. The measured return loss (S11)
of the antenna for a typical design is plotted in Figure 2. In
this case, L, La, and
WV, are chosen to be 12, 10, and
1.5 mm, respectively. From the plot, it is observed that two
distinct operating frequencies are excited. Here, the slot
fE- Ls -->I
Figure 1 Geometry of the proposed dual-frequency dual-polarized
slotted square microstrip antenna. Antenna parameters are given in
feed point %V.
TICAL TECHNOLOGY LETTERS / Vol. 25, No. 6, June 20 2000 395
400.9 1.0 1.1 1.2 1.3 1.4 1. 5 1.6 1 .7 1.8 1 .9 2.0 2.1
Figure 2 Measured return loss (S11) for the proposed dual-
frequency dual-polarized microstrip antenna having slot arm length
L = 10 mm. - S11 of the slot antenna (L = 10 mm), ...... S11 of
standard square patch. h = 1.6 mm, e, = 4.5, Lp = 40 mm, LS =
12 mm, La= 10 mm,W.-1.5mm
creates another resonance near the fundamental resonance
of the antenna, which will result in dual-frequency operation
(TM,11 and TM 111 modes). The fundamental resonance
frequency of the conventional unslotted square patch is
-1.8 GHz (f ). With a square slot alone (L° - 0), it is
observed that the antenna is resonating at 1.611 GHz,
whereas the introduction of the slot arm initiates an addi-
tional resonance frequency at 1.411 GHz. It can be con-
cluded that the slot effectively increases the patch dimen-
sions, and ;fence we have the result. Both of these two
resonant frequencies are well below the resonant frequency
of the standard square patch.
By changing the length of the slot arm (La), the frequency
ratio of the proposed antenna can be tuned in the range of
about 1.06-1.2. The results also indicate that, by increasing
the slot arm length, only the lower resonant frequency (f1) is
decreased, and it is much lower than the fundamental fre-
quency of the square patch (f = 1.8 GHz), whereas the higher
resonance frequency (f2) is only slightly affected by the slot
arm length variations. The optimum feed position remains
practically the same, even when the arm slot length is changed.
The area requirements of ordinary square patches operat-
ing at f1 and f2 frequencies of the new design are found to
be more; typically, when f1 = 1.39 GHz and f2 = 1.61 GHz,
the reduction in patch areas is 51 and 35%, respectively. The
variation of the two resonant frequencies, and hence the
frequency ratio of the antenna for different lengths of the
slot arm, are given in Table 1. The frequency ratio is found to
vary from 1.06 to 1.2. From the table, it can be seen that, by
changing the slot arm dimensions, we can merge or shift
apart the resonating frequencies [5, 6]. The percentage band-
width remains almost invariant, even when the slot arm
dimensions are changed to reduce the operating frequency
ratio (f2/fl). Figure 3 shows the measured resonant frequen-
cies f1 and f2 and frequency ratio (f2/f1) against the slot
arm length for the proposed antenna.
The transmission characteristic (S21) of the antenna in the
band for horizontal and vertical polarizations is shown in
Figure 4. Finally, from the polarization planes, we can infer
that the polarization planes of the two resonant frequencies
are orthogonal to each other .
Radiation patterns for the proposed antenna for two tun-
ing stubs are shown in Figure 5. The antenna also offers good
cross-polarization characteristics, in contrast to ordinary
square patch antennas. Like a rectangular or square patch, it
is observed that the beam width is also large for the proposed
antenna. The beam width along the E- and H-planes at
1.611 GHz are 112 and 82°, respectively; the corresponding
values at 1.41 GHz are 124 and 88°, respectively. Bandwidths
of 1.8 and 2.1%, respectively, have been obtained in the two
A novel design of a dual-frequency dual-polarized square
microstrip antenna embedded with a shaped slot is presented.
Figure 3 Measured resonant frequencies f, and f2 and frequency
ratio (f2/fl) against slot arm length . --.- f,, ---o--- f2, -•-'
TABLE I Dual-Frequency Performance for Proposed Antenna with a Shaped Slot with Various Slot Arm Lengths ; Antenna
Parameters are Given in Figure 2
(MHz) % Bandwidth
MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 25, No. 6, June 20 2000
1400 1440 1480 1520 1580 1600 1640
Flgure4 Variation of received power with frequency for the two
orthogonal polarization planes. ----------- vertical, horizontal
90, 1 r 1 J N/ 1 90
-901 ' rv r
r ^i 1
Figure 5 Measured E-plane and H-plane
for the proposed antenna. (a) f1 - 1411 MHz. (b) f2 - 1611 GHz.
- ------- E-plane, ---- H-plane
The proposed design uses the fundamental TM10 mode as
well as the new resonant mode TMOI excited due to the slot
for dual-frequency operation. This novel compact antenna
design is suitable in applications where reduced-size antennas
are needed, such as GPS and WLAN. The frequency ratio
(f2/fl) of the two frequencies can be made as low as 1.06,
which makes the proposed antenna suitable for applications
where a low-frequency ratio is required.
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MICROWAVE AND OPT
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0 2000 John Wiley & Sons, Inc.