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High-gain S-band slotted waveguide antenna arrays with elliptical slots and low sidelobe levels

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  • Beirut Research & Innovation Center and American University of Beirut
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High-gain S-band slotted waveguide antenna arrays with elliptical slots and low sidelobe levels

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Slotted waveguide antenna arrays offer clear advantages in terms of their design, weight, volume, power handling, directivity and efficiency. Slots with rounded corners are more robust for high power applications. This paper presents a slotted waveguide antenna with elliptical slots made to one broadwall of an S-band rectangular waveguide. The antenna is designed for operation at 3 GHz. The slots length and width are optimized for this frequency, and their displacements are determined for a 20 dB sidelobe level ratio. Two rectangular metal sheets are then symmetrically added as reflectors to focus the azimuth plane beam and increase the gain.
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Progress In Electromagnetics Research Symposium Proceedings, Stockholm, Sweden, Aug. 12-15, 2013 1821
High-gain S-band Slotted Waveguide Antenna Arrays with Elliptical
Slots and Low Sidelobe Levels
M. Al-Husseini1,A. El-Hajj2,and K. Y. Kabalan2
1Lebanese Center for Studies and Research, Beirut 2030 8303, Lebanon
2American University of Beirut, Beirut 1107 2020, Lebanon
AbstractSlotted waveguide antenna arrays offer clear advantages in terms of their design,
weight, volume, power handling, directivity and efficiency. Slots with rounded corners are more
robust for high power applications. This paper presents a slotted waveguide antenna with ellip-
tical slots made to one broadwall of an S-band rectangular waveguide. The antenna is designed
for operation at 3 GHz. The slots length and width are optimized for this frequency, and their
displacements are determined for a 20 dB sidelobe level ratio. Two rectangular metal sheets are
then symmetrically added as reflectors to focus the azimuth plane beam and increase the gain.
1. INTRODUCTION
Slotted waveguide antennas (SWAs) [1] radiate energy through slots cut in a broad or narrow wall
of a rectangular waveguide. They are attractive due to their design simplicity, since the radiating
elements are an integral part of the feed system, that is the waveguide itself. This removes the need
for baluns or matching networks. They also offer significant advantages in terms of weight, volume,
high power handling, high efficiency and good reflection coefficient [2]. Thus, they have been ideal
solutions for many radar, communications, navigation, and high power microwave applications [3].
SWAs can be realized as resonant or non-resonant according to the wave propagation inside
the waveguide (respectively standing or traveling wave) [4, 5]. The design of a resonant SWA is
generally based on the procedure described by Elliot [4, 6, 7], by which the waveguide end is short-
circuited at a distance of a quarter-guide wavelength from the center of the last slot, and the
inter-slot distance is one-half the guide wavelength. For rectangular slots, the slot length should
be about half the free-space wavelength. Slot shapes that avoid sharp corners are more suitable for
high power applications, since sharp corners aggravate the electrical breakdown problems. Elliptical
slots are an excellent candidate for such applications [8].
As with all antenna arrays, the resulting sidelobe level is related to the excitations of the
individual elements. In SWAs, the excitation of each slot is proportional to its conductance. For
the case of longitudinal slots in the broadwall of a waveguide, a slot conductance is controlled by its
displacement from the broadface centerline [9]. Thus, for a desired sidelobe level, the corresponding
set of slots displacements should be determined.
In this paper, an SWA designed for operation at 3 GHz is presented, where ten elliptical slots
are made to one broadface of an S-band rectangular waveguide. The slot displacements from the
centerline are determined to obtain a sidelobe level ratio of 20 dB. Later, two metals sheets are
attached to the SWA edges to focus its azimuth plane beam. The reflection coefficient, pattern
plots and gain results of the antenna are reported.
2. ANTENNA CONFIGURATION
The target frequency is 3 GHz, so a WR-284 waveguide having a= 2.8400 and b= 1.3700 is used
to construct the SWA. The waveguide is shorted at one end and fed at the other. Ten elliptical
slots are cut into one of its broadsides. The slots are spaced at half the guide wavelength, center to
center, where in this case the guide wavelength λg= 138.5 mm. The slots are positioned such that
the center of the first one, Slot1, is at a distance of λg/4 from the waveguide feed, and the center
of the last slot, Slot10, is at λg/4 from the waveguide’s short-circuited side. The total length of
the waveguide is thus 5λg.
The width of each slot, which is 2 times the minor radius of the ellipse, is fixed at 5mm. This
is calculated as follows: for X-band SWAs, which the literature if full of, the adopted width of a
rectangular slot is 0.062500, corresponding to a= 0.900. By proportionality, the width of the elliptical
slot for this S-band SWA is computed from 2.8400 ×0.0625/0.9, which is 0.19700 or 5 mm. Because
of their elliptical shape, the length of the slots (double the major radius) is expected to be larger
than half the free space wavelength. Simulations using ANSYS HFSS are done to optimize the slot
1822 PIERS Proceedings, Stockholm, Sweden, Aug. 12–15, 2013
length for resonance at 3 GHz. For these simulations, it is assumed that all slots are at the same
spacing from the broadside centerline, in an alternating fashion. The resonant slot length is found
to be 54.25 mm.
For a desired sidelobe level ratio (SLR) of 20 dB, a heuristic method is used to obtain the
required set of slots displacements. The slots near the two waveguide edges are closest to the
broadface centerline, whereas those toward the waveguide center have the largest displacement.
The detailed displacements values are given in Section 3.
Two metal sheets are then attached symmetrically, as shown in Fig. 1, at an angle of 60with
respect to the XZ plane. These 2 sheets act as reflectors, thus leading to beam focusing in the
azimuth plane and as a result to a gain increase. The width of each metal sheet, L, is 300.
Figure 1: Slotted waveguide with 10 elliptical slots with two re ectors added.
3. RESULTS
The uniform slots displacement that leads to a good reflection coefficient at 3 GHz is calculated
using
du=a
πsarcsin ·1
N×G¸,(1)
where
G= 2.09 ×a
b×λ0
λg×[cos(0.464π×λ0g)cos(0.464π)]2.(2)
In (1), Nis the number of slots, which is equal to 10, and in (2), λ0is the free-space wavelength.
At 3 GHz, λ0= 100 mm. For this SWA, du= 7.7mm. This displacement value is used in the
HFSS simulations to obtain the resonant elliptical slot length, which is found to be 54.25 mm. For
this slot length and this uniform displacement of all ten slots, the resulting SLR is around 13 dB,
which is as expected. The reflection coefficient S11 and the Y Z-plane gain pattern in this case are
given in Fig. 2. A peak gain of about 17 dB and an SLR of 13.2 dB are recorded. The half-power
beamwidth (HPBW) in this plane is 7.2 degrees. These values are obtained using CST Microwave
Studio, but are also verified with HFSS.
Since better SLRs are desirable, the slots displacements are changed, to non-uniform, using a
heuristic method, which will not be detailed in this paper. For an example SLR of 20 dB, the
displacement values are given in Table 1. The alternating pattern about the centerline is respected.
The length of all slots is kept at 54.25 mm, as in the uniform case. Simulations have proven that the
resonating length of these elliptical slots is not very sensitive to the distance from the centerline.
For these values, the antenna still resonates at 3 GHz, the SLR is 20 dB, the peak gain is 16.8 dB,
and the Y Z-plane HPBW increases to 8.4 degrees. The broadening of the main beam is expected
when the sidelobes are forced to go lower.
When the two reflectors are added, a gain increase of about 3 dB is obtained due to a focus of
the azimuth plane beam. The antenna retains its resonance at 3 GHz, and the SLR remains around
Progress In Electromagnetics Research Symposium Proceedings, Stockholm, Sweden, Aug. 12-15, 2013 1823
(a) S for uniform slots displacement (b) Pattern in the YZ plane
11
Figure 2: Antenna’s reflection coefficient and Y Z-plane pattern for the case of uniform slot displacement
and before attaching the two reflectors.
Table 1: Displacement of slot centers for an SLR of 20 dB.
Slot number 1 2 3 4 5 6 7 8 9 10
Displacement (mm) 3.74 5.42 7.11 8.4 9.11 9.11 8.4 7.11 5.42 3.74
Figure 3: Reflection coefficient without and with the reflectors.
(a) Patterns in the azimuth plane (b) Patterns in the YZ plane (c) Patterns in the XZ plane
Figure 4: Antenna’s gain patterns in the three principal planes (red line: no reflectors, blue line: with
reflectors).
1824 PIERS Proceedings, Stockholm, Sweden, Aug. 12–15, 2013
20 dB. The back lobe level stays about the same, so the main-to-back lobe level ratio also increases
by about 3 dB. The S11 and pattern results of the two cases, with and without the reflectors,
are shown in Fig. 3 and Fig. 4, respectively. All results generated in HFSS were verified in CST
Microwave Studio, where a good match is observed.
4. CONCLUSION
A 3 GHz slotted waveguide antenna was presented. It has 10 elliptical slots, with optimized dimen-
sions, made to one broadwall and displaced around its centerline so as to obtain a 20 dB sidelobe
level ratio. The antenna has a very broad azimuth plane beam and a peak gain of about 17 dB.
Upon adding two reflectors to the antenna’s edges, the beam is focused and the gain is increased
to about 20 dB.
REFERENCES
1. Gilbert, R. A., “Waveguide slot antenna arrays,” Antenna Engineering Handbook, 4th Edition,
J. L. Volakis, Ed., McGraw-Hill, 2007.
2. Mailloux, R. J., Phased Array Antenna Handbook , Artech House, 2005.
3. Rueggeberg, W., “A multislotted waveguide antenna for high-powered microwave heating sys-
tems,” IEEE Trans. Ind. Applicat., Vol. 16, No. 6, 809–813, 1980.
4. Elliott, R. S. and L. A. Kurtz, “The design of small slot arrays,” IEEE Trans. Antennas
Propagat., Vol. 26, 214–219, Mar. 1978.
5. Elliott, R. S., “The design of traveling wave fed longitudinal shunt slot arrays,” IEEE Trans.
Antennas Propagat., Vol. 27, No. 5, 717–720, Sep. 1979.
6. Elliott, R. S., “An improved design procedure for small arrays of shunt slots,” IEEE Trans.
Antennas Propagat., Vol. 31, 48–53, Jan. 1983.
7. Elliott, R. S. and W. R. O’Loughlin, “The design of slot arrays including internal mutual
coupling,” IEEE Trans. Antennas Propagat., Vol. 34, 1149–1154, Sep. 1986.
8. Baum, C. E., “Sidewall waveguide slot antenna for high power,” Sensor and Simulation Note
503, Aug. 2005.
9. Stevenson, A. F., “Theory of slots in rectangular waveguides,” Journal of Applied Physics,
Vol. 19, 24–38, 1948.
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... Slotted waveguide with two reflectors added[32]. ...
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An existing design procedure, applicable to standing-wave-fed longitudinal shunt slot arrays, and which accounts for the variability of mutual coupling, is extended to situations in which a traveling-wave feed is employed, i.e., the slots are not λg/2 on centers. The extension permits the design of slot arrays to give an input match and a sum pattern for which the main beam is not at broadside. The technique is also amenable to difference pattern applications. Copyright © 1979 by The Institute of Electrical and Electronics Engineers, Inc.
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Antenna Engineering Handbook , 4th Edition
  • R A Gilbert
Gilbert, R. A., "Waveguide slot antenna arrays," Antenna Engineering Handbook, 4th Edition, J. L. Volakis, Ed., McGraw-Hill, 2007.