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Controllable-sidelobe slotted waveguide antennas with corrugations for frequency selectivity

Authors:
  • Beirut Research & Innovation Center and American University of Beirut

Abstract

A simple method for the design of rectangular slotted waveguide antennas (SWAs) with specified sidelobe levels is briefly described, and is used to design an example SWA with 7 broadwall elliptical slots and sidelobes lower than −20 dB. A special corrugation geometry is then added to the SWA, where the uniform height of the corrugations controls the SWA's operation frequency. This height can be changed by either pushing the corrugations through the non-slotted broadwall, or by implementing them on a conducting thin plate that can be replaced as needed, leading to some form of mechanical frequency reconfigurability.
Controllable-Sidelobe Slotted Waveguide Antennas
with Corrugations for Frequency Selectivity
Mohammed Al-Husseini
Beirut Research & Innovation Center
Lebanese Center for Studies & Research
Beirut 2030 8303, Lebanon
husseini@ieee.org
Hilal M. El Misilmani, Karim Y. Kabalan,
and Ali El-Hajj
ECE Department
American University of Beirut
Beirut 1107 2020, Lebanon
Xuyuan Pan and
Christos G. Christodoulou
ECE Department
University of New Mexico
Albuquerque, NM 87131, USA
Abstract—A simple method for the design of rectangular
slotted waveguide antennas (SWAs) with specified sidelobe levels
is briefly described, and is used to design an example SWA
with 7broadwall elliptical slots and sidelobes lower than -
20 dB. A special corrugation geometry is then added to the
SWA, where the uniform height of the corrugations controls the
SWA’s operation frequency. This height can be changed by either
pushing the corrugations through the non-slotted broadwall, or
by implementing them on a conducting thin plate that can be
replaced as needed, leading to some form of mechanical frequency
reconfigurability.
I. INTRODUCTION
Slotted Waveguide Antennas (SWAs) have been ideal solu-
tions for high power microwave applications as well as radar,
communications, and navigation. They have the advantages
of a simple design, since their radiating elements (the slots)
are an integral part of the feed system (the wavegude itself),
in addition to their relatively low weight and small volume,
their high power handling, high efficiency, and good reflection
coefficient [1].
Resonant or standing-wave SWAs have the end of the
waveguide terminated with a short circuit. As a result, they
have a higher efficiency compared to their traveling-wave
counterparts, which use a matching-load termination, but they
also have a narrower frequency band. The design of resonant
rectangular SWAs is usually based on the use of the graphs
produced in 1951 by Stegen for X-band SWAs [2], or on
numerical techniques, such as the Method of Moments (MoM).
The design goal is to find the slots lengths and offsets to obtain
a desired radiation pattern. The design procedure by Elliott
[3] computes these slots lengths and offsets after setting the
following guidelines: 1) the waveguide is short-circuited at a
distance of a quarter-guide wavelength (λg/4) from the center
of the last slot, and the inter-slot distance is λg/2.
In [4], the authors add two identical sets of metallic
corrugations inside a low-sidelobe S-band SWA to either
improve the reflection coefficient, or for waveguide length
reduction. The two used corrugation geometries differ by both
the corrugations height and spacing.
In this paper, a simple method for the design of SWAs with
specified sidelobe level (SLL) is presented. The method uses
similar guidelines as Elliott’s procedure, but only requires the
computation of the slots offsets, since they all have the same
length. Two symmetrical sets of corrugations are then inserted
into the SWA. Their design is such that only their height has
to be modified to change the SWA’s resonance frequency.
II. DESIGN ME TH OD
For the use of SWAs in high-power microwave applications,
slot shapes that avoid sharp corners are more suitable. Thus,
elliptical slots are considered in this paper. An edge-fed SWA
is also considered. As per Elliott’s procedure, the first slot
is placed at a distance of λg/4(or 3λg/4) from the feed,
measured from its center. The last slot is also at λg/4(or
3λg/4) from the shorted end. The distance between 2 adjacent
slots is λg/2, and the slots are placed in alternating order
around the broadface centerline.
By proportionality to the width of rectangular slots used
in X-band SWAs available in the literature, the width of the
elliptical slot, for a waveguide of aperture dimensions aand
b, is taken as a×0.0625/0.9. The resonating length of a
rectangular slot is about half the free-space wavelength (λ0/2).
However, that of the elliptical slot is slightly larger. To find it,
the SWA is first designed with a uniform slot offset giving all
slots a length of λ0/2. The formula for the optimal uniform
slot offset is available in the literature. The uniform slot
displacement case results in a sidelobe level around 13 dB.
The length of the slots is then optimized (increased slowly),
using an antenna design software, to obtain a low reflection
coefficient S11 at the design frequency. This obtained length
will be used for all the slots in the non-uniform displacement
case to obtain sidelobes lower than 13 dB. So unlike other
methods, only the slots offsets will have to be computed.
The displacement of the nth slot is related to its normalized
conductance gnby:
dn=a
πarcsin v
u
u
u
t
gn/2.09
λg
λ0
a
bcos2πλ0
2λg,where gn=cn
N
P
n=1
cn
.
Nis the number of slots, cnsare the distribution coefficients
that should be determined to achieve the desired SLL, and
N
P
n=1
gn= 1. A design trick lies in the selection of the cns.
For a desired SLL, these coefficient should be taken from a
214978-1-4799-7815-1/15/$31.00 ©2015 IEEE AP-S 2015
distribution (e.g. Chebyshev) that is about 15 dB lower, i.e.
to guarantee an SWA SLL not higher than 20 dB, the cns
should be computed using a 35-dB Chebyshev distribution.
III. EXA MP LE A ND RE SU LTS
To validate the above procedure, an S-band SWA with 7
elliptical elements is designed for an SLL no higher than 20
dB. For fabrication purposes, the distance between each of the
two slots and the nearby waveguide edge is taken as 3λg/4.
The overall SWA length is then 4.5λg. In order not to cut the
available 50-cm WR-284 waveguide, the design was done for
a frequency of 3.4045 GHz, where λg= 111.11 mm. The
slot width is calculated at 5mm. The optimized slot length
is found to be 48 mm. A 35-dB Chebyshev distribution is
used, where the normalized cnsrespectively are: 1.0,2.588,
4.262,4.981,4.262,2.588, and 1.0. The corresponding slots
displacements are in mm: 6.759,11.149,14.746,16.171,
14.746,11.149, and 6.759.
A photo of the fabricated prototype is shown in Fig.
1(a). The measured and computed S11 plots are compared
in Fig. 1(b). There is satisfactory agreement, and the slight
discrepancy is due to fabrication inaccuracies.
(a)
-30
-25
-20
-15
-10
-5
0
2.8 3 3.2 3.4 3.6
Reflection Coefficient [dB]
Frequency [GHz]
Simulated (CST)
Simulated (HFSS)
Measured
(b)
Fig. 1. (a) SWA photo. (b) The computed and measured S11 plots
-40
-30
-20
-10
0
10
20
0 20 40 60 80 100 120 140 160 180
Gain [dB]
Elevation Angle [Degree]
CST
HFSS
Fig. 2. Compared elevation-plane gain pattern results from HFSS and CST.
The elevation-plane gain pattern computed in ANSYS HFSS
and CST Studio Suite is given in Fig. 2. The SLL is about
24 dB, clearly below the 20 dB level. The azimuth plane
pattern (not shown) is broadside, as expected in a 1D SWA.
IV. CORRUGATIONS AND FREQUE NC Y SEL EC TI ON
Two symmetrical sets of uniform-height conducting corru-
gations are implemented inside the SWA, as shown in Fig.
3(a). They span a length of 3.5λgleaving 0.5λguncorrugated
near each waveguide edge. The gap between the 2 sets is made
using an ellipse with a major radius of 4.5λg/2and a minor
one of 19 mm. It serves to clear any corrugations directly
below the slots and thus minimize their effect on the SLL.
There are 36 corrugations in each set, having each a length of
3.5λg/71, which is also the value of the distance between two
consecutive corrugations. Increasing the corrugations height
leads to a decrease in the SWA’s resonance frequency, as
shown in Fig. 3(b). The SLL remains below the specified level
over these operations bands, although the main lobe broadens
with decreasing frequency. In a practical scenario, the corruga-
tions’ height can be changed by pushing the corrugations forth
or back through holes in the back non-radiating broadwall, or
by implementing different-height corrugations on several thin
metal plates and inserting one of them inside the SWA as
needed.
(a)
-35
-30
-25
-20
-15
-10
-5
0
3.1 3.15 3.2 3.25 3.3 3.35 3.4 3.45 3.5 3.55
Reflection Coefficient [dB]
Frequency [GHz]
Height = 0.65 in
Height = 0.5 in
Height = 0.4 in
Height = 0.26 in
Height = 0 in
(b)
Fig. 3. (a) Corrugations geometry. (b) The computed S11 for different
corrugations heights.
V. CONCLUSION
The reported simple method proved well suitable for the
design of SWAs with suppressed sidelobes. Corrugations of a
special geometry can be used to change the SWA’s operation
frequency without exceeding the allowed SLL.
ACKNOWLEDGEMENT
This work is partially supported by an Associated Research
Unit (ARU) fund from the Lebanese National Council for
Scientific Research.
REFERENCES
[1] R. J. Mailloux, Phased Array Antenna Handbook. Artech House, 2005.
[2] R. J. Stegen, “Longitudinal Shunt Slot Characteristics”, Hughes Technical
Memorandum No. 261, Culver City, CA. November 1951.
[3] R. S. Elliott and W. R. O’Loughlin, “The Design of Slot Arrays Including
Internal Mutual Coupling”, IEEE Trans. Antennas Propagat., vol. 34, pp.
1149–1154, September 1986.
[4] X. Pan, C.G. Christodoulou and M. Al-Husseini, “Miniaturized Slotted
Waveguide Antennas with Periodic Structures for HPM Applications”, in
AMEREM 2014, Albuquerque, NM, USA, 27–31 July 2014.
215
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Article
Improved array performance (lower sidelobe levels, a better input match) requires elimination of approximations in earlier design procedures. One important change is to include internal higher order mode coupling. That is done here and is found to cause a simple but significant change in existing design formulas.
Miniaturized Slotted Waveguide Antennas with Periodic Structures for HPM Applications
  • X Pan
  • C G Christodoulou
  • M Al-Husseini
X. Pan, C.G. Christodoulou and M. Al-Husseini, "Miniaturized Slotted Waveguide Antennas with Periodic Structures for HPM Applications", in AMEREM 2014, Albuquerque, NM, USA, 27-31 July 2014.
Longitudinal Shunt Slot Characteristics
  • R J Stegen
R. J. Stegen, "Longitudinal Shunt Slot Characteristics", Hughes Technical Memorandum No. 261, Culver City, CA. November 1951.