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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

Abstract—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 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 60◦with

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×λ0/λg)−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.