A Bandwidth-Enhanced Cavity-Backed Slot Array
Antenna for mmWave Fixed-Beam Applications
Wai Yan Yong, Student Member, IEEE, Abolfazl Haddadi, Thomas Emanuelsson and
Andr´es Alay´on Glazunov, Senior Member, IEEE
Abstract—A bandwidth-enhanced 8 × 8 cavity-backed slot array antenna for fixed beam applications is presented. The antenna consists of four 2 × 2 subarrays fed by ridge gap waveguide and a modified bow-tie coupling slot in the cavity layer. The proposed bow-tie coupling slot provides additional 10 % impedance bandwidth as compared to the traditional rectangular coupling slot. The fabricated prototype demonstrates an |S11| ≤ −10 dB fractional impedance bandwidth and a
3 dB gain-drop bandwidth of approximately 28 % and 25 %, respectively, with peak gain of 26.4 dBi, aperture efficiency > 60 % and cross-polar discrimination > 40 dB. Measurements show good agreement with simulations. Moreover, we show the impact of the dimensions and shape of the cavity coupling slot on the bandwidth enhancement. The bandwidth enhancement is explained by the double-ridge slot behavior of the bow-tie coupling slot, which shows a more wideband behavior than the traditional rectangular coupling slot behaving like a cross-section of a waveguide.
Index Terms—5G, corporate-feed network, gap-waveguide, millimeter-wave, wideband antenna
I. INTRODUCTION
G
AP waveguide (GW) technology is a promising low-loss and cost-effective transmission line technology for the millimeter-wave (mmWave) band. The physical principle behind the GW is based on the concept of the parallel-plate waveguide [1]. By combining the parallel-plate waveguide with the periodic artificial magnetic conductor structure, the GW is able to control the direction of propagation of the quasi-TEM mode within the desired frequency band. The GW offers several advantages as compared to the conventional microstrip and hollow waveguide transmission lines. For example, the losses of the GW and the conventional hollow waveguide are comparable [2]. Moreover, the GW transmission line allows having a gap between the upper and lower metallic layers with maintained performance [1]. Hence, no need for strict electrical contact requirements for multi-layer antenna struc-tures, which alleviates the fabrication tolerance requirements and therefore may reduce manufacturing costs significantly. To date, a number of array antennas have been designed based on the GW technology at different operating frequency bandsThis project has received funding from the European Union’s Horizon 2020 research and innovation programm under the Marie Sklodowska-Curie grant agreement No. 766231 — WAVECOMBE — H2020-MSCA-ITN-2017.
W.Y.Yong and A.A.Glazunov are with the Department of Electrical En-gineering, University of Twente, Enschede, Netherlands. A.A.Glazunov is also affiliated with the Department of Electrical Engineering, Chalmers University of Technology, Gothenburg, Sweden (e-mail: (w.y.yongwaiyan, a.alayongglazunov)@utwente.nl).
A.Haddadi and T.Emanuelsson are with Gapwaves AB, Gothenburg, Swe-den (e-mail: (abolfazl.haddadi, thomas.emanuelsson)@gapwaves.com).
with excellent performance [3]–[7]. However, their fractional impedance bandwidths are limited to approximately 15−20 %. Recently, [8] reported a bandwidth performance of 30 % of the fully metallic GW-based antenna by increasing the number of tuning pins in the cavity layer. However, its main limitation is that the number, the dimensions and the positions of the additional tuning pins are fully determined through numerical simulations incurring a heavy computational burden. In [9], an ’8-shaped’ radiating slot resulted in an impedance bandwidth of 17% for a single layer corporate feed array antenna. Alternatively, the substrate-integrated GW technology has been shown to increase bandwidth in a simple yet effective way [10]–[12]. However, the losses of the antenna increase due to the presence of the dielectric substrate [2].
In this letter, an alternative approach to achieve substan-tial fractional impedance bandwidth enhancement of cavity-backed slot arrays, e.g., from 18 % to 28 % is presented. The main idea is replacing the conventional rectangular cavity slot (RSL) by a modified bow-tie cavity slot (BTSL). The main function of the cavity coupling slot is to maximize the energy coupled from the RGW feed into the cavity layer [13]. Hence its importance to the overall performance of the subarray and the array antenna system. We show by means of simulations that the size and shape of the cavity coupling slot also plays a fundamental role in the bandwidth performance enhancement of the cavity-backed slot array antenna. The proposed array antenna is a corporate-fed 8 × 8 planar array consisting of four 2 × 2 slot-backed subarrays fed by a ridge GW (RGW). The bandwidth-enhanced design shows good overall performance in terms compactness, realized gain 26.4 dBi, cross-polar discrimination > 40 dB and efficiency > 60 % while fabrication costs can be kept low using well-established fabrication methods.
II. ANTENNACONFIGURATION ANDPARAMETRICDESIGN
Fig. 1 shows an exploded view of the proposed 2 × 2 subarray antenna with three layers. The top layer contains four radiating slots. Each radiating slot is surrounded by a rectangular cavity to reduce the mutual coupling between the slots [4]. In the mid layer, these radiating slots are then backed by a rectangular air-filled cavity to distribute the input power equally among the radiating slots. The bottom layer is the feeding layer containing the RGW. The unit cell dimension of the proposed subarray is 18.4 × 18.4 mm2. The distances between the slots in the x− and the y−directions are equal to 0.95λ at the highest operating frequency (31 GHz) to avoid
Radiating Layer Cavity Layer Ground Ws Ls Hr Hc Wg Feeding Layer (a) Radiating Layer Feeding Layer Ground Hg Lrid Wrid Wpin Hpin (b)
Fig. 1. Exploded view of the proposed 2 × 2 subarray antenna (a) front and (b) back views.
grating lobes. Two subarrays were designed to evaluate their impact on bandwidth performance. The only difference was the type of cavity coupling slot used. All other parts of the three layer design was the same Fig. 1. The top view of the cavity layer containing the rectangular cavity slot (RSL) and the modified bow-tie cavity slot (BTSL) are shown in Fig. 2(a) and Fig. 2(b), respectively. The large array performance is simulated under the periodic boundary condition applied to each subarray (later, in order to improve simulation accuracy for the 8 × 8 array antenna, the open boundary condition is used). The optimized parameters are tabulated in Table I. It is worthwhile to note that the cavity layer shown in Fig. 1 contains the BTSL. Firstly, the subarray based on the
conven-Wcav Wcsl Lcp Wcp Lcsl (a) Wcav WBTSLA Lcp Wcp WBTSLB (b)
Fig. 2. Top view of the cavity layer with different cavity coupling slot (a) Conventional rectangular cavity slot (RSL) and modified bow-tie cavity slot (BTSL)
tional RSL was designed. In order to evaluate its impact on the bandwidth performance of the antenna, a parametric study of the impact of its dimensions was performed. Varied were the length, Lcsl and the width, Wcsl of the RSL (see Fig. 2(a)).
Corresponding simulation results are shown in Fig. 3. As can be seen in Fig. 3(a), when the Lcslis decreased, the lower
cut-off frequency of antenna is considerably shifted. On the other hand, while varying Wcsl, only a slight shifting on the high
cut-off frequency is observed. Thus, this slot is behaves similar to the cross-section of a waveguide, where its dimensions determine the cut-off frequency.
Secondly, the subarray with the modified BTSL is shown to improve the bandwidth performance of the antenna. The proposed BTSL coupling slot behaves like a double-ridge slot [9], [14]. This feature allows the cut-off frequency in the dominant modes to shift. The lower cut-off frequency can be shifted to a lower frequency and the resonant frequency of the next higher-order modes will be altered to a lower frequency
(a)
(b)
Fig. 3. Effects of the rectangular cavity slot dimensions over the bandwidth performance, (a) Variation of Length of RSL with fixed Wcsl= 1.6 mm and
(b) variation of width of the RSL with fixed Lcsl= 3mm
TABLE I. Dimensions of 2 × 2 Cavity-Backed Slot Subarray Antenna (Refers to Fig. 1 and 2)
Parameters Dimension [mm]
Subarray width in x- and y-direction, Wg 18.4
Gap between RGW feeding and ground, g 0.2
Width of the slot, Ws 2.4
Length of the slot, Ls 5.2
Thickness of the radiating layer, Hr 3.8
Height of the cavity tuning pin, Hc 2.2
Width of the cavity tuning pin, Wcp 2.4
Length of the cavity tuning pin, Lcp 2.7
Width of the BTSL, Wcsla 2.6
Middle width of the BTSL, Wcslb 2
Width of cavity, Wcav 12.9
Width of the RSL, Wcsl 2.4
Length of the RSL, Lcsl 5.8
Pins period, p 3
Width of pin, Wpin 2
Height of pin, Hpin 2.5
Width of ridge, Wrid 1.9
Length of ridge, Lrid 6.1
Height or ridge, Hrid 1.3
Thickness of ground, Hg 1
Fig. 4. Reflection coefficient S11as function of frequency f for the subarray
with RSL and BTSL cavity coupling slots and the 8 × 8 array antenna with BTSL cavity coupling slot.
Fig. 5. Impact of the width of bow-tie slot over the bandwidth performance. The upper and lower width of the BTSL is fixed as WBT SLA= 2.8 mm
and length of the BTSL is fixed as Lcsl= 5.8mm.
region [14]. As can be seen from Fig. 4, the lower cut-off frequency of the subarray is shifted from 24.2 GHz to 23 GHz when the RSL is replaced with the BTSL cavity coupling slot. Similarly, the resonant frequency of the next higher-order mode occurs at 32 GHz or above for the RSL subarray, which is shifted to 30 GHz and combined with the resonant frequency of the lower order modes. Hence, by properly designing and optimizing the dimension of the cavity coupling slot, the bandwidth performance of the subarray antenna has been increased significantly from 17.8 % (24.2 − 29 GHz) to 28 % (23 − 30.5 GHz). A parametric study supporting the above results explaining the impact of the proposed bow-tie cavity slot (BTSL) on the impedance bandwidth performance is shown next. The length of the proposed BTSL is fixed to a similar length of the RSL with Lcsl= 5.8 mm. Hence, the
shape of the BTSL is then mainly determined by the parameter WBT SLB. Therefore, the WBT SLB could affect the cut-off
frequency of the slot which could in turn help to enhance the impedance bandwidth of the antenna. To show this, the value of WBT SLB is initially chosen to be similar to WBT SLA,
which in this case behaves as the RSL. The value of WBT SLB
is then decreased to obtain the bow-tie shape of the BTSL. As can be seen from Fig. 5, when the BTSL is employed, the lower cut-off frequency is moved towards lower frequencies. A similar observation can be done at the high cut-off frequency. Thus, from the above results we clearly see that the BTSL leads to the impedance bandwidth enhancement.
III. MEASUREMENTRESULTS ANDDISCUSSION
Fig. 6 shows the fabricated 8 × 8 array antenna with dimen-sions 100 × 100 × 20 mm3. A Computer Numerical Control (CNC) milling machine was employed for manufacturing. As can be seen from Fig. 7, there is a slight discrepancy between the simulated and measured results, which can be mainly attributed to the manufacturing tolerances. This is illustrated by the simulated S11 performance with a 50µ m air gap in
between the radiating and cavity layer. In this design, the conventional hollow waveguide cavity is employed. It required a good electrical contact between layers. When there is an air gap of 50 µm, the S11 performance is slightly deteriorated.
However, both the measured and simulated S11have remained
below −10 dB from 23 − 30.5 GHz and the measured results are in accord with the simulated results. Fig. 8 and 9 show a comparison of the normalized radiation pattern at different frequency points for E- and H-plane, respectively. As can be
(a) (b)
Fig. 6. Prototype of the fabricated 8 × 8 array antenna (a) front and (b) back views.
Fig. 7. Simulated and measured reflection coefficient S11of the 8 × 8 array
antenna with BTSL coupling slot as a function of frequency f .
seen, the measured results are in good agreement with the simulated results. Hence, the proposed antenna is operating as per design over the desired wide bandwidth. Fig. 10 shows the comparison of the simulated and the measured gain. As can be seen, the proposed antenna demonstrates a simulated aperture efficiency of more than 60 % for frequency between 24 to 31 GHz with a bandwidth of approximately 25.2 %. The measured gain performance is in rather good agreement with the simulated gain performance even with an air gap in between the radiating and cavity layer. For the frequency band from 23 − 24 GHz, the difference between simulated and measured results is more significant. This can be explained by the mismatched losses of the antenna and the mismatch of the WR-28 waveguide connector that was used during the measurements as these frequency bands are closed to the cut-off frequency of the connector. Furthermore, discrepancies can also be attributed to fabrication errors. For instance, the air gap between the radiating and cavity layers contributes to losses. Also, the losses are more significant due to the surface roughness of the fabricated prototype.
As can be seen from Table II, most of cavity-backed slot array antennas designs published until now have an impedance bandwidth limited to approximately 11 − 15 % depending on the transmission line technology employed [4], [15]. In [13], a T-junction is employed in the cavity layer to tune its inductance and capacitance properties to generate more cavity modes leading to approximately a 36% impedance bandwidth. In [8], it was proposed to employ more cavity pins to tune the capacitance resulting in a bandwidth of ap-proximately 30 %. However, these techniques, a prohibitively large simulations are required to determine the number, the dimensions, and locations of pins for the capacitive and inductive tuning resulting in bandwidth enhancement [8], [13].
TABLE II. Performance comparison of existing High Gain antennas for mmWave applications. f is the frequency band and IBW is the fractional impedance bandwidth.
Antenna f [GHz] Feeding No.of Aperture IBW [%] 3 dB Gain Peak Gain Measured
Types Technology Elements Size [λ2
0] Drop BW [%] [dBi] Efficiency[%]
Cavity-Backed Slot [3] 60 RGW 8 × 8 6.93 × 6.93 14 13.1 25.8 > 65
Cavity-Backed Slot [8] 60 RGW 8 × 8 7.6 × 6.9 30 29.1 27.5 > 80
ME-Dipole [11] 28 Printed RGW 4 × 4 3.5 × 3.4 16.5 14.4 21.5 > 60
Cavity-Backed Slot [13] 12 Hollow Waveguide 8 × 8 7.4 × 7.4 36.9 31.9 23.4 > 60
Cavity-Backed Slot [15] 60 Hollow Waveguide 16 × 16 15.4 × 15.6 11.4 11 33.5 > 83.6
Bow-tie Slot [16] 30 Printed RGW 1 × 4 4 × 4.2 22 22 15.5 > 87
This Work 28 RGW 8 × 8 7.6 × 7.6 28 25.2 26.4 > 60
(a) (b)
(c) (d)
Fig. 8. Simulated and measured E-plane (xoz-plane) radiation pattern at (a) 24 GHz, (b) 26 GHz, (c) 28 GHz and (d) 30 GHz, where G0/Gmaxis the
normalized antenna gain in dBi, θ is the polar angle in degrees.
(a) (b)
(c) (d)
Fig. 9. Simulated and measured H-plane (yoz-plane) radiation pattern at (a) 24 GHz, (b) 26 GHz, (c) 28 GHz and (d) 30 GHz, where G0/Gmaxis the
normalized antenna gain in dBi, θ is the polar angle in degrees.
Alternatively, it has been proposed in [11], [16] to substitute the fully metallic waveguide-based antenna with the substrate-integrated waveguide (SIW) antenna offering a variety of
Fig. 10. Simulated and measured realized gain G0 and directivity D0 for
an aperture of the same size when the aperture efficiencies are 80 %, 70 %, and 60 %.
radiating elements to choose from. Wideband radiating ele-ments such as magneto-electric dipole (MED) and bow-tie slot (BTS) have been proposed which led to significant bandwidth enhancement [11], [16]. Similarly to the conventional SIW antenna, the losses of these antennas are more significant. However, the antenna solution proposed here shows that a larger impedance bandwidth can be achieved based on a fully metallic waveguide-based GW antenna while maintaining antenna efficiency performance.
IV. CONCLUSIONS
An alternative approach to considerably enhance the band-width cavity-backed slot antennas is presented. The large impedance bandwidth 28 %(23 − 31 GHz) at |S11| ≤ −10 dB
has been achieved by replacing the conventional rectangular cavity slot (RSL) with the bow-tie cavity slot (BTSL). The increase was from 17.8 % with the RSL to 28 % with the BTSL. The fabricated fixed-beam 8×8 cavity-backed slot array antenna based on the ridge gap waveguide technology has a 3 dB gain-drop bandwidth of 25.2 % with aperture efficiency larger than 60 %. A parametric study of the shape and size of the coupling slots shows that the cavity coupling slot has a major impact on the bandwidth performance of the cavity-backed slot array antenna. Future work will further investigate the bandwidth improvement by combining the proposed bow-tie coupling slot together with wideband radiating elements.
ACKNOWLEDGMENTS
The authors would like to thank Erik Johnsson and Alireza Bagheri from Gapwaves AB for the antenna fabrication and measurements as well as Prof. Narcis Cardona and Prof. Miguel Ferrando Bataller from the iTEAM at the UPV for facilitating antenna measurements.
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