Highly Miniaturized Self-Diplexed U-Shaped Slot Antenna Based on Shielded QMSIW

This article presents an efficient yet simple design approach to highly miniaturized cavity-backed self-diplexing antenna (SDA) with high-isolation. The structure employs a shielded quarter-mode substrate integrated waveguide (QMSIW). Two U-shaped slots are engraved on the top conducting plane, which realize two frequency bands and significant size reduction. The slots are excited by two independent 50Ω orthogonal feed-lines to achieve high isolation while enabling radiation at 3.5 GHz and 5.4 GHz. The proposed design approach allows us to design the two frequency bands independently from each other. The SDA structure is verified using an equivalent circuit model, full-wave electromagnetic (EM) analysis, and experimental validation of the antenna prototype. Excellent consistency between simulated and measured responses has been demonstrated. According to the measurement, the SDA has peak gains of 5.26/5.60 dBi at 3.5/5.4 GHz, and the return loss better than 21.6 dB. The isolation between ports is greater than 40 dB, whereas excellent front-to-back-ratio and co-to-cross polarization are obtained for the proposed SDA. Furthermore, the total size of the SDA is only 0.129 λ2, with λ being the guided wavelength at 3.5 GHz.


I. INTRODUCTION
With the accelerated evolution of advanced wireless networking technologies, low cost, low-profile, and highperformance planar dual-band antennas are in greater demand. In multi-standard communication systems, a single antenna with multiband characteristic offers attractive features, including reduction of the overall cost and size. However, integration of transceiver circuitry with antennas imposes isolation limitations on multiple radiators, which requires additional frequency selective surfaces to improve isolation, increasing the design complexity and the size of the system. To preserve dual-band operation of the antenna, along with high isolation and compact size is a challenging endeavor. One of possible solution approaches are selfdiplexing antennas (SDAs), which have become useful in achieving high isolation, size miniaturization, and the best overall microwave components for many applications [1].
Another potentially attractive approach is the substrateintegrated waveguide (SIW) technology. It has become of a paramount importance in the implementation of planar microwave components based on waveguide. SIW-based antennas are capable of rendering dual-band characteristics, small size, high gain, and excellent far-field performances [2]. As a matter of fact, many self-diplexing antennas that ensure sufficient isolation, size reduction, and radiation performance, are being developed in the SIW technology [3][4][5][6][7][8][9][10][11][12]. A rectangular-slot loaded SIW antenna excited by two orthogonal feed-lines has been demonstrated in [3]. In [4], a lightweight self-diplexing inverted U-shaped slot antenna has been realized on the SIW cavity for high isolation. In [5], a bow-tie-shaped slot has been engraved on the top conductor of SIW cavity for development of a SDA. In [6], a SDA has been realized by applying two rectangular-slots on the top surface of the SIW cavity for operation at 8.26 and 10.46 GHz; nevertheless, this structure offers low isolation, which hinders its application. In [7], a rectangular-ring slot has been loaded on the SIW cavity to build a compact SDA that radiates at 9.5 GHz and 10.5 GHz. A cavity-backed SIW antenna with an asymmetric cross-slot has been suggested to achieve SDA with circular polarization in [8]. In [9], a novel SIW cavity topology has been suggested for the development of a lowprofile, extremely tunable SDA with frequency tuning range of 3.77 to 4.59 GHz in the lower frequency band, and 4.96 to 6.1 GHz in the upper band. In [10], a SDA has been implemented using SIW cavity loaded by a cross-shaped slot. A lightweight SIW-based SDA has been realized for Wi-Fi (5.2 GHz to 5.29 GHz) and ISM band (5.78 GHz to 5.83 GHz) applications in [11]. With 45° linear polarized (LP) and dual-band CP antennas, [12] develops a dual-band SIW antenna. A novel dual-frequency antenna architecture has been proposed utilizing HMSIW cavities for Ka-band applications in [13].
In the context of size reduction, self-diplexing characteristic, and isolation, the aforementioned design topologies have been often capable of achieving satisfactory performance. Considerable size reduction is possible by choosing the cavity structure at the beginning of the topology development process to ensure high isolation between the ports. Therein, appropriate design modifications are applied to the cavity topology, feed-lines and radiating slots. Quartermode SIW (QMSIW) derived from the full-mode SIW resonator, and including a shielded mechanism that offer highly size miniaturization has been reported in the literature [14][15][16]. Furthermore, the conventional half-mode SIW (HMSIW) [17] and QMSIW [18][19] cavities are employed to design compact self-diplexing antennas. The abovementioned approach has been demonstrated successful in the development of miniaturized self-diplexing antennas but offers low isolation and does not provide equivalent model to validate their designs. Therefore, a new selfdiplexing antenna is realized by using shielded QMSIW to achieve further size reduction, high isolation and equivalent model to validate the antenna. This paper proposes a novel design procedure for highly-miniaturized self-diplexing antenna (SDA) with high isolation. Our approach relies on applying a shielded quarter-mode SIW (QMSIW). Two radiating patches are formed on the top of the shielded QMSIW by incorporating two U-shaped slots for radiation at 3.5 GHz and 5.4 GHz. These frequency bands can be tuned independently. The proposed SDA is validated through the lumped circuit model, EM simulation, as well as experimental validation.

II. DESIGN PROCEDURE
This section provides a description of the antenna configuration, and the eigenmode analysis. Subsequently, the topology evolution and the design process flowchart are discussed. In order to verify the design, the RLC circuit model is derived for the proposed SDA and analyzed, along with full-wave EM simulation. The tunability of the proposed SDA is realized in a versatile manner by designing the two operating frequency bands independently from each other. The experimental validation and benchmarking against stateof-the-art antenna structures are discussed in Section III. Figure 1 shows the layout of the proposed self-diplexing antenna (SDA) featuring the size of 0.129λ 2 at 3.5 GHz. The substrate used for this antenna has a thickness of 0.787 mm and permittivity 2.33. The proposed configuration comprises a shielded QMSIW, two U-shaped slots, and two orthogonal 50Ω lines. Figure 2 shows the design steps for realizing the proposed SDA. Initially, a full-mode SIW is developed for operating in the TE 110 mode. The electric field distribution is shown in Fig. 3(a). Subsequently, the FMSIW is divided into four equal parts and each part is defined as QMSIW. Compared to full-mode SIW cavity, the conventional QMSIW cavity suffers from low Q factor due to leakage of wave through the magnetic walls shown in Fig. 3(b). To minimize this extra loss, a shielded QMSIW is developed by employing two rows of metallic vias near the open-ended edges of the conventional QMSIW, and the magnetic wall are recreated by engraving an open slot near the vias, as shown in Fig. 3(c). The wave propagation can be blocked partially by this shield walls, consequently, minimizing the loss [14][15][16]. Table 1 shows the Q-factor for different SIW cavities at TE110 mode. As it can be seen in Table 1, the Q factor of conventional QMSIW is smaller than the shielded QMSIW due to low loss [14][15][16]. Hence, the shielded QMSIW not only achieves compact size but also provides good overall performance.

A. ANTENNA TOPOLOGY
In order to minimize the energy leakage, the diameter d and the distance p between the vias are chosen using the following relationship: d/λ ≤ 0.1 and d/p ≥ 0.5 [2]. The openended edges of the QWSIW causes the unwanted radiation loss, which can be mitigated by enclosing the open-ended edges by two rows of metallic vias and open-slot. Finally, two U-shaped slots are created on the top conductor to realize dual-band operation.  Figure 4 shows an equivalent RLC circuit model of the proposed SDA, where each cavity resonator is represented by a parallel RLC tank, and each U-shaped slot is modeled by a shunt capacitance. Size reduction is accounted for by increasing the extra capacitance produced due to the slots. Furthermore, the transformers are used in order to achieve good matching between the cavity resonators and the input signals. The coupling between the two cavity resonators is represented by M 12 , and modeled by an LC circuit. The resonant frequency of the QMSIW, which operates below the waveguide cutoff frequency is expressed as

B. CIRCUIT MODEL ANALYSIS
The input impedance can be determined as Equation (4) indicates that the operating frequency f 0i of the antenna depends on L ai and C bi , where i = 1, 2.
The capacitance C bi increases by enlarging the slot dimension and shifting operating frequency toward lower values, which is beneficial from miniaturization standpoint. The lumped circuit model is optimized using Keysight Advanced Design System (ADS), and the optimized values of the components are given in Table 2. The circuitsimulated S-parameters are also obtained from EM simulation. The comparison thereof is shown in Fig. 5.  Figures 6 and 7 portray the electric and magnetic field distributions of the proposed antenna, respectively. The excitation is applied to one port (ON), whereas the other port is terminated (OFF) by a 50-ohm load, and vice versa. The maximum field strength is observed at the inner-edges of the slots by exciting the ports as shown in Fig. 6, whereas the magnetic field strength is maximum at the inside area enclosed by the slots, which is depicted in Fig. 7.

D. TUNABILITY AND PARAMETRIC ANALYSIS
The proposed antenna configuration offers a flexibility of designing each frequency band independently. It has been found in the previous section that the frequency is in a direct relationship with the slot dimension. The allocation of the frequency bands can be controlled by increasing/decreasing the total capacitance, which is achieved by varying the slot length. The first (f 01 ) and the second (f 02 ) operating bands are controlled by varying the parameters L 1 and L 3 , respectively. As shown in Fig. 8(a), the first band frequency f 01 is shifted towards higher values as L 1 decreases due to the decrease in capacitance. Therefore, f 01 can be designed independently in the range of 3.5-3.87 GHz by varying the parameter L 1 from 12.0 mm to 12.9 mm. Similarly, the second operating frequency is moved towards higher values by decreasing the value of L 3 , which is shown in Fig. 8(b). The frequency f 02 can be tuned independently in the range of 5.4-6.1 GHz by varying the parameter L 3 from 7.0 mm to 7.9 mm. It is noted that when L 3 varies, the capacitive loading due to the slot changes significantly, which makes the second operating frequency tunable to a larger extent than the first band. Consequently, the proposed SDA offers excellent flexibility in terms of frequency tunability, which is essential in many applications.
(a) (b) Figure 9. Variation of peak gain with respect to (a) L1 and (b) L3. Figure 9 shows the variation of the peak gain with respect to the parameters L 1 and L 3 . As shown in Fig. 9(a), the peak gain is obtained at f 01 by applying excitation to Port 1 and terminating Port 2 with 50-ohm load. Similarly, the peak gain is computed at f 02 by applying excitation to Port 2 and terminating Port 2 with 50-ohm load, as shown in Fig. 9(b). From this study, it is found that the variation of peak gain is about 5 to 7% at both operating bands.
The proposed shielded QMSIW SDA is realized on a single-layer printed circuit board using the metallic vias, which results in a significant reduction of surface-wave transmission within the substrate. This improves the antenna performance due to the suppression of the edge diffraction effects, and reduction of power dissipation [2]. The SIW based antenna shows unidirectional radiation, which results in elimination of the back side radiation, leading to a good far field performance [2]. This SIW antenna is designed at TE 110 mode, which offers a reasonable amount of gain. However, the gain of this antenna can be increased when operated at higher order modes without increasing the physical size of the antenna. Also, the gain can be increased by thickening the substrate height [20].
It should be noted that the antenna gain has a direct relationship with the effective aperture of antenna. Considering the above facts, the gain of the antenna based on SIW does not only depend on the size, but also other parameters such as mode of operation and substrate height. In this design, we achieved a reasonable gain while ensuring a compact size of the antenna, which is realized at TE 110 mode. As mentioned before, the gain of this particular VOLUME XX, 2017 9 antenna can be increased by realizing at higher order mode without increasing the physical size of the antenna. Based on the above study, a detailed design flowchart is provided in Fig. 10, which includes optimization and characterization of the proposed QMSIW-based.

III. FABRICATION, RESULTS AND DISCUSSION
To validate the proposed design approach, EM simulation and circuit model analysis, a prototype of the shielded QMSIW-based SDA has been fabricated and demonstrated for WiMAX and WLAN applications. The antenna is fabricated on the RT/Duroid substrate (ε r = 2.33, H = 0.787 mm, tanδ = 0.0012). The photograph of the manufactured SDA is shown in Fig. 11(a). All of the tests are carried out by applying the excitation at Port 1 and terminating Port 2 by a 50Ω load, and vice-versa. The S-parameters are recorded by a two-port Rohde & Schwarz vector network analyzer. Figure 11(b) illustrates the EM-simulated and measured return losses (| 11 | and | 22 |) and port-isolation (| 12 |). The farfield responses of the manufactured SDA are recorded inside an anechoic chamber in two-orthogonal planes of Φ = 0° and Φ = 90° at 3.5 GHz and 5.4 GHz. Figures 12 and 13 show the EM-simulated and measured peak gains and radiation efficiency, respectively. The simulated and measured radiation patterns of the manufactured SDA are shown in     dBi and 5.60 dBi at 3.5 GHz and 5.4 GHz, respectively.  The measured radiation efficiencies of the fabricated SDA are 84% and 86.8% at 3.5 GHz and 5.4 GHz, respectively. A comparison between the proposed SDA and the previously reported state-of-the-art antennas based on fullmode SIW [3][4][5][6][7][8][9][10][11], HMSIW [17] and QMSIW [18][19] has been discussed based on performance indicators (size, isolation, peak gain and FTBR) as shown in Table 3.

V. CONCLUSION
In this paper, a novel design of highly miniaturized cavitybacked self-diplexing antenna (SDA) employing shielded quarter-mode SIW for WiMAX and WLAN applications has been presented. The design employs a shielded QMSIW, two U-shaped slots, and two orthogonal 50Ω feed lines. The Ushaped slots are loaded on the top conductor plane of the shielded QMSIW to produce two radiating patches for the antenna to operate at 3.5 GHz and 5.4 GHz. These frequency bands can be designed individually by varying the dimensions of the U-shaped slots. The slots are excited by two orthogonal 50 Ω feed lines, which creates a weak crosscoupling path resulting in high element isolation. An equivalent LC circuit model is used to validate design along with full-wave EM analysis. Furthermore, the SDA has been fabricated and measured. Good alignment between simulations and measurements has been observed. The SDA prototype offers peak gains of 5.26/5.60 dBi at 3.5/5.4 GHz and a return loss of better than 21.6 dB, according to the tested results. The proposed SDA exhibits a better than 40 dB isolation between ports, as well as outstanding front-to-back ratio and co-to-cross polarization.