Abstract
This paper presents an antipodal linearly tapered slot antenna (ALTSA) array with the newly emerging substrate-integrated coaxial line (SICL) technology. Each element of the proposed array consists of two vertically stacked antipodal tapered slot antennas (TSA) with the inner conductor of a SICL as the shared fin. The vertical symmetry of this radiating element structure significantly suppresses the cross-polarization level. By employing a SICL-based power splitter with differential outputs, a two-element array with horizontal symmetry is formed. With the inner conductors of two parallel SICLs combined as the flaring metal shared by adjacent tapered slots, the main beam squint is effectively suppressed. Experimental results show an actual −10-dB impedance bandwidth (BW) of 24.5∼34.4 GHz, and the realized gain range measured is 16.1∼18.7 dBi. The cross-polarization levels are −39.4 dB/−40.7 dB at 30 GHz for the E-/H-planes. The measured results agree well with the simulated data which demonstrates the feasibility of the proposed SICL-based ALTSA scheme in the potential 5 G·millimeter-wave (mmWave) applications.
1. Introduction
Nowadays, the growing demands for high data rate communication and short-range detection have stimulated the applications of millimeter wave (mmWave) [1–3]. However, transmission in the mmWave band is susceptible to environmental factors such as raindrops and oxygen absorption [4, 5], which calls for high-gain and low-cross-polarization antenna designs. Tapered slot antenna (TSA), also known as the Vivaldi antenna, is one of the promising candidates [6–9] featuring broadband, low weight, low profile, ease of fabrication, and integration with microwave circuits [10]. Recently, the development of substrate-integration technologies has enabled using the substrate-integrated waveguide (SIW) technology as the feed of the antipodal TSAs [11–19]. Compared with conventional microstrip-based feed, the SIW-based feed achieves a broader bandwidth and can provide a phase reversal excitation for the antipodal TSA without using a balun. However, the performances of these SIW-based schemes are susceptible to substrate thickness, permittivity, and antenna geometry. Specifically, two issues need to be fixed. One is that the cross-polarization level of the antipodal TSA increases with the substrate thickness as it is vertically asymmetric. The other is that the dispersive characteristic of SIW restrains bandwidth enhancement and structural miniaturization. Lately, a substrate-integrated technology, termed substrate-integrated coaxial line (SICL), has been proposed. It is a two-wire nondispersive transmission line with ultra-slim size, which is suitable to meet the design requirement of wideband and miniaturization [20]. As shown in Figure 1, SICL comprises a conductive signal strip sandwiched between two grounded substrates and bounded bilaterally by two rows of metallic vias, which constrains the undesired parallel-plate mode. The nondispersive characteristic of SICL makes it competitive in designing broadband feeds [21] and compact antennas [22, 23].
In this paper, a balanced antipodal linearly taper slot antenna comprising two vertically stacked antipodal TSAs is proposed. As shown in Figure 2, SICL is employed as the feed with its inner conductor as a shared fin of the two antipodal TSAs.
The vertical symmetry of the proposed structure significantly suppresses the vertical E-field and the cross-polarization level. Furthermore, a two-element antenna array is implemented with considerable gain and cross-polarization enhancement.
The remaining contents are organized as follows. Section 2 elaborates on the designs of both the radiating element and the array. Section 3 details the experimental verification. Section 4 concludes the whole work.
2. Antenna Design
The design procedure comprises two phases: design of the SICL-based TSA radiating element and design of the 1 × 2 array.
2.1. Design of the Radiating Element
ALTSA is known to have a high input impedance, which causes an impedance mismatch to the SICL feed, as the characteristic impedance of the SICL is relatively low. To solve this problem, we propose a SICL-based tapered slot element with which the inner strip of SICL protrudes and is expanded into the trapezoidal fin of the tapered slot. As depicted in Figure 3, the element consists of three plates and two dielectric substrates with an intermediate prepreg for bonding. The fin in the middle plate is ungrounded and antipodal to the top/bottom fins etched in the top/bottom plates. The top/middle/bottom fins gradually flare at a constant angle, making up the shunt up/low TSAs, and consequently lower the input impedance. Thus, the impedance matching between the ALTSA and the SICL can be realized without additional transition.
Conventional TSAs, such as SIW-based schemes, are realized by flaring the grounded plates on both sides of the substrate in the opposite direction with the electric field intensity vector () inclined to the horizontal plane. As is illustrated in Figure 4(a), while the horizontal component of the electric field () contributes to the effective radiation, a vertical component () due to the structural asymmetry cannot be eliminated and causes the unwanted cross-polarization.
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Unlike the SIW-based scheme with all fins grounded, the proposed SICL-based scheme herein has a vertical symmetry as sketched in Figure 4(b). Since the top/bottom fins are grounded, the E-vector inside the triple-plate structure consists of two symmetric components, and . With the thickness of the prepreg to be small enough (0.1 mm in this design), the difference between the magnitudes of and is subtle and can be neglected. So can be considered as the negative vector of , and . As a consequence, the source that causes cross-polarization is eliminated.
Once the impedance matching condition is satisfied, the dimensions of the tapered slot can be determined with the well-studied method [24–33]: Firstly, the length of the tapered slot line () is determined. A typical value is , where is the free-space wavelength. Secondly, the width of the open end of the tapered slot line () is determined, which is usually more than . Thirdly, since the performance of the ALTSA is quite sensitive to the relative permittivity (), the substrate thickness () can be determined in the range of . Given the designed center frequency at 30 GHz, we can have and , respectively. The corrugation structures are etched in the outer edges of the fins to alter the phases of edge currents so that both the gain drop and the near-field radiation caused by the undesired surface currents at the outer edges can be suppressed [27, 28]. In the meanwhile, for corrugation lengths (rcl) under , small changes will cause an obvious impact on the E-plane beamwidth, and the H-plane beamwidth variations in this region are less severe.
The simulated performance of the proposed TSA element is demonstrated in Figure 5. As is seen in the figure, the −10-dB impedance bandwidth is 25.3∼32.9 GHz, and the in-band gain is flat and has a peak value of 17.1 dBi. The E- and H-plane patterns at 30 GHz are illustrated in Figure 5(b), which shows a gain of 16.98 dBi and a cross-polarization level of −37.2 dB. Although the radiating element has effectively suppressed the cross-polarization, the asymmetry of its structure causes a beam squint. The issue can be fixed by using asymmetric substrate cutouts and dual-scale slotted edges. In our work, the beam squint is eliminated by employing an array with a differential feed, as is discussed in the following section.
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2.2. Design of the Array
To feed a TSA array, a SICL-based equal-split power divider is designed first. It consists of a microstrip-to-SICL transition and a pair of differential outputs. As shown in Figure 6, two trapezoid slots of length and with widths , , and are, respectively, etched in the top plate. These two slots are used to mitigate the abrupt E-field variation aroused by the junction of transition. The microstrip line is connected to the SICL inner conductor with a tapered transition for impedance matching. The metallic vias on both sides of the microstrip line prevent the excitation of spurious parallel plate waveguide mode between the conducting strip and the bottom plate. Meanwhile, complementary split-ring resonators (CSRRs) are etched in the top and bottom plates on both sides of the SICL to suppress the surface currents.
With the phase-reversal excitation and the TSA element designed above, a 1 × 2 array is formed as illustrated in Figure 7 with the geometrical specifications. The trapezoidal metal sheets connected to the inner and outer conductors of the SICL build a balanced structure of ALTSA. The corresponding E-field distribution is shown in Figure 8(a). A semielliptical-shaped dielectric guiding structure is added as a dielectric lens in front of the array to narrow the beamwidth in E-plane, enhancing the gain as discussed in [33]. Meanwhile, according to the E-field plotted in Figure 8(b), we can safely assume that the proposed array is equivalent to an array. The simulated beam squints about the element and array are shown in Figure 9, where we can find that the proposed array fed by odd-mode excitation has a lower beam squint than that of the element designed above.
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3. Experimental Verification
To verify the feasibility of the proposed SICL-based ALTSA array, we have fabricated it using the standard PCB process, as presented in Figure 10. The prototype consists of two dielectric layers (Taconic TLY-5, , , 0.254 mm in thickness) bonded by a prepreg (Taconic FR-27, , , 0.1 mm in thickness), and two metallic layers (35 µm in thickness). The overall size is 89.4 mm × 27 mm × 0.608 mm. A 2.40-mm end launch connector [34] is used in the measurement.
The reflection coefficient is measured over the frequency range of 24∼35 GHz by using a vector network analyzer (R&S ZVA50). As illustrated in Figure 11, the −10-dB impedance bandwidth is 9.9 GHz (24.5∼34.4 GHz, 33%@30 GHz). Since TSA usually includes none resonant structures, manufacturing tolerances do not strongly influence its operating frequency or radiation. The measured center frequency is 29.45 GHz, which is 550 MHz lower than the designed value of 30 GHz.
The radiation performance is measured by using the far-field method in an anechoic chamber (see Figure 12). The gain is measured from 24∼33 GHz, as shown in Figure 13. A measured peak gain of 18.77 dBi is observed at 33 GHz (the insertion loss of the end launch connector, which is about 0.84 dB at 30 GHz, has been excluded), while the maximum gain in simulation is 18.81 dBi. The corresponding details are illustrated in Figure 13. In the meanwhile, the simulated antenna efficiency in the entire operating frequency band is greater than 75%. The cross-polarization performance versus frequency is plotted in Figure 14. According to the configuration of the array, both the E-plane and the H-plane have the same cross-polarizations, and the simulated values are −33.2 dB @ 27 GHz, −45.0 dB @ 30 GHz, and −24.0 dB @ 33 GHz, respectively. The measured cross-polarizations at 27 GHz, 30 GHz, and 33 GHz are−34.7 dB, −39.4 dB, and −24.7 dB for the E-plane and −31.6 dB, −40.7 dB, and −26.1 dB for the H-plane.
There is a small difference between the two principal planes’ values. This is because the main-lobe direction of the antenna does not point exactly toward the direction with zero degree angle due to the alignment error when the antenna was mounted on the turntable. The cross-polarization levels increase slightly as the operating frequency deviates from the center frequency of 30 GHz. This is caused by the limited bandwidth of the SICL balun. Figure 15 shows the E-plane and the H-plane radiation patterns of the proposed array at 27 GHz, 30 GHz, and 33 GHz. It can be found that the E-plane beamwidth is narrower than that of the H-plane, which is caused by the horizontal arrangement of the two radiating elements in the array.
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A comprehensive performance comparison between the proposed antenna and other related antennas in the open literature is listed in Table 1, indicating that the proposed ALTSA design does have an apparent enhancement in cross-polarization performance.
4. Conclusions
This paper presents a balanced ALTSA fed by using the SICL technology, which provides a method to get a high gain with lower cross-polarization. The proposed ALTSA has a measured −10-dB impedance bandwidth of 33% centered at 30 GHz with a peak gain of 18.77 dBi observed in the measurement. The good agreement between the simulated and measured results demonstrates the feasibility of the proposed scheme with features of high gain and low cross-polarization, making it a possible candidate for antennas in millimeter-wave wireless communication.
Data Availability
The simulated and measured data of the proposed antenna used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This work was supported by the National Natural Science Foundation of China under Grant 62171221.