Connected slots antenna array feeding a high-gain lens for wide-angle beam-steering applications

Abstract This paper presents a 60 GHz connected slots linear-phased array feeding a high-gain semi-symmetric lens antenna. This design provides high gain, broadband, and beam-steering capabilities for gigabit rate access and backhaul communications. The connected slots antenna array (CSAA) is excited at 16× equidistant points which not only yields spatial power combining but also allows the progressive phase changes to steer the beam in ±45° in azimuth plane. To characterize the CSAA-fed lens antenna, four different power splitters are fabricated which steer the main beam in 0, 15, 30, and 45°. The lens is designed in a way to overcome the scan loss and get comparatively higher gain when beam is steered away from the broadside. The measured results show 25.4 dBi maximum gain with 3 dB gain bandwidth covering the full band 57–66 GHz whereas 3 dB beam-steering range is ±45° for all frequencies. Besides, the half power beamwidth is 6 and 10° in elevation (E-plane) and azimuth plane (H-plane), respectively.


Introduction
The exponentially increasing wireless data demands have called for millimeter-wave frequency bands where the 60 GHz unlicensed band has especially attracted the researchers' and industry's attention to realize several Gbps throughput. Higher atmospheric losses and oxygen absorption at 60 GHz ask for high-gain antennas which can be addressed by using dielectric lenses. Moreover, beam-steering can be employed to extend the coverage area of these highgain narrow beam antennas. The feed-switchable antenna arrays feeding the lens have been largely reported in the literature [1][2][3][4][5], however, the beam can only be steered to the predefined angles with this technique, and the link can break if the orientation of the antenna changes from its predefined position. This is true especially if we talk about the narrow-beam backhaul communications. This paper instead proposes a linear-phased antenna array to feed a dielectric lens antenna, where the feed will not only have significantly higher gain compared to single element used in feed-switching, but will also allow to steer the beam to any arbitrary angle in ± 45°. Moreover, excitation of 16-paths at the same time results in spatial power combining and will increase the effective isotropicradiated power (EIRP) by 12 dB compared to single-element excitation, as doubling the number of excitations results in 3 dB rise.
The idea of self-complementary ultra-wide band antenna arrays goes back to the 70s [6], however, the concept of a periodically fed long slot was presented in [7]. The further analysis of such an array was carried out in [8,9] with Green's function derivations. In this paper, the same idea of connected slots is used to design a broadband linear connected slots antenna array (CSAA) at 60 GHz. The mutual coupling in this coherent-phased array is used as an advantage to increase the impedance bandwidth [10]. Moreover, this linear-phased array is fed periodically along its axis which allows to steer the beam in the azimuth plane (H-plane). The CSAA is used as a feed for the high-gain dielectric lens, bringing advantages of connected slots and lens antenna together in one place, resulting in a broadband and beamsteerable design. Keeping in view the applications where communication nodes are in the same plane, e.g. wireless mesh network, track-to-train, etc., the dielectric lens is designed to provide a fixed narrow-beam in the elevation plane while CSAA steers the beam in the azimuth plane for broad coverage. The beam-steering capabilities of the design are tested using four different power splitters that allow the beam to steer at 0, 15, 30, and 45°. The 3 dB beam-steering range is at least ± 45°as higher angles are not tested. The proposed solution is planned to be integrated with 16-path RFIC [11] that can steer the beam at any arbitrary angle in the beamsteering range. Besides, this paper is an extension of the research presented in [12].

Connected slots array design
The design of the CSAA presented in this paper is based upon the unit cell shown in Fig. 1(a). The inset shows the zoom-in view of the slot, which is narrowed in the center to lower the localized reactive energy and improve the impedance bandwidth. Two faces of the linear array are assigned as master/slave pair in Ansys HFSS to create the infinite periodic boundaries. A single layer low loss PCB material Astra MT77 (ϵ r = 3 and tanδ = 0.0017) is used to design the connected slots array. As shown in Fig. 1(b), the slot plane is fed from other side of the PCB with the help of a microstrip line by a via pin.
As the propagation constant for a slot is b = (k 2 1 + k 2 2 )/2 , where k 1 and k 2 are propagation constants of two medium separated by the slot plane, the leaky wave angle cos −1 (β/k 2 ) is only 45°for maximum contrast between ϵ r values of the two medium, thus giving 90°beam separation [13]. To reduce this beam separation, i.e. focus more power in the broadside direction, leaky wave angle can be increased by modifying the structure in a way that propagation constant β ≈ k 1 , where k 1 corresponds to lesser dense medium. This can either be done by adding a low ϵ r superstrate layer between the slot plane and dense dielectric lens [14] or by introducing a small airgap [13], which is a cost effective and easy to realize alternative to superstrate. Figure 2(a) shows that this airgap (h 2 ) acts as a guiding structure resulting in a directivity improvement of more than 2 dB. Besides, among other modes, a cylindrical expanding TM wave is excited by the slot which spreads homogeneously above the slot when there is a free space above it. However, if we look at the elevation plane (E-plane view, Fig. 1(b)) for better understanding, the waves bend away from the broadside when high ϵ r lens is in direct contact with the slot plane. These waves illuminate the lens areas away from the center, and thus giving rise to side lobe levels (SLLs). This is explained in the elevation plane radiation pattern shown in Fig. 2(b), where the SLLs are much higher when no airgap is placed between the slot plane and dielectric lens (i.e. h 2 = 0) compared to when h 2 = 100 μm. A more detailed comparison of maximum directivity as a function of h 2 is shown in panel (c) of the figure. It can be seen that maximum value of 25.1 dB is achieved when airgap is 100 μm and it reduces with increase of this h 2 value. This gap can be realized by placing a 100 μm thick shim between the PCB and lens. Besides, the slot is placed at the center of the lens which gives broadside beam in the elevation plane. If the slot/lens is offset from the center, the beam is steered few degrees away from the broadside, i.e. an offset of 200 μm results in 1.5°beam offset from the broadside without affecting the realized gain, SLL, and impedance matching.

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Imran Aziz et al. Fig. 3(a) shows the active reflection coefficient comparison for different values of the airgap between the slot plane and lens. Less than − 10 dB impedance bandwidth is more than 33% when h 2 = 100 μm. The impedance is reactive when there is no airgap (h 2 = 0) and moves toward lower impedance for values higher than 100 μm.

Moreover,
As it is obvious from the name of connected slots that there is no discontinuity between individual slots, thus the mutual coupling is very high between each element, which is however used to increase the impedance bandwidth in this coherent phase array. The pitch "p" between the feed points is optimized and observed that impedance is more reactive when the pitch is closer to half wavelength, which becomes more real toward the quarter wavelength. Reflection coefficient for different values of the pitch are shown in Fig. 3(b). Besides, the radiation patterns for different values of the pitch are similar to comparable gains, so a value closer to quarter wavelength is used that provides impedance matching from 55 to 75 GHz. This also makes CSAA significantly compact compared to if pitch is half wavelength.

Power splitters
A 50 Ω coaxial based RPC-1.85 Rosenberger connector 1 is used as a launching pad for the RF signal. As shown in Fig. 4, the pad is shorted with the slot plane with the help of PCB VIAs. The signal pin in the connector connects with the 50 Ω microstrip line while 50-70.7-100 Ω impedance transformation is used in designing the 1:16 splitter networks. For power splitter shown in Fig. 4(a), lengths of all 16 paths is the same, i.e. all feed points on the CSAA are fed with the same phase, thus giving rise to the broadside beam. However, for Fig. 4(b), 4(c), and 4(d), the lengths of the lines increase gradually when moving from left to right on the CSAA no y-axis, which results in progressive phase delay. This progressive phase delay steers the beam away from the  broadside in the elevation plane (H-plane). More is the phase delay, higher is the beam-steering angle. Figures 4(b), 4(c), and 4(d) present the power splitters designed to steer the beam at 15, 30, and 45°, respectively. These splitters are being used only for characterization of the array, as the plan is to integrate this array with 16-path RFIC [11].

Lens design
The higher dielectric constant values of lens material provide better power coupling into the lens body from the feed [15], so a high dielectric constant low loss material Preperm PPE950 2 (ϵ r = 9.5 and tan δ = 0.0025) is used as the lens. As the eccentricity e of an ellipse is inversely proportional to the dielectric constant of the material, e = 1/ e √ r , an elliptical lens can be well approximated as extended hemispherical lens for high dielectric constant values [16]. In other words, the extension height will be higher for lower ϵ r values, which causes more power to be spill over, i.e. fields leaking out from the extension part of lens [17]. This extension ext in the Z-direction is calculated and optimized to have the feed at the lower focus of the lens, which provides a plane wave above the lens. Figure 5 shows the maximum directivity as a function of ext and it can be seen that the value is maximum (25.1 dB) when 7 mm extension length is used.
The 24 mm long CSAA, shown in Fig. 4(a), is used as feed for the lens. Figure 6(a) shows the elevation view of the lens antenna. As the CSAA is extended along YY ′ , this feed can be viewed as a point source in this plane. It is mainly this plane where the lens plays its major role to increase the gain and reduce beam width. This is evident from the elevation plane radiation pattern in Fig. 2(b) as the half power beamwidth (HPBW) is 6°in this plane. Besides, keeping in view the length of the feed extended along YY ′ , rotationally symmetric hemispherical lens cannot be used in this case. Instead a half cylindrical lens with hemispheric terminations is employed as shown in the azimuth plane view in Fig. 6(b). When all 16 feed points in the CSAA are excited in phase (0°beam), it is mainly the array itself which results in 10°H PBW is the azimuth plane and the lens doesn't contribute in it. However, when the CSAA is excited in a way to steer the beam away from the broadside in this azimuth plane, the hemispherical terminations play their role to enhance the gain. For usual phased arrays, the gain reduces with higher beam-steering angles because of angular dependence of element radiation pattern, called scan loss [18]. However, in our case, the gain increases up to certain beam-steering angles because of hemispherical terminations, which is further discussed with the measurement results in next section.
The design dimensions for the feed and the lens are tabulated in Table 1.

Fabrication and measurements
The microstrip-fed CSAA is fabricated on a single layer Astra PCB with four different power splitters (1:16) to measure the beam-steered radiation patterns at 0, 15, 30, and 45°. The PCB with the power splitter for 0°beam excites all 16-feed points with the same phase and amplitude, while below relation is used to calculate the progressive phase increment between adjacent columns to achieve the beam steering at any angle θ: where β = 2π/λ o and p is the distance between two adjacent feed points.
The dielectric lens antenna is fabricated from a block of Preprem PPE950 to the shape shown Fig. 7(a) by milling machine. The base of the lens is made rectangular (in xx ′ ) with some thickness to mechanically support the Rosenberger connector. This rectangular base, which is extended beyond the circular base, does not affect the performance of the lens as rays impinging below the "ext" height do not get focused by the lens. These rays will be scattered in pointless directions even if the lens is shaped as shown in Fig. 6. Figure 7(b) shows the CSAA fed through 1:16 power splitter to achieve the main beam at broadside (0°). A coax-based Rosenberger connector is used to feed the signal from network analyzer. The reflection coefficient measurements are presented in Fig. 8 when different power splitters are in place to steer the main beam. All the arrays almost cover the 60 GHz license free band 57-66 GHz. The − 10 dB impedance bandwidth is 10 GHz (55-65 GHz), 12 GHz (55-67 GHz), 12 GHz (55-67 GHz), and 10 GHz (55-65 GHz) for 0, 15, 30, and 45°beam-steering arrays, respectively.
The radiation patterns of this CSAA-fed lens antenna is measured with the help of standard horn antennas at 2°steps.    show the elevation plane (E-plane) and azimuth plane (H-plane) normalized radiation patterns centered at 0°for different frequencies from 56 to 70 GHz. The dashed line at 60 GHz presents the simulation results obtained by using the symmetry boundary as full structure simulation is not possible due to RAM limitations. The simulations agree very well in the azimuth plane but in the elevation plane there are higher side lobes at ± 35°, which might be a shift of lobes as some higher lobes are also present in the measurements at ± 50°. The HPBW in the elevation and azimuth planes is around 6 and 10°, respectively, which nicely agrees with the simulation results. Moreover, looking at the elevation plane view, all the beams are centered at 0°with SLL less than 10 dB, which can be further improved by power tapering to the edge elements. Figures 10(a), 10(b), and 10(c) present the beam-steered radiation patterns in the azimuth plane for 15°, 30°and 45°. The beams are centered around given angles with couple of degrees stretch. Comparatively higher side lobes being observed in Fig. 10(a) for 62 and 64 GHz seem to be due to some localized effect of the PCB as side lobes are much lower even for high steering angles. Figure 11 shows the gain over the band 55-70 GHz. Gain is measured with the help of comparison method, where S 21 is measured for two standard horn antennas and then one of them is replaced with the test antenna. The measurement results at 0°f airly agree to the simulation curve where the measured values range between 22.8 and 24.4 dBi in 57-66 GHz bandwidth. No comparison of measurements and simulations is shown for other beam-steering angles as master/slave or symmetry boundary conditions are used in the simulations which either do not take into consideration the hemi-spherical terminations on the YY ′ axis or doesn't allow beam-steering (as symmetry boundary condition doesn't provide option for progressive phase shift on other side of the boundary). These hemi-spherical terminations play a role in improving the gain of the array when beam is steered away from the broadside. The slight increment in the gain can be observed in the 15°curve where gain values lie between 23.5 and 24.5 dBi in the 57-66 GHz band. This increment is more   GHz band for all beam-steering angles. Moreover, the 3 dB beamsteering range for the presented antenna array is at least ± 45°as higher beam-steering angles are not measured.
The results presented in this paper are limited to 0, 15, 30, and 45°as only four different power splitters are fabricated to characterize the performance of the array. However, this array shall be integrated with the 16-path 57-71 GHz beam-forming Si-Ge transceiver chip [11] which can excite each feed point of CSAA with appropriate phase and amplitude to steer the beam at any arbitrary angle in ± 45°and beyond.

Conclusion
This paper presents an antenna design having 25.4 dBi maximum gain and 3 dB gain bandwidth covering the entire 60 GHz band. The beam-steering range (3 dB) is at least ± 45°while the hemispherical terminations on the azimuth plane play a role in improving the gain when beam is steered away from broadside. Table 2     table utilizes feed switching to achieve the beam-steering except [19] which mechanically steers the lens above a horn feed. To our knowledge, this is the first solution which integrates the phased array with the dielectric lens to achieve the beam-steering. The 16-feed point CSAA enables to steer the beam at any arbitrary angle in ± 45°or beyond with the help of 16-path RFIC. The performance of the presented solution makes it a good candidate for high data rate 60 GHz backhaul and access communications.