A Compact Gap-Waveguide Dual-Polarized Ka-Band Feed for 50dBi Reflector Antennas With Tracking Function

A dual-polarized Ka-band feed based on gap waveguide (GW) technology for an ultra-high-gain reflector antenna is presented. The feed provides SUM-beams for data transfer and DIFF-beams for tracking. The whole reflector antenna is composed of the feed and a dual-reflector Cassegraine antenna. The feed has been prototyped, and the measured reflection coefficients for the horizontally and the vertically polarized SUM-beam ports are below −10 dB and −8 dB over 30.8–38 GHz, respectively. The measured feed radiation patterns agree well with the simulated ones. The dual-reflector Cassegraine antenna has been designed and simulated by using GRASP with the simulated far-field function of the proposed feed, showing that the reflector antenna achieves SUM-beam gains above 50 dBi and the null depth of the DIFF-beams are more than 30 dB below the maximum of SUM-beams.


I. INTRODUCTION
5G and beyond 5G wireless communication systems will provide ultra-fast data transfer up to 10 Gbps with an ultra-low latency of 1 ms, which can be realized only inevitably with the support of millimeter wave (mmWave) systems. Delivering high data rate in mmWave backhauling (point-to-point) systems over long distances is a big challenge due to the severe path loss and other limitations. Since it is not economically viable to deploy backhaul systems with station towers within every hundred meters, ultra-high gain (≥ 50 dBi) antennas operating at mmWave regime are the core to implement The associate editor coordinating the review of this manuscript and approving it for publication was Kai-Da Xu .
However, the ultra-high antenna gain means ultra-narrow beamwidth (50dBi gain requires a narrow 3-dB beamwidth of about 0.6 • ), where vibrations and wind loads on the antenna may cause the antenna beam swing and result in disruption of the link. Therefore, the beam tracking becomes a critical requirement for mmWave backhauling ultra-high-gain antennas.
The purpose of our work is to design a dual-polarized feed for a dual-reflector Cassegraine antenna of an ultra-high gain above 50 dBi with a compact geometry and a low manufacture cost at Ka-band. The GW technology is characterized by low losses, contactless layered structures and therefore low manufacture costs and easy assembling. Many mmWave GW antennas and passive components have been reported [18], [19], [20], [21], [22], [23]. As for monopulse antennas, radial line slot arrays (RLSAs) combined with a GW feeding network achieved monopulse characteristics but with only a very narrow band [22]. In [23] and [24], a monopulse network using Butler matrix was implemented based on the GW technology. However, all reported GW monopulse antennas including aforementioned are either single linearly or single circularly polarized. In [25] and [26], dual-polarized array antennas based on GW technology at Ka-band and W-band were presented respectively without monopulse function.
In this paper, a new dual-polarized broadband monopulse GW feed for a dual-reflector Cassegraine antenna at Ka-band is proposed. The dual-reflector Cassegraine antenna was preliminarily designed in order to verify that the feed can operate well in the antenna system with good radiation patterns. The performance of the whole antenna is simulated with a commercial solver, GRASP [27], using the CST [28] simulated far-field functions of the feed, verified by measurements, to show preliminary results of the ultra-high-gain antenna for 5G backhauling systems. The novelties of this work include: 1) A square radiation horn with new feeding structure of dual polarization with good isolation between polarizations, low cross polar level and good impedance matching realized by GW technology; 2) A new compact planar magic tee with three ridged gap waveguide (RGW) branches and one E-plane groove gap (E-GGW) branch has been proposed, and the magic tee with three E-GGW branches and one RGW branch proposed in [29] and [30] has been simplified for easy manufacture; 3) Two single-layered comparators, each for one polarization, realized by three contactless metal plates with several new GW transitions between layers for low cost manufacturing and easy assembling; 4) A dual-polarized simple layered structured monopulse feed. This paper is organized as follows. Section II presents the feed design with simulation and measurement data. Section III describes the preliminary design of the dualreflector Cassegraine antenna with the designed feed and presents simulated performance. Section IV draws conclusions of the work.

II. DESIGN, FABRICATION AND MEASUREMENTS OF THE MONOPULSE FEED
In order to describe the geometry of the feed more clearly, we define that a plate is a metal one with texture patterns on one side or both sides and a layer is the space between two plates. The waves propagate in layers (or in other words, between plates). The feed is of a geometry with two layers made by three plates.

A. TWO TYPES OF GAP WAVEGUIDES
For a compact geometry, the combination of two types of GWs is used in the comparator: the E-GGW and the RGW. The geometries of the E-GGW and the RGW are given in Fig. 1, with dimensions in the caption.The dispersion diagrams are calculated by using the Eigen-mode solver of CST Microwave Studio. It is seen that the E-GGW and the RGW achieve a single Quasi-TEM mode transmission over 25.6 − 62.5 GHz and 26.1 − 70.7 GHz, respectively.

B. MONOPULSE COMPARATOR
Two types of wideband planar magic-Ts based on the GW technology, the 3(RGW)+(E-GGW) Magic-T and the VOLUME 10, 2022  3(E-GGW)+(RGW) Magic-T, are proposed in order to form a compact monopulse comparator. Fig. 2 shows the geometry of the 3(RGW)+(E-GGW) planar magic-T that is composed of three RGWs and an E-plane GGW. The work principle is illustrated in Fig. 3. When Port 1f is excited, the signal from Port 1f will be split with the equal magnitude and in-phase into Port 3f and Port 4f, where a capacitive coupling is applied. When Port 2f is excited, the signal from Port 2f will be split with the equal magnitude and out-of-phase into Port 3f and Port 4f via the capacitive gap and help of a cavity below the extended ridge. By adjusting the parameters l s1 and h s1 of the junction, a good impedance matching performance can be obtained. Fig. 3 shows the simulated results of the 3(RGW)+(E-GGW) magic-T. Within the frequency band of 26−40 GHz, the reflection coefficients at Port 1f and Port 2f are below −10 dB, the isolation between  the two ports is better than 45 dB, and the phase imbalance due to numerical errors in CST is found less than 0.8 • . Fig. 4 illustrates the geometry of the 3(E-GGW)+(RGW) planar magic-T, correspondingly consisted of three E-GGWs and an RGW. The work principle illustrated in Fig. 5 is similar to that of the 3(RGW)+(E-GGW) magic-T. A similar performance, i.e., below −10dB reflection coefficients at Ports 1s and 2s, better than −45dB isolation between the two ports, 0.3 • phase imbalance error is also obtained over 26−40 GHz, as shown in Fig. 5.
The monopulse comparator is realized by combining one 3(RGW)+(E-GGW) magic-T, two 3(E-GGW)+(RGW) magic-Ts and an E-plane T-junction divider, as shown in Fig. 6, with three ports: SUM Port , E-plane DIFF Port E , and H-plane DIFF Port H . Fig. 7 shows the electric   field distribution of the proposed monopulse comparator at 34 GHz. The signal from SUM Port propagates through the 3(RGW)+(E-GGW) magic-T first, and then the 3(E-GGW)+(RGW) magic-T to Ports 4c-7c with equal magnitude and same phase, while the signal from Port E will arrives at Ports 4c-7c with equal magnitude and phases of (0 • , 0 • , 180 • , 180 • ) respectively, and the signal from Port H will arrives at Ports 4c-7c with equal magnitude and phases of (0 • , 180 • , 0 • , 180 • ) respectively. Fig. 8 exhibits the simulated reflection coefficients of all ports of the comparator: below −14 dB and −10 dB for the SUM port and the DIFF ports, respectively, over 30-38 GHz, where the performance of Port is more important.

C. DUAL-POLARIZED RADIATION ELEMENT
The geometry of the radiation element is shown in Fig. 9. The element consists of three layers: the radiation layer, the FIGURE 9. Geometry of the radiation element, with dimensions as h a1 = 3 mm, h a2 = 1.5 mm, h a3 = 4 mm, h a4 = 1.6 mm, h a5 = 2.7 mm, w a1 = 6 mm, w a2 = 4 mm, w a3 = 3.8 mm, w a4 = 1.5 mm. V-pol layer and the H-pol layer. The radiation aperture in the radiation layer is a square horn with a size of w a1 × w a1 . By using a structure similar to the orthomode transducer in the V-pol layer and the H-pol layer, horizontal polarization and vertical polarization can be achieved when Port 1r and Port 2r are excited, respectively. In order to broaden the bandwidth, the dimensions of the steps in these two layers are optimized.
The simulated results of the proposed dual-polarized radiation element are shown in Fig. 10. It is seen that the reflection coefficients are lower than −15 dB, and the isolation between the ports of both polarizations is larger than 55 dB within the frequency band of 30-38 GHz.

D. DUAL-POLARIZED MONOPULSE FEED
The proposed dual-polarized monopulse feed, shown in Fig. 11, consists of the afore-described planar comparators VOLUME 10, 2022  and a 2×2 array of the afore-described radiation element with an element spacing of 9 mm in both x− and y−axis. The three plates of the monopulse feed can be easily assembled by using several screws without leakage due to the GW characteristics.
Step transitions from the RGW and the E-GGW to standard waveguide WR-28 are used for the ports of the feed with simulated reflection coefficients better than −20 dB.

E. PROTOTYPING AND MEASURED RESULTS
The proposed dual-polarized monopulse feed was fabricated by computerized numerical control machining technique. The prototype is shown in Fig. 13, which is made of copper (with electric conductivity 5.8 × 10 7 S/m) and gold-plated. The manufacture tolerance is 0.02 mm. The total size is 75 × 75 × 20 mm 3 . The S-parameters were measured by using an Agilent E8363B vector network, and the radiation characteristics of the antenna were measured in a far-field range test setup, at Gapwaves AB and Chalmers University of Technology, both in Gothenburg. The simulated and measured gains of the SUM-patterns are shown in Fig. 14, where the measured gain G mea is obtained by comparison  method with a standard horn antenna. Then, the measured antenna efficiency e ant mea is obtained by e ant mea =G mea /D max , where D max is the theoretical maximum available directivity of the antenna with the aperture size of 18 × 18 mm 2 . The measured gains follow the simulated ones, though there are 2 dB gain differences (about 10% difference in total antenna efficiency) on average. We believe that this difference is due to that the surface roughness of the prototype which was not included in the simulation. The overall measured antenna efficiency is above 60% except at few points. The simulated and measured S-parameters are compared in Fig. 15. It is    seen that the measured reflection coefficients at SUM-ports are below −10 dB and −8 dB over the frequency band of 30.8 − 38 GHz for horizontal and vertical polarizations, respectively. For Diff ports, the reflection coefficient specification is relaxed to −5 dB and the measured data has fulfilled the specification, because the deep difference patterns' null dips are more important. The simulated mutual couplings between different ports within the entire frequency band are below −30 dB. It is noted that the measured mutual couplings between the SUM-ports and the DIFF-ports are worse than the simulations, especially between Port H and Port EH with the values about −15 dB, due to an asymmetrical fabrication    vertical polarizations. The simulated relative null depth in DIFF patterns (the dB value below the maximum of SUM patterns) is around −40 dB. All measured DIFF patterns' relative null depths are below −30 dB. These measured results indicate that the proposed feed achieves a good monopulse performance in dual-polarization and in both E-and H-planes within the whole band.
Key performance of some reported planar monopulse antennas and monopulse feeds have been summarized in Table 1 as a comparison with our proposed feed. Compared with the single polarized feeds in [5] and [6], our feed introduces a new planar magic-T and simplifies an existing planar magic-T by combining E-GGW with RGW to make a compact monopulse structure for dual polarization with a wide bandwidth. Due to the utilization of the combined GW technology, the proposed feed exhibits a good performance over a wide bandwidth with a simpler structure for assembling and lower manufacturing cost than those feeds in [5] and [6].

III. PRELIMINARY DESIGN OF ULTRA-HIGH-GAIN REFLECTOR ANTENNA
A dual-reflector Cassegraine antenna, consisting of a paraboloidal main reflector, a hyperboloidal sub-reflector and the dual-polarized monopulse GW feed as shown in Fig. 18, is employed in this work to achieve 50 dBi antenna gain. A preliminary design is done by using GRASP, and antenna parameters are listed in the caption of Fig. 18.
The simulated radiation performance of the whole reflector antenna is obtained by using GRASP with the CST simulated (verified by measurements) far-field function of the feed as the input to GRASP. The radiation patterns in E-and H-planes for horizontal and vertical polarizations are shown in Figs. 19 and 20 at 30, 34 and 38 GHz. The gain of the SUM-beam within the operating frequency range of 30-38 is shown in Fig. 21. It is seen that the simulated gains of SUM-beams are higher than 50 dBi and is 53.4 dBi at 34 GHz, and the first relative sidelobe level is below −22 dB. The DIFF-beam null depth is below −34.7 dB relative to the maximum of SUM-beams. The gain difference between SUM-and DIFF-patterns is about 6 dB. All the mentioned data are the results after considering the blockage of the sub-reflector, modelled in GRASP. The simulated results verify the design of the dual-polarized monopulse feed preliminarily and numerically.

IV. CONCLUSION
In this paper, a dual-polarized monopulse feed, composed of three contactless plates, has been proposed based on gap waveguide technology with two types of planar GW magic tees to form a monopulse comparator. Due to the separate plates, the manufacture cost has been reduced a lot with a low-loss characteristic, where the assembly tolerance is relatively easy to be guaranteed. The feed has been designed and manufactured. The measurements of the prototype have verified the design of the feed. Finally, the designed feed is used to feed a dual-reflector Cassegraine antenna with the simulations of numerical modeling in GRASP to achieve an ultra-high gain of above 50 dBi with 2D tracking functions. The proposed antenna is aiming to be used in 5G mmWave backhauling systems.