Multiport Beamforming System Based on Reconfigurable Waveguide Phased Antenna Array for Satellite Communication Applications

In this article, a multiport beamforming system based on the reconfigurable waveguide phased antenna array is presented, which is fed by the microstrip single-layer multiport feeding network. The reconfigurable waveguide transitions and waveguide linear-polarized/circular-polarized antenna arrays have high flexibility to meet different requirements in beamforming applications according to the design purposes, which is also suitable for satellite communication applications with separate transmitted and received systems. Both simulated and experimental results are presented for the demonstration. Results indicate that the multiport beamforming system based on the reconfigurable waveguide phased antenna array could provide wideband performance at both K/Ka-band with flexible functions compared with the conventional single-layer beamforming systems.


I. INTRODUCTION
The designs with dual-band operation frequencies functions are popular in recent years, especially for modern satellite communication applications with two separated receiving/transmitting systems to achieve lower interference and influence between uplink/downlink [1] and better downlink margin [2]. To serve the end-users and mobile devices, the beamforming technology based on the phased antenna array is usually applied to communication applications and devices for the ''user-beam''. In a beamforming system, both the structures of the feeding network and antenna array, and the connections between them are very critical for the system designs and functions. The waveguide transition between the feeding network in PCB The associate editor coordinating the review of this manuscript and approving it for publication was Mohammad Zia Ur Rahman . and the phased antenna array is more appropriate than the coaxial connections and transitions due to its simple installation and maintenance. Some recent research also demonstrated the advantages of waveguide transitions in RF communication applications in its low loss, wideband performance and simple structure [3], [4], [5], [6], [7], [8].
From some recent dual-band waveguide transition designs and research [9], [10], to achieve a dual K/Ka-band operation performance, they will commonly restrict and sacrifice the bandwidth and the performance. In article [11], a substrate integrated coaxial line (SICL) K/Ka-band reconfigurable structure waveguide transitions design could provide wideband performance, whose reconfigurable structure is also suitable for microstrip-to-waveguide transition in this paper.
In this article, the novel beamforming system design which consists of reconfigurable antenna arrays, waveguide transitions and feeding networks provides high flexibility and adaptability for more design requirements and purposes both in designs and applications. These two kinds of microstripto-waveguide transitions have the same waveguide dimension for the same operation mode. Furthermore, the two K/Ka-band modes waveguides will share the same PCB feeding network in beamforming applications. It only needs to easily install or replace the metallic waveguide parts by using screws to achieve K/Ka-band (Tx/Rx) operation modes switching, which will benefit to save cost, simplify the installation and maintenance, and provide flexible functions. The feeding network in PCB is connected by the waveguide transitions with the waveguide antenna array. The waveguide antenna array could be easily installed and replaced on waveguide transitions.
Linear polarization and circular polarization are the two main polarization modes for the antenna array designs in a beamforming system. The linear-polarized (LP) antenna array and circular-polarized (CP) antenna array are both widely used in communication applications. The LP antenna array could provide good cross-polar isolation, simple structure, low cost and low profile [12]. The CP antenna array is popular in satellite communication applications due to its merits in terms of multi-path interference and polarization mismatch [13]. In this article, the reconfigurable structure could provide reconfigurability and replaceability for the LP/CP waveguide antenna arrays and waveguide transitions to meet the requirements for different design purposes. More functional options could be achieved by replacing the different waveguide parts, which are installed and connected by the screws to the same feeding network in PCB.
Some recent designs and research about the dual LH/RH CP waveguide antenna and reconfigurable antenna demonstrated their high flexibility in functions and structures [14], [15]. Some dual LH/RH CP waveguide antenna designs use triangular-shaped stepped transformers or slots to generate a CP wave [15], [16] with around 10% wideband operational performance. Nevertheless, to design an antenna array for the beamforming system, it is desired to achieve wideband performance both at K/Ka-band (around 20GHz and 30 GHz). Therefore, two different waveguide antenna arrays with different waveguide dimensions need to be designed unless the waveguide antenna array could provide ultra-wideband performance over 40% (20 GHz-30 GHz). In this article, both the reconfigurable LP and dual LH/RH CP waveguide antenna arrays could provide ultra-wide operational bandwidth at K/Ka-band simultaneously. Compared with the conventional waveguide antenna array designs for beamforming systems, the characteristic of the reconfigurable structures contributes to the advantages of simpler installation and maintenance, more cost reduction and more functional flexibility.
In recent articles, there are some single-layer beamforming designs [12], [17], [18], [19], [20], [21], however, they commonly work at a single operational frequency band and could only provide a single species of polarization mode. To work   in different situations and requirements in a beamforming application or a satellite application, the flexible functions of linear-polarized, LH and RH CP modes could be achieved  by the replaceable linear polarized and the dual LH/RH CP waveguide antennas with the alternative waveguide inputs, which will be fed by the same waveguide port from the waveguide transition.

II. DESIGN OF THE MULTIPORT BEAMFORMING SYSTEM BASED ON RECONFIGURABLE WAVEGUIDE ANTENNA ARRAY
In this beamforming system, the microstrip-to-waveguide transitions connect the feeding network in PCB and the waveguide antenna array. An example of a 2×2 beamforming antenna array is illustrated in Fig. 1 to show the conceptual beamforming system network. The phase and gain control section would be installed at the bottom of the PCB feeding network.
To get some preliminary results of the beamforming system, the reconfigurable K/Ka-band beamforming samples with a 2×2 waveguide antenna array are designed to demonstrate the beamforming operational performance. To that end, a cost-effective 1-4 microstrip feeding network is deployed for the demonstration, where the microstripto-waveguide transition designs are similar to that for the SICL-to-waveguide transition which is mentioned in [11]. The rectangular waveguide transition structure with stepped transformers is used in this 2×2 beamforming array, due to the rectangular waveguide structure is suitable to connect the waveguide antenna in the beamforming array system. The 2×2 beamforming system sample is designed which consists of the single-layer 1-4 microstrip PCB feeding network, reconfigurable 2×2 microstrip-to-waveguide transitions array and reconfigurable 2×2 waveguide LP/CP antenna arrays. The one-input-four-output feeding network in PCB and the 2×2 waveguide antenna array are connected by the 2×2 reconfigurable waveguide transitions array. The  reconfigurable CP waveguide antenna array are demonstrated in Fig. 3 and Fig.4 respectively with dimensions.
In this design, the antenna element distance is 11 mm, the height of the waveguide transitions array is 15 mm and the height of the waveguide LP antenna array and CP antenna array is 8 mm and 34 mm respectively.
The antenna array beam angle could be calculated from the formula [22]: where, ϕ is the signal phase shifting between each antenna element, d is the distance from each element, λ is the wavelength and θ is the beam angle. The simulated antenna array structure and far-field patterns for the beamforming system when there is a 60-degree phase shift between the 2 antenna array elements along the Y-axis at 20/30GHz are shown in Figs. 5 (a) (b) and (c) respectively [23], [24].

III. FABRICATIONS AND EXPERIMENTAL RESULTS
To support the investigation of the waveguide transitions array performance in the beamforming system sample, the back-to-back structure was used in the measurement to demonstrate the single rectangular microstripto-waveguide transition with stepped transformers, whose measured and simulated results at the K/Ka-band are shown in Figs. 6 (a) and (b) respectively. The measured return loss These results also demonstrated the wideband performance of the single rectangular microstrip-to-waveguide transition with stepped transformers designs at the required K/Ka-band. Some discrepancy between the simulation and measurement would attribute to the tolerance in material and fabrication.
The single-layer 1-4 microstrip feeding network used PCB RO4003C with 0.8mm thickness, and the material of all the waveguide transitions array and waveguide antenna array parts are aluminum. The fabricated PCB feeding network and the LP/CP waveguide antenna array are illustrated in Figs. 7. (a) and (b). The 1-4 microstrip feeding network will provide the phase difference between the antenna array elements. Fig. 8. (a) illustrated the 1-4 feeding network to feed the antenna array and provide phase shift, the Fig. 8. (b) showed the fabricated 1-4 feeding network sample.
Compared with the simulated results from Figs. 9. (a) and (b), although the connected adapters and connectors in measurements changed the phase, the measured phase results still showed similar phase differences and demonstrated that the 1-4 feeding network could provide a 60-degree phase difference around 20 GHz and a 90-degree phase difference around 30 GHz respectively. The measured and simulated S11 in Figs. 10. (a), (b), (c) and (d) illustrated that both the beamforming system samples with reconfigurable waveguide LP antenna array and CP antenna array provided wideband performance for K/Ka-band. Some small differences between measured and simulated S11 are caused by the connected cables and connectors in measurements and tolerance in fabrications. Compared with the beamforming system samples with no element phase shift, the measured and simulated far-field patterns of the LP 2×2 antenna array beamforming system samples for 60-degree element phase shift at K-band mode and 90-degree element phase shift at Ka-band mode are demonstrated in Fig. 11 and Fig. 12 respectively; The measured and simulated far-field patterns of the right-handed and left-handed CP 2×2 antenna array beamforming system samples for 60-degree element phase shift at K-band mode and 90-degree element phase shift at Ka-band mode are demonstrated in Figs. 13, 14, 15 and Fig. 16 respectively. Both the LP and CP antenna array beamforming systems achieved around 10-12 degrees beam angle shifts when there is a 60-degree antenna element phase shift for K-band mode or a 90-degree antenna element phase shift for Ka-band mode along the Y-axis, which are similar to the simulated results [23], [24] and calculated results from the formula [22]. The measured gains of the LP 2×2 antenna array beamforming system sample are around 13.5-13.9 dBi at the K-band which are similar to the simulated gains of around 14.2-14.4 dBi; At Ka-band, the measured gains of  respectively. Both at the required K-band and Ka-band frequency ranges, the axial ratios are less than 3 dB, which meets the requirement of a circular-polarized antenna at the operational frequencies for this design.

IV. CONCLUSION
A K/Ka-band beamforming system based on the reconfigurable waveguide antenna arrays and waveguide transitions has been presented in this article. The achieved linear-polarized/right-handed circular-polarized/left-handed circular-polarized operations by the reconfigurable structure were designed and tested to demonstrate the design concept. The measured gain is around 13.5-13.9 dBi at K-band and 14.4-15.1 dBi at Ka-band for the LP 2×2 antenna array beamforming system; The measured gain is around 14.2-15.1 dBi at K-band and 14.2-14.9 dBi at Ka-band for the CP 2×2 antenna array beamforming system. Measured results agreed relatively well with simulated results which both provided wideband performance in K/Ka-band. The flexible functions and properties of the reconfigurable structures are promising for applications in satellite communications.

ACKNOWLEDGMENT
The author Yifang Wei would like to thank the collaboration and technical support from Tesat-Spacecom GmbH & Company KG.