A dual-line polarized antenna feed array for Ku-band satellite communication is introduced. The feed array can be used for satellite communication antenna with single reflector or double reflector to realize small angle, high speed and high precision electron beam scanning and tracking of communication satellite, which reduces the requirements of satellite antenna for mechanical servo structure precision and dynamic tracking, thus greatly reducing the cost of servo system and expanding the application of mobile satellite antenna in civil field.
Keywords: feed array; Move through; microstrip antenna
1 Introduction
Satellite-to-ground communication antenna system meets the needs of users to transmit broadband data information through satellites in dynamic motion, enabling mobile carriers such as cars, ships and airplanes to track satellites in real time and transmit information such as voice, data and images without interruption [1][2]. At present, the mobile communication antenna mainly uses Ku frequency band to communicate with fixed-orbit satellites [3], which needs to cover two frequency bands, namely, the uplink frequency band is 13.75- 14.5GHz, and the downlink frequency band is10.95-1.75 GHz and/. In order to ensure the smooth communication between the satellite and the ground mobile equipment, the mobile communication antenna should point to the communication satellite in real time, and at the same time, in order to avoid the interference to the adjacent satellites when the antenna is transmitted, the tracking error of the moving mobile equipment should be less than 0. 1, and the feed should also be tracked in rotation, and the polarization isolation between transmission and reception should be greater than 30dB[4][5]. Many enterprises at home and abroad have introduced mobile communication antenna products, such as the multi-chip antenna of RaySat Company in Israel and the IMVS450M product of TracStar Company in the United States [6]. In order to meet the requirements of high-precision real-time tracking and alignment of satellites by antennas, all the above mobile communication antennas include automatic tracking systems. In the initial static state, GPS, theodolite and strapdown inertial navigation system are used to measure the heading angle, latitude and longitude of the carrier position and the initial angle relative to the horizontal plane, and then the antenna elevation angle based on the horizontal plane is automatically determined according to its attitude, geographical position and satellite longitude, and the azimuth angle is rotated on the premise of keeping the elevation angle and the horizontal plane unchanged, so that the satellite can automatically aim at the maximum signal. During the movement of the carrier, the change of the attitude of the carrier is measured and converted into the error angle of the antenna through mathematical operation. The azimuth angle, pitch angle and polarization angle of the antenna are adjusted through the servo mechanism to ensure that the antenna is within the specified range during the change of the carrier, so that the satellite transmitting antenna can track the geosynchronous satellite in real time during the movement of the carrier. High-precision servo system has always been the key part of traditional mobile communication antenna system. Usually, the high-precision servo system has a high cost because of the large aperture (generally about 0.8~ 1.2m) and heavy weight of the moving antenna. At present, the cost of high-precision servo system applied to mobile communication antenna is tens of thousands or even hundreds of thousands, accounting for a large part of the cost of the whole mobile communication antenna system, which limits the wide application of mobile communication satellite antenna in civil fields [5].
Two-wire polarized antenna feed array
In order to overcome the shortcomings of the existing antenna tracking servo system in mobile communication, such as high precision and high cost, a dual-line polarized antenna feed array is developed, which can be applied to single reflection or Cassegrain satellite communication antennas, and the electromechanical fusion tracking of the antenna system is realized by combining the back-end digital beamforming (DBF) technology. Finally, through the combination of "large-angle low-precision mechanical tracking" and "small-angle multi-channel DBF precise tracking", the high-precision tracking and alignment of the antenna system to the satellite is realized, and the accuracy requirements for the servo system are reduced, thus reducing the cost of the servo system. This feed array is a central symmetric structure, and the center of the array is located at the focus of the single reflection or Cassegrain antenna. When feeding different elements in the array, the antenna will radiate high gain beams in different directions. At this time, combined with the high-precision DBF technology at the back end, high-precision beam pointing control in a small angle range can be realized. The feed array takes the form of Fabry-Perot antenna based on microstrip printed circuit board. The array consists of three layers, in which the bottom layer is a microstrip reflector with a metal floor, the middle layer is an antenna structure in microstrip form, and the top layer is a pure dielectric plate that can enhance directivity.
2. 1 bottom structure
The bottom layer of the feed array is a dielectric plate with copper attached to one side and eight feed holes. SSMA and hollow copper column are welded on the bottom dielectric plate through feed holes, and the feed port of the transmitting antenna and the feed port of the receiving antenna have four feed holes respectively. Fig. 2 is a schematic diagram of the bottom circuit board structure.
2.2 Top structure
The top dielectric plate is a dielectric plate that completely etches away the copper-clad laminate that constitutes the Fabry-Perot superstructure. Fig. 3 is a schematic diagram of the top circuit board structure.
2.3 Intermediate layer structure
The transmitting antenna, the receiving antenna and their attached feeders are etched on both sides of the middle layer circuit board, wherein, in order to facilitate welding, the pads are all on one side. In order to isolate the influence of surface waves on the antenna pattern, the antenna array is divided by grid-like metal strips, and there are metal strips on both sides of the circuit board, which are connected with each other through metallized through holes. Fig. 4 is a schematic structural diagram of an intermediate layer circuit board. The microstrip array unit on the middle layer circuit board adopts a pair of crossed metal dipoles to realize the receiving/transmitting function respectively. Two metal dipoles are printed on the front and back of the microstrip dielectric plate in the middle layer, which work in the receiving/transmitting (downlink/uplink) frequency band respectively, and the cross dipole structure can correspond to two orthogonal linear polarizations required for receiving/transmitting. The array unit is fed by coaxial bottom feed, in which the two arms of the dipole are respectively connected with the inner core and the outer wall of the coaxial interface through a section of printed thin wire. The thin wire is used here to reduce the influence of the feed structure on the isolation between the receiver and the transmitter. In order to further reduce the influence of the feed structure on the isolation of the transceiver, two dipole structures in the same position are connected by a piece of printed thin wire, and the high isolation of the transceiver is realized by appropriate cancellation means according to its length, thickness and other parameters. By introducing a circle of dense metallized through-hole structures around the array units and designing a metal additional structure on the circuit board to isolate the surface waves in the medium, the mutual coupling between the array units is reduced.
2.4 Assembly of Feed Array
The three-layer circuit board of the feed array is fixed by several nylon studs. Fig. 5 is a schematic diagram of three-dimensional decomposition and overall assembly of the feed array. In the feed array structure, the working frequency of the antenna can be adjusted by adjusting the arm length of the metal dipole. By adjusting the distance between the top dielectric substrate and the intermediate circuit board, the radiation gain can be easily adjusted to meet the requirements of different reflector sizes and focal lengths.
3 simulation and measurement results
Ports 1, 3, 5 and 7 of the feed array are receiving ports, and ports 2, 4, 6 and 8 are transmitting ports. Fig. 6 shows the simulation and test results of the return loss of the feed array. As can be seen from fig. 6, the echoes of the receiving port and the transmitting port are less than-12.25- 12.75GHz and 13.75- 14.5GHz, respectively, thus achieving good matching. Fig. 7 is the simulated and measured receiving direction diagram of the feed array at the operating frequency of 12.5GHz. As can be seen from fig. 7, when operating at 12.5GHz, the gain of the zenith antenna is 15dB, and the sidelobe is lower than the main lobe by 10dB (simulation)//kloc-0. Fig. 8 is the simulated and measured emission pattern of the feed array at the operating frequency of 14. 1GHz. As can be seen from Figure 8, when operating at 14. 1GHz, the antenna zenith gain is 15dB, and the sidelobe is lower than the main lobe 1 1dB (simulation)/10dB (actual measurement).
4 conclusion
The feed array adopts microstrip printed circuit board structure, which is simple and compact, mature in technology, simple in processing and low in cost, and suitable for mass production. Compared with the traditional feed structure such as waveguide port and waveguide horn, it can realize multiple units and multiple receiving/transmitting channels in a smaller area, which is beneficial to realize more accurate beam pointing control. At the same time, the cancellation technology adopted by the feed array can achieve 30dB isolation between the receiving/transmitting channels in the same position at the antenna structure end, thus reducing the pressure on the back-end devices. From the practical application point of view, the antenna array cooperates with the main reflector to realize the small-angle, high-speed and high-precision electron beam scanning and tracking of Ku-band communication satellite through the mobile communication satellite antenna. Using this technology, the requirements of the antenna for the accuracy and dynamic response speed of the servo system are greatly reduced, and the cost of the servo system is reduced by an order of magnitude, which is helpful to promote the large-scale application of satellite antenna in the integrated communication between heaven and earth.
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