Design of ultra-high performance microwave antenna feed system
In recent years, with the rapid development of my country's communication industry, microwave relay communication antennas have also been continuously developed and improved. The transmission network function of the satellite communication system is mainly completed through optical fiber, ground microwave, air satellite and other communication methods. From the perspective of the new technology and transmission capacity adopted by the microwave transmission system, the new generation of synchronous digital series SDH microwave communication system has replaced the traditional PDH microwave communication. In order to adapt to the emerging development of frequency reuse in SDH microwave communications, we need to develop ultra-high performance microwave antennas. It should have a high front-to-back ratio (F/D), a high cross-polarization discrimination rate (XPD) and a very low voltage standing wave ratio (VSWR). Therefore, the ultra-high performance microwave antenna system has a low voltage standing wave ratio (VSWR better than 1.06 or reflection loss greater than 30.7dB) and high cross-polarization discrimination rate (greater than 38dB).
2. System composition
The feed system of the ultra-high performance microwave antenna is composed of horn, orthogonalizer, twisted waveguide, bent waveguide and waveguide feeder. Among them, the horn and the quadrature device are the key components.
There are many kinds of horns suitable for the feed of ultra-high performance microwave antennas. This feed uses a flat corrugated horn with three choke grooves. This flat corrugated horn has a rotationally symmetrical pattern, low side lobes, low cross polarization and stable phase center. It is composed of a circular waveguide and three concentric rings. In order to improve the standing wave characteristics of the horn, we symmetrically place the adjustment block near the horn mouth. To prevent foreign objects from entering the horn, the horn must be sealed. Usually a dielectric film is added to the mouth of the horn. Generally, the dielectric film will deteriorate the standing wave of the horn. We use high-frequency simulation software to adjust the position and thickness of the medium to improve the standing wave characteristics. The optimized horn standing wave is better than 1.05.
In modern antenna feeder systems, frequency reuse technology is one of the most economical ways to utilize frequency resources, which can achieve the purpose of expanding communication capacity. Orthogonal polarization frequency multiplexing technology is realized by dual-polarization antennas, that is, on the same frequency, two independent signals are transmitted using polarization orthogonal characteristics. There are two orthogonal polarization frequency reuse technologies, namely dual-line polarization and dual-circular polarization. The combination and separation of orthogonal polarization is realized in the feed system. The dual-line polarization frequency multiplexing is accomplished by using an orthogonal mode coupler (OMT), also called a polarization splitter (quadrature for short).
The quadrature device is a commonly used microwave component, but there are few documents describing its design method. Although an ordinary quadrature device only presents three physical ports, it is an electrical four-port device. This is because there are two orthogonal main modes in the common port (TE11/TE*11 mode in a circular waveguide or TE10/TE01 mode in a square waveguide) and respective fundamental modes in the other two ports (TE10 mode in a rectangular waveguide). Or TEM mode in the coaxial line) matching.
The function of the quadrature device is to separate the independent signals of the two orthogonal main modes in the common port and transmit them to the fundamental mode of a single signal port, so that all electrical ports are matched and there is a high cross polarization between the two independent signals Discernment. Therefore, the scattering matrix of the ideal orthogonalizer is
Here, ports 1 and 2 represent the main mode located at the physical common port, and ports 3 and 4 are base mode interfaces. For example, direct connections are provided between port 1 and port 3 and port 2 and port 4, respectively. The phase shift lags are respectively φ1 and φ2.
There are many forms of quadrature, and their performance is slightly different. Generally, the main waveguide has a circular waveguide and a square waveguide, and a quad-ridge waveguide can also be used in broadband applications. The position of the coupling hole coupled with the branch waveguide (also called the side arm) is in the tapered (gradient or step) part, and it is also used for short-circuit coupling with a diaphragm or isolation barrier. The quadrature device introduced in this article meets the requirements of high performance and low cost in a narrow operating frequency band (10%~20%). For high performance, small reflection loss (VSWR) and high isolation (port isolation and polarization isolation) are required; low cost requires simple structure and convenient processing.
In order to ensure the performance of the quadrature device, its minimum operating frequency should meet fmin>1.1fc. Thus, the maximum working bandwidth of the circular waveguide orthogonalizer is about 17%, and the maximum working bandwidth of the square waveguide orthogonalizer is about 25%. In such a bandwidth, the isolation performance of the quadrature device is only affected by the structural size and processing symmetry. If it is greater than the maximum operating frequency, the isolation performance of the quadrature device will deteriorate due to the influence of higher-order modes. The design criterion of the quadrature device is to suppress the generation of high-order modes, simplify the structure, ensure the symmetry of the structure, and use fewer matching components to achieve the matching of each port.
The key to quadrature design is the structure of the square or circular waveguide branch coupler and the matching part of the two fundamental mode ports. In the whole design process, the size of the square waveguide is first determined, and then the step transition of the rectangular waveguide with the straight opening is designed. Finally, determine the position of the side arm coupling hole. The size and position of the coupling hole should be selected to minimize the impact on the straight arm and to couple the polarized signal well. Since there are many side-arm coupling structural variables, which have a great impact on performance, it is very necessary to optimize the side-arm size.
For microwave components, it is difficult to obtain their characteristics by solving the classical method of Maxwell equation. Due to the emergence of high-speed and large-capacity computers. Promoted the development of various numerical analysis methods. Various methods have emerged in the field of numerical calculation of electromagnetic field problems, such as finite time domain difference method (FDTD), mode matching method (MMT), transmission line matrix method (TLM) and finite element method (FEM). These methods are partially effective for dealing with various electromagnetic field problems, but all have limitations. Relatively speaking, the application of finite element method is relatively mature, and it can deal with many types of electromagnetic field problems. Of course, the requirements for computer resources are also higher. The high-frequency structure simulation software HPHFSS based on the finite element method provides an effective method for solving the analysis method of microwave components.
Using software to optimize the design process is actually a simulation process of processing and debugging, and the size determined by experimental methods in the past can be obtained by computer analysis. Side arm optimization requires a large amount of calculation. Since the size of the side arm has little effect on the performance of the through port and the matching of the side arm is more difficult, the matching effect on the through port can be reduced by selecting specific components. The model of the optimized side arm can use its symmetry to reduce the amount of calculation. The standing wave after optimization of the curved waveguide is better than 1.02. The standing wave after optimization of the twisted waveguide is better than 1.04.
The stability of microwave component performance is another important design goal. Generally, for microwave components with non-resonant structures, the size has a gentle (not drastic change) effect on performance. The method of perturbating the structure size can achieve the purpose of verifying calculation results and determining manufacturing tolerances. In particular, it is necessary to determine the dimensional tolerances that have a great impact on performance, which can provide a scientific basis for rationally distributing tolerances and reducing manufacturing costs.
3. Optimal design method of feed system
The performance optimization of the feed system is a very complex problem, and the size changes of each part will affect the performance. Due to the limitation of computer resources, it is difficult to optimize the design of the entire feed system. After optimizing the design of each microwave component, and then optimizing the connection relationship (interface position) of each microwave component, a better system can be obtained. performance. For example, the maximum return loss of the horn is -34dB, and the maximum return loss of the quadrature device is -32dB. After optimizing the connection size of the horn and the quadrature device, the maximum return loss after the combination of the quadrature device and the horn is- 32.5dB.
3. Calculated and measured performance
The optimized VSWR and pattern results of the horn, the optimized VSWR result of the square waveguide orthogonalizer, and the VSWR calculated after perturbing the main structure size in the orthogonalizer (size plus tolerance). From the simulation results, the tolerance requirements of the main structural dimensions in the quadrature device are appropriate at +0.2%~+0.4%. The VSWR result of the entire feed system, its cross-polarization discrimination rate.
This article introduces the design method of the C-band ultra-high performance microwave antenna feed system. The calculation and measurement results are given, and a method to determine the manufacturing tolerances of microwave components using high-frequency structure simulation software is proposed. The standing wave of the whole system is better than 1.05, and the cross-polarization isolation is better than 40dB. The feed system has been well applied to the 3.2m microwave relay antenna.