I. Overview
In recent years, my country's communication industry has developed, microwave relay communication antenna has also continuously developed and improved, and the transfer network function of the satellite communication system is mainly accomplished by communication methods such as fiber, ground microwave, air satellite. From the perspective of new technologies and transfer capacity employed by the microwave transfer system, the new generation of synchronous digital series SDH microwave communication systems replaces PDH microwave communication in the traditional sense. In order to adapt to the development of frequency reuse in the XDH microwave communication, we need to develop ultra-high performance microwave antennas. It should have a very high front and rear ratio (f / d), very high cross-polarization Identification (XPD) and extremely low voltage standby (VSWR). Therefore, the ultra-high performance microwave antenna system has a low voltage stationary wave ratio (VSWR is more than 1.06 or the reflection loss greater than 30.7dB) and a high cross-polarization identification (greater than 38dB).
Second, the system composition
The feed system of the ultra-high performance microwave antenna is composed of a speaker, an orthoenter, a twisted waveguide, a bending waveguide, and a waveguide feeder. Where a speaker and an orthogon are critical components.
Trumpet
Various types of fed sources suitable for ultra-high performance microwave antennas. This feed is a flat corrugated speaker with three chokes. This flat corrugated speaker has a rotational symmetrical pattern, a low sub-flap, a low cross-polarization, and a stable phase center. The structure of the speaker is shown in Figure 1. It consists of a circular waveguide and three concentric ring. In order to improve the standing wave characteristics of the speaker, we placed the adjustment blocks in the vicinity of the flange. In order to prevent foreign matter from entering the speaker, the flare is closed. Usually adding the medium film on the flatk, the general medium film will make the horn's standing wave, and we use high-frequency simulation software to adjust the position of the medium to the thickness, so that it has the characteristics of the standing wave. Optimized speaker standing is better than 1.05.
Figure 1 speaker structure
2. Optical
In modern natural feed systems, frequency multiplexing techniques are one of the most economical methods of frequency resources to achieve the purpose of expanding communication capacity. The orthogonal polarization frequency multiplexing technique is to be implemented with a bipolarization antenna, i.e., on the same frequency, using polarized orthogonal characteristics to transmit two separate signals. There are two types of orthogonal polarization frequency multiplexing techniques, namely double-wire polarization and double circulation. The synthesis and separation of orthogonal polarization is achieved in the feed system. Double-wire polarization frequency multiplexing is done with a orthogonal dielectric coupler (OMT) also known as the polarization separator (abbreviated album).
The orthogon is a commonly used microwave element, but it introduces less literature in its design method. Ordinary orthogon (as shown in Figure 2) although only three physical ports, it is four-port devices for electrical. This is due to two orthogonal main models (TE10 / TE01 model in round waveguides in circle waveguide) and other two ports (TE10 model of rectangular waveguides) in other two ports Or TEM mode in the same axis) matches.
The role of the orthogon is to isolate the independent signals of two orthogonal main models in the common port and pass them to the base mode of a single signal port, which matches all electrical ports and has high cross-polarization between the two independent signals. Identification. Therefore, the scattering matrix of the ideal orthogonal
Here, ports 1 and 2 represent the main mode, ports 3, and 4 located in the physical common port, for example, to provide direct connections between port 1 and port 3 and port 2 and port 4, respectively. It is φ1 and φ2 after its phase shif.
There are many forms in the form of orthogonality, and the performance is slightly different. The general main waveguide has a circular waveguide and square waveguide, and a quadruple waveguide can also be used when the wideband is applied. The position of the coupling hole coupled to the branch waveguide (also known as the side arm) is coupled with a diaphragm or an isolation gate to a diaphragm or an isolation gate. The orthogon described herein is a narrow operating frequency band (10% to 20%) to meet high performance and low cost requirements. For high performance, there is a small reflection loss (VSWR) and high isolation (port isolation and polarization isolation); low cost requires simple structure and convenient processing.
In order to ensure the performance of the orthogon, the minimum operating frequency should meet Fmin>1.1fc. Such a maximum working bandwidth of the round waveguide orthogonal is approximately 17%, and the maximum working bandwidth of square waveguide is approximately 25%. In such bandwidth, orthogonal isolation performance is only affected by structural dimensions and symmetry. If it is greater than the maximum operating frequency, the isolation performance of the orthogon will be deteriorated due to the impact of the high-time mode. The criteria for the design of the actress is to suppress the generation, simplification of the structure, to ensure the symmetry of the structure, and realize the matching of each port with less matching elements.
The key to the intersection of the orthogonal design is the structure of the square or the circular waveguide branch coupler and the matching portion of the two base ports. Our orthogon we design uses as shown in Figure 2. The size of the square waveguide is first determined during the entire design process, and then the square torque waveguide of the direct port is designed. Finally, the side arm coupling hole position is determined. Selecting the size and position of the coupling hole should be appropriately coupled to the influence of the straight arm as much as possible. Since the side arm coupling structure variables are large, the performance has a large impact, and the optimized side arm size is necessary.
Figure 2 C band orthogon
For microwave components, it is difficult to obtain its characteristics by solving the classical approach this MaxWell Equation. Due to the emergence of high speed large capacity computers. Promote the development of various numerical analysis methods. There are a variety of methods in the field of electromagnetic field problems, such as a limited time domain difference method (FDTD), a modulation method (MMT), a transmission line matrix method (TLM), and a finite element method (FEM). These parties have partially effective in handling various electromagnetic field problems, but they have restrictions. Relatively speaking, the finite element method is mature, and more types of electromagnetic fields can be handled, and of course the requirements for computer resources are higher. High-frequency structural simulation software based on finite element method provides an effective means to solve the analysis method of microwave components.
Using the software optimization design process is actually a simulation process of processing and commissioning, it can be obtained by computer analysis of the size determined by the experimental method. The amount of calculated side arm optimization is large. Since the side arm size affects the performance of the straight-through, the matching effect of the side arm matches is large, and the matching effect on the straight opening can select a specific component to achieve a reduction purpose. The model of the optimized side arm can reduce the amount of symmetry to reduce the amount of calculation, and the bending waveguide is excellent in the bending wave than 1.02. The stationary wave is better than 1.04 after the torsion waveguide optimization.
The stability of the performance of the microwave element is one of the other important goals of the design. Typically, for non-resonant structural microwave components, the size of the size is gentle (non-fiercely changed), and the method of detecting the test calculation can be achieved, and the purpose of the manufacturing tolerance can be determined. In particular, it is necessary to determine the size tolerance of performance, which can provide a scientific basis for reasonable distribution tolerances, reducing manufacturing costs.
3. Optimized design method for feed system
The performance optimization of the feed system is a very complex issue, and the dimensional changes in each part will affect performance. Due to the restriction of computer resources, it is difficult to optimize the entire feed system. After optimizing the various microwave components, the connection relationship (interface position) of each microwave element can be obtained, and a better system can be obtained. performance. For example, the maximum echo loss of the speaker is -34dB, the maximum echo loss of the orthogon is -32dB, and after the connection size of the horn and the orthogon, the maximum echo loss is the maximum echo loss after the orthogonal consumption 32.5dB.
Third, calculation and measurement performance
The VSWR and direction graph results after the horn optimized, as shown in Figure 3, the VSWR results after the square waveguide orthogonal executive are optimized, as shown in FIG. 4, the primary structure size is calculated after the main structure size (dimensional additional difference) The VSWR is shown in Figure 5. From the simulation results, the main structural dimensions in the orthogonal requirements are appropriate in +0.2% to +0.4%. The VSWR result of the entire feed system is shown in Figure 6, and its cross-polarization identification is shown in Figure 7.
Figure 3 VSWR and direction maps after the horn optimization
Figure 4 Fang waveguide orthogonal optimized VSWR
Figure 5 VSWR after the main structural size of the actress is added to the VSWR
Figure 6 VSWR of the feed system
Figure 7 Cross polarization Identification of Feed System
Fourth, theory
This paper introduces the design method of the feed system of the C-band super high performance microwave antenna. The calculation and measurement results are given, and the method of determining the manufacturing tolerance of the microwave element using high frequency structure simulation software is proposed. The entire system is more than 1.05, and the cross polarization is better than 40 dB. The feed system is well applied to a 3.2M microwave relay antenna.
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