Design of Ultra-High Performance Microwave Antenna Feed System

This paper introduces the design method of the C-band ultra-high performance feed system used in the microwave relay antenna feed, and optimizes its design using high-frequency structure simulation software. The deviation method is used to determine the processing accuracy of some important and difficult-to-adjust dimensions. The calculated results are in good agreement with the measured results. In the frequency band of 4.4 to 5 GHz, the standing wave of the entire feed system is better than 1.05, and the cross-polarization discrimination rate is better than -40dB.
Keywords: ultra-high performance feed system high-frequency structure simulation software

1. Overview

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, aerial 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 development of frequency reuse in the emerging SDH microwave communication, we need to develop ultra-high performance microwave antennas. It should have a very high front-to-back ratio (F/D), a very high cross-polarization discrimination (XPD) and an extremely 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 a high cross-polarization discrimination (greater than 38dB).

2. System composition

The feed system of the ultra-high performance microwave antenna is composed of a horn, an orthogonalizer, a twisted waveguide, a curved waveguide and a waveguide feed line. The horn and the orthogonalizer are the key components.
1. Horn
There are many types of horns suitable for the feed of ultra-high performance microwave antennas [1] [2]. This feed source adopts a planar corrugated horn with three choke slots. This planar corrugated horn has a rotationally symmetrical radiation pattern, low side lobes, low cross polarization and a stable phase center. The structure of the horn is shown in Figure 1. It consists of a circular waveguide and three concentric rings. In order to improve the standing wave characteristics of the horn, we place the adjustment blocks symmetrically near the horn mouth. In order to prevent foreign objects from entering the horn, the horn mouth needs to be sealed. Usually, a dielectric film is added to the horn mouth. 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 dielectric to improve the standing wave characteristics. The standing wave of the optimized horn is better than 1.05.

Figure 1 Speaker structure

2. Orthogonalizer
In modern antenna and feeder systems, frequency reuse technology is one of the most economical methods to utilize frequency resources and can achieve the purpose of expanding communication capacity. Orthogonal polarization frequency reuse technology is implemented using dual-polarization antennas, that is, at the same frequency, two independent signals are transmitted using the orthogonal characteristics of polarization. There are two types of orthogonal polarization frequency reuse technology, namely dual-linear polarization and dual-circular polarization [3]. The synthesis and separation of orthogonal polarizations are implemented in the feeder system. Dual-linear polarization frequency reuse is accomplished using an orthogonal mode coupler (OMT), also known as a polarization separator (orthogonalizer for short).
Orthogonals are commonly used microwave components, but there are few literatures introducing their design methods [4]. Although a common orthogonalizer (as shown in Figure 2) only appears as three physical ports, it is a four-port device electrically. This is because there are two orthogonal main modes in the common port (TE11/TE*11 mode in circular waveguide or TE10/TE01 mode in square waveguide) matching the respective fundamental modes in the other two ports (TE10 mode in rectangular waveguide or TEM mode in coaxial line).
The function of the orthogonal filter is to separate the independent signals of the two orthogonal main modes in the common port and pass them to the fundamental mode of a single signal port, so that all electrical ports are matched and there is high cross-polarization discrimination between the two independent signals. Therefore, the scattering matrix of the ideal orthogonal filter is

Here, ports 1 and 2 represent the main mode located at the physical common port, and ports 3 and 4 are fundamental mode interfaces, for example, direct connections are provided between port 1 and port 3 and between port 2 and port 4, respectively. Their phase shift lags are φ1 and φ2, respectively.
There are many forms of orthogonal devices, and their performances are slightly different. Generally, the main waveguides are in the form of circular waveguides and square waveguides. Quad-ridge waveguides can also be used in wide-band applications. The coupling holes that couple with the branch waveguides (also called side arms) are located in the tapered (gradient or step) part, and there are also short-circuit couplings with diaphragms or isolation barriers. The orthogonal device introduced in this article meets the requirements of high performance and low cost within a narrow working frequency band (10% to 20%). For high performance, it requires 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 orthogonal device, its minimum operating frequency should meet fmin＞1.1fc. In this way, the maximum working bandwidth of the circular waveguide orthogonal is about 17%, and the maximum working bandwidth of the square waveguide orthogonal is about 25%. Within such a bandwidth, the isolation performance of the orthogonal is only affected by the structural size and processing symmetry. If it is greater than the highest operating frequency, the isolation performance of the orthogonal will deteriorate due to the influence of high-order modes.
The design principle of the orthogonal 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 the design of the orthogonal is the structure of the square or circular waveguide branch coupler and the matching part of the two fundamental mode ports. The orthogonal we designed is in the form shown in Figure 2. In the entire design process, the size of the square waveguide is first determined, and then the square waveguide step transition of the straight-through port is designed. Finally, the position of the side arm coupling hole is determined. The size and position of the coupling hole should be selected to minimize the impact on the straight arm and can couple the polarization signal well. Since there are many variables in the side arm coupling structure, which has a great impact on the performance, it is very necessary to optimize the side arm size.

Figure 2 C-band orthogonalizer

For microwave components, it is difficult to obtain their characteristics by solving the classical method of Maxwell's equations. The emergence of high-speed and large-capacity computers has promoted the development of various numerical analysis methods. In the field of numerical calculation of electromagnetic field problems, there are many methods, 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 in dealing with various electromagnetic field problems, but they are all limited. Relatively speaking, the application of finite element method is relatively mature and can deal with more types of electromagnetic field problems. Of course, the requirements for computer resources are also higher. HPHFSS, a high-frequency structure simulation software based on finite element method, provides an effective means to solve the analysis method of microwave components. The
design optimization process using software is actually a simulation process of processing and debugging. The dimensions determined by experimental methods in the past can be obtained by computer analysis. The calculation amount of side arm optimization is large. Since the side arm size has little effect on the performance of the through-port and the difficulty of side arm matching is relatively large, 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 of the optimized curved waveguide is better than 1.02. The standing wave of the optimized twist waveguide is better than 1.04.
The stability of the performance of microwave components is another important design goal. Generally speaking, for non-resonant structure microwave components, the effect of size on performance is gentle (non-dramatic changes). The method of perturbing the size of the structure can be used to verify the calculation results and determine the manufacturing tolerance. In particular, it is necessary to determine the dimensional tolerance that has a great impact on performance, which can provide a scientific basis for the reasonable allocation of tolerances and the reduction of manufacturing costs.
3. Optimization design method of feed system
The performance optimization of the feed system is a very complex problem. 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, the connection relationship (interface position) of each microwave component is optimized to obtain better system performance. For example, the maximum return loss of the speaker is -34dB, and the maximum return loss of the orthogonal amplifier is -32dB. After optimizing the connection size between the speaker and the orthogonal amplifier, the maximum return loss of the orthogonal amplifier and the speaker is -32.5dB.

3. Calculation and Measured Performance

The VSWR and directional diagram results after the horn optimization are shown in Figure 3, the VSWR results after the square waveguide orthogonal device optimization are shown in Figure 4, and the VSWR calculated after adding perturbations (size plus tolerance) to the main structural dimensions in the orthogonal device is shown in Figure 5. From the simulation results, the tolerance requirement of the main structural dimensions in the orthogonal device is appropriate at +0.2% to +0.4%. The VSWR results of the entire feed system are shown in Figure 6, and its cross-polarization discrimination rate is shown in Figure 7.

Figure 3 VSWR and directivity diagram of the speaker after optimization

Figure 4 VSWR after optimization of square waveguide orthogonal filter

Figure 5. VSWR of the main structural dimensions of the orthogonal filter after adding perturbation.

Figure 6 VSWR of the feed system

Figure 7 Cross-polarization discrimination of the feed system

IV. Conclusion

This paper introduces the design method of the feed system for C-band ultra-high performance microwave antennas. Calculation and measurement results are given, and a method for determining the manufacturing tolerance of microwave components using high-frequency structure simulation software is proposed. The standing wave of the entire system is better than 1.05, and the cross-polarization isolation is better than 40dB. The feed system has been well applied to 3.2m microwave relay antennas.