I. Overview
In recent years, the rapid development of China's communication industry has led to continuous improvements in microwave relay communication antennas. The transmission network function of satellite communication systems is mainly achieved through fiber optics, ground-based microwave, and satellite communication methods. With the emergence of new technologies and higher transmission capacities in microwave transmission systems, the next generation of Synchronous Digital Hierarchy (SDH) microwave communication systems has gradually replaced traditional Pulse Code Modulation (PDH) microwave systems. To adapt to the frequency reuse developments in modern SDH microwave communications, ultra-high performance microwave antennas are essential. These antennas must exhibit a high front-to-back ratio (F/D), excellent cross-polarization discrimination (XPD), and a very low voltage standing wave ratio (VSWR). As a result, an ultra-high performance microwave antenna system typically features a VSWR better than 1.06 or a reflection loss greater than 30.7 dB, along with a cross-polarization discrimination of over 38 dB.
II. System Composition
The feed system of an ultra-high performance microwave antenna consists of a horn, an orthogonal device, a twisted waveguide, a curved waveguide, and a waveguide feed line. Among these components, the horn and the orthogonal device are particularly critical.
Horn
There are various types of horns suitable for feeding ultra-high performance microwave antennas [1][2]. In this design, a planar corrugated horn with three chokes is used. This type of horn provides a rotationally symmetric radiation pattern, low side lobes, minimal cross-polarization, and a stable phase center. The structure of the horn is shown in Figure 1. It comprises a circular waveguide and three concentric rings. To enhance the standing wave characteristics of the horn, a matching block is symmetrically placed near the aperture. Additionally, to prevent foreign objects from entering the horn, the aperture is usually covered with a dielectric film. However, this can degrade the standing wave performance. To mitigate this, high-frequency simulation software is employed to fine-tune the position and thickness of the dielectric material, resulting in an optimized horn with a VSWR better than 1.05.
Figure 1: Horn Structure
2. Orthogonal Device
In modern antenna feed systems, frequency reuse technology is one of the most cost-effective ways to utilize frequency resources, enabling increased communication capacity. Orthogonal polarization frequency multiplexing is achieved using dual-polarized antennas, where two independent signals are transmitted on the same frequency by exploiting the orthogonality of polarization. There are two main techniques: two-line polarization and double-circle polarization [3]. The synthesis and separation of orthogonal polarizations occur within the feed system. Two-line polarization frequency multiplexing is implemented using an orthogonal mode coupler (OMT), also known as a polarization separator.
Orthogonal devices are common microwave components, but there is limited literature on their design methods [4]. A standard orthogonal device, although having only three physical ports, is electrically a four-port device. This is because the common port contains two orthogonal main modes (such as TE11/TE*11 in a circular waveguide or TE10/TE01 in a square waveguide), while the other two ports operate in their fundamental modes (e.g., TE10 in a rectangular waveguide or TEM in a coaxial line).
The primary function of the orthogonal device is to separate the two independent signals from the orthogonal main modes in the common port and route them to the fundamental mode of the respective signal ports. This ensures that all electrical ports are matched and that there is high cross-polarization discrimination between the two signals. Therefore, the ideal scattering matrix of the orthogonal device is as follows:
Ports 1 and 2 represent the main modes at the physical common port, while ports 3 and 4 are the base mode interfaces. The phase shift hysteresis is φ1 and φ2, respectively.
There are various forms of orthogonal devices, each with slightly different performance characteristics. Typically, they are constructed using circular or square waveguides, and a four-ridge waveguide may be used for wideband applications. The coupling hole, which connects to the branch waveguide (also known as the side arm), is located in the tapered section and is often short-circuited by a diaphragm or isolation barrier. The orthogonal device described in this paper meets the requirements for high performance and low cost in a narrow operating frequency band (10% to 20%). For high performance, it requires low reflection loss (VSWR) and high isolation (port and polarization isolation). For low cost, it needs a simple structure and easy fabrication.
To ensure the performance of the orthogonal device, its minimum operating frequency should exceed 1.1 times the cutoff frequency (fmin > 1.1fc). As a result, the maximum operating bandwidth of a circular waveguide orthogonal device is approximately 17%, and that of a square waveguide orthogonal device is about 25%. Within this bandwidth, the isolation performance is primarily affected by structural size and processing symmetry. If the operating frequency exceeds the maximum, the isolation performance will deteriorate due to the influence of higher-order modes.
The design of the orthogonal device focuses on suppressing higher-order modes, simplifying the structure, ensuring symmetry, and achieving port matching with fewer components. The key to the design lies in the structure of the square or circular waveguide branch coupler and the matching of the two fundamental mode ports.
The orthogonal device we designed is shown in Figure 2. During the entire design process, the size of the square waveguide is first determined, followed by the design of the rectangular waveguide step transition for the through port. Finally, the position of the side arm coupling hole is decided. It is advisable to select the size and position of the coupling hole to minimize interference with the straight arm and ensure efficient coupling of the polarization signal. Due to the large number of variables in the side arm coupling structure, the performance of the side arm is crucial.
Figure 2: C-band Orthogonal Device
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