Design of X-Band Monopulse Feed Network Design of X-Band Monopulse Feed Network

Feed is an important component used in wireless communication systems, and it plays a key role in wireless transmission. This article will introduce the basic concepts, working principles, and design details of the x-band feed.

The x-band feed refers to a feed device that works in the x-band frequency range.

The x-band is an important frequency band in wireless communication systems, and its frequency range is generally 8GHz to 12GHz. The x-band feed is usually composed of components such as amplifiers, filters, and mixers, which can provide stable and efficient signal transmission.

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The working principle of the x-band feed mainly includes signal generation, signal amplification, and signal filtering.

First, the signal generator generates a reference signal, and then the signal is amplified by the amplifier to increase the power of the signal.

Next, the signal is filtered by the filter to remove unnecessary frequency components, making the signal purer.

Finally, after processing by the mixer, the signal is converted to the x-band frequency range and output to the wireless communication system.

The x-band frequency is higher and the transmission bandwidth is larger, which can support higher-speed data transmission. Secondly, the propagation loss of the x-band signal in the atmosphere is smaller and the transmission distance is longer. However, the x-band will also face some challenges. Due to the high frequency of X-band, signal transmission is greatly interfered by factors such as weather and buildings, and effective interference prevention measures are required. Therefore, the design and manufacturing technology of X-band feed also puts higher requirements on engineers.

The main technical parametersof X-band monopulse feed network

Working frequency band: Transmit: 7.1~7.3G Receive: 7.7~7.9G

Standing wave: 1.5

Axis ratio: 1.5db

Differential beam null depth: -35dB

Spurious mode suppression: -40dB

Port isolation: -35dB

Interface mode: The main channel is BJ100 waveguide, dual-channel, corresponding to left and right circular polarization

Solution Designof X-band monopulse feed network

2.1 Background

Precision tracking antennas are widely used in communication equipment such as measurement and control radars, ground reception of satellite signals, and vehicle-mounted mobile communications. They meet technical requirements such as long target tracking distance and high tracking angle indication accuracy. Specifically for antenna design, the tracking antenna is required to have the dual advantages of high sum beam efficiency and high difference beam sensitivity. However, for traditional precision tracking antennas, these two performances are contradictory (called the sum-difference contradiction) and cannot be optimized at the same time. In order to improve or solve the sum-difference contradiction, improve the feed pattern, improve antenna efficiency, and increase the working bandwidth, the precision tracking antenna introduces differential mode tracking technology, and its feed source uses a corrugated horn. The key to the design of differential mode tracking antenna technology lies in the differential beam forming and operation network, and its key component is the differential mode coupler.

The single pulse feed technology is one of the core technologies of the single pulse antenna. Commonly used single pulse feeds include multi-horn feeds, single-port multi-mode horn feeds, and multi-horn multi-mode feeds. Multi-mode feeds mostly adopt circular waveguide feeding, generally with the lowest TE11 as the main mode transmission to form and beam radiation, while the differential mode is realized by TM01 mode or TE21 mode. The TM01 mode is the first high-order mode in the circular waveguide. This mode is not easy to couple with other high-order modes, and its coupling structure is simple and easy to implement, but it is only suitable for circularly polarized wave tracking. In order to meet the requirements of linear polarization and circular polarization tracking at the same time, the TE21 mode is often used to realize the differential beam. In current engineering applications, multi-mode tracking mostly adopts a single-channel single pulse tracking system with circular waveguide TE11 main mode transmission and TE21 mode differential mode transmission.

2.2 Theoretical basis

TE21 differential mode tracking is a high-precision zero-value automatic tracking system, which uses the characteristics of the antenna radiation pattern of the differential mode electromagnetic field to be zero in the axial direction and to have gain when deviating from the axial direction to realize the source angle measurement and automatic tracking. Compared with the multi-horn (i.e., multi-antenna) single pulse system, differential mode tracking has the advantages of high precision, small antenna and difference contradiction, and small sum channel signal loss. Differential mode tracking must use the sum channel mode (for circular waveguide, the sum channel mode is the TE11 mode) as the reference signal, and the differential mode can choose TM01, TE01, TE21, etc. The present invention uses a single pulse system with TE11 as the sum signal mode and TE21 as the difference signal mode, which can realize the tracking of any polarization source. Compared with the multi-channel single pulse tracking receiver, the single channel has the advantages of small size, light weight, low power consumption, good maintainability, good adaptability, etc. It can be adapted to various forms of single pulse antenna feed sources.

This project involves four parts: circular waveguide corrugated horn, TE21 mode coupler, circular polarization converter and differential mode active and differential operation network. The circular waveguide corrugated horn is used to radiate circular polarized waves, and has the advantages of rotationally symmetrical radiation pattern, almost overlapping phase centers of each radiation surface, low cross-polarization level and sidelobe level. The function of TE21 mode coupler is to fully couple and receive the TE21 high-order mode generated by the deviation of the antenna axis from the signal source, calculate the signal source deviation angle according to its amplitude, and then drive the servo circuit to realize antenna tracking and alignment. The circular polarization converter realizes the conversion of rectangular waveguide TE10 mode to circular waveguide TE11 mode, realizes equal amplitude but orthogonal phase TE10 and TE01 modes in the step waveguide transformation section, and realizes circular polarized waves of antenna feed source. The differential mode active and differential operation network is realized by the front low noise amplifier, 180 degree and 90 degree bridge network, and superimposes 8 differential mode signals to one output.

2.3 Design of corrugated feed horn

The corrugated horn has a rotationally symmetric radiation pattern, and each radiating surface has an almost overlapping phase center. The corrugated horn has a lower cross-polarization level and sidelobe level. The corrugated groove depth of the corrugated horn is about λ/4, which can effectively suppress the longitudinal current in the corrugated horn, and the transverse components of the electric field and magnetic field are zero, so that the boundary conditions of the transverse electric wave and the transverse magnetic wave are constant. In addition, due to the presence of the corrugated groove, the TE11 mode in the horn is transformed into the HE11 mode, in which the TE component and the TM component have the same cutoff frequency and phase velocity, and are independent of frequency. Therefore, the corrugated horn has a wider working bandwidth than the multi-mode horn.

10: Corrugated horn

11: Circular waveguide horn radiation surface

12: Ripple Transformation Structure

13: Circular waveguide feed port

Figure 1 Corrugated horn structure

2.4 TE21 mode coupler

The basic function of the TE21 mode coupler is to use the TE21 mode to measure the elevation difference and azimuth difference signals. The output (difference signal) of the TE21 mode and the sum signal together form a single-channel single-pulse tracking system. When the incoming electromagnetic wave is in the azimuth direction, a path difference is generated at the near and far ends of the circular waveguide (as shown in Figure 2, the directions of the electric fields excited at the near and far ends of the circular waveguide are opposite, that is, the phase difference is 180 degrees). When the electric field is perpendicular to the incident azimuth, the TE21 mode and TE01 mode will be excited in the circular waveguide. This project performs full coupling extraction on TE21. The following analyzes the response of the TE21 mode of the circular waveguide under the conditions of linear polarization and circular polarization of the incident electromagnetic field.

Figure 2 Schematic diagram of the internal field of a circular waveguide excited by far-field azimuth incoming waves

The four waveguides distributed horizontally and vertically cannot couple the TE21 mode from the 45-degree polarized wave incident at 45 degrees. Therefore, an 8-arm waveguide with a 45-degree interval is required to fully couple the TE21 mode. The 8-arm waveguide can completely couple two orthogonal degenerate TE21 modes. The phase of the TE21 mode relative to the main mode TE11 of the circular waveguide reflects the azimuth of the incident wave. The incident wave elevation angle determines the response amplitude of the TE21 mode. When the elevation angle is θ, the path difference is, as shown in Figure 3, where d is the diameter of the circular waveguide. The larger the path difference, the stronger the excited TE21 response. The specific relationship is relatively complex, but it is a monotonic function in the area near θ=90°, and the TE21 response is a minimum value at θ=90° (corresponding to the zero depth of the TE21 mode). The elevation angle can be determined based on the TE21 response amplitude. When the TE21 mode response is a minimum value, it can be determined that the antenna elevation angle has been aligned with the signal source.

Figure 3 Elevation angle path difference

24: TE21 mode coupling hole;

25: Circular waveguide;

26: rectangular waveguide;

27: outer wall of circular waveguide;

28: Rectangular waveguide cover fastening screws;

29: External fastening screws for the eight-arm waveguide coupling structure;

Figure 4 Schematic diagram of eight-arm waveguide coupling structure

2.5 Circular Polarization Converter

The function of the circular polarization converter is to convert the TE10 mode linear polarization wave in the rectangular waveguide into a right-handed or left-handed circularly polarized TE11 mode in the circular waveguide. The present invention uses a step impedance converter to first convert the TE10 mode in the rectangular waveguide into a square waveguide merging the TE10 mode and the TE01 mode, and uses a suitable impedance step conversion to make the TE10 mode phase lag behind or lead the TE01 mode by 90 degrees, thereby realizing the right-handed or left-handed circularly polarized TE mode electromagnetic wave in the square waveguide. The circular polarization converter structure of the present invention is processed and assembled in two parts, as shown in Figures 5 and 6, and the overall installation structure is shown in Figure 7.

31: Converter structure 1;

310: rectangular waveguide input port;

311: Step transformation structure;

312: Square waveguide to circular waveguide transformation structure;

313: Fastening screw hole;

314: Fastening screw hole for connecting to TE21 coupler;

315: TE21 module output SMA fastening screw hole;

316: Flange surface for connection with TE21 coupler;

317: Watertight groove;

318: Rectangular waveguide flange fastening screw hole;

319: Rectangular waveguide butt flange surface;

Figure 5 Circular polarization converter structure 1

32: Converter structure 2;

320: short-circuited rectangular waveguide cavity;

322: Square waveguide to circular waveguide transformation structure;

323: Transformer structure fastening screw hole;

Figure 6 Circular polarization converter structure 2

30: Circular polarization converter structure;

324: Circular waveguide surface;

Figure 7 Circular polarization converter structure

We believe that you will learn something useful from above brief introduction to Design of X-Band Monopulse Feed Network. If you have any related questions about design of feed network, please feel free to consult us. Antesky is professional antenna suppliers from China for years.please contact by sales@antesky.com or you can WhatsApp us directly. we will offer you high quality antenna with the best price for satellite communication antennas and accessories.