A millimeter-wave reflection-free waveguide bandpass filter based on a balanced structure

Abstract

This invention relates to a millimeter-wave reflection-free waveguide bandpass filter based on a balanced structure, consisting of a pair of orthogonal couplers and a pair of symmetrically arranged waveguide bandpass filters. The isolation terminals of the couplers are connected to the load. Out-of-band reflected energy phases are superimposed at the isolation ports and absorbed by the load connected to the ports. The reflected energy phases are canceled out at the input and output ports, thus achieving excellent port-free reflection performance. This invention utilizes the high intrinsic quality factor, low loss, and high rectangular coefficient characteristics of waveguide resonators to solve the high dielectric loss problem of traditional microstrip and lumped element-based reflection-free bandpass filters, making high-frequency design easier and suitable for high-power applications.

Description

Technical Field

[0001] This invention relates to the field of wireless communication technology, and in particular to a millimeter-wave non-reflective waveguide bandpass filter based on a balanced structure. Background Technology

[0002] Bandpass filters, as key frequency-selective components, play a crucial role in radio frequency (RF) communication systems and modern communication systems. However, traditional reflective RF bandpass filters suffer from out-of-band signals that are reflected back to adjacent stages, affecting their operation and thus impacting the overall system stability. Simultaneously, with the continuous development of 5G communication and the strategic deployment of 6G communication, the number of users and various electronic devices is rapidly increasing, placing higher demands on RF circuits in terms of performance and concurrency, including low loss, miniaturization, and high stability. To mitigate the impact of out-of-band reflected signals, many scholars have focused on the research of reflection-free bandpass filters in recent years. Based on their construction methods, they can be broadly classified into two categories: First, reflection-free bandpass filters based on complementary duplex structures, where the transfer functions of the bandpass filtering section and the absorption bandstop section have a complementary matching relationship. This allows the out-of-band reflected energy of the bandpass filtering section to be dissipated by the lossy resistance of the absorption bandstop section, achieving a reflection-free port effect. Secondly, there are reflection-free bandpass filters based on balanced structures. The most common construction of a balanced structure consists of a pair of identical 3dB quadrature couplers and a pair of identical bandpass filters. The specific port absorption effect is achieved by the phase cancellation of reflected energy at the input port. In addition, there are reflection-free bandpass filter circuits based on balanced structures designed using power dividers. These methods all involve constructing the circuit so that the phase of the reflected signal is out of phase across the isolation resistor, thus allowing the reflected energy to be absorbed by the isolation resistor. The above-mentioned reflection-free bandpass filter methods often employ PCB circuit designs, such as microstrip and lumped elements. However, circuits based on PCB fabrication inevitably suffer from dielectric loss problems, which are fatal for high-power applications. Specifically, taking microstrip resonators as an example, their quality factor is limited, causing their in-band transmission loss to increase as the required bandwidth decreases and the frequency increases. Furthermore, microstrip designs also suffer from considerable conductor losses, especially noticeable in high-frequency applications. Specifically, as the frequency increases, the skin depth of the metal microstrip decreases, leading to an increase in circuit conductor losses. Meanwhile, the losses caused by parasitic capacitance and resistance in lumped element designs are not negligible. Especially as the frequency increases, the overload of lumped elements causes their surface temperature to rise continuously, potentially leading to circuit overload and affecting normal operation. Furthermore, although high frequency is the future trend in communication and high-power applications are numerous, research on millimeter-wave reflection-free bandpass filters for high-frequency and high-power applications is currently lacking. Summary of the Invention

[0003] The purpose of this invention is to overcome the shortcomings of the prior art by proposing a millimeter-wave reflection-free waveguide bandpass filter based on a balanced structure. Utilizing the high quality factor, low loss, high rectangular coefficient, and high power capacity characteristics of waveguide filters, this invention effectively solves the high-loss problem inherent in PCB-based reflection-free bandpass filter designs, meeting the requirements of high-power applications and facilitating high-frequency design. Furthermore, the filter circuit of this invention features a small cavity design and a folded circuit structure, effectively reducing the planar size of the circuit.

[0004] To achieve the objectives of this invention, a millimeter-wave reflection-free waveguide bandpass filter based on a balanced structure is provided, characterized by comprising: a first coupler having a first rectangular cavity and a second rectangular cavity arranged and coupled laterally; a second coupler having a third rectangular cavity and a fourth rectangular cavity arranged and coupled laterally; a first reflective filter having a first resonant cavity and a second resonant cavity arranged and coupled vertically; and a second reflective filter having a third resonant cavity and a fourth resonant cavity arranged and coupled vertically.

[0005] The first rectangular cavity is coupled to the first resonant cavity through a first input coupling window; the second rectangular cavity is coupled to the third resonant cavity through a second input coupling window; the third rectangular cavity is coupled to the second resonant cavity through a first output coupling window; and the fourth rectangular cavity is coupled to the fourth resonant cavity through a second output coupling window. The outer end face of the first rectangular cavity is the input port of the reflection-free waveguide bandpass filter; the outer end face of the second rectangular cavity is the isolation end of the first coupler; the outer end face of the third rectangular cavity is the isolation end of the second coupler; and the outer end face of the fourth rectangular cavity is the output port of the reflection-free waveguide bandpass filter. The isolation ends of the first and second couplers are grounded via the load.

[0006] In this invention, the input signal is input through the input port of the non-reflective waveguide bandpass filter, and split into two signals with equal amplitude and a phase difference of 90° via the first coupler. The two signals are filtered by the first reflective filter and the second reflective filter respectively and then transmitted to the second coupler. At the isolation end of the second coupler, phase cancellation of 0° and 180° is generated. At the output port of the non-reflective waveguide bandpass filter, two 90° phase superposition signals are generated and output. The out-of-band reflected signals generated by the first and second reflective filters have equal amplitudes and a 90° phase difference. After passing through the first coupler, the out-of-band reflected signals from the first and second reflective filters produce 180° phase cancellation at the input of the reflectionless waveguide bandpass filter, and 90° phase superposition at the isolation end of the first coupler, which is then absorbed by the load. After passing through the second coupler, the out-of-band reflected signals from the first and second reflective filters produce 180° phase cancellation at the output of the reflectionless waveguide bandpass filter, and 90° phase superposition at the isolation end of the second coupler, which is then absorbed by the load, thus enabling the reflectionless waveguide bandpass filter to obtain symmetrical reflectionless characteristics at its input and output ports.

[0007] Furthermore, the first coupler and the second coupler are waveguide 3dB orthogonal couplers, and the first reflective filter and the second reflective filter are waveguide bandpass filters.

[0008] As can be seen, this invention employs a construction method based on a balanced structure. Specifically, it consists of a pair of waveguide 3dB orthogonal couplers and a pair of symmetrically arranged waveguide bandpass filters. The isolation terminals of the couplers are connected to the load. The out-of-band reflected energy phase is superimposed at the isolation port and absorbed by the load connected to the port. The reflected energy phase is canceled out at the input and output ports, thus achieving a good port-free reflection effect.

[0009] Furthermore, the first rectangular cavity and the second rectangular cavity are coupled through a first wide-slit coupling window; the third rectangular cavity and the fourth rectangular cavity are coupled through a second wide-slit coupling window.

[0010] Furthermore, the first resonant cavity and the second resonant cavity are coupled through a first half-wavelength coupling window; the third resonant cavity and the fourth resonant cavity are coupled through a second half-wavelength coupling window. To achieve good filtering characteristics while reducing circuit size, this invention employs a resonant coupling window design, creating a small-cavity third-order waveguide bandpass filter circuit. This waveguide bandpass filter route consists of two identical rectangular waveguide resonant cavities and a centrally located half-wavelength resonant coupling window. The half-wavelength coupling window can be reused as a first-stage resonator to provide a transmission pole within the passband. Compared to traditional cascaded cavity filter designs, this eliminates the need for a first-stage resonant cavity and two coupling windows, resulting in a compact circuit size. In addition, the half-wavelength resonant coupling window can also form an additional cross-coupling path. Specifically, because the half-wavelength coupling window, acting as a first-stage resonator, makes the distance between two non-adjacent resonant cavities particularly close, it naturally introduces a cross-coupling path between the two non-adjacent resonant cavities, resulting in a transmission zero in the upper stopband. Traditional cavity filter design often employs cross-coupling to achieve out-of-band transmission zeros and improve out-of-band rejection or passband selectivity. However, existing designs frequently require additional circuitry to achieve this cross-coupling. The reusable half-wavelength coupling window of this invention naturally creates a cross-coupling path, eliminating the need for complex structural designs and avoiding an increase in overall circuit size, thus enabling the miniaturization of the filter. The circuit design parameters can be fitted using a Chebyshev low-pass prototype filter circuit to calculate the theoretically required coupling coefficient and quality factor. Subsequently, electromagnetic simulation software can be used to obtain the specific physical parameters of this reflective bandpass filter.

[0011] The waveguide 3dB orthogonal coupler employs a wide-slot coupling structure. Its amplitude and phase imbalances have a certain impact on the anti-reflection effect of this invention. Furthermore, the operating bandwidth of the anti-reflection waveguide bandpass filter described in this invention is determined by this waveguide coupler; that is, the out-of-band absorption bandwidth of the anti-reflection waveguide bandpass filter is equivalent to the operating bandwidth of the waveguide coupler minus the bandwidth of the reflective bandpass filter. During the design of the waveguide 3dB orthogonal coupler, the amplitude imbalance of the coupler and its operating bandwidth can be well balanced by adjusting the height of its side wide-slot coupling groove.

[0012] Finally, the aforementioned independently designed pair of small cavity waveguide bandpass filters are connected to a pair of waveguide 3dB quadrature couplers. The two output signals of the 3dB quadrature couplers have the characteristics of equal amplitude and opposite phase. The reflected signals are superimposed at the isolation port and then absorbed by the load of the connected port. The signals reflected back to the port exhibit mutual cancellation at 0° and 180°, thus achieving excellent reflection-free characteristics. Because this invention adopts a symmetrical circuit structure, the design exhibits symmetrical reflection-free characteristics, meaning that theoretically, there are no reflected signals output from either the input or output ports, thereby effectively ensuring system stability in practical applications.

[0013] The main innovations of this invention are as follows:

[0014] 1. By utilizing the high inherent quality factor, low loss, and high rectangular coefficient of waveguide resonators, the problem of high dielectric loss in traditional microstrip and reflectionless bandpass filters based on lumped elements is solved, overcoming the obstacle of difficulty in realizing high-power applications in the millimeter-wave band.

[0015] 2. By utilizing the characteristic of equal amplitude and phase inversion of the two outputs of the 3dB quadrature coupler, the reflection-free effect of the output port can be easily achieved. Compared with the reflection-free filter design based on planar PCB circuit, there is no need to add additional lossy component resistors, which can effectively avoid the additional loss problems caused by lossy components.

[0016] This invention offers the following advantages: It leverages the high intrinsic quality factor, low loss, and high rectangularity coefficient of waveguide resonators to solve the high dielectric loss problem inherent in traditional microstrip and lumped-element-based reflectionless bandpass filters. This makes high-frequency design easier to achieve and meets the requirements of high-power applications. The millimeter-wave reflectionless waveguide bandpass filter based on a balanced structure described in this invention achieves a center frequency of 27.9 GHz, a relative bandwidth of 6.7%, and a minimum in-band insertion loss of 0.21 dB. Compared to existing microstrip high-frequency reflectionless bandpass filter designs, this invention exhibits superior loss characteristics. Furthermore, this invention demonstrates good passband flatness and achieves excellent reflection-free performance across the entire operating frequency band of the coupler. Attached Figure Description

[0017] The invention will now be further described with reference to the accompanying drawings;

[0018] Figure 1 This is a perspective perspective view of a millimeter-wave non-reflective waveguide bandpass filter based on a balanced structure according to the present invention.

[0019] Figure 2 This is a side view of a millimeter-wave non-reflective waveguide bandpass filter based on a balanced structure according to the present invention.

[0020] Figure 3This is an exploded view of a millimeter-wave non-reflective waveguide bandpass filter based on a balanced structure according to the present invention.

[0021] Figure 4 This is a simulation result of the scattering parameters of a reflective waveguide bandpass filter in a millimeter-wave non-reflective waveguide bandpass filter based on a balanced structure, according to the present invention.

[0022] Figure 5 The figure shows the simulation results of the scattering parameters of the waveguide 3dB orthogonal coupler in a millimeter-wave non-reflective waveguide bandpass filter based on a balanced structure according to the present invention. (a) Amplitude; (b) Phase.

[0023] Figure 6 This is a simulation result of the scattering parameters of a millimeter-wave non-reflective waveguide bandpass filter based on a balanced structure according to the present invention. Detailed Implementation

[0024] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0025] To provide a clearer understanding of the technical features, objectives, and effects of the present invention, specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0026] See Figures 1 to 3 As shown, a millimeter-wave non-reflective waveguide bandpass filter based on a balanced structure according to an embodiment of the present invention includes: a first coupler 3, a second coupler 4, a first reflective filter 5, and a second reflective filter 6. The first coupler 3 and the second coupler 4 are waveguide 3dB orthogonal couplers. The first reflective filter 5 and the second reflective filter 6 are waveguide bandpass filters. Specifically, the first coupler 3 has a first rectangular cavity 3-1 and a second rectangular cavity 3-2 arranged laterally and coupled, with the first rectangular cavity 3-1 and the second rectangular cavity 3-2 coupled through a first wide-slit coupling window 3-3. The second coupler 4 has a third rectangular cavity 4-1 and a fourth rectangular cavity 4-2 arranged laterally and coupled, with the third rectangular cavity 4-1 and the fourth rectangular cavity 4-2 coupled through a second wide-slit coupling window 4-3. The first reflective filter 5 has a first resonant cavity 5-2 and a second resonant cavity 5-4 arranged vertically and coupled; the second reflective filter 6 has a third resonant cavity 6-2 and a fourth resonant cavity 6-4 arranged vertically and coupled. In this example, the first resonant cavity 5-2 and the second resonant cavity 5-4 are coupled through the first half-wavelength coupling window 5-3; the third resonant cavity 6-2 and the fourth resonant cavity 6-4 are coupled through the second half-wavelength coupling window 6-3.

[0027] like Figure 1 , Figure 3As shown, the first rectangular cavity 3-1 is coupled to the first resonant cavity 5-2 through the first input coupling window 5-1; the second rectangular cavity 3-2 is coupled to the third resonant cavity 6-2 through the second input coupling window 6-1; the third rectangular cavity 4-1 is coupled to the second resonant cavity 5-4 through the first output coupling window 5-5; and the fourth rectangular cavity 4-2 is coupled to the fourth resonant cavity 6-4 through the second output coupling window 6-5. The first input coupling window 5-1 is located on the inner end face of the first resonant cavity 5-2, and the first output coupling window 5-5 is located on the inner end face of the second resonant cavity 5-4; the second input coupling window 6-1 is located on the inner end face of the third resonant cavity 6-2, and the second output coupling window 6-5 is located on the inner end face of the fourth resonant cavity 6-4.

[0028] The outer end face of the first rectangular cavity 3-1 is the input terminal 1-1 of the first coupler 3, and also serves as the input port of the non-reflective waveguide bandpass filter. The inner end face of the first rectangular cavity 3-1 is the through end of the first coupler 3, which is the connection surface between the first coupler 3 and the first reflective filter 5. The outer end face of the second rectangular cavity 3-2 is the isolation terminal 1-2 of the first coupler 3, and the inner end face of the second rectangular cavity 3-2 is the coupling end of the first coupler 3, which is the connection surface between the first coupler 3 and the second reflective filter 6.

[0029] The outer end face of the third rectangular cavity 4-1 is the isolation terminal 2-2 of the second coupler 4, and the inner end face of the third rectangular cavity 4-1 is the coupling terminal of the second coupler 4, which is the connection surface between the second coupler 4 and the first reflective filter 5. The outer end face of the fourth rectangular cavity 4-2 is the input terminal 2-1 of the second coupler 4, which serves as the output port of the non-reflective waveguide bandpass filter. The inner end face of the fourth rectangular cavity 4-2 is the through terminal of the second coupler 4, which is the connection surface between the second coupler 4 and the second reflective filter 6. The isolation terminals of the first coupler 3 and the second coupler 4 are grounded via the load.

[0030] The first reflective filter 5 and the second reflective filter 6, respectively connected to the through end and the coupling end of the coupler, determine the filtering characteristics of the waveguide reflectionless bandpass filter of the present invention. The first coupler 3, the second coupler 4, and the bandwidth determine the operating bandwidth and absorption bandwidth of the waveguide reflectionless bandpass filter of the present invention. The input signal is input through the input port (input end 1-1 of the first coupler 3) of the reflectionless waveguide bandpass filter, and is split into two signals with equal amplitude and a phase difference of 90° by the first coupler 3. The two signals are filtered by the first reflective filter 5 and the second reflective filter 6, respectively, and then transmitted to the second coupler 4. At the isolation end of the second coupler 4, 0° and 180° phase cancellation is generated, and at the output port of the reflectionless waveguide bandpass filter, the two 90° phase superpositions are generated and output.

[0031] Similarly, the out-of-band reflected signals generated by the first reflective filter 5 and the second reflective filter 6 have equal amplitudes and a phase difference of 90°. After passing through the first coupler 3, the out-of-band reflected signals generated by the first reflective filter 5 and the second reflective filter 6 produce 180° phase cancellation at the input end of the non-reflective waveguide bandpass filter, and 90° phase superposition at the isolation end of the first coupler 3, which is absorbed by the load. After passing through the second coupler 4, the out-of-band reflected signals generated by the first reflective filter 5 and the second reflective filter 6 produce 180° phase cancellation at the output end of the non-reflective waveguide bandpass filter, and 90° phase superposition at the isolation end of the second coupler 4, which is absorbed by the load, so that the non-reflective waveguide bandpass filter obtains symmetrical non-reflective characteristics at the input and output ports.

[0032] This invention relates to a millimeter-wave non-reflective waveguide bandpass filter based on a balanced structure, employing a folded layout. Both the first reflective filter 5 and the second reflective filter 6 are third-order bandpass filters. Their three transmission poles within the passband are contributed by the first resonant cavity 5-2, the first half-wavelength coupling window 5-3, the second resonant cavity 5-4, the third resonant cavity 6-2, the second half-wavelength coupling window 6-3, and the fourth resonant cavity 6-4, respectively. Taking the first reflective filter 5 as an example, the energy passing through the first resonant cavity 5-2 is coupled through the first half-wavelength coupling window 5-3 and transferred to the second resonant cavity 5-4, and then to the coupling end of the second coupler 4. In summary, in the design of the reflective filter, the half-wavelength coupling window not only serves as a coupling window between resonant cavities to couple energy, but also acts as a first-stage resonator providing a transmission pole within the passband. Furthermore, it provides a cross-coupling path between two non-adjacent resonant cavities, thus generating additional high-frequency transmission zeros. When the design center frequency is determined, a resonant cavity of appropriate size can be selected based on the cutoff frequency of the waveguide. A half-wavelength coupling window is etched at the center of the rectangular resonant cavity (i.e., where the field strength of the TE101 mode, the dominant mode of the rectangular waveguide, is strongest), which controls the strength of the coupling energy between cavities. The length of the half-wavelength coupling window is half the wavelength at the center frequency, while its width and thickness can further control the coupling strength between it and adjacent resonant cavities, as well as the coupling magnitude between two non-adjacent resonant cavities. The input and output coupling windows are the same size, and their size and thickness control the coupling strength of the input and output energy, i.e., the external quality factor. By setting the filter's ripple coefficient, center frequency, relative bandwidth, and filter order, the theoretically required coupling coefficient and quality factor can be calculated through Chebyshev low-pass prototype filter circuit fitting, which can guide the determination of the required physical dimensions of each component of the waveguide bandpass filter. The simulation results of the scattering parameters of the reflective waveguide bandpass filter in the millimeter-wave non-reflective waveguide bandpass filter based on the balanced structure of this invention are shown in the figure below. Figure 4 As shown, the third-order bandpass filter response exhibits good filtering performance as expected, with the out-of-band transmission zero located at 32.12 GHz. The coupler's energy is coupled through a wide-slit coupling window. Adjusting the size of the wide-slit coupling window allows for a trade-off between the coupler's bandwidth, amplitude, and phase imbalance performance. The final simulation results of the scattering parameters are shown in the figure. Figure 5 As shown, its effective operating frequency range is defined as 21.84-31.03 GHz. Within this frequency range, it achieves excellent performance with a return loss of less than -12 dB, an amplitude imbalance of less than 2 dB, and a phase imbalance of less than 5°. Combining the discrete design of the coupler and filter described above, the simulation results of the scattering parameters of the finally realized waveguide-based reflection-free bandpass filter are shown in the figure below. Figure 6As shown, a third-order waveguide reflection-free bandpass filter design with a center frequency of 27.9 GHz and a relative bandwidth of approximately 6.7% was ultimately achieved. This invention achieves a minimum in-band insertion loss of 0.21 dB (compared to the typical insertion loss of 3.5 dB for traditional microstrip designs around 28 GHz), while simultaneously achieving excellent reflection-free performance across the entire operating frequency band (21.84-31.03 GHz). 11 <-10 dB).

[0033] The embodiments of the present invention have been described above with reference to the accompanying drawings. In addition to the embodiments described above, the present invention may have other implementations. All technical solutions formed by equivalent substitution or equivalent transformation fall within the protection scope claimed by the present invention.

Claims

1. A millimeter-wave reflection-free waveguide bandpass filter based on a balanced structure, characterized in that... include: The first coupler (3) has a first rectangular cavity (3-1) and a second rectangular cavity (3-2) arranged laterally and coupled. The second coupler (4) has a third rectangular cavity (4-1) and a fourth rectangular cavity (4-2) arranged laterally and coupled. The first reflective filter (5) has a vertically arranged and coupled first resonant cavity (5-2) and second resonant cavity (5-4); The second reflective filter (6) has a vertically arranged and coupled third resonant cavity (6-2) and fourth resonant cavity (6-4). Specifically, the first rectangular cavity (3-1) is coupled to the first resonant cavity (5-2) through the first input coupling window (5-1), the second rectangular cavity (3-2) is coupled to the third resonant cavity (6-2) through the second input coupling window (6-1), the third rectangular cavity (4-1) is coupled to the second resonant cavity (5-4) through the first output coupling window (5-5), and the fourth rectangular cavity (4-2) is coupled to the fourth resonant cavity (6-4) through the second output coupling window (6-5). The outer end face of the first rectangular cavity (3-1) is the input port of the non-reflective waveguide bandpass filter, the outer end face of the second rectangular cavity (3-2) is the isolation end of the first coupler (3), the outer end face of the third rectangular cavity (4-1) is the isolation end of the second coupler (4), and the outer end face of the fourth rectangular cavity (4-2) is the output port of the non-reflective waveguide bandpass filter; the isolation ends of the first coupler (3) and the second coupler (4) are respectively grounded through the load.

2. The millimeter-wave reflection-free waveguide bandpass filter based on a balanced structure according to claim 1, characterized in that: The input signal is input through the input port of the non-reflective waveguide bandpass filter and split into two signals with equal amplitude and a phase difference of 90° by the first coupler (3). The two signals are filtered by the first reflective filter (5) and the second reflective filter (6) respectively and then transmitted to the second coupler (4). The phase cancellation of 0° and 180° is generated at the isolation end of the second coupler (4). The two signals are superimposed at 90° and output at the output port of the non-reflective waveguide bandpass filter.

3. The millimeter-wave reflection-free waveguide bandpass filter based on a balanced structure according to claim 1, characterized in that: The out-of-band reflected signals generated by the first reflective filter (5) and the second reflective filter (6) have equal amplitudes and a phase difference of 90°. After passing through the first coupler (3), the out-of-band reflected signals generated by the first reflective filter (5) and the second reflective filter (6) generate 180° phase cancellation at the input end of the non-reflective waveguide bandpass filter. At the isolation end of the first coupler (3), they generate 90° phase superposition and are absorbed by the load. After passing through the second coupler (4), the out-of-band reflected signals generated by the first reflective filter (5) and the second reflective filter (6) generate 180° phase cancellation at the output end of the non-reflective waveguide bandpass filter. At the isolation end of the second coupler (4), they generate 90° phase superposition and are absorbed by the load, so that the non-reflective waveguide bandpass filter obtains symmetrical non-reflective characteristics at the input and output ports.

4. The millimeter-wave reflection-free waveguide bandpass filter based on a balanced structure according to claim 1, characterized in that: The first rectangular cavity (3-1) and the second rectangular cavity (3-2) are coupled through a first wide-slit coupling window (3-3); the third rectangular cavity (4-1) and the fourth rectangular cavity (4-2) are coupled through a second wide-slit coupling window (4-3).

5. The millimeter-wave reflection-free waveguide bandpass filter based on a balanced structure according to claim 1, characterized in that: The first resonant cavity (5-2) and the second resonant cavity (5-4) are coupled through a first half-wavelength coupling window (5-3); the third resonant cavity (6-2) and the fourth resonant cavity (6-4) are coupled through a second half-wavelength coupling window (6-3).

6. The millimeter-wave reflection-free waveguide bandpass filter based on a balanced structure according to claim 1, characterized in that: The first input coupling window (5-1) is disposed on the inner end face of the first resonant cavity (5-2), and the first output coupling window (5-5) is disposed on the inner end face of the second resonant cavity (5-4); the second input coupling window (6-1) is disposed on the inner end face of the third resonant cavity (6-2), and the second output coupling window (6-5) is disposed on the inner end face of the fourth resonant cavity (6-4).

7. The millimeter-wave reflection-free waveguide bandpass filter based on a balanced structure according to claim 1, characterized in that: The outer end face of the first rectangular cavity (3-1) is the input end (1-1) of the first coupler (3), which serves as the input port of the non-reflective waveguide bandpass filter. The inner end face of the first rectangular cavity (3-1) is the through end of the first coupler (3), which serves as the connection surface between the first coupler (3) and the first reflective filter (5). The outer end face of the second rectangular cavity (3-2) is the isolation end (1-2) of the first coupler (3), and the inner end face of the second rectangular cavity (3-2) is the coupling end of the first coupler (3), which serves as the connection surface between the first coupler (3) and the second reflective filter (6).

8. The millimeter-wave reflection-free waveguide bandpass filter based on a balanced structure according to claim 7, characterized in that: The outer end face of the third rectangular cavity (4-1) is the isolation end (2-2) of the second coupler (4), and the inner end face of the third rectangular cavity (4-1) is the coupling end of the second coupler (4). The coupling end of the second coupler (4) is the connection surface between the second coupler (4) and the first reflective filter (5). The outer end face of the fourth rectangular cavity (4-2) is the input end (2-1) of the second coupler (4). The input end (2-1) of the second coupler (4) serves as the output port of the non-reflective waveguide bandpass filter. The inner end face of the fourth rectangular cavity (4-2) is the through end of the second coupler (4). The through end of the second coupler (4) is the connection surface between the second coupler (4) and the second reflective filter (6).

9. The millimeter-wave reflection-free waveguide bandpass filter based on a balanced structure according to claim 1, characterized in that: The first coupler (3) and the second coupler (4) are waveguide 3dB orthogonal couplers.

10. The millimeter-wave reflection-free waveguide bandpass filter based on a balanced structure according to claim 1, characterized in that: The first reflective filter (5) and the second reflective filter (6) are waveguide bandpass filters.

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