A dual-band reflectionless filter based on loaded composite spur-line structure

Abstract

The application discloses a double-frequency band non-reflection filter based on a loaded composite spur line structure, adopts a ring-shaped planar microstrip cascaded coupling microstrip line structure to realize double-frequency band filtering response, loads two pairs of complementary absorption branches to generate multiple reflection zero points, and realizes full-frequency band absorption of input / output port reflected signals. A composite spur line structure is loaded at the input / output port to realize high harmonic suppression, the spur line generates a stop band effect through line-to-line coupling, thereby realizing suppression of a parasitic passband. In addition, the spur line structure is embedded in the microstrip line, which not only effectively reduces the circuit size, but also effectively reduces the insertion loss in the passband. The circuit structure of the application not only has the characteristics of simple and compact structure, planarization and easy integration, but also has the advantages of double-frequency band, band-pass filtering, full-frequency band non-reflection and high harmonic suppression.

Description

Technical Field

[0001] The present invention relates to a microwave communication device, in particular to a dual-band non-reflection filter. Background Art

[0002] With the rapid development of wireless communication technology, communication equipment is evolving towards higher frequencies, higher speeds, and smaller sizes, placing increasing demands on the performance of RF and microwave filters. In modern wireless communication systems, filters select and process signals in specific frequency bands while suppressing unwanted spurious frequencies and harmonics. To address the demand for dual-band or even multi-band filters, numerous design solutions have been proposed. Most of these solutions utilize two independent single-band filters connected in parallel or series. While this design can achieve multi-band filtering, it struggles to guarantee overall system performance due to reflection-free characteristics, harmonic interference, and out-of-band spurious passband interference. Especially in high-frequency bands, filters must simultaneously exhibit low insertion loss, high frequency selectivity, and high stopband rejection, posing significant challenges to filter circuit design. To address this challenge, recent research has focused on spurline structures with slow-wave characteristics and bandgap properties.

[0003] A spur line is a microstrip defect structure embedded in a traditional microstrip line. Its compact, coplanar design effectively perturbs the filter's current distribution and alters its transmission characteristics. Due to its unique capacitance and inductance properties, a spur line can significantly increase the propagation constant of a microstrip transmission line, thereby shortening the transmission line wavelength and achieving filter miniaturization and harmonic suppression. A traditional single spur line structure can produce a notch at a specific frequency. The specific frequency location and depth are primarily determined by geometric parameters such as the slot width, length, and depth. However, in practical applications, a single spur line's harmonic suppression capability over a wide frequency band is limited, making it unable to meet the requirements for efficient isolation between multi-band passbands. To overcome this limitation, various improved spur line structures have been developed, such as asymmetric spur lines for dual notching and curved spur lines for enhanced capacitance and inductance. These improved structures further optimize filter performance by modifying the spur line's geometry and arrangement.

[0004] In summary, while existing spur line structures offer advantages in harmonic suppression and miniaturization, they still suffer from limitations in signal transmission loss and broadband suppression. Furthermore, it's difficult to achieve both harmonic suppression and zero-reflection characteristics. Further development of existing technologies urgently requires a new spur line structure that can maintain miniaturization while improving harmonic suppression and multi-band isolation performance, and is compatible with zero-reflection dual-band filtering. Summary of the Invention

[0005] Purpose of the invention: In view of the above-mentioned existing technologies, a dual-band reflectionless filter based on a loaded composite spur line structure is proposed, which has the characteristics of dual-band, bandpass filtering, full-band reflectionlessness, wide stopband and high harmonic suppression.

[0006] Technical solution: A dual-band non-reflection filter based on a loaded composite spur line structure, the overall structure is bilaterally symmetrical, including a dual-band filtering part, an out-of-band signal absorption part, and an input end and an output end loaded with the composite spur line structure; wherein, the dual-band filtering part includes a pair of quarter-wavelength coupled microstrip lines and another quarter-wavelength coupled microstrip line connected in series, and the three-quarter-wavelength microstrip line of the square ring and the quarter-wavelength coupled microstrip line located in the middle are connected in parallel; the input end and the output end loaded with the composite spur line structure are respectively connected to the two ends of the series structure; the out-of-band signal absorption part includes complementary absorption branches loaded at the input end and the output end respectively.

[0007] Furthermore, the composite spur line structure includes seven L-shaped defect structures with the same slot width located on the horizontal microstrip line, and the width of the two ends of the horizontal microstrip line is smaller than the width of the middle section; wherein, L-shaped defect structures one to three are located on the upper side of the horizontal microstrip line, and L-shaped defect structures four to seven are located on the lower side of the horizontal microstrip line; the upper side from right to left is: L-shaped defect structure one extending to the left, L-shaped defect structure two extending to the right, and L-shaped defect structure three extending to the left on the left side of L-shaped defect structure two; the lower side from right to left is: L-shaped defect structure five extending to the left; L-shaped defect structure four is an I-shaped defect structure extending to the left by reusing the short side of L-shaped defect structure five; L-shaped defect structure six extends to the right; L-shaped defect structure seven extends to the left on the left side of defect structure six.

[0008] Furthermore, in the dual-band filtering part, the upper left end of the quarter-wavelength coupled microstrip line located in the middle is connected to the upper right end of the quarter-wavelength coupled microstrip line on the left, the upper right end of the quarter-wavelength coupled microstrip line located in the middle is connected to the upper left end of the quarter-wavelength coupled microstrip line symmetrically located on the right, and the lower left end and the lower right end of the quarter-wavelength coupled microstrip line located in the middle are both short-circuited to ground; the two ends of the three-quarter wavelength microstrip line of the square ring are respectively connected in parallel at the two connection points of the series structure.

[0009] Furthermore, the complementary absorption branch is composed of two absorption branches 1 and two absorption branches 2; wherein, the absorption branch 1 is composed of a resistor R2 and a quarter-wavelength coupled microstrip line four; the absorption branch 2 is composed of quarter-wavelength microstrip lines one to four and a resistor R1; in the absorption branch 1 located on the left, the upper left end of the quarter-wavelength coupled microstrip line four is connected to the lower right end of the quarter-wavelength coupled microstrip line on the left through the resistor R2, the upper right end of the quarter-wavelength coupled microstrip line four is short-circuited to ground, and the terminal of the quarter-wavelength coupled microstrip line four is short-circuited; in the absorption branch 2 located on the left, the microstrip line one, the resistor R1 and the microstrip line three are connected in series in sequence, the other end of the microstrip line one is connected to the lower left end of the quarter-wavelength coupled microstrip line on the left, the other end of the microstrip line three is grounded, the microstrip line one is connected to the resistor R1 connection end in parallel with the microstrip line two with one end open, and the microstrip line three is connected to the resistor R1 connection end in parallel with the microstrip line four with one end open.

[0010] Furthermore, one end of a composite spur line structure is connected to the lower left end of the quarter-wavelength coupled microstrip line on the left; one end of another composite spur line structure is connected to the lower right end of the quarter-wavelength coupled microstrip line on the right.

[0011] Beneficial effects: Existing dual-band filters have out-of-band reflection signals that interfere with system signals. A small number of non-reflective dual-band filters have problems such as complex structure, narrow absorption bandwidth, and poor absorption effect. The dual-band filters based on the spur line structure have problems such as high insertion loss, insufficient out-of-band suppression capability, difficulty in taking into account the non-reflective characteristics, low size and integration, and high design complexity and cost. The present invention loads a composite spur line structure on the input / output microstrip line, which forms a notch response with complementary characteristics without affecting the full-band non-reflective dual-band filtering characteristics, thereby achieving the suppression of high-order harmonics.

[0012] Specifically, a pair of quarter-wavelength coupled microstrip lines and another quarter-wavelength coupled microstrip line are connected in series, and a three-quarter-wavelength microstrip line in a square ring is introduced in parallel with the coupled microstrip line in the middle to construct a dual-band bandpass filter response with good frequency selection characteristics. A pair of parallel absorption branches are loaded at the input / output ends, and another pair of absorption branches are loaded inside the circuit. Through the two complementary absorption branches, three reflection-free areas are formed in the three out-of-band signal areas, achieving a full-band reflection-free response. On this basis, a composite spur line structure is loaded at the input / output port, ultimately achieving the fusion of dual-band, full-band reflection-free, bandpass filtering, and high-order harmonic suppression functions, with a simple, compact, planar structure that is easy to integrate.

[0013] Based on the traditional L-shaped spur line, a composite spur line structure composed of seven L-shaped spur lines is proposed. This structure not only achieves a good low-pass response, but also forms a periodically distributed notch response that complements higher harmonics, effectively suppressing higher harmonics without affecting the main circuit performance.

[0014] In addition, a composite spur line structure is etched on the input / output microstrip line, which suppresses the high-order harmonics of the main circuit without adding additional circuit structure. The structure is simple, compact and easy to process. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is a schematic diagram of the traditional spur line structure;

[0016] Figure 2 This is a schematic structural diagram of a composite spur line involved in the present invention;

[0017] Figure 3 Schematic diagram of a dual-band non-reflective filter based on a loaded composite spur line structure;

[0018] Figure 4 This is a schematic diagram of the structural dimensions of the composite spur line;

[0019] Figure 5 The frequency responses of the composite spur line (a) and (b) are shown;

[0020] Figure 6 A schematic diagram of the dimensions of the upper microstrip structure of a reflectionless dual-band filter based on a composite spur line structure;

[0021] Figure 7 Schematic diagram of the cross-sectional dimensions of the circuit structure of an embodiment of the present invention;

[0022] Figure 8 Frequency response of the composite spur line structure and the reflectionless dual-band filter;

[0023] Figure 9 This is the frequency response of a dual-band reflectionless filter based on a composite spur line structure. DETAILED DESCRIPTION

[0024] The present invention will be further explained below with reference to the accompanying drawings.

[0025] like Figure 1 As shown, the traditional spur line is composed of an L-shaped defect structure etched on the microstrip line. This slot structure will cause a disturbance in the current distribution and affect the transmission. The L-shaped spur line will produce a notch at a specific frequency point, and the position and depth of the notch are mainly determined by the slot width W of the spur line. S , slot length L g And the slot depth Ld These three parameters are related, and they will affect the frequency position of the notch.

[0026] The present invention relates to a composite spur wire structure, such as Figure 2 As shown in (a), the composite structure is composed of seven L-shaped defect structures 01 to 07 with the same slot width, of which L-shaped defect structures 01 to 03 are located on the upper side of the horizontal microstrip line, and L-shaped defect structures 04 to 07 are located on the lower side of the horizontal microstrip line. From right to left on the upper side, they are: L-shaped defect structure 01 extends to the left, L-shaped defect structure 02 extends to the right, and L-shaped defect structure 03 extends to the left on the left side of L-shaped defect structure 02. From right to left on the lower side, they are: L-shaped defect structure 05 extends to the left; L-shaped defect structure 04 is an I-shaped defect structure that extends to the left by reusing the short side of L-shaped defect structure 05; L-shaped defect structure 06 extends to the right; L-shaped defect structure 07 extends to the left on the left side of defect structure 06. Figure 2 Based on (a), the ends of the horizontal microstrip line are cut off by a certain width to form Figure 2 The composite spur wire structure of the present invention shown in (b).

[0027] like Figure 3 As shown, a dual-band non-reflection filter based on a loaded composite spur line structure has a bilaterally symmetrical overall structure. Its dual-band filtering portion comprises a three-quarter wavelength microstrip line 101, a quarter-wavelength coupled microstrip line 102, and a pair of quarter-wavelength coupled microstrip lines 104. The upper left end of the quarter-wavelength coupled microstrip line 102 is connected to the upper right end of the quarter-wavelength coupled microstrip line 104 on the left, and the upper right end of the quarter-wavelength coupled microstrip line 102 is connected to the upper left end of the quarter-wavelength coupled microstrip line 104 symmetrically located on the right. The lower left and right ends of the quarter-wavelength coupled microstrip line 102 are both short-circuited to ground. The two ends of the square ring of the three-quarter wavelength microstrip line 101 are respectively connected to the connection points between the quarter-wavelength coupled microstrip line 102 and the pair of symmetrical quarter-wavelength coupled microstrip lines 104.

[0028] The out-of-band signal absorption section primarily consists of two absorption branches 1 and two absorption branches 2. Absorption branch 1 comprises resistor R2 and a quarter-wavelength coupled microstrip line 103; absorption branch 2 comprises four quarter-wavelength microstrip lines 105-108 and resistor R1. Taking the symmetrical left-side structure as an example, in absorption branch 1 on the left, the upper left end of coupled microstrip line 103 is connected to the lower right end of the quarter-wavelength coupled microstrip line 104 on the left via resistor R2. The upper right end of coupled microstrip line 103 is short-circuited to ground, and the terminals of coupled microstrip line 103 are short-circuited. In the absorption branch 2 on the left, the microstrip line 105, the resistor R1 and the microstrip line 107 are connected in series in sequence. The other end of the microstrip line 105 is connected to the lower left end of the quarter-wavelength coupled microstrip line 104 on the left, and the other end of the microstrip line 107 is grounded. The connection end of the microstrip line 105 and the resistor R1 is connected in parallel to the microstrip line 106 with one end open, and the connection end of the microstrip line 107 and the resistor R1 is connected in parallel to the microstrip line 108 with one end open.

[0029] Figure 2 The composite spur line structure shown in (b) is loaded onto the input and output branches 109 of the dual-band filter. Specifically, at input port 1 on the left, one end of a composite spur line structure is connected to the lower left end of the quarter-wavelength coupled microstrip line 104 on the left; at output port 2 on the right, one end of another composite spur line structure is connected to the lower right end of the quarter-wavelength coupled microstrip line 104 on the right. Ultimately, a dual-band reflectionless filter circuit structure based on loaded composite spur line structures is formed, with complete left and right symmetry.

[0030] Under the condition that the composite spur line structure is not loaded, the RF signal enters directly from the input end and is divided into two paths after passing through the quarter-wavelength coupled microstrip line 104. One path passes through the three-quarter-wavelength microstrip line 101, and the other path passes through the quarter-wavelength coupled microstrip line 102. Since the electrical length of the three-quarter-wavelength microstrip line 101 and the quarter-wavelength coupled microstrip line 102 differ by half a wavelength, the two signals are cancelled at the intersection, and at the center frequency f A transmission zero is formed at 0, splitting the original bandpass filter response into two, thereby achieving a dual-band filtering effect. In addition, two transmission poles are generated in each passband, and two transmission zeros are generated between the two passbands. Finally, the RF signal is output from the output terminal.

[0031] In the above process, absorbing branch 1 forms a complementary dual-stopband structure, conducting at center frequency ƒ0 and outside the filter's upper and lower passbands, while cutting off at the center frequencies of the upper and lower passbands, creating a reflection zero at ƒ0. Therefore, reflected signals in the stopband between the two passbands are directed into absorbing branch 1 and dissipated by resistor R2, creating a reflection-free region for all three stopbands, particularly the region between the two passbands. Similarly, the added complementary absorbing branch 2 generates a complementary wide-stopband filter response with a center frequency of ƒ0. This directs reflected signals in the upper and lower frequency bands of the two passbands into the absorbing branch 2 circuit for absorption by resistor R1, ultimately creating two reflection-free regions in the upper and lower frequency bands of the two passbands. Thus, the combined action of absorbing branches 1 and 2 achieves a full-band reflection-free response.

[0032] Because the loaded composite spur line not only produces a low-pass response but also generates a periodic notch response in the high-frequency band, the periodic notch complements the high-frequency harmonics and suppresses them. Therefore, the loaded composite spur line structure not only does not affect the low-frequency reflectionless dual-bandpass filter response but also effectively suppresses higher-order harmonics. Ultimately, the entire circuit structure forms a reflectionless dual-band filter response with harmonic suppression.

[0033] The aforementioned dual-band non-reflection filter based on a loaded composite spur line structure not only has dual-band, bandpass filtering, full-band non-reflection, and high-order harmonic suppression, but also has better frequency selection characteristics, lower loss, simple and compact structure, planarization, easy integration, low loss, and high suppression compared to the existing technology. The center frequency of the dual-band non-reflection filter based on the composite spur line structure of this embodiment is at 2 GHz. In the composite spur line structure used, the slot width W of the spur lines 01 to 07 of the L-shaped defect structure is 2 GHz. S All are 0.3mm; other sizes of composite spur wire structures are as follows Figure 4 As shown in (a), Spur Line 01: L g1 =6mm, L d1 =0.2mm; Spur line 02: L g2 =5.6mm, L d2 =0.6mm; Spur line 03: L g3 =6.06mm, L d3 =0.2mm; Spur line 04: L g4 =8mm, L d4 =0.2mm; Spur line 05: L g5 =4.9mm, L d5 =0.7mm; Spur line 06: L g6 =2.41mm, L d6=0.7mm; Spur line 07: L g7 =9mm, L d7 =0.2mm; the distance between spur line 01 and spur line 02 is G 12 =10.2mm, the distance between spur line 02 and spur line 03 is G 23 =0.1mm, the distance between spur line 04 and spur line 06 is G 46 =0.2mm, the distance G between spur line 07 and spur line 06 67 =0.1mm. Figure 4 As shown in (b), the width of the microstrip line is W0 = 3.38 mm, and the length of the composite spur line is L SL =18mm, the width of the microstrip line cut inward is W K =0.5mm.

[0034] Figure 5 The frequency responses obtained by CST MWS 3D simulation software in this embodiment correspond to Figure 4 Structures of the composite spur line (a) and the composite spur line (b). The solid line is the reflection coefficient |S 11 |, the dotted line is the transmission coefficient |S 21 |. It can be seen that the composite spur line generates four stopbands at high frequencies, namely: center frequency 5.36GHz, 10dB relative bandwidth of 7.5%, generated by spur lines 04, 06 and 07; center frequency 6.64GHz, 10dB relative bandwidth of 13.4%, generated by spur lines 01 and 02; center frequency 7.865GHz, 10dB relative bandwidth of 3.4%, generated by spur line 03; center frequency 9.58GHz, 10dB relative bandwidth of 7.25%, generated by spur line 05. Figure 5 It is not difficult to find that the composite spur line (a) causes a certain reflection of the signal within the working frequency band, affecting signal transmission. If the input / output section of the composite spur line (a) is cut inward with a width of W K The frequency response of the composite spur line (b) obtained after the distance is as follows Figure 5 As shown in (b), within 0-4GHz |S 11 The values ​​of | are all greater than 16.54dB, which can ensure that harmonics are suppressed without affecting signal transmission within the working frequency band.

[0035] Figure 6 This is a schematic diagram of the dimensions of the upper microstrip structure of the reflectionless dual-band filter based on the loaded composite spur line structure in this embodiment. The overall size is 13.1cm×8.65cm. The specific parameters are as follows: W0=3.38mm, L port =30.5mm, L in1= 5 mm, W1 = 1.46 mm, L1 = 73 mm, W2 = 0.68 mm, S2 = 0.19 mm, L2 = 21 mm, W3 = 0.93 mm, S3 = 0.13 mm, L3 = 22.4 mm, W g = 0.26 mm, W4 = 1.1 mm, S4 = 0.4 mm, L4 = 23 mm, W5 = 0.3 mm, L5 = 24.92 mm, W6 = 1.1 mm, L6 = 23.7 mm, W7 = 4 mm, L7 = 21 mm, W8 = 0.2 mm, L8 = 25.8 mm, W hp = 0.4 mm, L hp = 1.23 mm, the radius of the metallized via R via = 0.2 mm, and the two loss resistors have resistances R1 = 110 Ω and R2 = 86 Ω, respectively.

[0036] Figure 7 is a schematic diagram of the cross-sectional dimensions of the circuit structure of the present embodiment. The middle dielectric layer structure 20 uses RO4003C board material, the relative dielectric constant ε r is 3.55, and the thickness H is 1.524 mm. The upper metal 10 structure and the lower metal ground 30 structure each have a thickness of 0.017 mm. The use of the reflectionless dual-band filter of the composite spur line in other frequency bands can be scaled in the present embodiment.

[0037] Figure 8 is the frequency response curve of the composite spur line and a reflectionless dual-band filter obtained by the CST MWS three-dimensional simulation software of the present embodiment. The operating frequencies of the reflectionless dual-band filter are 1.4 GHz and 2.62 GHz, respectively, the minimum insertion loss in the passband is 0.97 dB and 1 dB, respectively, the reflection coefficient |S 11 | is less than -10.6 dB within 0-4 GHz, and the return loss in the passband is 33.2 dB and 13.7 dB, respectively. There are two periodic high harmonics within 5-10 GHz, which are: the center frequency 5.37 GHz, the 10 dB relative bandwidth is 4.47%; the center frequency 6.5 GHz, the 10 dB relative bandwidth is 6.61%. It can be seen that the trap wave generated by the composite spur line and the high harmonics of the dual-band form a complement, which can effectively suppress the harmonics of the high frequency band.

[0038] Figure 9 is the frequency response of the dual-band filter based on the composite spur line structure and the signal absorption rate of the reflectionless dual-band filter obtained by the CST MWS three-dimensional simulation software of the present embodiment. The absorption rate can be calculated by the formula AR = 100 x (1 - |S 11 | 2 - |S 21 | 2) is calculated. After the input and output ports are loaded with composite spur lines, the transmission coefficient |S 21 The values ​​of | are all less than -19.5dB, and the high-order harmonics in the dual-band are well suppressed. 11 The value of | is less than -10.2dB, the out-of-band signal absorption rate is better than 91%, and the minimum insertion loss in the passband is 0.95dB and 1.11dB respectively, which is only 0.01dB higher than that of the unloaded composite spur line. The return loss is 19.25dB and 17.77dB respectively.

[0039] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A dual-band non-reflective filter based on a loaded composite spur line structure, characterized in that: The overall structure is bilaterally symmetrical and includes a dual-band filtering portion, an out-of-band signal absorption portion, and an input end and an output end loaded with the composite spur line structure; wherein the dual-band filtering portion includes a series structure formed by a pair of quarter-wavelength coupled microstrip lines and another quarter-wavelength coupled microstrip line in series, and the three-quarter-wavelength microstrip line of the square ring and the quarter-wavelength coupled microstrip line located in the middle are connected in parallel; the input end and the output end loaded with the composite spur line structure are respectively connected to the two ends of the series structure; the out-of-band signal absorption portion includes complementary absorption branches loaded at the input end and the output end respectively; The composite spur line structure includes seven L-shaped defect structures with the same slot width located on the horizontal microstrip line, and the width of the two ends of the horizontal microstrip line is smaller than the width of the middle section; wherein, L-shaped defect structures one to three are located on the upper side of the horizontal microstrip line, and L-shaped defect structures four to seven are located on the lower side of the horizontal microstrip line; the upper side, from right to left, is as follows: L-shaped defect structure one extends to the left, L-shaped defect structure two extends to the right, and L-shaped defect structure three extends to the left on the left side of L-shaped defect structure two; the lower side, from right to left, is as follows: L-shaped defect structure five extends to the left; L-shaped defect structure four extends to the left to form an I-shaped defect structure by reusing the short side of L-shaped defect structure five; L-shaped defect structure six extends to the right; L-shaped defect structure seven extends to the left on the left side of defect structure six.

2. The dual-band reflectionless filter according to claim 1, wherein: In the dual-band filtering part, the upper left end of the quarter-wavelength coupled microstrip line (102) located in the middle is connected to the upper right end of the quarter-wavelength coupled microstrip line (104) on the left, the upper right end of the quarter-wavelength coupled microstrip line (102) located in the middle is connected to the upper left end of the quarter-wavelength coupled microstrip line (104) symmetrically located on the right, and the lower left end and the lower right end of the quarter-wavelength coupled microstrip line (102) located in the middle are short-circuited and grounded; the two ends of the three-quarter-wavelength microstrip line (101) of the square ring are respectively connected in parallel at the two connection points of the series structure.

3. The dual-band reflectionless filter according to claim 2, wherein: The complementary absorption branch is composed of two absorption branches close to the symmetry line and two absorption branches away from the symmetry line; wherein, the absorption branch close to the symmetry line is composed of a resistor R2 and a quarter-wavelength coupled microstrip line four (103); the absorption branch away from the symmetry line is composed of a quarter-wavelength microstrip line one to four (105-108) and a resistor R1; in the absorption branch close to the symmetry line on the left, the upper left end of the quarter-wavelength coupled microstrip line four (103) is connected to the lower right end of the quarter-wavelength coupled microstrip line (104) on the left through the resistor R2, and the quarter-wavelength coupled microstrip line four (103) is connected to the lower right end of the quarter-wavelength coupled microstrip line (104) on the left through the resistor R2. ) is short-circuited to ground at the upper right end, and the terminal of the quarter-wavelength coupled microstrip line four (103) is short-circuited; in the absorption branch located on the left side away from the symmetry line, the microstrip line one (105), the resistor R1 and the microstrip line three (107) are connected in series in sequence, the other end of the microstrip line one (105) is connected to the lower left end of the quarter-wavelength coupled microstrip line (104) on the left side, the other end of the microstrip line three (107) is grounded, the microstrip line one (105) and the resistor R1 connection end are connected in parallel to the microstrip line two (106) with one end open, and the microstrip line three (107) and the resistor R1 connection end are connected in parallel to the microstrip line four (108) with one end open.

4. The dual-band reflectionless filter according to claim 3, wherein: One end of a composite spur line structure is connected to the lower left end of the quarter-wavelength coupled microstrip line (104) on the left side; and one end of another composite spur line structure is connected to the lower right end of the quarter-wavelength coupled microstrip line (104) on the right side.

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