Dual-band non-reflection filter based on loading composite spur line structure

Abstract

The invention discloses a dual-band non-reflection filter based on a loaded composite spur line structure, which adopts an annular plane microstrip cascade coupling microstrip line structure to realize dual-band filtering response, and loads two pairs of complementary absorption branches to generate a plurality of reflection zero points to realize full-band absorption of signals reflected by an input / output port. A structure based on a composite spur line is loaded at an input / output port to realize higher harmonic suppression, and the spur line generates a stop band effect through coupling between lines, so that suppression of a parasitic passband is realized. In addition, the spur line structure is embedded into the microstrip line, so that the circuit size is effectively reduced, and the insertion loss in a passband can be effectively reduced. The circuit structure provided by the invention not only has the characteristics of simple and compact structure, planarization and easy integration, but also has the advantages of dual-band, band-pass filtering, full-band non-reflection and higher harmonic suppression.

Description

Technical Field

[0001] The 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 high frequency, high speed and miniaturization, and the performance requirements for RF and microwave filters are also increasing. In modern wireless communication systems, filters are used to select and process signals in a specific frequency band while suppressing unwanted spurious frequencies and harmonics. In order to meet the needs of dual-band or even multi-band, many design schemes for dual-band or multi-band filters have been proposed. Most of these schemes are implemented by connecting two independent single-band filters in parallel or in series. Although this design can achieve multi-band filtering functions, the overall performance of the system is difficult to be effectively guaranteed due to the presence of non-reflective characteristics, harmonic interference and out-of-band parasitic passbands. Especially in the high frequency band, the filter needs to have the characteristics of low insertion loss, high frequency selectivity and high stopband suppression at the same time, which poses a severe challenge to the circuit structure design of the filter. To solve this problem, in recent years, spurline structures with slow-wave characteristics and bandgap characteristics have been studied in depth.

[0003] The spur line is a microstrip defect structure embedded in the traditional microstrip line. It has the characteristics of compactness and coplanarity, which can effectively disturb the current distribution of the filter and change its transmission characteristics. Due to its unique capacitance and inductance characteristics, the spur line can significantly improve the propagation constant of the microstrip transmission line, thereby shortening the wavelength of the transmission line and realizing the miniaturization of the filter size and harmonic suppression. The traditional single spur line structure can produce a notch at a specific frequency point. The specific frequency position and depth mainly depend on its geometric parameters such as slot width, length and depth. However, in practical applications, the harmonic suppression capability of a single spur line over a wide band is limited and cannot meet the requirements of efficient isolation between multi-band passbands. In order to overcome this limitation, a variety of improved spur line structures have been developed, such as asymmetric spur lines to achieve dual notch functions, curved spur lines to enhance capacitance and inductance effects, etc. These improved structures further optimize the performance of the filter by changing the geometric shape and arrangement of the spur line.

[0004] In summary, although the existing spur line structure has certain advantages in harmonic suppression and miniaturization, it still has shortcomings in signal transmission loss and wide-band suppression effect, and it is difficult to take into account both harmonic suppression and non-reflection characteristics. The further development of the existing technology urgently requires a new spur line structure that can improve the harmonic suppression capability and multi-band isolation performance while maintaining the miniaturization characteristics, and can be compatible with the non-reflection dual-band filtering characteristics. Summary of the invention

[0005] Purpose of the invention: In view of the above-mentioned prior art, 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 a three-quarter-wavelength microstrip line of a 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 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: L-shaped defect structure five extends to the left; L-shaped defect structure four extends a section of I-shaped defect structure to the left to reuse 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 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 side, 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 side 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 side, 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 side, 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 reflectionless 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 reflectionless 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 reflectionless 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 of 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 end, and another pair of absorption branches are loaded inside the circuit. Through two complementary absorption branches, three non-reflection areas are formed in three out-of-band signal areas to achieve full-band non-reflection response. On this basis, a composite spur line structure is loaded at the input / output port, and finally the fusion of dual-band, full-band non-reflection, bandpass filtering and high-order harmonic suppression functions is realized, and the structure is simple, compact, planar and 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 forms a good low-pass response, but also forms a periodically distributed notch response to complement the high-order harmonics, which can effectively suppress the high-order harmonics without affecting the performance of the main circuit.

[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; Figure 2 This is a schematic diagram of the structure of a composite spur wire involved in the present invention; Figure 3 A schematic diagram of a dual-band non-reflection filter based on a loaded composite spur line structure; Figure 4 It is a schematic diagram of the structure and dimensions of the composite spur line; Figure 5 The frequency responses of the composite spur line (a) and (b); Figure 6 A schematic diagram of the dimensions of the upper microstrip structure of a reflection-free dual-band filter based on a composite spur line structure; Figure 7 It is a schematic diagram of the cross-sectional dimensions of the circuit structure of an embodiment of the present invention; Figure 8 The frequency response of the composite spur line structure and the reflection-free dual-band filter; Fig. 9 Frequency response of a dual-band reflectionless filter based on a composite spur line structure. DETAILED DESCRIPTION

[0016] The present invention will be further explained below in conjunction with the accompanying drawings.

[0017] like Figure 1 As shown in the figure, the traditional spur line is formed by etching a length of L-shaped defect structure on the microstrip line. This slot structure will cause a disturbance to the current distribution and affect the transmission. The L-shaped spur line will produce a notch at a certain 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 L d These three parameters are related, and they will affect the frequency position of the notch.

[0018] The present invention relates to a composite spur wire structure, such as Figure 2As shown in (a), the composite structure is composed of seven L-type defect structures 01~07 with the same slot width, wherein L-type defect structures 01~03 are located on the upper side of the horizontal microstrip line, and L-type defect structures 04~07 are located on the lower side of the horizontal microstrip line. From right to left on the upper side, they are: L-type defect structure 01 extends to the left, L-type defect structure 02 extends to the right, and L-type defect structure 03 extends to the left on the left side of L-type defect structure 02. From right to left on the lower side, they are: L-type defect structure 05 extends to the left; L-type defect structure 04 extends a section of I-type defect structure to the left by reusing the short side of L-type defect structure 05; L-type defect structure 06 extends to the right; L-type defect structure 07 extends to the left on the left side of defect structure 06. Figure 2 Based on (a), the two ends of the horizontal microstrip line are cut off by a certain width to form Figure 2 (b) shows the composite spur wire structure of the present invention.

[0019] 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, and its dual-band filtering part is composed of a three-quarter wavelength microstrip line 101, a quarter wavelength coupled microstrip line 102, and a pair of quarter wavelength coupled microstrip lines 104. Among them, the upper left end of the coupled microstrip line 102 is connected to the upper right end of the coupled microstrip line 104 on the left, the upper right end of the coupled microstrip line 102 is connected to the upper left end of the coupled microstrip line 104 symmetrically located on the right, and the lower left and right ends of the coupled microstrip line 102 are short-circuited to ground; the two ends of the square ring microstrip line 101 are respectively connected to the connection points of the coupled microstrip line 102 and the symmetrical pair of coupled microstrip lines 104.

[0020] The out-of-band signal absorption part is mainly composed of two absorption branches 1 and two absorption branches 2. Among them, the absorption branch 1 is composed of a resistor R2 and a quarter-wavelength coupled microstrip line 103; the absorption branch 2 is composed of four quarter-wavelength microstrip lines 105-108 and a resistor R1. Taking the symmetrical left structure as an example, in the absorption branch 1 on the left, the upper left end of the coupled microstrip line 103 is connected to the lower right end of the coupled microstrip line 104 on the left through a resistor R2, the upper right end of the coupled microstrip line 103 is short-circuited to ground, and the terminal of the coupled microstrip line 103 is short-circuited. In the absorption branch 2 located on the left side, 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 coupled microstrip line 104 on the left, the other end of the microstrip line 107 is grounded, the microstrip line 105 and the resistor R1 connecting end are connected in parallel to the microstrip line 106 with one end open, and the microstrip line 107 and the resistor R1 connecting end are connected in parallel to the microstrip line 108 with one end open.

[0021] Figure 2The composite spur line structure shown in (b) is loaded on the input and output branches 109 of the dual-band filter. Specifically, at the input port 1 on the left, one end of a composite spur line structure is connected to the lower left end of the coupled microstrip line 104 on the left; at the output port 2 on the right, one end of another composite spur line structure is connected to the lower right end of the coupled microstrip line 104 on the right. Finally, a dual-band non-reflection filter circuit structure based on a loaded composite spur line structure is formed with complete symmetry on the left and right sides.

[0022] Without loading the composite spur line structure, the RF signal enters directly from the input end and is divided into two paths after passing through the coupled microstrip line structure 104, one path passing through the three-quarter wavelength microstrip line structure 101, and the other path passing through the quarter wavelength coupled microstrip line 102. Since the electrical length of the microstrip line structure 101 and the coupled microstrip line 102 differ by half a wavelength, the two signals are offset at the intersection, and at the center frequency point f A transmission zero is formed at 0, which splits 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 end.

[0023] In the above process, the absorption branch 1 is a complementary dual stopband structure, which is turned on at the center frequency ƒ0 and outside the upper and lower passbands of the filter, and is cut off at the center frequency points of the upper and lower passbands, and a reflection zero point is generated at ƒ0. Therefore, the reflection signal of the stopband part between the two passbands is introduced into the absorption branch 1, dissipated by the resistor R2, and a non-reflection characteristic area is formed for the three stopband parts, especially for the stopband area between the two passbands. Similarly, the loaded complementary absorption branch 2 can generate a complementary wide stopband filter response with a center frequency of ƒ0, which can introduce the reflection signals of the upper and lower frequency bands of the two passbands into the absorption branch 2 circuit to be absorbed by the resistor R1, and finally form two non-reflection areas in the upper and lower frequency bands of the two passbands. So far, under the joint action of the absorption branch 1 and the absorption branch 2, a full-band non-reflection response is achieved.

[0024] Since the loaded composite spur line can not only produce a low-pass response, but also produce a periodic notch response in the high-frequency band, and the periodic notch can just complement the harmonics in the high-frequency band, it can suppress the high-frequency harmonic signal. Therefore, the loaded composite spur line structure not only does not affect the reflectionless dual-band pass filter response in the low-frequency band, but also can effectively suppress high-order harmonics. Finally, the entire circuit structure forms a dual-band filter response with a reflectionless characteristic and harmonic suppression function.

[0025] The above-mentioned dual-band non-reflection filter based on the 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 with the prior art. 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 adopted, the slot width W of the L-shaped defect structure 01~07 is S All are 0.3mm; other sizes of composite spur wire structures are as follows Figure 4 As shown in (a), Spurs 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.

[0026] Figure 5 The frequency responses obtained by CST MWS 3D simulation software in this embodiment correspond to Figure 4 Structure 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 to a width of W K The frequency response of the composite spur line structure (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.

[0027] 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 =5mm, W1=1.46mm, L1=73mm, W2=0.68mm, S2=0.19mm, L2=21mm, W3=0.93mm, S3=0.13mm, L3=22.4mm, W g =0.26mm, W4=1.1mm, S4=0.4mm, L4=23mm, W5=0.3mm, L5=24.92mm, W6=1.1mm, L6=23.7mm, W7=4mm, L7=21mm, W8=0.2mm, L8=25.8mm, W hp =0.4mm, L hp =1.23mm, metallized through hole radius R via =0.2mm, the resistance values ​​of the two loss resistors are R1=110Ω and R2=86Ω respectively.

[0028] Figure 7 The intermediate dielectric layer structure 20 is made of RO4003C sheet material with a relative dielectric constant of ε rThe thickness H is 3.55 mm, the thickness H is 1.524 mm, and the thickness of the upper metal 10 structure and the lower metal bottom 30 structure are both 0.017 mm. The reflection-free dual-band filter using the composite spur line in other frequency bands can be obtained by scaling up the embodiment.

[0029] Figure 8 The frequency response curves of the composite spur line and a non-reflection dual-band filter obtained by CST MWS three-dimensional simulation software in this embodiment. The operating frequencies of the non-reflection dual-band filter are 1.4 GHz and 2.62 GHz, respectively. The minimum insertion losses in the passband are 0.97 dB and 1 dB, respectively. The reflection coefficient |S 11 | are all less than -10.6dB, and the return losses within the band are 33.2dB and 13.7dB respectively. There are two periodic higher harmonics within 5-10GHz, namely: center frequency 5.37GHz, 10dB relative bandwidth 4.47%; center frequency 6.5GHz, 10dB relative bandwidth 6.61%. It can be seen that the notch generated by the composite spur line complements the higher harmonics of the dual-band, which can effectively suppress the harmonics in the high-frequency band.

[0030] Fig. 9 The frequency response of the dual-band filter based on the composite spur line structure and the signal absorption rate of the non-reflective dual-band filter obtained by CST MWS three-dimensional simulation software in this embodiment. The absorption rate can be obtained by the formula AR=100×(1-|S 11 | 2 -|S 21 | 2 ) is calculated. After the composite spur line is loaded on the input and output ports, the transmission coefficient |S 21 The values ​​of | are all less than -19.5dB, and the high-order harmonics of the dual-bands are well suppressed. The reflection coefficient |S in the 0-4GHz passband 11 The value of | is less than -10.2dB, the signal absorption rate out of the band 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 losses are 19.25dB and 17.77dB respectively.

[0031] 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 principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A dual-band non-reflection filter based on a loaded composite spur line structure, characterized in that: The overall structure is bilaterally symmetrical, and includes 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.

2. The dual-band reflectionless filter according to claim 1, characterized in that: The composite spur line structure includes seven L-shaped defect structures with the same slot width located on a 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; from right to left on the upper side, they are: 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; from right to left on the lower side, they are: L-shaped defect structure five extending to the left; L-shaped defect structure four extending to the left a section of I-shaped defect structure for reusing the short side of L-shaped defect structure five; L-shaped defect structure six extending to the right; and L-shaped defect structure seven extending to the left on the left side of defect structure six.

3. The dual-band reflectionless filter according to claim 1 or 2, characterized in that: 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; and 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.

4. The dual-band reflectionless filter according to claim 3, characterized in that: The complementary absorption branch node is composed of two absorption branches 1 and two absorption branches 2; wherein the absorption branch node 1 is composed of a resistor R2 and a quarter-wavelength coupled microstrip line four (103); the absorption branch node 2 is composed of quarter-wavelength microstrip lines one to four (105-108) and a resistor R1; in the absorption branch node 1 located on the left side, 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 side through the resistor R2, and the upper right end of the quarter-wavelength coupled microstrip line four (103) is short-circuited and grounded. The terminal of the quarter-wavelength coupled microstrip line four (103) is short-circuited; in the absorption branch 2 located on the left, 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, 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.

5. The dual-band reflectionless filter according to claim 4, characterized in that: 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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