JP4491001B2 - filter - Google Patents

Description

  The present invention relates to a filter in which coaxial resonators are connected in multiple stages.

  A base station apparatus or the like in mobile communication requires a band-pass filter having a wide pass band and a sharp attenuation characteristic when using noise reduction or sharing an antenna. As such a band-pass filter, a filter in which coaxial resonators are connected in multiple stages has been conventionally used.

  In bandpass filters with coaxial resonators connected in multiple stages, the electrical characteristics are wide in the passband and the attenuation characteristics are steep, so adjustments must be made during filter manufacture to obtain the desired electrical characteristics. There was a need to do. This adjustment is performed by adjusting the resonance frequency of each stage in the coaxial resonator and adjusting the coupling amount between stages. When adjusting the coupling amount between the stages, the lid is removed from the case of the band-pass filter, the coupling amount between the stages is changed, and then the lid is attached to the case to obtain the desired electrical characteristics. I try to verify that I can get it. By repeatedly performing such operations, the desired electrical characteristics are obtained. However, the removal and installation of the lid is a complicated operation due to the large number of screws fixing them. was there.

Here, FIG. 7 shows an example of a conventional configuration for varying the coupling amount between stages in the band-pass filter. However, FIG. 7 does not show the front and top of the case of the band-pass filter.
In FIG. 7, the first stage coaxial resonator constituting the band pass filter 100 includes a center conductor 104a, and the second stage coaxial resonator includes a center conductor 104b. A slit 103 is formed in a wall portion between the first-stage coaxial resonator and the second-stage coaxial resonator. The leakage electromagnetic field from the slit 103 causes the first-stage coaxial resonator and the second-stage coaxial resonator to be electrically coupled. The amount of coupling between the first-stage coaxial resonator and the second-stage coaxial resonator can be adjusted by inserting / removing a metal adjusting rod 102 into / from the slit 103. Thereby, the electrical characteristics of the band pass filter 100 can be adjusted. However, in this adjustment structure, not only the variable coupling capacity is small, but if the length of the adjustment rod 102 is increased, an undesired secondary resonance point is generated in the high range due to the influence of the adjustment rod 102, and transmission characteristics are reduced. There is a problem that the insertion loss increases with deterioration.

Next, FIG. 8 shows another example of a conventional configuration for varying the coupling amount between stages in a band-pass filter. However, FIG. 8 does not show the front and top of the case of the bandpass filter.
In FIG. 8, the first-stage coaxial resonator constituting the band-pass filter 110 includes a center conductor 114a, and the second-stage coaxial resonator includes a center conductor 114b. A window 113 is formed in the wall portion between the first-stage coaxial resonator and the second-stage coaxial resonator. The leakage electromagnetic field from the window 113 causes the first-stage coaxial resonator and the second-stage coaxial resonator to be electrically coupled. The amount of coupling between the first-stage coaxial resonator and the second-stage coaxial resonator can be adjusted by closing a part of the window 113 with a metal adjusting plate 112 and changing the size of the window 113. Thereby, the electrical characteristics of the band pass filter 110 can be adjusted. However, in this adjustment structure, since the position of the adjustment plate 112 cannot be adjusted from the outside, it is necessary to remove the lid from the case 111 for adjustment each time adjustment is performed. For this reason, there is a problem that adjustment work is complicated and requires a long time and fine adjustment becomes difficult.

Next, FIG. 9 shows still another example of a conventional configuration in which the amount of coupling between stages in a band-pass filter is variable. However, FIG. 9 does not show the front and upper portions of the band-pass filter case.
In FIG. 9, the first-stage coaxial resonator constituting the band-pass filter 120 includes a center conductor 124a, and the second-stage coaxial resonator includes a center conductor 124b. A window 123 is formed in a wall portion between the first-stage coaxial resonator and the second-stage coaxial resonator. The leakage electromagnetic field from the window 123 causes the first-stage coaxial resonator and the second-stage coaxial resonator to be electrically coupled. The amount of coupling between the first-stage coaxial resonator and the second-stage coaxial resonator can be adjusted by closing a part of the window 123 with a metal adjustment plate 112 and changing the size of the window 123. In this case, the adjustment plate 122 is sandwiched between the pair of holding members 125 and can slide along the wall portion in which the window 123 is formed. Thereby, the electrical characteristics of the band pass filter 120 can be adjusted. However, in this adjustment structure, although the position of the adjustment plate 122 can be adjusted from the outside, a gap is formed in the notch provided in the case 121 for sliding the adjustment plate 122, and an electromagnetic field leaks from this gap and is unnecessary. There was a problem that radiation was generated or foreign substances were mixed. Further, the contact state between the end portion of the adjustment plate 122 located in the case 121 and the wall portion of the case 121 is unstable, and the contact resistance is difficult to stabilize. there were.

Next, FIG. 10 shows still another example of a conventional configuration in which the amount of coupling between stages in a band-pass filter is variable. However, FIG. 10 does not show the front and top of the case 131 of the band-pass filter.
In FIG. 10, the first-stage coaxial resonator constituting the band-pass filter 130 includes a center conductor 134a, and the second-stage coaxial resonator includes a center conductor 134b. A coupling loop 132 is provided between the first-stage coaxial resonator and the second-stage coaxial resonator. In this case, a through hole 133 is formed in a wall portion between the first-stage coaxial resonator and the second-stage coaxial resonator, and a coupling loop 132 is inserted into the through-hole 133. The central conductor 134a and the coupling loop 132 are magnetically coupled. As a result, an electromotive current flows through the coupling loop 132, and a magnetic field is generated in the second-stage coaxial resonator by the electromotive current. The coaxial resonator and the second-stage coaxial resonator are electrically coupled by the coupling loop 132. The amount of coupling between the first-stage coaxial resonator and the second-stage coaxial resonator can be adjusted by changing the size of the coupling loop 132. However, in this adjustment structure, since the size of the coupling loop 132 cannot be adjusted from the outside, it is necessary to remove the lid from the case 131 for adjustment each time the adjustment is made. For this reason, there is a problem that adjustment work is complicated and requires a long time and fine adjustment becomes difficult.

Next, FIG. 11 shows still another example of a conventional configuration in which the amount of coupling between stages in a band-pass filter is variable. However, FIG. 11 does not show the front surface and the upper portion of the case 141 of the band-pass filter.
The band-pass filter 140 shown in FIG. 11 has an adjustment structure obtained by modifying the adjustment structure shown in FIG. 10, and the size of the coupling loop is changed by changing the ground point of the coupling loop. In other words, the first-stage coaxial resonator constituting the band-pass filter 140 includes the center conductor 144a, and the second-stage coaxial resonator includes the center conductor 144b. A coupling loop 142 is provided between the first-stage coaxial resonator and the second-stage coaxial resonator. In this case, a through hole 143 is formed in a wall portion between the first-stage coaxial resonator and the second-stage coaxial resonator, and a coupling loop 142 is inserted into the through-hole 143. The principle that the first-stage coaxial resonator and the second-stage coaxial resonator are electrically coupled by the coupling loop 142 is as described above. The amount of coupling between the first-stage coaxial resonator and the second-stage coaxial resonator can be adjusted by changing the ground point on the same plane as the through-hole 143 of the coupling loop 142. However, in this adjustment structure, since the ground point of the coupling loop 142 cannot be adjusted from the outside, it is necessary to remove the lid from the case 141 every time it is adjusted. For this reason, there is a problem that adjustment work is complicated and requires a long time and fine adjustment becomes difficult.

An example of the electrical characteristics after adjustment of the band-pass filter 140 having the adjustment structure shown in FIG. 11 is shown in FIG.
FIG. 13 shows the frequency characteristics of the VSWR (voltage standing wave ratio) and attenuation amount in the operating frequency band of the band-pass filter 140. Referring to FIG. 13, a value of about 1.15 is obtained for VSWR within the used frequency band, and the amount of attenuation is a slight amount of attenuation. However, since it is necessary to remove the lid from the case 141 for adjustment every time adjustment is performed, the adjustment time takes several hours.

Next, FIG. 12 shows still another example of a conventional configuration in which the coupling amount between stages in a band-pass filter is variable. However, FIG. 12 shows the bandpass filter case with its front and top surfaces open.
A band-pass filter 150 shown in FIG. 12 has an adjustment structure obtained by modifying the adjustment structure shown in FIG. 10, and the ground point of the coupling loop can be changed from the outside. That is, the first-stage coaxial resonator constituting the band-pass filter 150 includes the center conductor 154a, and the second-stage coaxial resonator includes the center conductor 154b. A coupling loop 152 in which a ground point is provided on the bottom surface of the case 151 is provided between the first-stage coaxial resonator and the second-stage coaxial resonator. In this case, a through hole 153 is formed in a wall portion between the first-stage coaxial resonator and the second-stage coaxial resonator, and a coupling loop 152 is inserted into the through-hole 153. The principle that the first-stage coaxial resonator and the second-stage coaxial resonator are electrically coupled by the coupling loop 152 is as described above. The amount of coupling between the first-stage coaxial resonator and the second-stage coaxial resonator can be adjusted by sliding support members 155 provided at both ends of the coupling loop 152 on the bottom surface of the case 151. . However, in this adjustment structure, although the ground point of the coupling loop 152 can be adjusted from the outside, a gap is formed in the case 151 by a hole provided for sliding the coupling loop 152, and an electromagnetic field leaks from the gap. There was a problem that unnecessary radiation was generated or foreign matter was mixed. Further, since the coupling amount is adjusted by deforming the coupling loop 152, there is a problem that the variable range becomes narrow and mechanical stress is applied to the coupling loop 152.

  Therefore, an object of the present invention is to provide a filter having an adjustment structure that can solve the problems of the conventional adjustment structure of a band-pass filter.

In order to achieve the above object, the filter of the present invention is a filter comprising a plurality of coaxial resonators connected to each other and having a metal case, and is arranged between each of the coaxial resonators. An interstage coupling loop formed in a U shape that can variably couple the coupling amount between the devices, and an open end of the interstage coupling loop is inserted into a sliding hole formed in the axial direction. In this way, a metal sliding holding portion that can hold the open end in a slidable manner and a pair of slides in which the open end of the interstage coupling loop is held slidably A notch formed in the case, the holding portion being fixed, and a plurality of types of metal substrates having different intervals between the fixed sliding holding portions. Before any one of multiple types The interstage coupling loop is fixed to the case by inserting the substrate through the interstage coupling loop held by the sliding holding portion and fixing the board to the case from the outside so as to close the notch. The interstage coupling loop is electrically connected to the case, and is replaced with another type of the substrate having a different interval between the pair of sliding holding portions and fixed to the case. By doing so, the coupling amount between the coaxial resonators can be adjusted, and the open end of the interstage coupling loop is slid from the outside of the case with respect to the sliding holding portion, and the interstage The amount of coupling between the coaxial resonators can be adjusted by changing the length of the open end in the case of the coupling loop.

In the filter of the present invention, an open end of the interstage coupling loop is inserted into a through hole formed in the axial direction of the sliding holding portion, and the slide in which the through hole is formed. A slit is formed in one end of the movement holding portion in a substantially axial direction, and the one end elastically holds the release end of the interstage coupling loop, and thereby the release end of the interstage coupling loop can slide. It may be held by the sliding holding portion.
Further, in the filter of the present invention, by selecting any one of a plurality of substrates, the interval between the sliding holding portions arranged between the coaxial resonators can be set for each coaxial resonator. It may be possible to make them different.

  According to the present invention, the amount of coupling between the coaxial resonators is adjusted and the open end of the interstage coupling loop is changed by exchanging with another type of substrate having a different interval between the pair of sliding holding portions. The amount of coupling between the coaxial resonators can be adjusted by sliding the sliding holding portion to change the length of the interstage coupling loop in the case. As a result, the amount of coupling between adjacent coaxial resonators can be adjusted from the outside, and desired electrical characteristics can be obtained by a short-time adjustment, and can be adjusted over a wide range. In this case, the resistance of the ground point of the interstage coupling loop can be stabilized by forming a slit at one end of the sliding holding portion. In addition, since the notch formed in the case is completely closed by the metal substrate, it is possible to prevent the electromagnetic field from leaking from the notch to generate unnecessary radiation and foreign matters.

FIG. 1 is a cross-sectional view of the structure of the band-pass filter according to the embodiment of the present invention, and FIG.
As shown in these drawings, the band-pass filter 1 according to the present invention is configured by five coaxial resonators connected in cascade. The case 10 of the five-stage coaxial resonator is formed by processing a metal plate having good electrical characteristics such as copper. The first-stage coaxial resonator has a central conductor 13a disposed substantially at the center, the lower end of the central conductor 13a is fixed to the bottom surface of the case 10, and the upper portion thereof is positioned approximately at the center by an insulating spacer 14a. To be held. One end of a pair of disc-shaped opposing adjustment plates 15 a is provided at the tip of the center conductor 13 a, and a screw portion 16 a is provided on the other of the adjustment plates 15 a, and this screw portion 16 a is the case 10. Screwed into the upper plate. For this reason, by rotating the screw portion 16a, the interval between the adjustment plates 15a is adjusted, and the resonance frequency of the coaxial resonator can be adjusted. The above configuration is the same in the second-stage to fifth-stage coaxial resonators.

  A coupling loop 12 having one end grounded to the case 121 is disposed adjacent to the center conductor 13a in the first-stage coaxial resonator, and the other end of the coupling loop 12 for magnetic field coupling is connected to the input terminal 11. Yes. As a result, a signal input from the input terminal 11 is supplied to the first-stage coaxial resonator via the coupling loop 12. Further, a coupling loop 18 having one end grounded to the case 121 is disposed near the center conductor 13e in the fifth stage coaxial resonator, and the other end of the coupling loop 18 for magnetic field coupling is connected to the output terminal 19. Has been. As a result, the output signal of the band-pass filter 1 is output from the output terminal 19 via the coupling loop 18.

  A slit is formed in the wall portion between each stage of the band-pass filter 1. For example, a vertically long slit 20a is formed in the wall between the first-stage coaxial resonator and the second-stage coaxial resonator, and the fourth-stage coaxial resonator and the fifth-stage coaxial resonator are formed. A vertically long slit 20d is formed in the wall between the two. Folded portions of interstage coupling loops 17a to 17d formed in a U-shape are inserted into slits 20a to 20d formed in the walls between the respective stages. As a result, the central conductor 13a and the interstage coupling loop 17a are magnetically coupled, whereby an electromotive current flows through the interstage coupling loop 17a, and a magnetic field is generated in the second-stage coaxial resonator due to the electromotive force. Thus, the first-stage coaxial resonator and the second-stage coaxial resonator are electrically coupled by the interstage coupling loop 17a. Similarly, the other stages are electrically coupled by the interstage coupling loops 17b to 17d. The coupling amount between the first-stage coaxial resonator and the second-stage coaxial resonator can be adjusted by inserting / removing the interstage coupling loop 17a into / from the slit 20a as shown in FIG. Similarly, the interstage coupling loops 17b to 17d can be adjusted by inserting and removing the interstage coupling loops 17b to 17d from the outside into the slits 20b to 20d. The ground points of these interstage coupling loops 17b to 17d are the bottom plate of the case 10, and are on the same plane as the ground points of the center conductors 13a to 13e.

Next, FIG. 3 shows a configuration in which the interstage coupling loop 17 is attached to the case 10, and FIG. 4 shows a configuration in which the interstage coupling loop 17 is attached to the case 10. However, in FIGS. 3 and 4, the configuration of the two-stage coaxial resonator of the five-stage band-pass filter is a perspective view showing the front and upper parts omitted.
As shown in FIGS. 3 and 4, a vertically long slit 20 a is formed in the wall portion between the first-stage coaxial resonator and the second-stage coaxial resonator, and the bottom plate of the case 10 has a vertically long slit. A notch 21 is formed. The open end of the U-shaped interstage coupling loop 17 a is slidably held by a pair of sliding holding portions 31 a and 31 b fixed to the substrate 30.

Here, the detailed structure of the sliding holding part 31 is shown in FIG. However, FIG. 5 is a half sectional view showing the configuration of the sliding holding portion 31.
As shown in FIG. 5, the sliding holding portion 31 is formed with a sliding hole 36 penetrating along its axis. A plurality of slits 32 are formed in the upper portion of the sliding hole 36 and have a plurality of holding pieces 33. Although not shown, the plurality of holding pieces 33 are drawn so that the upper ends of the holding pieces 33 are narrowed. As a result, the interstage coupling loops 17a to 17d inserted through the sliding hole 36 are stably and electrically connected to the sliding holding portion 31, and are securely fixed slidably. Further, a flange portion 34 is formed at a substantially central portion of the sliding holding portion 31, and a screw portion 35 is formed at a lower portion of the flange portion 34.

  Although the sliding holding portion 31 is made of metal, it is preferable to use a spring metal material typified by beryllium copper. The surface of the sliding holding portion 31 is preferably plated with a highly conductive plating material such as gold, silver, or copper. Here, when the screw portion 35 in the sliding holding portion 31 is inserted into the mounting hole formed in the substrate 30, the flange portion 34 comes into contact with the upper surface of the substrate 30. In this state, the sliding holding portion 31 is fixed to the substrate 30 by screwing a nut onto the screw portion 35 protruding from the lower surface of the substrate 30.

  3 and 4, the open ends of the interstage coupling loops 17 are respectively inserted into the pair of sliding holding portions 31 a and 31 b fixed to the rectangular substrate 30, and the interstage coupling fixed to the substrate 30 is inserted. The loop 17 is inserted into the notch 21 formed on the bottom surface of the case 10, and the substrate 30 is fixed to the bottom surface of the case 10 from the outside. This state becomes the state shown in FIG. 4, and the interstage coupling loop 17 is positioned in the slit 20. The interstage coupling loop 17 can be inserted into and removed from the case 10 of the bandpass filter 1 from the outside by holding the portion protruding downward from the bottom plate of the case 10 with a hand or a tool. become able to. Thereby, the amount of coupling between adjacent coaxial resonators can be adjusted from the outside, and desired electrical characteristics can be obtained by a short-time adjustment.

  In this case, since the interstage coupling loop 17 is electrically connected to the sliding holding portions 31a and 31b and is grounded to the bottom plate of the case 10, the contact resistance is stable and the electrical characteristics are stable over a long period of time. can do. In addition, the bottom plate of the case 10 has a notch 21, but the notch 21 is completely closed when the metal substrate 30 is fixed, so that an electromagnetic field leaks from the notch 21 and is unnecessary. It is possible to prevent radiation and foreign matter from entering. Further, since the slit 20 is formed in an elongated shape and the mechanical variable range in which the interstage coupling loop 17 is taken in and out of the case 10 is increased, the adjustment range of the coupling amount between the coaxial resonators can be increased. As a result, by adjusting the interstage coupling loops 17a to 17d in the band-pass filter 1, desired electrical characteristics can be obtained. After the adjustment, the boundary point between the metal fitting of the sliding holding portion 31 and the interstage coupling loop 17 is fixed by soldering, and the remaining interstage coupling loop 17 is cut to obtain the adjusted electrical characteristics. Will be able to maintain.

Here, FIG. 6 shows an example of electrical characteristics after adjustment of the band-pass filter 1 according to the present invention shown in FIGS.
FIG. 6 shows the frequency characteristics of the VSWR (voltage standing wave ratio) and the attenuation amount in the operating frequency band of the band-pass filter 1. Referring to FIG. 6, a good value of about 1.03 is obtained for VSWR within the frequency band used, and a good characteristic is obtained with a slight amount of attenuation. And it is not necessary to remove the lid from the case 10 every time it is adjusted, and the contact resistance of the ground point in the interstage coupling loops 17a to 17d becomes stable contact resistance by the sliding holding portions 31a and 31b. Adjustment is completed in a few minutes to a few minutes.

  As the distance between the pair of sliding holding portions 31a and 31b is increased, the distance between the interstage coupling loop 17 and the center conductor 13 is reduced, so that the coupling amount between the coaxial resonators is increased, and the pair of sliding holdings is performed. The amount of coupling decreases as the interval between the portions 31a and 31b decreases. And since the board | substrate 30 with which the pair of sliding holding parts 31a and 31b which attached the interstage coupling loop 17 slidably was fixed can be easily replaced | exchanged from the outside, a pair of sliding holding parts 31 By preparing several kinds of substrates 30 with different intervals and replacing them as necessary, the coupling amount between the coaxial resonators can be adjusted over a wider range. Further, the interstage coupling loop 17 is configured to be bent in a U-shape, but is not limited thereto, and may be configured to be U-shaped or bent in a polygon.

  As described above, the present invention adjusts the coupling amount between coaxial resonators by exchanging with another substrate of a plurality of types, and at the same time opens the open end of the interstage coupling loop with respect to the sliding holding portion. The amount of coupling between the coaxial resonators can be adjusted by sliding and changing the length of the interstage coupling loop in the case. As a result, the amount of coupling between adjacent coaxial resonators can be adjusted from the outside, and desired electrical characteristics can be obtained by a short-time adjustment, and can be adjusted over a wide range. In this case, the resistance of the ground point of the interstage coupling loop can be stabilized by forming a slit at one end of the sliding holding portion. In addition, since the notch formed in the case is completely closed by the metal substrate, it is possible to prevent the electromagnetic field from leaking from the notch to generate unnecessary radiation and foreign matters.

It is sectional drawing which shows the structure of the band pass filter of embodiment of this invention. It is an external view which shows the structure of the band pass filter of embodiment of this invention. It is a perspective view which shows the structure which attaches an interstage coupling loop to a case in the band pass filter of embodiment of this invention. FIG. 3 is a perspective view showing a configuration in which an interstage coupling loop is attached to a case in the band-pass filter according to the embodiment of the present invention. It is a figure which shows the structure of the sliding holding | maintenance part in the band pass filter of embodiment of this invention. It is a figure which shows an example of the electrical property in the band pass filter of embodiment of this invention. It is a perspective view which shows an example of a structure in the conventional band pass filter. It is a perspective view which shows another example of the structure in the conventional band pass filter. It is a perspective view which shows another example of the structure in the conventional band pass filter. It is a perspective view which shows another example of the structure in the conventional band pass filter. It is a perspective view which shows another example of the structure in the conventional band pass filter. It is a perspective view which shows another example of the structure in the conventional band pass filter. It is a figure which shows an example of the electrical property in the conventional band pass filter.

Explanation of symbols

1 band pass filter, 10 case, 11 input terminal, 12 coupling loop, 13a center conductor, 13e center conductor, 14a spacer, 15a adjustment plate, 16a screw part, 17-stage coupling loop, 17a inter-stage coupling loop, 17b stage Inter-coupling loop, 18 coupling loop, 19 output terminal, 20 slit, 20a slit, 20d slit, 21 notch, 30 substrate, 31 sliding holding portion, 31a, 31b sliding holding portion, 32 slit, 33 holding piece, 34 鍔Part, 35 thread part, 36 sliding hole, 100 band-pass filter, 102 adjustment rod, 103 slit, 104a center conductor, 104b center conductor, 110 band-pass filter, 111 case, 112 adjustment plate, 113 window, 114a center Conductor, 114b Center conductor, 120 Band-pass fill , 121 case, 122 adjustment plate, 123 window, 124a center conductor, 124b center conductor, 125 holding member, 130 band pass filter, 131 case, 132 coupling loop, 133 through-hole, 134a center conductor, 134b center conductor, 140 Bandpass filter, 141 case, 142 coupling loop, 143 through-hole, 144a center conductor, 144b center conductor, 150 bandpass filter, 151 case, 152 coupling loop, 153 through-hole, 154a center conductor, 154b center conductor, 155 Support member

Claims (3)

A filter comprising a plurality of coaxial resonators connected to each other and having a metal case,
An interstage coupling loop that is arranged between the coaxial resonators and is formed in a U shape that can variably couple the coupling amount between the coaxial resonators;
A sliding holding portion made of metal capable of slidably holding the open end by inserting the open end of the interstage coupling loop into the sliding hole formed in the axial direction;
A pair of the sliding holding portions, in which the open ends of the interstage coupling loops are slidably held, are fixed, and the intervals between the fixed sliding holding portions are different from each other. It is equipped with a metal substrate that is prepared in multiple types,
One of a plurality of types of the substrates is inserted into the notch formed in the case through the interstage coupling loop held by the sliding holding part, so that the notch is closed. By fixing to the case, the interstage coupling loop is slidably held with respect to the case, and the interstage coupling loop is electrically connected to the case,
The amount of coupling between the coaxial resonators can be adjusted by replacing with another type of the substrate having a different interval between the pair of sliding holding portions and fixed to the case, and the interstage coupling loop By sliding the open end in the case from the outside of the case with respect to the sliding holding portion, and changing the length of the open end in the case of the interstage coupling loop, between the coaxial resonators A filter characterized in that the amount of coupling is adjustable.   A slit is formed in a substantially axial direction at one end of the sliding holding portion in which the sliding hole is formed, and the one end elastically holds the open end of the interstage coupling loop. 2. The filter according to claim 1, wherein the open end of the interstage coupling loop is slidably held by the sliding holding portion.   By selecting one of a plurality of types of the substrates, the interval between the sliding holding portions arranged between the coaxial resonators can be made different between the coaxial resonators. The filter according to claim 1, wherein the filter is a filter.

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