Patent Grant
11114732
The present invention relates to a waveguide non-reflective terminator and a waveguide circuit for transmitting signals of microwaves or millimeter waves.
A waveguide non-reflective terminator, which is disclosed in Patent Literature 1, for example, has an opening being rectangular in a plane perpendicular to the propagation direction of radio waves, and includes a waveguide unit opened at one end thereof in the propagation direction of radio waves and closed at the other end thereof by a terminating metallic inner wall, and a radio wave absorber disposed inside the waveguide unit.
In recent years, a layering fabrication technique of piling up resin material or metallic material to form a shape has been rapidly developed, and components formed by the layering fabrication are used in various fields.
However, since the waveguide non-reflective terminator disclosed in Patent Literature 1 has a configuration in which the radio wave absorber is disposed inside the waveguide unit, manufacturing based on the layering fabrication is difficult.
For example, when a radio wave absorber is disposed in a middle of the layering fabrication of the waveguide unit, it is necessary to carry out the layering fabrication for closing the waveguide unit after the radio wave absorber is disposed, which is technically difficult.
Furthermore, after the waveguide unit is completely formed by the layering fabrication, it is required to dispose the radio wave absorber inside the waveguide unit from its rectangular opening, which is a highly complex process.
The present invention has been conceived to solve the problems described above, and an object of the present invention is to provide a waveguide non-reflective terminator and a waveguide circuit suitable for manufacturing based on layering fabrication.
A waveguide non-reflective terminator according to the present invention includes: a waveguide unit whose one end is closed by a short circuit plane provided with a plurality of through holes; and a plurality of radio wave absorbers absorbing a frequency signal being a non-reflective target in a state of being inserted through the plurality of through holes toward an inside of the waveguide unit and contacting with inner surfaces of the plurality of through holes, respectively.
According to the present invention, a function as a terminator can be realized by disposing a radio wave absorber in each of the plurality of through holes in the short circuit plane of the waveguide unit. Separately from the fabrication process of the radio wave absorber, only the waveguide unit can be formed by layering fabrication, so that it is possible to provide a terminator suitable for manufacturing based on layering fabrication.
Hereinafter, some embodiments for carrying out the present invention will be described to explain the present invention in more detail with reference to the accompanying drawings.
As illustrated in
The waveguide unit 2 is a rectangular waveguide closed at one end thereof by a short circuit plane 2a. The traveling direction of radio waves in the waveguide unit 2 is a direction perpendicular to the short circuit plane 2a. The short circuit plane 2a is one end face of the waveguide unit 2, which is electrically short-circuited to close the waveguide unit 2.
Note that, although the rectangular waveguide unit 2 is described, the waveguide unit 2 may be a cylindrical waveguide. That is, a waveguide unit 2 having a rectangular or cylindrical shape may be used as long as the waveguide has a short circuit plane 2a provided with a plurality of through holes.
Each of through holes 3-1 to 3-6 is a rectangular hole piercing through the short circuit plane 2a, and has a dimension in which signals do not propagate in a signal frequency band.
The dimension in which signals do not propagate is, for example, in a rectangular waveguide, by representing the long side length of the through hole by a and the short side length thereof by b, a dimension in which the long side length a is equal to or less than λc/2, in which λc represents a cutoff frequency.
Although an example where six through holes are provided in the short circuit plane 2a and the radio wave absorber is inserted into each of the six through holes is illustrated in
Although the rectangular through holes 3-1 to 3-6 are described, the through holes 3-1 to 3-6 may be circular holes. That is, it is only required for the through holes 3-1 to 3-6 to be in hole shapes corresponding to the shapes of the radio wave absorbers 4-1 to 4-6 as long as signals do not propagate to the outside of the waveguide unit 2 through the through holes 3-1 to 3-6.
In addition, although an example where each of the through holes 3-1 to 3-6 has the same shape and the same dimension is illustrated in
For example, as long as signals do not propagate to the outside of the waveguide unit 2 through the through holes 3-1 to 3-6, the respective through holes 3-1 to 3-6 may have different shapes corresponding to the shapes of the radio wave absorbers 4-1 to 4-6, or may have different dimensions.
Each of the radio wave absorbers 4-1 to 4-6 is a tabular member made of lossy material that absorbs radio waves. As the lossy material, metallic powder solidified with epoxy resin as a resistance component, or ceramic material represented by ferrite is used.
The radio wave absorbers 4-1 to 4-6 have inclined surfaces 4′-1 to 4′-6 each forming a tapered shape inclined from one end toward the other end, respectively.
The radio wave absorbers 4-1 to 4-6 are inserted, from their respective ends having the inclined surfaces 4′-1 to 4′-6 each forming a tapered shape, into the through holes 3-1 to 3-6, respectively.
The ends of the radio wave absorbers 4-1 to 4-6 having the inclined surfaces 4′-1 to 4′-6 each forming a tapered shape are disposed inside the waveguide unit 2 along the propagation direction of radio waves.
The radio wave absorbers 4-1 to 4-6 are inserted through the through holes 3-1 to 3-6, respectively, from the outside to the inside of the waveguide unit 2, and set to a state of contacting with inner surfaces 3′-1 to 3′-6 of the through holes 3-1 to 3-6. In other words, the through holes 3-1 to 3-6 are closed by the radio wave absorbers 4-1 to 4-6 without any gap, respectively.
As illustrated in
In this manner, the waveguide non-reflective terminator 1 functions as a terminator by disposing the radio wave absorbers 4-1 to 4-6 through the through holes 3-1 to 3-6, respectively. As a result, it is possible to form only the waveguide unit 2 by layering fabrication separately from the radio wave absorbers 4-1 to 4-6, so that it is possible to provide a terminator suitable for manufacturing based on layering fabrication.
Furthermore, the through hole and the radio wave absorber may have various shapes.
A waveguide non-reflective terminator 1A illustrated in
Note that, in
The waveguide unit 2A is a waveguide closed at one end thereof by the short circuit plane 2a provided with through holes 3A-1 and 3A-2. The through holes 3A-1 and 3A-2 are cross-shaped holes piercing through the short circuit plane 2a.
The radio wave absorbers 4A-1 and 4A-2 are cross-shaped members made of lossy material that absorbs radio waves. As the lossy material, metallic powder solidified with epoxy resin as a resistance component, or ceramic material represented by ferrite is used.
Note that, although the rectangular waveguide unit 2A is described, the waveguide unit 2A may be a cylindrical waveguide. That is, a waveguide unit 2A having a rectangular or cylindrical shape may be used as long as the waveguide has a short circuit plane 2a provided with the through holes 3A-1 and 3A-2.
Each of four portions of the radio wave absorber 4A-1, which projects outward from the axis, has an inclined surface 4A′-1 forming a tapered shape inclined from one end toward the other end.
Likewise, each of four portions of the radio wave absorber 4A-2, which projects outward from the axis, has an inclined surface 4A′-2 forming a tapered shape.
As illustrated in
With the electromagnetic field analysis simulation being carried out on the waveguide non-reflective terminator 1A, a result similar to that in
Moreover, although the configuration in which the through holes 3-1 to 3-6 are arranged along one direction in the short circuit plane 2a is described, the through holes may be disposed to form a zigzag arrangement.
A waveguide non-reflective terminator 1B illustrated in
The waveguide unit 2B is a waveguide closed at one end thereof by the short circuit plane 2a provided with through holes 3B-1 to 3B-5. As illustrated in
Note that, although the rectangular waveguide unit 2B is described, the waveguide unit 2B may be a cylindrical waveguide. That is, a waveguide unit 2B having a rectangular or cylindrical shape may be used as long as the waveguide has a short circuit plane 2a provided with the through holes 3B-1 to 3B-5.
The radio wave absorbers 4B-1 to 4B-5 have inclined surfaces 43-1 to 43-5 each forming a tapered shape inclined from one end toward the other end, respectively.
The radio wave absorbers 4B-1 to 4B-5 are inserted, from their respective ends having the inclined surfaces 43-1 to 43-5 each forming a tapered shape, into the through holes 3B-1 to 3B-5, respectively.
After the insertion, the radio wave absorbers 4B-1 to 4B-5 are set to a state of contacting with inner surfaces 33-1 to 33-5 of the through holes 3B-1 to 3B-5, respectively.
In other words, the through holes 3B-1 to 3B-5 are closed by the radio wave absorbers 4B-1 to 4B-5 without any gap, respectively.
With the electromagnetic field analysis simulation being carried out on the waveguide non-reflective terminator 1B, a result similar to that in
Although the inclined surfaces 4′-1 to 4′-6, 4A′-1, 4A′-2, and 43-1 to 4B′-5, each forming a tapered shape, are described in the above, no limitation is intended.
For example, the inclined surface of the radio wave absorber may be a stepped inclined surface as long as the radio wave absorber can be inserted through the through hole in the short circuit plane 2a and the through hole can be closed without any gap.
In addition, although an example where all of the radio wave absorbers have the same shape has been described, no limitation is intended by this example. For example, radio wave absorbers in different shapes may be attached to the waveguide unit depending on the positions of the through holes.
As described above, the waveguide non-reflective terminator 1 according to the first embodiment includes the waveguide unit 2 and a plurality of radio wave absorbers 4-1 to 4-6. The waveguide unit 2 has one end closed by the short circuit plane 2a provided with the plurality of through holes 3-1 to 3-6. The radio wave absorbers 4-1 to 4-6 absorb a frequency signal being a non-reflective target in a state of being inserted through the plurality of through holes 3-1 to 3-6 toward the inside of the waveguide unit 2 and contacting with the inner surfaces 3′-1 to 3′-6 of the plurality of through holes 3-1 to 3-6.
With this configuration, the function as a terminator can be obtained by disposing the radio wave absorbers 4-1 to 4-6 in the through holes 3-1 to 3-6, respectively. It is possible to form only the waveguide unit 2 by layering fabrication separately from the radio wave absorbers 4-1 to 4-6, so that it is possible to provide a terminator suitable for manufacturing based on layering fabrication. In addition, by forming the waveguide unit 2 by layering fabrication, it is not necessary to separately manufacture a plurality of components for forming the waveguide unit 2, so that flexibility in the design of the waveguide unit 2 can be improved.
Note that the effects similar to those described above can be obtained also in the waveguide non-reflective terminators 1A and 1B.
The waveguide non-reflective terminator 1C includes a waveguide unit 2 and radio wave absorbers 4C-1 to 4C-6, and causes the radio wave absorbers 4C-1 to 4C-6 to absorb (terminate) signals that propagate inside the waveguide unit 2. The signals to be absorbed by the radio wave absorbers 4C-1 to 4C-6 are frequency signals being non-reflective targets. Note that, in
Each of the radio wave absorbers 4C-1 to 4C-6 is a tabular member made of lossy material that absorbs radio waves. As the lossy material, metallic powder solidified with epoxy resin as a resistance component, or ceramic material represented by ferrite is used.
Note that, although the rectangular waveguide unit 2 is described, the waveguide unit 2 may be a cylindrical waveguide. That is, a waveguide unit 2 having a rectangular or cylindrical shape may be used as long as the waveguide has a short circuit plane 2a provided with a plurality of through holes.
As illustrated in
Further, the radio wave absorber 4C-1 has a stopper 4C″-1 whose dimension is larger than the dimension of a through hole 3-1. As illustrated in
The radio wave absorbers 4C-2 to 4C-6 are configured to have the same shape as the shape of the radio wave absorber 4C-1 illustrated in
The radio wave absorbers 4C-1 to 4C-6 are inserted through the through holes 3-1 to 3-6, respectively, toward the inside of the waveguide unit 2.
When the stoppers 4C″-1 to 4C″-6 come into contact with the short circuit plane 2a, the insertion direction of the radio wave absorbers 4C-1 to 4C-6 is restricted. At this time, the radio wave absorbers 4C-1 to 4C-6 become a state of contacting with inner surfaces 3′-1 to 3′-6 of the through holes 3-1 to 3-6. In other words, the through holes 3-1 to 3-6 are closed by the radio wave absorbers 4C-1 to 4C-6 without any gap, respectively.
As described above, since all of the radio wave absorbers 4C-1 to 4C-6 have the same shape, insertion amounts of the radio wave absorbers 4C-1 to 4C-6 inserted to the through holes 3-1 to 3-6, respectively, are all the same. Accordingly, the insertion amount of each of the radio wave absorbers 4C-1 to 4C-6 is adjusted to be constant, so that the termination effect of signals due to the radio wave absorbers 4C-1 to 4C-6 can be kept constant.
Furthermore, since positions at which the stoppers 4C″-1 to 4C″-6 come into contact with the short circuit plane 2a are appropriate insertion positions of the radio wave absorbers 4C-1 to 4C-6, the radio wave absorbers 4C-1 to 4C-6 can be easily installed and the operation time required for installation can be shortened.
With the electromagnetic field analysis simulation being carried out on the waveguide non-reflective terminator 1C, a result similar to that in
The insertion portions 4D-1 to 4D-6 are portions of the radio wave absorber 4D to be inserted into the through holes 3-1 to 3-6, respectively, which have inclined surfaces 4D′-1 to 4D′-6 each forming a tapered shape, respectively.
A stopper 4″ is a portion to be exposed to the outside of the waveguide unit 2 from the through holes 3-1 to 3-6 at the time when the insertion portions 4D-1 to 4D-6 are inserted into the through holes 3-1 to 3-6, respectively.
As described above, the radio wave absorber 4D has a structure in which the stoppers 4C″-1 to 4C″-6 of the radio wave absorbers 4C-1 to 4C-6 illustrated in
Accordingly, the insertion amount of each of the insertion portions 4D-1 to 4D-6 in the radio wave absorber 4D becomes are adjusted to be constant, whereby the termination effect of signals due to the radio wave absorber 4D can be kept constant.
Furthermore, it is only necessary to insert the insertion portions 4D-1 to 4D-6 to the position at which the stopper 4D″ abuts on the short circuit plane 2a to attach the radio wave absorber 4D to the waveguide unit 2, whereby the operation time for installation of the radio wave absorber 4D can be shortened compared with that of the radio wave absorbers 4C-1 to 4C-6.
Note that, although the rectangular waveguide unit 2 is described, the waveguide unit 2 may be a cylindrical waveguide. That is, a waveguide unit 2 having a rectangular or cylindrical shape may be used as long as the waveguide has the short circuit plane 2a provided with a plurality of through holes.
As described above, in the waveguide non-reflective terminator 1C according to the second embodiment, the insertion amounts of the radio wave absorbers 4C-1 to 4C-6 inserted through the through holes 3-1 to 3-6 are all the same. Accordingly, the termination effect of signals due to the radio wave absorbers 4C-1 to 4C-6 can be kept constant.
In the waveguide non-reflective terminator 1C according to the second embodiment, the radio wave absorbers 4C-1 to 4C-6 have the stoppers 4C″-1 to 4C″-6, whereby the termination effect of signals due to the radio wave absorbers 4C-1 to 4C-6 can be kept constant.
Furthermore, since positions at which the stoppers 4C″-1 to 4C″-6 come into contact with the short circuit plane 2a are appropriate insertion positions of the radio wave absorbers 4C-1 to 4C-6, the radio wave absorbers 4C-1 to 4C-6 can be easily installed and the operation time for installation can be shortened.
In the waveguide non-reflective terminator 1C according to the second embodiment, portions of the radio wave absorber 4D exposed to the outside of the waveguide unit 2 from the respective through holes 3-1 to 3-6 are integrated.
With this configuration, the insertion amount of the insertion portions 4D-1 to 4D-6 in the radio wave absorber 4D are adjusted to be constant, whereby the termination effect of signals due to the radio wave absorber 4D can be kept constant.
Furthermore, since it is only necessary to insert the insertion portions 4D-1 to 4D-6 to the position at which the stopper 4D″ abuts on the short circuit plane 2a to attach the radio wave absorber 4D to the waveguide unit 2, the operation time for installation of the radio wave absorber 4D can be shortened compared with that of the radio wave absorbers 4C-1 to 4C-6.
Note that, in
The waveguide unit 2C is a rectangular waveguide closed at one end thereof by a short circuit plane 2a. The traveling direction of radio waves of the waveguide unit 2C is a direction perpendicular to the short circuit plane 2a. The short circuit plane 2a is one end face of the waveguide unit 2C, which is electrically short-circuited to close the waveguide unit 2C.
Note that, although the rectangular waveguide unit 2C is described, the waveguide unit 2C may be a cylindrical waveguide. That is, a waveguide unit 2 having a rectangular or cylindrical shape may be used as long as the waveguide has a short circuit plane 2a provided with a plurality of through holes.
Each of through holes 3C-1 to 3C-6, 3D-1 to 3D-6, and 3E-1 to 3E-6 is a rectangular hole piercing through the short circuit plane 2a, and has a dimension in which signals do not propagate in a signal frequency band. Although the rectangular through holes are illustrated in
That is, it is only required for the through holes 3C-1 to 3C-6, 3D-1 to 3D-6, and 3E-1 to 3E-6 to be in hole shapes corresponding to the shapes of the radio wave absorbers 4E-1 to 4E-6, 4F-1 to 4F-6, and 4G-1 to 4G-6 as long as signals do not propagate to the outside of the waveguide unit 2C through the through holes.
Each of the radio wave absorbers 4E-1 to 4E-6, 4F-1 to 4F-6, and 4G-1 to 4G-6 is a rod-shaped member made of lossy material that absorbs radio waves. As the lossy material, metallic powder solidified with epoxy resin as a resistance component, or ceramic material represented by ferrite is used.
The radio wave absorbers 4E-1 to 4E-6 are each formed to have the same length, and those lengths are longer than the lengths of the radio wave absorbers 4F-1 to 4F-6 and 4G-1 to 4G-6. The radio wave absorbers 4F-1 to 4F-6 are each formed to have the same length, and those lengths are longer than the length of the radio wave absorbers 4G-1 to 4G-6. The radio wave absorbers 4G-1 to 4G-6 are each formed to have the same length, and those lengths are shorter than the length of the radio wave absorbers 4E-1 to 4E-6 and 4F-1 to 4F-6.
The radio wave absorbers 4E-1 to 4E-6 are inserted through the through holes 3C-1 to 3C-6, respectively, toward the inside of the waveguide unit 2C, and set to a state of contacting with inner surfaces 3C′-1 to 3C′-6 of the through holes 3C-1 to 3C-6.
The radio wave absorbers 4F-1 to 4F-6 are inserted through the through holes 3D-1 to 3D-6, respectively, toward the inside of the waveguide unit 2C, and set to a state of contacting with inner surfaces 3D′-1 to 3D′-6 of the through holes 3D-1 to 3D-6.
The radio wave absorbers 4G-1 to 4G-6 are inserted through the through holes 3E-1 to 3E-6, respectively, toward the inside of the waveguide unit 2C, and set to a state of contacting with inner surfaces 3E′-1 to 3E′-6 of the through holes 3E-1 to 3E-6.
In this manner, the through holes 3C-1 to 3C-6, 3D-1 to 3D-6, and 3E-1 to 3E-6 are closed by the radio wave absorbers 4E-1 to 4E-6, 4F-1 to 4F-6, and 4G-1 to 4G-6 without any gap, respectively.
As illustrated in
In other words, in the waveguide non-reflective terminator 1D, the insertion amounts of the radio wave absorbers are different from one another depending on the position of the through hole.
With the electromagnetic field analysis simulation being carried out on the waveguide non-reflective terminator 1D, a result similar to that in
Furthermore, the waveguide non-reflective terminator 1D functions similarly to the waveguide non-reflective terminator 1 by attaching the radio wave absorbers 4E-1 to 4E-6, 4F-1 to 4F-6, and 4G-1 to 4G-6 to the through holes 3C-1 to 3C-6, 3D-1 to 3D-6, and 3E-1 to 3E-6, respectively.
That is, the waveguide non-reflective terminator 1D functions in a similar manner to a structure in which a radio wave absorber has a shape tapered from the third row toward the first row inside the waveguide unit 2C.
By adjusting the insertion amount of the radio wave absorber in accordance with the position of each through hole, the waveguide non-reflective terminator 1D having appropriate reflection characteristics can be implemented.
Note that, although the rod-shaped radio wave absorbers 4E-1 to 4E-6, 4F-1 to 4F-6, and 4G-1 to 4G-6 are described, the radio wave absorbers 4E-1 to 4E-6, 4F-1 to 4F-6, and 4G-1 to 4G-6 may be tapered, or may be in a shape tapered with a stepped inclined surface. Also in such a configuration, the waveguide non-reflective terminator 1D having an appropriate reflection characteristic can be implemented.
In addition, although the structure in which six radio wave absorbers are attached in each of three rows aligned with one another in the lateral direction is described as the waveguide non-reflective terminator 1D, no limitation is intended by this structure.
For example, a structure in which only one row is disposed as a structure corresponding to the above three rows which are aligned with one another in the lateral direction, and two or more radio wave absorbers are attached to the one row may be adopted. Further, a structure in which two rows are disposed in the lateral direction and one or more radio wave absorbers are attached to each of the two rows may be adopted. Namely, any waveguide non-reflective terminator may be used as long as the insertion amounts of the radio wave absorbers are different from one another depending on the positions of the through holes.
As described above, in the waveguide non-reflective terminator 1D according to the third embodiment, the insertion amounts of the radio wave absorbers 4E-1 to 4E-6, 4F-1 to 4F-6, and 4G-1 to 4G-6 are different from one another depending on the positions of the through holes 3C-1 to 3C-6, 3D-1 to 3D-6, and 3E-1 to 3E-6.
In this manner, by adjusting the insertion amount of the radio wave absorber depending on the position of the through hole, the waveguide non-reflective terminator 1D having appropriate reflection characteristics can be implemented.
The waveguide circuit 5 is a waveguide terminated at positions A to C surrounded by broken lines, and a waveguide non-reflective terminator 1 described in the first embodiment is provided at those positions.
As described in the first embodiment, the waveguide non-reflective terminator 1 functions as a terminator only by attaching the radio wave absorbers 4-1 to 4-6 in the through holes 3-1 to 3-6.
Therefore, in the waveguide circuit 5 provided with the waveguide non-reflective terminator 1, a routing circuit and a plate or lid for short circuiting are not required, and it is also not required to provide a choke structure.
As described above, the waveguide circuit 5 according to the fourth embodiment includes the waveguide non-reflective terminator 1.
With this configuration, the waveguide circuit 5 can be implemented by a simple circuit.
Further, the waveguide circuit can also be implemented by a simple circuit when the waveguide circuit is provided with, instead of the waveguide non-reflective terminator 1, any one of the waveguide non-reflective terminators 1A to 1D indicated in the first to third embodiments.
Furthermore, the waveguide circuit can also be implemented by a simple circuit when the waveguide circuit is provided with a combination of the waveguide non-reflective terminators 1, 1A to 1D.
Note that, in the present invention, the respective embodiments can be freely combined, any constituent element of each embodiment can be modified, and any constituent element of each embodiment can be omitted within the scope of the invention.
The waveguide non-reflective terminator according to the present invention can be used in a communication apparatus that uses signals of microwaves or millimeter waves.
1, 1A to 1D: Waveguide non-reflective terminator, 2, 2A to 2C: Waveguide unit, 2a: Short circuit plane, 3-1 to 3-6, 3A-1, 3A-2, 3B-1 to 3B-5, 3C-1 to 3C-6, 3D-1 to 3D-6, 3E-1 to 3E-6: Through hole, 3′-1 to 3′-6, 3A′-1, 3A′-2, 3B′-1 to 3B′-5, 3C′-1 to 3C′-6, 3D′-1 to 3D′-6, 3E′-1 to 3E′-6: Inner surface, 4-1 to 4-6, 4A-1, 4A-2, 4B-1 to 4B-5, 4C-1 to 4C-6, 4E-1 to 4E-6, 4F-1 to 4F-6, 4G-1 to 4G-6: Radio wave absorber, 4D-1 to 4D-6: Insertion portion, 4′-1 to 4′-6, 4A′-1, 4A′-2, 4B′-1 to 4B′-5, 4C′-1 to 4C′-6, 4D′-1 to 4D′-6: Inclined surface, 4C″-1 to 4C″-6, 4D″: Stopper, 5: Waveguide circuit.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2017/019043 | 5/22/2017 | WO | 00 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2018/216072 | 11/29/2018 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 20050017815 | Asao et al. | Jan 2005 | A1 |
| 20140077901 | Uhm | Mar 2014 | A1 |
| Number | Date | Country |
|---|---|---|
| 2005-045341 | Feb 2005 | JP |
| 2007-259046 | Oct 2007 | JP |
| 2297697 | Jan 2007 | RU |
| Entry |
|---|
| Extended European Search Report issued in corresponding European Application No. 17910947.5 dated Mar. 18, 2020. |
| Number | Date | Country | |
|---|---|---|---|
| 20200274218 A1 | Aug 2020 | US |
