Patent Application
20190296416
The present invention relates to an electromagnetic wave transmission cable.
A hollow waveguide tube made of a dielectric such as a resin has a light weight, high flexibility, a low transmission loss, and high transmission efficiency compared to metal cables such as a coaxial cable and metal waveguide tubes, and is thus effective as a cable for mainly transmitting millimeter waves (30 to 300 GHz) to THz-band (0.1 to 100 THz) electromagnetic waves.
While a hollow waveguide tube made of a single-layer dielectric tube has a confinement effect with respect to the surrounding air, the confinement effect can be hampered and electromagnetic waves leak and scatter off if a metal or another dielectric, such as a human body in particular, comes into contact with the outer wall of the waveguide tube.
A transmission path that confines and transmits electromagnetic waves by stacking two types of dielectrics in layers to construct a Bragg mirror on the outer periphery of a waveguide tube has thus been conceived (for example, Patent Literature 1).
Patent Literature 1: United States Patent Application Publication No. 2009/0097809
According to the foregoing conventional technique, different materials need to be stacked in layers to constitute the Bragg mirror. There have thus been problems of increased man-hours and high manufacturing cost. In addition, the thicknesses of the respective layers need to be designed on the basis of the wavelength of transmission, and there has been a problem of wavelength dependence.
Among examples of problems to be solved by the present invention is that the electromagnetic wave confinement effect is hampered when another object is in contact with the hollow waveguide tube.
The invention according to claim 1 is an electromagnetic wave transmission cable for transmitting an electromagnetic wave, comprising: a hollow waveguide tube including a hollow dielectric layer formed in a tubular shape; and a foamed resin member that is provided over a predetermined length in a longitudinal direction of the hollow waveguide tube and covers a surface of the dielectric layer to surround an outer periphery of the hollow waveguide tube.
Embodiments of the present invention will be described below with reference to the drawings. In the following description and the accompanying drawings, substantially the same or equivalent parts are designated by the same reference numerals.
The dielectric layer 11 is formed in a tubular shape to surround the hollow region 12 with a center axis CA at the center. In other words, the dielectric layer 11 has a rotationally symmetrical shape about the center axis CA. For example,
The hollow region 12 is formed in a rotationally symmetrical shape about the center axis CA along the inner diameter of the hollow waveguide tube 10 (i.e., the inner diameter of the dielectric tube made of the dielectric layer 11). Note that the dielectric waveguide does not necessarily need to have a rotationally symmetrical shape as long as desired performance is obtained. For example, while
There is a close relationship between the wavelength of the electromagnetic waves EW flowing through the hollow waveguide tube 10 and an optimum tube shape. For example, to enhance adherence to the HE11 mode of low transmission loss, the inner diameter of the hollow waveguide tube 10 is desirably set to be smaller than the wavelength. On the other hand, the inner diameter of the hollow waveguide tube 10 is desirably set to be greater than the half-wavelength since the electromagnetic wave confinement effect weakens as the hollow region 12 decreases. The inner diameter of the hollow waveguide tube 10 according to the present embodiment is therefore set to be equal to or greater than the half-wavelength and not greater than the wavelength.
The outer diameter of the hollow waveguide tube 10 is desirably greater than a wavelength equivalent (wavelength×refractive index). On the other hand, to make the transmission in the HE11 mode dominant, the outer diameter of the hollow waveguide tube is desirably not so large. Specifically, the outer diameter is desirably less than or equal to twice the wavelength equivalent. The outer diameter of the hollow waveguide tube 10 according to the present embodiment is therefore set to be equal to or greater than the wavelength equivalent and not greater than twice the wavelength equivalent.
To enhance the adherence to the HE11 mode of low transmission loss, the thickness of the dielectric layer 11 of the hollow waveguide tube 10 is desirably set to be smaller than the wavelength. However, since too thin a dielectric layer 11 fails to provide strength needed for the waveguide, the thickness of the dielectric layer 11 is desirably set to be greater than 1/10 the wavelength. The thickness of the dielectric layer 11 according to the present embodiment is therefore set to be equivalent to or greater than 1/10 the wavelength and not greater than the wavelength.
The foamed resin member 20 extends in the longitudinal direction of the hollow waveguide tube 10 (i.e., waveguide direction) and covers the outside surface of the dielectric layer 11 (i.e., surface opposite from the hollow region 12) to surround the outer periphery of the hollow waveguide tube 10 with the center axis CA of the hollow waveguide tube 10 at the center. The foamed resin member 20 in its simplest form has a rotationally symmetrical cross-sectional shape about the center axis CA of the hollow waveguide tube 10. However, the foamed resin member 20 may have any shape as long as desired performance is obtained. For example, while
For example, the foamed resin member 20 is made of foamed polystyrene. Foamed polystyrene has a fine intricate structure of polystyrene having a refractive index of 1.6 and air having a refractive index of 1, and includes a lot of fine reflection interfaces between polystyrene that is the dielectric and air. For example, low-expansion-ratio foamed polystyrene used as a packaging material was measured in a bulk state and found to have an average refractive index (refractive index on the assumption that the bulk material was a uniform medium of a single substance) of approximately 1.1 at 0.1 to 0.5 THz. This result demonstrates that a large amount of air is mixed in foamed polystyrene.
As described above, the foamed resin member 20 contains a large amount of air, and the ratio of polystyrene which is in contact with the surface of the hollow waveguide tube 10 is extremely low. This can significantly reduce the leakage of electromagnetic waves even in situations where a metal, a human body, or the like comes into contact with the outside surface of the foamed resin member 20 (i.e., surface opposite from the surface which is in contact with the hollow waveguide tube 10).
Too small a thickness of the foamed resin member 20 lowers the electromagnetic wave confinement effect. For example, in an experiment performed by using a PTFE hollow waveguide tube having a waveguide frequency of 300 GHz, an outer diameter of 0.9 mm, and an inner diameter of 0.5 mm, a sufficient effect was not obtained if the thickness of the foamed resin member 20 was less than 1 mm. The thickness of the foamed resin member 20 (thickness of the covering portion) is therefore desirably greater than or equal to a thickness equivalent to the wavelength of the electromagnetic waves to be transmitted (wavelength×average refractive index). Since too great a thickness results in poor handleability, the thickness of the foamed resin member 20 is desirably set to 50 mm or less, preferably 10 mm or less.
As described above, in the electromagnetic wave transmission cable 100 according to the present embodiment, the surface of the hollow waveguide tube 10 is covered with the foamed resin member 20. The foamed resin member 20 contains a large amount of air and has an average refractive index lower than the refractive index of the dielectric layer 11 of the hollow waveguide tube 10.
With such a configuration, the electromagnetic wave confinement effect of the hollow waveguide tube 10 can be maintained even in situations where a metal, human body, or other object comes into contact therewith.
The embodiment of the present invention is not limited to the foregoing one. For example, in the foregoing embodiment, the foamed resin member 20 is described to be made of foamed polystyrene. However, the material of the foamed resin member 20 is not limited thereto, and the foamed resin member 20 may be made of foamed polyurethane, foamed polyolefin, foamed polyolefin (foamed polyethylene, foamed polypropylene), foamed polytetrafluoroethylene (PTFE), or the like.
If the outer diameter of the hollow waveguide tube 10 is smaller than the wavelength equivalent (wavelength×refractive index), there occurs a component propagating over the outer periphery of the waveguide tube. This can cause an adverse effect if the covering foamed resin has an attenuation factor higher than that of air. If the material of the foamed resin member 20 is polystyrene (PS), polyethylene (PE), or fluorocarbon resin (PTFE), the foamed material has an average attenuation factor of 0.1 cm−1 or less, and the problem does not matter much.
In the foregoing embodiment, the foamed resin member 20 is described to be made of foamed polystyrene and have an average refractive index of approximately 1.1 at 0.1 to 0.5 THz, for example. However, the average refractive index of the foamed resin member 20 is not limited thereto. Since the electromagnetic wave confinement effect results from a low refractive index, the foamed resin member 20 desirably has a high expansion ratio. However, too high an expansion ratio increases softness and results in poor handleability. The foamed resin member 20 is therefore desirably foamed to an extent such that the average refractive index in the transmission frequency band in a bulk state falls below 1.4.
Unlike the foregoing embodiment, the dielectric layer 11 of the hollow waveguide tube 10 and the foamed resin member 20 may be made of the same type of material by using a foam of foamed fluorocarbon resin (polytetrafluoroethylene) (PTFE) as the material of the foamed resin member 20. In other words, the hollow waveguide tube 10 and the foamed resin member 20 can be constituted by changing the expansion ratio of the same material.
A three-dimensional structure may be formed on the surface of the foamed resin member 20. For example, the electromagnetic wave transmission cable 100 can be made flexible by forming a protruding structure on the outside surface opposite from the inside surface that is in contact with the dielectric layer 11 as shown in
To protect the electromagnetic wave transmission cable 100 from collapsing, as shown in
As shown in
As shown in
As shown in
In the electromagnetic wave transmission cable 100 according to the present embodiment, the dielectric layer 11 may be made of e-PTFE (expanded polytetrafluoroethylene) which is PTFE given stretch processing. e-PTFE can be obtained, for example, by stretching a PTFE material at least in one direction to provide continuous porosity (structure including a large number of continuous pores) and then sintering-fixing (fixing by sintering) the resultant at high temperature. The stretched porous resin (e-PTFE) used in the present embodiment has characteristic fine nodes and fine fiber structures in the stretching direction, and can thus function as a medium having a low average refractive index without increasing the electromagnetic wave transmission loss.
The porosity of the stretched porous resin (the proportion of porous portions in the resin) can be selected from among 30% to 90% depending on the intended use. To cover the outside with the foamed resin member 20 as in the present embodiment, there needs to be a refractive index difference from the foamed resin member 20. To suppress the electromagnetic wave transmission loss, a refractive index difference of at least 0.01 or so is need. The desirable porosity derived therefrom is 70% or less. The optimum range of the porosity of the stretched porous resin of the dielectric layer 11 according to the present embodiment is therefore 30% to 70%.
The dielectric layer 11 is made of a resin material having a low refractive index or a low complex refractive index, such as PTFE (polytetrafluoroethylene), e-PTFE (expanded polytetrafluoroethylene) formed by stretching PTFE to provide continuous porosity and fixing the same by sintering, and PE (polyethylene). To reduce a transmission loss, the tube (i.e. dielectric layer 11) desirably has a small thickness.
The foamed resin member 20 is arranged to surround the outer periphery of the hollow waveguide tube 10. The foamed resin member 20 needs to have a refractive index lower than that of the dielectric constituting the dielectric layer 11 of the hollow waveguide tube 10. The closer to 1 the refractive index is, the better.
The foamed resin member 20 is formed on the outer peripheral surface of the hollow waveguide tube 10, for example, by a method of inserting the hollow waveguide tube 10 into the inside of a foamed resin of hollow shape, a method of winding foamed resin around the hollow waveguide tube 10, or other methods.
Like the hollow waveguide tube 10 and the foamed resin member 20, the metal film 50 has a rotationally symmetrical shape about the center axis CA. The metal film 50 extends in the longitudinal direction of the hollow waveguide tube 10 and the foamed resin member 20, and further covers the outside surface of the foamed resin member 20 covering the hollow waveguide tube 10 to surround the outer periphery of the foamed resin member 20 with the center axis CA at the center. Note that the metal film 50 may have any shape as long as desired performance is obtained. While
The metal film 50 is made of a metal having a relatively high conductivity, such as gold, silver, and copper. The metal film 50 may have a thickness of about 1 μm or more. The metal film 50 is formed, for example, by a method of directly forming a metal film on the surface of the foamed resin member 20, a method of forming a metal film on the surface of a dielectric sheet to fabricate a metal-coated sheet in advance and then winding the metal-coated sheet on the foamed resin member 20 with the metal surface toward the foamed resin member 20, or other methods.
The outer peripheral portion of the metal film 50 is covered with a protective film (not shown) made of a dielectric or the like. The protective film has only a protective function and does not contribute to electromagnetic wave transmission. For example, if the metal film 50 is formed by the method of winding a metal-coated sheet on the foamed resin member 20, the dielectric sheet can be left unremoved and used as a protective film.
As described above, in the electromagnetic wave transmission cable 200 according to the present embodiment, the metal film 50 is formed to further cover the outside surface of the foamed resin member 20 covering the hollow waveguide tube 10. The effect of the metal film 50 will be described with reference to
If the hollow waveguide tube is formed alone without any outer covering, as shown by a broken line in
If the outside surface of the dielectric layer of the hollow waveguide tube is covered with a foamed polystyrene resin having an expansion ratio of approximately 30 times, as shown by a dot-dashed line in
In contrast, if the outside surface of the foamed resin covering the hollow waveguide tube is further covered with a metal film, as shown by a solid line in
If the outside surface of the dielectric layer of the hollow waveguide tube is covered with a foamed resin, as shown by a dot-dashed line in
In contrast, if the outside surface of the foamed resin is covered with the metal film, as shown by a solid line in
As described above, in the electromagnetic wave transmission cable 200 according to the present embodiment, the outer covering of the hollow waveguide tube 10 has a double structure including the foamed resin member 20 and the metal film 50. This can suppress a drop in the transmission loss occurring if the covering is the foamed resin alone. More specifically, if the dielectric layer of the hollow waveguide tube is covered with only the foamed resin, electromagnetic wave transmission can no longer be transmitted once the transmission loss falls below a lower limit value determined by the physical property values of the dielectric waveguide and the foamed resin (for example, the effective refractive index falls below the refractive index of the foamed resin). However, if the foamed resin (foamed resin member 20) is further covered with the metal film as in the present embodiment, transmission losses below the lower limit value (lower limit value of the transmission loss in the case of covering with only the foamed resin) can be achieved.
In the electromagnetic wave transmission cable 200 according to the present embodiment, the metal film 50 functions as a shield against external electromagnetic waves (noise). Since the hollow waveguide tube 10 is shielded from outside, electromagnetic waves can be stably transmitted.
The thickness of the entire covering portion can be reduced for miniaturization, compared to when the hollow waveguide tube is covered with only a foamed resin.
The metal film 50 does not necessarily need to cover the entire outside surface of the foamed resin member 20. For example, the metal film 50 may be patterned to partly cover the outside surface of the foamed resin member 20 at intervals less than or equal to the wavelength of the electromagnetic waves EW. The patterning of the metal film may be implemented, for example, by a method of winding a metal thin wire or a method of putting a net such as a braided shield. The patterning of the metal film may be implemented by forming a wire grid or a metamaterial structure by using a mask during film formation.
The needed thickness of the foamed resin member 20 can be reduced by adjusting the structures and materials of the dielectric waveguide 10 and the metal film 50.
While the metal film 50 is described to be made of a metal such as gold, silver, and copper, the metal film 50 may be made of aluminum or an alloy. The metal constituting the metal film 50 desirably has a high conductivity, whereas any metal having some conductivity can be used since even a metal having a low conductivity has the effect of improving the transmission characteristic.
Instead of the metal film 50, a dielectric film may be used as the outer coating that covers the foamed resin member 20. For example, a dielectric film having a refractive index of about 1.4 or more can provide a sufficient refractive index difference at the interface.
The electromagnetic wave transmission cable according to the present embodiment can be used, for example, as a cable for in-vehicle high capacity high speed information communication to replace a vehicle information harness of an automobile and the like, or as a cable intended for a data center, moving image transmission, and the like where high capacity communication is needed.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2016-232105 | Nov 2016 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2017/037700 | 10/18/2017 | WO | 00 |
