Patent Description:
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 (<NUM> to <NUM>) to THz-band (<NUM> to <NUM> 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 <CIT>).

<CIT> discloses an electromagnetic wave transmission cable.

Also <CIT>, <CIT>, <CIT>, and <CIT> respectively disclose transmission cables for electromagnetic waves.

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 <NUM> is an electromagnetic wave transmission cable for transmitting an electromagnetic wave.

In the following description and the accompanying drawings, substantially the same or equivalent parts are designated by the same reference numerals.

<FIG> is a diagram schematically showing a configuration of an electromagnetic wave transmission cable <NUM> according to the present embodiment. The electromagnetic wave transmission cable <NUM> includes a hollow waveguide tube <NUM> including a hollow dielectric layer in which a waveguide of tubular shape is formed, and a foamed resin member <NUM> covering the outside surface of the hollow waveguide tube <NUM>. The dielectric layer and the hollow portion of the hollow waveguide tube <NUM> form a waveguide for transmitting electromagnetic waves EW.

<FIG> is a longitudinal cross-sectional view of the electromagnetic wave transmission cable <NUM>. The hollow waveguide tube <NUM> includes a dielectric layer <NUM> and a hollow region <NUM>.

The dielectric layer <NUM> is formed in a tubular shape to surround the hollow region <NUM> with a center axis CA at the center. In other words, the dielectric layer <NUM> has a rotationally symmetrical shape about the center axis CA. For example, <FIG> shows a case where the cross section of the dielectric layer <NUM> in a direction perpendicular to the center axis CA has a circular outer rim, whereas the outer rim may have an oblong circular shape, an elliptical shape, a rectangular shape, etc. The dielectric layer <NUM> is made of a fluorocarbon resin PTFE (polytetrafluoroethylene), for example. The dielectric layer <NUM> thus has a refractive index of approximately <NUM>.

The hollow region <NUM> is formed in a rotationally symmetrical shape about the center axis CA along the inner diameter of the hollow waveguide tube <NUM> (i.e., the inner diameter of the dielectric tube made of the dielectric layer <NUM>). 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 <FIG> shows a case where the cross section of the hollow region <NUM> (i.e., the cross section of the waveguide) in a direction perpendicular to the center axis CA is circular, the cross section may have an oblong circular shape, an elliptical shape, a rectangular shape, etc..

There is a close relationship between the wavelength of the electromagnetic waves EW flowing through the hollow waveguide tube <NUM> 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 <NUM> is desirably set to be smaller than the wavelength. On the other hand, the inner diameter of the hollow waveguide tube <NUM> is desirably set to be greater than the half-wavelength since the electromagnetic wave confinement effect weakens as the hollow region <NUM> decreases. The inner diameter of the hollow waveguide tube <NUM> 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 <NUM> 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 <NUM> 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 <NUM> of the hollow waveguide tube <NUM> is desirably set to be smaller than the wavelength. However, since too thin a dielectric layer <NUM> fails to provide strength needed for the waveguide, the thickness of the dielectric layer <NUM> is desirably set to be greater than <NUM>/<NUM> the wavelength. The thickness of the dielectric layer <NUM> according to the present embodiment is therefore set to be equivalent to or greater than <NUM>/<NUM> the wavelength and not greater than the wavelength.

The foamed resin member <NUM> extends in the longitudinal direction of the hollow waveguide tube <NUM> (i.e., waveguide direction) and covers the outside surface of the dielectric layer <NUM> (i.e., surface opposite from the hollow region <NUM>) to surround the outer periphery of the hollow waveguide tube <NUM> with the center axis CA of the hollow waveguide tube <NUM> at the center. The foamed resin member <NUM> in its simplest form has a rotationally symmetrical cross-sectional shape about the center axis CA of the hollow waveguide tube <NUM>. However, the foamed resin member <NUM> may have any shape as long as desired performance is obtained. For example, while <FIG> shows a case where the cross section of the foamed resin member <NUM> in the direction perpendicular to the center axis CA has a circular outer rim, the outer rim may be an oblong circle, an ellipse, a rectangular, etc. The outer rim of the cross section of the foamed resin member <NUM> may have the same shape as or a different shape from that of the outer rim of the cross section of the dielectric layer <NUM>.

For example, the foamed resin member <NUM> is made of foamed polystyrene. Foamed polystyrene has a fine intricate structure of polystyrene having a refractive index of <NUM> and air having a refractive index of <NUM>, 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 <NUM> at <NUM> to <NUM> THz. This result demonstrates that a large amount of air is mixed in foamed polystyrene.

As described above, the foamed resin member <NUM> contains a large amount of air, and the ratio of polystyrene which is in contact with the surface of the hollow waveguide tube <NUM> 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 <NUM> (i.e., surface opposite from the surface which is in contact with the hollow waveguide tube <NUM>).

Too small a thickness of the foamed resin member <NUM> lowers the electromagnetic wave confinement effect. For example, in an experiment performed by using a PTFE hollow waveguide tube having a waveguide frequency of <NUM>, an outer diameter of <NUM>, and an inner diameter of <NUM>, a sufficient effect was not obtained if the thickness of the foamed resin member <NUM> was less than <NUM>. The thickness of the foamed resin member <NUM> (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 <NUM> is desirably set to <NUM> or less, preferably <NUM> or less.

As described above, in the electromagnetic wave transmission cable <NUM> according to the present embodiment, the surface of the hollow waveguide tube <NUM> is covered with the foamed resin member <NUM>. The foamed resin member <NUM> contains a large amount of air and has an average refractive index lower than the refractive index of the dielectric layer <NUM> of the hollow waveguide tube <NUM>.

With such a configuration, the electromagnetic wave confinement effect of the hollow waveguide tube <NUM> 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 <NUM> is described to be made of foamed polystyrene. However, the material of the foamed resin member <NUM> is not limited thereto, and the foamed resin member <NUM> 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 <NUM> 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 <NUM> is polystyrene (PS), polyethylene (PE), or fluorocarbon resin (PTFE), the foamed material has an average attenuation factor of <NUM>-<NUM> or less, and the problem does not matter much.

In the foregoing embodiment, the foamed resin member <NUM> is described to be made of foamed polystyrene and have an average refractive index of approximately <NUM> at <NUM> to <NUM> THz, for example. However, the average refractive index of the foamed resin member <NUM> is not limited thereto. Since the electromagnetic wave confinement effect results from a low refractive index, the foamed resin member <NUM> desirably has a high expansion ratio. However, too high an expansion ratio increases softness and results in poor handleability. The foamed resin member <NUM> is therefore desirably foamed to an extent such that the average refractive index in the transmission frequency band in a bulk state falls below <NUM>.

Unlike the foregoing embodiment, the dielectric layer <NUM> of the hollow waveguide tube <NUM> and the foamed resin member <NUM> 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 <NUM>. In other words, the hollow waveguide tube <NUM> and the foamed resin member <NUM> 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 <NUM>. For example, the electromagnetic wave transmission cable <NUM> 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 <NUM> as shown in <FIG>. The electromagnetic wave confinement effect can be enhanced by forming a protruding structure on the inside surface that is in contact with the dielectric layer as shown in <FIG>. The three-dimensional structure formed on the surface(s) of the foamed resin member <NUM> may have a notch shape or other pit-and-projection shape.

To protect the electromagnetic wave transmission cable <NUM> from collapsing, as shown in <FIG>, an outer coating <NUM> covering the outside surface of the foamed resin member <NUM> may be provided along the longitudinal direction of the hollow waveguide tube <NUM> and the foamed resin member <NUM>.

As shown in <FIG>, instead of covering the entire hollow waveguide tube <NUM> with the foamed resin member <NUM>, only parts that can come into contact with other members, such as connector units for connecting waveguide tubes to each other and supports for holding the waveguide tube(s) at a predetermined height, may be configured to be covered with the foamed resin member <NUM>. In other words, the foamed resin member <NUM> may be formed over a predetermined length (distance) in the longitudinal direction of the hollow waveguide tube, and may be provided at a plurality of positions.

As shown in <FIG>, the expansion ratio of the foamed resin member <NUM> may be changed stepwise between regions closer to a contact surface that is in contact with the dielectric layer <NUM> (i.e., inside) and regions farther from the contact surface (i.e., outside). For example, in the inside regions closer to the contact surface with the dielectric layer <NUM>, the expansion ratio can be increased to reduce the refractive index and enhance the electromagnetic wave confinement effect. In the outside regions farther from the contact surface with the dielectric layer <NUM>, the expansion ratio can be reduced to increase cable rigidity. This can be implemented, for example, by dividing the regions closer to and farther from the contact surface of the foamed resin member <NUM> with the dielectric waveguide <NUM> into a plurality of areas, and reducing the expansion ratios of the respective areas stepwise from the areas closer to the contact surface to the farther areas.

As shown in <FIG>, a foamed resin member <NUM> can be used as a connector <NUM> in a connector unit for connecting waveguide tubes to each other. For example, as shown in <FIG>, by utilizing the property of being deformable of the foamed resin member <NUM>, foamed resin members <NUM> are provided on the inner sides of a pair of holding members 41A and 41B to form a tapered gap. As shown in <FIG>, the hollow waveguide tube <NUM> can thus be inserted into the connector <NUM> so that the hollow waveguide tube is supported. This enables easy and reliable cable positioning while preventing the electromagnetic wave confinement effect from being hampered by contact with the holding members 41A and 41B.

In the electromagnetic wave transmission cable <NUM> according to the present embodiment, the dielectric layer <NUM> 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 <NUM>% to <NUM>% depending on the intended use. To cover the outside with the foamed resin member <NUM> as in the present embodiment, there needs to be a refractive index difference from the foamed resin member <NUM>. To suppress the electromagnetic wave transmission loss, a refractive index difference of at least <NUM> or so is need. The desirable porosity derived therefrom is <NUM>% or less. The optimum range of the porosity of the stretched porous resin of the dielectric layer <NUM> according to the present embodiment is therefore <NUM>% to <NUM>%.

<FIG> is a diagram schematically showing a configuration of an electromagnetic wave transmission cable <NUM> according to the present embodiment. The electromagnetic wave transmission cable <NUM> includes a hollow waveguide tube <NUM> including a hollow dielectric layer in which a waveguide of tubular shape, a foamed resin member <NUM> covering the outside surface of the hollow waveguide tube <NUM>, and a metal film <NUM> covering the outside surface of the foamed resin member <NUM>.

The dielectric layer <NUM> 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 <NUM>) desirably has a small thickness.

The foamed resin member <NUM> is arranged to surround the outer periphery of the hollow waveguide tube <NUM>. The foamed resin member <NUM> needs to have a refractive index lower than that of the dielectric constituting the dielectric layer <NUM> of the hollow waveguide tube <NUM>. The closer to <NUM> the refractive index is, the better.

The foamed resin member <NUM> is formed on the outer peripheral surface of the hollow waveguide tube <NUM>, for example, by a method of inserting the hollow waveguide tube <NUM> into the inside of a foamed resin of hollow shape, a method of winding foamed resin around the hollow waveguide tube <NUM>, or other methods.

Like the hollow waveguide tube <NUM> and the foamed resin member <NUM>, the metal film <NUM> has a rotationally symmetrical shape about the center axis CA. The metal film <NUM> extends in the longitudinal direction of the hollow waveguide tube <NUM> and the foamed resin member <NUM>, and further covers the outside surface of the foamed resin member <NUM> covering the hollow waveguide tube <NUM> to surround the outer periphery of the foamed resin member <NUM> with the center axis CA at the center. Note that the metal film <NUM> may have any shape as long as desired performance is obtained. While <FIG> shows a case where the cross section of the metal film <NUM> in a direction perpendicular to the center axis CA has a circular outer rim, the outer rim may be an oblong circle, an ellipse, a rectangular, etc. The outer rim of the cross section of the metal film <NUM> may have the same shape as or a different shape from that of the cross section of the hollow waveguide tube <NUM> or the foamed resin member <NUM>. In other words, the metal film <NUM> may have any shape covering the foamed resin member <NUM>.

The metal film <NUM> is made of a metal having a relatively high conductivity, such as gold, silver, and copper. The metal film <NUM> may have a thickness of about <NUM> or more. The metal film <NUM> is formed, for example, by a method of directly forming a metal film on the surface of the foamed resin member <NUM>, 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 <NUM> with the metal surface toward the foamed resin member <NUM>, or other methods.

The outer peripheral portion of the metal film <NUM> 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 <NUM> is formed by the method of winding a metal-coated sheet on the foamed resin member <NUM>, the dielectric sheet can be left unremoved and used as a protective film.

As described above, in the electromagnetic wave transmission cable <NUM> according to the present embodiment, the metal film <NUM> is formed to further cover the outside surface of the foamed resin member <NUM> covering the hollow waveguide tube <NUM>. The effect of the metal film <NUM> will be described with reference to <FIG> and <FIG>.

<FIG> is a graph showing a relationship between the thickness of the dielectric layer <NUM> (dielectric tube) made of e-PTFE and the transmission loss, depending on the presence or absence of the outer covering.

If the hollow waveguide tube is formed alone without any outer covering, as shown by a broken line in <FIG>, the transfer loss decreases as the thickness of the dielectric layer decreases (dielectric tube becomes thinner).

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 <NUM> times, as shown by a dot-dashed line in <FIG>, the transmission loss is lower than without an outer covering, and the transmission loss decreases with the decreasing thickness of the dielectric layer within a range where the dielectric layer (dielectric tube) has a certain thickness or more (in <FIG>, approximately <NUM> to <NUM>). However, if the thickness of the dielectric layer falls below a certain value (<NUM>), the transmission loss degrades to a nontransmissible degree (in <FIG>, shown as immeasurable).

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 <FIG>, the transmission loss decreases in proportion to the thickness of the dielectric layer. The transmission loss does not degrade even in the range where the thickness of the dielectric layer is less than the certain value (<NUM>).

<FIG> is a graph showing a relationship between the thickness of the dielectric layer and an effective refractive index, depending on the presence or absence of the outer covering. The effective refractive index is a parameter that represents an average refractive index of the medium contributing to the electromagnetic wave transmission and characterizes the transmission state of the electromagnetic waves.

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 <FIG>, the effective refractive index asymptotically approaches the refractive index of the foamed resin (<NUM>) (in the case of foamed polystyrene having an expansion ratio of approximately <NUM> times) as the thickness of the dielectric layer decreases (dielectric tube becomes thinner). This indicates that the propagation medium of the electromagnetic waves shifts from the dielectric to the foamed resin as the thickness of the dielectric layer decreases. The transmission with the foamed resin as the propagation medium is unstable in terms of the confinement of the electromagnetic waves, and eventually becomes incapable of transmission.

In contrast, if the outside surface of the foamed resin is covered with the metal film, as shown by a solid line in <FIG>, the effective refractive index does not asymptotically approach the refractive index of the foamed resin, but the effective refractive index decreases with the decreasing thickness of the dielectric layer. This indicates that the propagation medium of the electromagnetic waves does not shift to the foamed resin and the electromagnetic waves are stably transmitted with the dielectric layer (dielectric tube) as the waveguide.

As described above, in the electromagnetic wave transmission cable <NUM> according to the present embodiment, the outer covering of the hollow waveguide tube <NUM> has a double structure including the foamed resin member <NUM> and the metal film <NUM>. 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 <NUM>) 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 <NUM> according to the present embodiment, the metal film <NUM> functions as a shield against external electromagnetic waves (noise). Since the hollow waveguide tube <NUM> 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 <NUM> does not necessarily need to cover the entire outside surface of the foamed resin member <NUM>. For example, the metal film <NUM> may be patterned to partly cover the outside surface of the foamed resin member <NUM> 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 <NUM> can be reduced by adjusting the structures and materials of the dielectric waveguide <NUM> and the metal film <NUM>.

While the metal film <NUM> is described to be made of a metal such as gold, silver, and copper, the metal film <NUM> may be made of aluminum or an alloy. The metal constituting the metal film <NUM> 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 <NUM>, a dielectric film may be used as the outer coating that covers the foamed resin member <NUM>. For example, a dielectric film having a refractive index of about <NUM> or more can provide a sufficient refractive index difference at the interface.

Claim 1:
An electromagnetic wave transmission cable (<NUM>, <NUM>) for transmitting an electromagnetic wave (EW), comprising:
a hollow waveguide tube (<NUM>) including a hollow dielectric layer (<NUM>) formed in a tubular shape, and
a foamed resin member (<NUM>) that is provided over a predetermined length in a longitudinal direction of the hollow waveguide tube (<NUM>) and covers an outer surface of the hollow waveguide tube (<NUM>),
characterized in that the dielectric layer (<NUM>) is made of polytetrafluoroethylene, expanded polytetrafluoroethylene, or polyethylene,
the foamed resin member (<NUM>) is made of foamed polystyrene, foamed polyurethane, foamed polyolefin, foamed polyethylene, foamed polypropylene, or foamed polytetrafluoroethylene and has a fine intricate structure of resin and air with different refractive indices,
and in that the foamed resin member (<NUM>) has different expansion ratios between a region close to a contact surface of the foamed resin member (<NUM>) that is in contact with the outer surface of the hollow waveguide tube (<NUM>) and a region far from the contact surface.