Patent Description:
A known antenna device is installed outdoors. Such an antenna device is fixed to an electric pole, a road, or the like using, for example, a support or a foundation. Patent Document <NUM> discloses an antenna device according to the preamble of claim <NUM>. Patent Document <NUM> discloses a mast arrangement comprising: a mast arranged for carrying a first module of a radio network node, a holding structure adapted to receive a lower portion of the mast, and a housing for a second module of the radio network node. An air inlet channel in the mast is connected to the housing via an inlet passage, and an air outlet channel in the mast is connected to the housing via an outlet passage. An interior of the housing is arranged to direct an airflow from the inlet passage along, and/or through, the second module to the outlet passage. Further, a radio network node and a method of cooling the second module in a radio network node are provided. Patent Document <NUM> discloses a modular monopole for wireless communications including: an antenna module having a floor, a ceiling and a side wall that form an antenna compartment, wherein at least one antenna resides within the antenna compartment; a radio module having a floor, a ceiling and a side wall that form a radio compartment, wherein at least one remote radio unit resides within the radio compartment; and a base. The base, the radio module, and the antenna module are arranged in vertically stacked relationship, with the base below the radio module and the antenna module above the radio module.

An antenna device according to an aspect of an embodiment includes an antenna portion, a connecting portion, and a support. The connecting portion connects the antenna portion and the support. The support is located above the antenna portion and includes a channel extending from an inflow opening located opposite to the antenna portion to an outflow opening located farther from the antenna portion than the inflow opening.

An embodiment of an antenna device disclosed in the present application will be described in detail below. The disclosure is not limited by the following embodiment.

First, a configuration of the antenna device according to the embodiment will be described with reference to <FIG> and <FIG>. <FIG> is a perspective view schematically illustrating the antenna device according to the embodiment. <FIG> is a side view schematically illustrating the antenna device according to the embodiment. <FIG> is a view of one side of a support as viewed in plan view in a perpendicular direction of the one side.

As illustrated in <FIG> and <FIG>, an antenna device <NUM> includes an antenna portion <NUM>, a support <NUM>, and connecting portions <NUM>. For the sake of clarity, <FIG> and <FIG> illustrate a three-dimensional orthogonal coordinate system including a Z axis in which a vertically upward direction is a positive direction. Such an orthogonal coordinate system may also be presented in other drawings used in the description below. In the following description, the Z axis positive direction side may be referred to as "above" for convenience. The same and/or similar components as those of the antenna device <NUM> illustrated in <FIG> and <FIG> are denoted by the same reference numerals, and descriptions thereof will be omitted or simplified.

The antenna portion <NUM> includes, for example, an antenna element mounted on a wiring board. The antenna element includes, for example, an insulation substrate, a patch, and a circuitry. The insulation substrate includes, for example, a dielectric material or other insulation materials. The patch is, for example, an electrical conductor film made of an electrical conductive material such as copper. The circuitry includes, for example, an integrated circuit such as a Radio Frequency Integrated Circuit (RFIC). The patch and the circuitry are, for example, electrically connected to each other via a feed line.

The antenna portion <NUM> may further include, for example, a support member that supports an antenna element and a heat dissipation member. The heat dissipation member includes, for example, Thermal Interface Material (TIM), and dissipates heat generated by the antenna element.

Such an antenna portion <NUM> is housed in a housing having a substantially spherical shape. The antenna portion <NUM> has an outer appearance having a substantially spherical shape as illustrated in <FIG> and <FIG>.

The support <NUM> is located above the antenna portion <NUM>. The support <NUM> supports the antenna portion <NUM> via the connecting portions <NUM>. The support <NUM> has a quadrangular prism shape elongated in the Z axis direction. The support <NUM> is fixed such that a first end 20a side closer to the antenna portion <NUM> is the lower side and a second end 20b side away from the antenna portion <NUM> is the upper side.

The connecting portions <NUM> are located between the antenna portion <NUM> and the support <NUM>, and connect the antenna portion <NUM> and the support <NUM>.

Here, configurations of the support <NUM> and the connecting portions <NUM> will be further described with reference to <FIG>. <FIG> is a cross-sectional view taken along II-II in <FIG>. <FIG> is a cross-sectional view taken along III-III in <FIG>. Note that in <FIG>, to facilitate understanding of a relationship between an outer shape of the support <NUM> and an outer shape of the antenna portion <NUM>, a circle having a shape corresponding to the outer shape of the antenna portion <NUM> is given.

As illustrated in <FIG> and <FIG>, the support <NUM> includes a plurality of channels <NUM>. Each of the plurality of channels <NUM> is a through hole that penetrates the inside of the support <NUM> from the first end 20a to the second end 20b. The plurality of channels <NUM> are located side by side in the X axis direction and the Y axis direction, and a partition <NUM> defining adjacent ones of the plurality of channels <NUM> is located between the adjacent ones of the plurality of channels <NUM>. In other words, the support <NUM> is located above the antenna portion <NUM>. The support <NUM> includes the channels <NUM> extending from an inflow opening located opposite to the antenna portion <NUM> to an outflow opening located farther from the antenna portion <NUM> than the inflow opening. The inflow opening is located at the first end 20a. The outflow opening is located at the second end 20b.

The support <NUM> includes heat dissipation portions <NUM>. The heat dissipation portions <NUM> extend from the first end 20a to the second end 20b of the support <NUM> in parallel with the plurality of channels <NUM>. In other words, the heat dissipation portions <NUM> are disposed to extend along the channels <NUM>.

Each of the heat dissipation portions <NUM> includes, for example, the plurality of channels <NUM> arranged in a lattice shape. The heat dissipation portions <NUM> extend in a height direction (Z axis direction) of the support <NUM> in a manner that some of the channels <NUM> among the plurality of channels <NUM> are blocked, and each of the plurality of channels <NUM> adjacent to the heat dissipation portion <NUM> is located in a manner that the periphery of the heat dissipation portion <NUM> is surrounded. The heat dissipation portions <NUM> may be located on the outer edge of the support <NUM>, and may serve as a part of a peripheral wall <NUM> of the support <NUM>.

The support <NUM> supports the antenna portion <NUM> via the connecting portions <NUM>, and has a heat dissipation function that contributes to the heat dissipation of the antenna portion <NUM>.

As illustrated in <FIG>, in a cross-sectional view of the connecting portions <NUM>, for example, members constituting the connecting portions <NUM> are present in an isolated state. That is, the connecting portions <NUM> are partially connected to both members of the antenna portion <NUM> and the support <NUM>. This point will be described with reference to <FIG>.

<FIG> is a cross-sectional view for explaining heat dissipation by the support. In <FIG>, only one connecting portion <NUM> is illustrated, but other connecting portions <NUM> also have the same and/or similar configuration. In this case, when heat generated at the antenna portion <NUM> is transmitted to the heat dissipation portion <NUM> of the support <NUM> via the connecting portion <NUM>, the temperature in the channels <NUM> located around the heat dissipation portion <NUM> rises. With the temperature rise in the channels <NUM>, the air in the channels <NUM> moves from the lower side to the upper side in the height direction (Z axis direction) of the support <NUM> in the channels <NUM> as indicated by arrows <NUM>, and the outside air is continuously taken in from the first end 20a of the channels <NUM> located at the lower end of the support <NUM>. According to the antenna device <NUM> according to the embodiment, such a chimney effect is generated, and thus the radiation characteristics of the antenna portion <NUM> can be enhanced.

As illustrated in <FIG>, the peripheral wall <NUM> (member located on the outermost side) constituting the support <NUM> may include a part where the thickness changes in the longitudinal direction of the support <NUM>. For example, a thinner part and a thicker part of the peripheral wall <NUM> may be alternately formed in the longitudinal direction of the support <NUM>. The peripheral wall <NUM> constituting the support <NUM> may be thinner than the partition <NUM> of the inside. The peripheral wall <NUM> constituting the support <NUM> may include many thinner parts than the partition <NUM> of the inside. By partially changing the thicknesses of the peripheral wall <NUM> and the partition <NUM>, the support <NUM> can be easily deformed. Thus, for example, even when the wall surface of a construction in which the support <NUM> is installed is curved or bent, the support <NUM> can be easily made to conform to the shape of the wall surface. The term "construction" includes not only houses and buildings, but also electric poles, traffic lights, roadside trees.

The support <NUM> may be, for example, a member made of a metal such as an aluminum alloy or the like. The support <NUM> may be integrally formed by, for example, extrusion molding or other methods, or may be formed by appropriately processing the support <NUM> that is individually formed for each portion.

Each of the connecting portions <NUM> is a solid rod shape body located between a respective one of the heat dissipation portions <NUM> of the support <NUM> and the antenna portion <NUM>. That is, the connecting portions <NUM> partially connect the antenna portion <NUM> and the support <NUM>. In this case, each of the connecting portions <NUM> may be connected to the heat dissipation portions <NUM> in the support <NUM>.

An area of a horizontal cross-section of the connecting portions <NUM> is smaller than an area of a horizontal cross-section of the support <NUM>. Thus, in the periphery of the connecting portions <NUM>, the outside air easily enters the channel <NUM> from the first end 20a side of the support <NUM>, and the radiation characteristics is further enhanced. An increase in the total weight of the antenna device <NUM> due to the connecting portions <NUM> can be suppressed. Here, the horizontal cross-section of the connecting portions <NUM> is a plane indicated by the line II-II in <FIG>.

The connecting portions <NUM> may be, for example, members made of a metal such as an aluminum alloy or the like. The connecting portions <NUM> may be integrally formed with the support <NUM> by, for example, extrusion molding or other methods, or may be formed by bonding an individually formed rod shape bodies to the support <NUM> and/or the antenna portion <NUM> by welding, adhesion, or the like.

A length L (see <FIG>) of the connecting portions <NUM> that define a distance between the antenna portion <NUM> and the support <NUM> can be, for example, from <NUM> to <NUM>, particularly from <NUM> to <NUM>, further from <NUM> to <NUM>. By defining the length L in this manner, the radiation characteristics of the antenna device <NUM> can be enhanced. The length L is not limited to the range described above, and can be appropriately set according to, for example, the number, positions, and sizes of the connecting portions <NUM>.

The number, positions, and sizes of the channels <NUM> and the heat dissipation portions <NUM> can be changed as appropriate depending on, for example, a material, a shape, and the like of the support <NUM>.

In the example illustrated in <FIG>, an outer shape of the support <NUM> as viewed in the Z axis direction is located to be inscribed in an outer shape of the antenna portion <NUM>. However, the outer shape of the support <NUM> may be larger than or smaller than the outer shape of the antenna portion <NUM>. As illustrated in <FIG>, when the outer shape of the support <NUM> is rectangular, and the outer shape of the antenna portion <NUM> is circular, and for example, the outer shape of the antenna portion <NUM> is small enough to fit into the outer shape of the support <NUM>, the outside air easily flows into the channels <NUM> of the support <NUM> from the antenna portion <NUM> side in the Z axis. Thus, the radiation characteristics of the antenna portion <NUM> is further enhanced.

A variation of the antenna device <NUM> will be described with reference to <FIG>. <FIG> is a cross-sectional view schematically illustrating the antenna device according to a first variation of the embodiment. <FIG> illustrates a cross-section at the same position as in <FIG>. As the same as or similar to <FIG>, in <FIG>, to facilitate understanding of the relationship between the outer shape of the support <NUM> and the outer shape of the antenna portion <NUM>, the circle having the shape corresponding to the outer shape of the antenna portion <NUM> is given.

The antenna device <NUM> illustrated in <FIG> differs from the antenna device <NUM> according to the embodiment in that the antenna device <NUM> includes heat dissipation portions <NUM> instead of the heat dissipation portions <NUM> of the support <NUM>. The heat dissipation portions <NUM> have a higher coefficient of thermal conductivity than other portions such as the peripheral wall <NUM> and the partition <NUM> of the support <NUM>. Thus, the radiation characteristics of the antenna portion <NUM> can be further enhanced. As a material of the heat dissipation portions <NUM>, for example, a metal material such as copper having a higher coefficient of thermal conductivity than the material of the support <NUM> can be used.

Note that a material of the connecting portions <NUM> may be the same as the material of the heat dissipation portions <NUM>. By making the connecting portions <NUM> from the same material as the material of the heat dissipation portions <NUM>, the radiation characteristics are further enhanced.

<FIG> is a cross-sectional view schematically illustrating the antenna device according to a second variation of the embodiment. <FIG> illustrates a cross-section at the same position as in <FIG>. Also in <FIG>, to facilitate understanding of the relationship between the outer shape of the support <NUM> and the outer shape of the antenna portion <NUM>, a circle having a shape corresponding to the outer shape of the antenna portion <NUM> is given. The antenna device <NUM> illustrated in <FIG> differs from the antenna device <NUM> according to the first variation in that the antenna device <NUM> includes heat pipes <NUM> instead of the heat dissipation portions <NUM>.

<FIG> is a cross-sectional view taken along VII-VII in <FIG>. As illustrated in <FIG>, the heat pipe <NUM> includes a hollow portion <NUM> at an inner portion thereof. The hollow portion <NUM> is sealed with a cooling medium <NUM>. The cooling medium <NUM> is vaporized when the heat pipe <NUM> is heated, and is condensed when the heat pipe <NUM> is cooled. A material of the heat pipe <NUM> may be, for example, copper. The cooling medium <NUM> may be, for example, water or a substitute for CFCs (e.g., HFC-134a).

The heat pipe <NUM> illustrated in <FIG> is located from the first end 20a to the second end 20b of the support <NUM>, and is not located in the connecting portion <NUM>. However, the heat pipe <NUM> may be located, for example, from an inner portion of the connecting portion <NUM> to the second end 20b of the support <NUM>. The heat pipe <NUM> need not be located, for example, up to the second end 20b of the support <NUM>. In such a case, above the heat pipe <NUM>, for example, the heat dissipation portion <NUM> or the heat dissipation portion <NUM> may be located up to the second end 20b of the support <NUM>. The heat pipe <NUM> is disposed along the channel <NUM> at an inner portion of at least one of the support <NUM> or the heat dissipation portion <NUM>.

The antenna device <NUM> according to the above-described embodiment and each of the variations is described as including any one of the heat dissipation portions <NUM> and <NUM> and the heat pipes <NUM>, but may include two or more types of heat dissipation mechanisms, such as, for example, the heat dissipation portions <NUM> and heat pipes <NUM>.

In the antenna device <NUM> according to the above-described embodiment and each of variations, the connecting portions <NUM> are described as the solid rod shape bodies, but may be, for example, hollow tubular bodies. Making the inner portion of each of the connecting portions <NUM> hollow makes it possible to contribute to weight reduction of the antenna device <NUM>. As the same as and/or similar to the heat pipes <NUM> described above, the hollow may be sealed with the cooling medium <NUM>, and thus the radiation characteristics in the connecting portions <NUM> can be further enhanced.

<FIG> and <FIG> are cross-sectional views each schematically illustrating the antenna device according to a third and a fourth variations of the embodiment. <FIG> and <FIG> also illustrate cross-sections at the same position as in <FIG>. Also, in <FIG> and <FIG>, to facilitate understanding of the relationship between the outer shape of the support <NUM> and the outer shape of the antenna portion <NUM>, the circle having a shape corresponding to the outer shape of the antenna portion <NUM> is given.

As illustrated in <FIG>, the antenna device <NUM> according to the third variation differs from each antenna device <NUM> described above in that the shape of the cross-section (outer shape) of the support <NUM> is circular. The support <NUM> constituting the antenna device <NUM> according to the third variation has an outer appearance having a cylindrical shape. The antenna device <NUM> according to the fourth variation illustrated in <FIG> differs from each antenna device <NUM> described above in that the shape of the cross-section (outer shape) of the support <NUM> is a hexagon. The support <NUM> constituting the antenna device <NUM> according to the fourth variation has an outer appearance having a hexagonal prism shape. In this way, the support <NUM> may have a pillar shape elongated in the Z axis direction, and the shape of the support <NUM> is not particularly limited.

The antenna device <NUM> illustrated in <FIG> includes the plurality of channels <NUM> that are through holes each having a cylindrical shape. The antenna device <NUM> illustrated in <FIG> includes the plurality of channels <NUM> that are through holes each having a cross-section of a hexagonal shape. The channels <NUM> formed in the support <NUM> may include the plurality of channels <NUM> extending in the Z axis direction. A shape of each of the channels <NUM> and a shape of each of the heat dissipation portions <NUM> associated with an array of the channels <NUM> are not particularly limited. For example, the shapes may be deformed by a shape of the wall surface of the construction in which the support <NUM> is installed. When the wall surface of the construction is, for example, curved, the support <NUM> may also be deformed to conform the wall surface of the construction. When the support <NUM> has a shape to conform the wall surface of a building, a portion of the support <NUM> protruding from the wall surface is reduced, and thus, a probability that the support <NUM> is damaged or deformed due to collision of an object or the like is reduced. Harmony with an outer appearance of the building is maintained.

<FIG> is a cross-sectional view schematically illustrating the antenna device according to a fifth variation of the embodiment. <FIG> also illustrates a cross-section at the same position as in <FIG>. Also in <FIG>, to facilitate understanding of the relationship between the outer shape of the support <NUM> and the outer shape of the antenna portion <NUM>, a circle having a shape corresponding to the outer shape of the antenna portion <NUM> is given.

The support <NUM> constituting the antenna device <NUM> illustrated in <FIG> includes a heat dissipation portion <NUM>, a plurality of first fin members <NUM>, and a plurality of second fin members <NUM>. The heat dissipation portion <NUM> is located at a center of the support <NUM>. A shape of the cross-section of the heat dissipation portion <NUM> is rectangular (square in the case of <FIG>). The plurality of first fin members <NUM> and the plurality of second fin members <NUM> are fixed to side surfaces of the heat dissipation portion <NUM> at approximately equal intervals.

The plurality of first fin members <NUM> are disposed on the side surface of the heat dissipation portion <NUM> in the Y direction. The plurality of second fin members <NUM> are disposed on the side surface of the heat dissipation portion <NUM> in the X direction. The plurality of first fin members <NUM> are fixed to the side surface of the heat dissipation portion <NUM> perpendicular to the X direction. The plurality of second fin members <NUM> are fixed to the side surface of the heat dissipation portion <NUM> perpendicular to the Y direction.

An end portion of each of the plurality of the first fin members <NUM> opposite to the heat dissipation portion <NUM> is oriented away from the side surface of the heat dissipation portion <NUM>. An end portion of each of the plurality of the second fin members <NUM> opposite to the heat dissipation portion <NUM> is oriented away from the side surface of the heat dissipation portion <NUM>. A length of each of the plurality of first fin members <NUM> from the side surface of the heat dissipation portion <NUM> to the end portion is the same, but the length may be changed along with the outer shape of the antenna portion <NUM>. For example, a space between two first fin members <NUM> is the channel <NUM>. A space between two second fin members <NUM> is the channel <NUM>. That is, the antenna device <NUM> illustrated in <FIG> includes the support <NUM> including the plurality of first fin members <NUM> whose adjacent ones sandwich the channel <NUM> and the plurality of second fin members <NUM> whose adjacent ones sandwich the channel <NUM>. The plurality of first fin members <NUM> are located at intervals in the Y axis direction. Each of the first fin members <NUM> extends along a ZX plane from the heat dissipation portion <NUM>, and the channel <NUM> is located between adjacent ones of the first fin members <NUM>.

On the other hand, the plurality of second fin members <NUM> are located at intervals in the X axis direction. Each of the second fin members <NUM> extends along a YZ plane from the heat dissipation portion <NUM> and the first fin members <NUM>, and the channel <NUM> is located between adjacent ones of the second fin members <NUM>.

The antenna device <NUM> according to the present variation differs from each antenna device <NUM> according to the embodiment and the variations described above in that a plurality of the channels <NUM> are located outside the support <NUM>. In this way, even when the plurality of channels <NUM> are located outside the support <NUM>, the antenna device <NUM> can be properly dissipated.

Although the embodiments of the present disclosure have been described above, the present disclosure is not limited to the embodiments described above, and various modifications can be made without departing from the scope of the claims.

<FIG> is a diagram for comparing radiation characteristics of antenna devices. In <FIG>, in experimental examples <NUM> to <NUM>, the influence of differences in shape of the support <NUM> on the heat dissipation performance was evaluated. In experimental examples <NUM> and <NUM> to <NUM>, the influence of differences in the length L of the connecting portions <NUM> illustrated in <FIG> on the heat dissipation performance was evaluated. In experimental examples <NUM> and <NUM>, differences of the heat dissipation performance due to the presence of the heat pipes <NUM> (see <FIG> and <FIG>) were compared.

In the experimental example <NUM>, in the antenna device <NUM> illustrated in <FIG>, the support <NUM> having a quadrangular prism shape, in which lengths in the X axis direction, the Y axis direction, and the Z axis direction are <NUM>, <NUM>, and <NUM>, respectively, was used. An aluminum alloy having a coefficient of thermal conductivity = <NUM> W/(m•K) was used as a material of the support <NUM> and the connecting portions <NUM>. A thickness of the partition <NUM> = <NUM>, a dimension of the channels <NUM> in a cross-sectional view along the XY plane = <NUM> x <NUM>, and the length of the connecting portions <NUM> illustrated in <FIG> L = <NUM>. In the experimental examples <NUM> to <NUM>, the antenna device <NUM> having the same dimensions as in the experimental example <NUM> was prepared except that the length L of the connecting portions <NUM> was changed.

In the experimental example <NUM>, in the antenna device <NUM> illustrated in <FIG>, the support <NUM> having a cylindrical shape was used, in which the first end 20a and the second end 20b along the XY plane were circular with a diameter of <NUM> and a length in the Z axis direction was <NUM>. A cross-sectional shape of each of the channels <NUM> along the XY plane was circular with a diameter of <NUM>, and the length L of each of the connecting portions <NUM> illustrated in <FIG> = <NUM>.

In the experimental example <NUM>, in the antenna device <NUM> illustrated in <FIG>, the support <NUM> having a hexagonal prism shape was used. in which the first end 20a and the second end 20b along the XY plane were equilateral hexagons having substantially the same cross-sectional area as that of the support <NUM> according to the experimental example <NUM>, and the length in the Z axis direction was <NUM>. The cross-sectional shape of each of the channels <NUM> along the XY plane was an equilateral hexagon having a cross-sectional area similar to that of each of the channels <NUM> according to the experimental example <NUM>, and the length L of each of the connecting portions <NUM> illustrated in <FIG> = <NUM>.

In the experimental example <NUM>, the antenna device <NUM> to which the heat pipes <NUM> having the coefficient of thermal conductivity = <NUM> W/(m•K) was applied was used instead of the heat dissipation portions <NUM> of the antenna device <NUM> according to the experimental example <NUM>.

Note that in <FIG>, results obtained by measuring each of the maximum temperatures of the antenna device <NUM> under the same energization condition of the antenna device <NUM> according to each experimental example are shown. Here, the term "maximum temperature" refers to a temperature at a site where a surface temperature is highest in the antenna element housed in the antenna portion <NUM>.

As shown in <FIG>, when comparing the experimental examples <NUM> to <NUM>, in the antenna device <NUM> according to the experimental example <NUM> using the support <NUM> having the quadrangular prism shape, the maximum temperature of the antenna portion <NUM> was reduced as compared with the antenna device <NUM> according to the experimental examples <NUM> and <NUM>. It is conceivable that this is because the areas of the first end 20a and the second end 20b are different, and the surface area of the partition wall <NUM> in contact with the channel <NUM> is different. Note that it was confirmed that the antenna device <NUM> according to the experimental examples <NUM> and <NUM> also had the radiation characteristics suitable for actual use.

It was confirmed that when comparing the experimental examples <NUM> and <NUM> to <NUM>, in the antenna device <NUM> according to the experimental example <NUM> with the length L = <NUM>, the maximum temperature of the antenna portion <NUM> was lowest and indicated the minimum value. Note that it was confirmed that the antenna device <NUM> according to the experimental examples <NUM>, <NUM> and <NUM> to <NUM> also had the radiation characteristics suitable for actual use.

When comparing the experimental examples <NUM> and <NUM>, the experimental example <NUM> to which the heat pipes <NUM> were applied further reduced the maximum temperature of the antenna portion <NUM> as compared with the experimental example <NUM>. It was confirmed that the antenna device <NUM> to which the heat pipes <NUM> were applied had higher radiation characteristics as compared with the antenna device <NUM> to which the heat pipes <NUM> were not applied.

Claim 1:
An antenna device (<NUM>) comprising:
an antenna portion (<NUM>);
a connecting portion (<NUM>); and
a support (<NUM>), wherein
the connecting portion (<NUM>) connects the antenna portion (<NUM>) and the support (<NUM>),
characterized in that the support (<NUM>) is located above the antenna portion (<NUM>) and comprises a channel (<NUM>) extending from an inflow opening located opposite to the antenna portion (<NUM>) to an outflow opening located farther from the antenna portion (<NUM>) than the inflow opening.