Directional coupler

Disclosed is a directional coupler including a first hollow portion that is disposed in a first ground conductor and is arranged directly above a first signal conductor and a second signal conductor, and that is constructed of a discontinuous structure that has a function of delaying the phase and that is small with respect to the one-quarter wavelength of an operating frequency.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a directional coupler used in a microwave band or the like.

2. Description of Related Art

A directional coupler is widely used in order to carry out monitoring of electric power. As a directional coupler, there is a directional coupler having a structure of broadside-coupling two lines (for example, refer to the following nonpatent reference 1). By broadside-coupling lines this way, a directional coupler can be implemented.

RELATED ART DOCUMENT

However, the following problems arise in conventional technologies. In a case in which a directional coupler is constructed of a microstrip line or a triplate line, there is a case in which the reflection characteristic and the isolation quantity of the directional coupler are minimized and the coupled line impedance maximizing the coupling amount is lower than the terminal impedance connected to each terminal of the coupler because of constraints on manufacturing, such as a substrate thickness and a line width. A problem is that because when the coupled line impedance is lower than the terminal impedance, the passing phase at the time of an even mode operation leads against that at the time of an odd mode operation, a phase difference occurs between the passing phase at the time of the even mode operation and that at the time of the odd mode operation, and hence the directivity degrades.

SUMMARY OF THE INVENTION

The present invention is made in order to solve the above-mentioned problems, and it is therefore an object of the present invention to provide a directional coupler that can provide good directivity even when its coupled line impedance is lower than a terminal impedance because of constraints on manufacturing.

In accordance with the present invention, there is provided a directional coupler including: a first signal conductor; a second signal conductor that is arranged on a plane different from that on which the first signal conductor is arranged, and that is arranged in parallel with the first signal conductor; a ground conductor that is isolated from the first signal conductor and the second signal conductor, and that is arranged in a direction which is identical with respect to the first signal conductor and the second signal conductor; and a reactive element that is disposed in the ground conductor and is arranged directly below the first signal conductor and the second signal conductor, and that is comprised of a discontinuous structure that has a function of delaying a phase and that is small with respect to the one-quarter wavelength of an operating frequency.

The directional coupler in accordance with the present invention includes the reactive element that is disposed in the ground conductor and is arranged directly below the first signal conductor and the second signal conductor, and that is comprised of a discontinuous structure that has a function of delaying the phase and that is small with respect to the one-quarter wavelength of the operating frequency. Therefore, even when the coupled line impedance is lower than the terminal impedance, the reactive element disposed directly below the first signal conductor and the second signal conductor makes it possible to make the passing phase at the time of an even mode operation match that at the time of an odd mode operation because a plane of symmetry between the first signal conductor and the second signal conductor serves as an electric wall at the time of the odd mode operation and hence the passing phases do not vary without being affected by the influence of the reactive element, while the phase is delayed while being affected by the influence of the reactive element at the time of the even mode operation as compared with a case in which the reactive element is not formed. Therefore, there is provided an advantage of being able to improve the directivity of the directional coupler.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereafter, the preferred embodiments of a directional coupler in accordance with the present invention will be explained with reference to the drawings. In each of the views, the same reference numerals refer to the same elements or like elements.

FIG. 1is an exploded perspective view showing a directional coupler in accordance with Embodiment 1, andFIG. 2is a perspective view showing the directional coupler in accordance with Embodiment 1. InFIGS. 1 and 2, reference character strings1000ato1000edenote dielectric substrates, a reference numeral1001denotes a first signal conductor disposed on a surface of the dielectric substrate1000c, a reference numeral1002denotes a second signal conductor disposed on a surface of the dielectric substrate1000d. A reference numeral1101denotes a first input output terminal disposed in the first signal conductor1001, a reference numeral1102denotes a second input output terminal disposed in the first signal conductor1001. A reference numeral1103denotes a third input output terminal disposed in the second signal conductor1002, and a reference numeral1104denotes a fourth input output terminal disposed in the second signal conductor1002.

A reference numeral1201denotes a first ground conductor disposed on a surface of the dielectric substrate1000b, and a reference numeral1202denotes a second ground conductor disposed on a surface of the dielectric substrate1000e. A reference numeral1301denotes a first hollow portion in which a part of the first ground conductor1201is removed, and a reference numeral1302denotes a second hollow portion in which a part of the second ground conductor1202is removed. The length of each side of the first and second hollow portions1301and1302is sufficiently smaller than one fourth of a free space wavelength at an operating frequency. For example, the length of each side of the first and second hollow portions is 1/10 or less of the free space wavelength.

FIG. 3is a perspective diagram showing the directional coupler in accordance with Embodiment 1, andFIG. 4is a cross-sectional view showing a cross section A-A′ ofFIG. 3. Referring toFIGS. 1 to 4, the first signal conductor1001and the second signal conductor1002are arranged in such a way that they can be seen overlapping each other with respect to a vertical direction, and they construct a broadside coupling portion.

Because the direction coupler in accordance with this Embodiment 1 is symmetrical with respect to a cross section B-B′ inFIG. 4, an even/odd mode analysis can be applied. A cross-sectional view in a case in which the cross section B-B′ shown inFIG. 4is made to serve as a magnetic wall/electric wall, i.e., at the time of an even/odd mode operation is shown inFIG. 5. The cross section B-B′ shown inFIG. 5serves as a magnetic wall at the time of an even mode operation, and serves as an electric wall at the time of an odd mode operation.

FIG. 6is an exploded perspective view showing a conventional directional coupler.FIG. 7is a perspective view showing the conventional directional coupler. InFIGS. 6 and 7, reference character strings9000ato9000edenote dielectric substrates, a reference numeral9001denotes a first signal conductor disposed on a surface of the dielectric substrate9000b, a reference numeral9002denotes a second signal conductor disposed on a surface of the dielectric substrate9000c. A reference numeral9101denotes a first input output terminal disposed in the first signal conductor9001, a reference numeral9102denotes a second input output terminal disposed in the first signal conductor9001. A reference numeral9103denotes a third input output terminal disposed in the second signal conductor9002, and a reference numeral9104denotes a fourth input output terminal disposed in the second signal conductor9002. A reference numeral9201denotes a first ground conductor disposed on a surface of the dielectric substrate9000a, and a reference numeral9202denotes a second ground conductor.

FIG. 8is a perspective diagram showing the conventional directional coupler, andFIG. 9is a cross-sectional view showing a cross section A-A′ ofFIG. 8. Referring toFIGS. 6 to 9, the first signal conductor9001and the second signal conductor9002are arranged in such a way that they can be seen overlapping each other with respect to a vertical direction, and they construct a broadside coupling portion.

Because the conventional direction coupler is symmetrical with respect to a cross section B-B′ shown inFIG. 9, an even/odd mode analysis can be applied. A cross-sectional view in a case in which the cross section B-B′ shown inFIG. 9is made to serve as a magnetic wall/electric wall, i.e., at the time of an even/odd mode operation is shown inFIG. 10. The cross section B-B′ shown inFIG. 10serves as a magnetic wall at the time of an even mode operation, and serves as an electric wall at the time of an odd mode operation.

When the impedance of the line in the broadside coupling portion at the time of the even mode operation in which the cross section B-B′ serves as a magnetic wall is expressed by Z′eand the impedance of the line in the broadside coupling portion at the time of the odd mode operation in which the cross section B-B′ serves as an electric wall is expressed by Z′o, the coupled line impedance Z′ is given by the following equation (1).
Z′=√{square root over (Z′eZ′o)}  (1)

Further, when the reflection characteristic at the time of the even mode operation is expressed by S11eand the pass characteristic at the time of the even mode operation is expressed by S21e, and the reflection characteristic at the time of the odd mode operation is expressed by S11oand the pass characteristic at the time of the odd mode operation is expressed by S21o, the reflection characteristic S11, the pass characteristic S21, the coupling characteristic S31, and the isolation characteristic S41of the directional coupler are given by the following equations (2) to (5) respectively.
S11=(S11e+S11o)/2  (2)
S21=(S21e+S21o)/2  (3)
S31=(S11e−S11o)/2  (4)
S41=(S21e−S21o)/2  (5)

Further, the directivity D of the directional coupler is calculated according to the following equation (6), and the larger value this directivity has, the better directivity the directional coupler has.
D=20×log10(|S31|)−20×log10(|S41|)  (6)
By designing the broadside coupling portion in such a way that the coupled line impedance Z′ expressed by the equation (1) becomes equal to the terminal impedance Zoof each of the first through fourth input output terminals9101to9104, the reflection characteristic and the isolation quantity of the directional coupler can be minimized while the coupling amount can be maximized.

FIG. 11shows examples of the calculation of the phase passing from the first input output terminal9101to the second input output terminals9102when the impedance Z′eof the line at the time of the even mode operation is 100Ω, the coupled line length is 30 degrees, and the terminal impedance of each of the first and second input output terminals9101and9102is 50Ω, and the phase passing from the first input output terminal9101to the second input output terminals9102when the impedance Z′oof the line at the time of the odd mode operation is 25Ω, the coupled line length is 30 degrees, and the terminal impedance of each of the first and second input output terminals9101and9102is 50Ω.

In this case, because it can be seen from the equation (1) that the coupled line impedance is 50Ω and is equal to the terminal impedance of each input output terminal, the passing phase at the time of the even mode operation matches that at the time of the odd mode operation, and the passing amount at the time of the even mode operation similarly matches that at the time of the odd mode operation, as shown inFIG. 11.

Although the isolation characteristic can be determined according to the equation (5), the isolation characteristic of the directional coupling coupler using the coupled line satisfying these conditions is 0 and the directivity of the directional coupler is infinite because the amplitudes of S21eand S21oare equal to each other and their passing phases are also equal to each other.

However, there is a case in which the coupled line impedance cannot be made to be equal to the terminal impedance because of constraints on manufacturing, such as a substrate thickness and a line width. It is assumed hereafter that the line width cannot be thinned because of constraints on manufacturing, and the impedance Z′oat the time of the even mode operation and the impedance Z′oat the time of the odd mode operation are 80Ω and 20Ω respectively. At this time, the coupled line impedance is 40Ω according to the equation (1). On the other hand, because the impedance of each of circuits connected before and after the directional coupler is typically 50Ω, the terminal impedance of the directional coupler at this time is 50Ω.

An example of the calculation of the phase passing from the first input output terminals9101to the second input output terminals9102is shown inFIG. 12. Although the amplitude of the passage S21eat the time of the even mode operation is nearly equal to that of the passage S21oat the time of the odd mode operation because the coupled line length is short, the passing phase at the time of the odd mode operation lags behind that at the time of the even mode operation and the passing phase difference becomes large, as shown inFIG. 12.

From this fact and the equation (5), in the conventional directional coupler, when the coupled line impedance becomes lower than the terminal impedance connected to each terminal of the coupler because of constraints on manufacturing, the passages (S21eand S21o) at the times of the even and odd mode operations do not cancel each other, and hence the isolation quantity increases and the directivity of the directional coupler degrades. More specifically, a problem is that when the coupled line impedance becomes lower than the terminal impedance because of constraints on manufacturing or the like, the directivity of the directional coupler degrades.

In contrast, in the directional coupler in accordance with this Embodiment 1, the first hollow portion1301that operates as a reactive element is disposed in the first ground conductor1201and the second hollow portion1302which operates as a reactive element is disposed in the second ground conductor1202. A reactive element represents a structure having an effect of delaying the passing phase of a signal passing through the reactive element, as compared with a typical straight line in which no reactive element exists. In accordance with this Embodiment 1, such reactive elements are implemented by a first hollow portion1301partially disposed in the first ground conductor1201and constructed of a discontinuous structure that has a function of delaying the phase and is sufficiently small with respect to the one-quarter wavelength of an operating frequency and a second hollow portion1302partially disposed in the second ground conductor1202and constructed of a discontinuous structure that has a function of delaying the phase and is sufficiently small with respect to the one-quarter wavelength of the operating frequency. A path through which a current flows in the first ground conductor1201disposed above the first signal conductor1001at the time of the even mode operation of the directional coupler in accordance with Embodiment 1 is shown inFIG. 13. Further, an electric field distribution in a cross section A-A′ shown inFIG. 13is shown inFIG. 14.

Because a cross section B-B′ shown inFIG. 14serves as a magnetic wall, electric lines of force occurring from the signal line are terminated at the first ground conductor1201. Therefore, as shown inFIG. 13, the current flowing through the first ground conductor1201flows in such a way as to bypass the first hollow portion1301. In contrast, in the conventional directional coupler in which no first hollow portion1301is disposed, the current flowing through the first ground conductor9201does not bypass. More specifically, in the directional coupler in accordance with this Embodiment 1, the passing phase at the time of the even mode operation can be delayed as compared with that at the time of the even mode operation of the conventional directional coupler. More specifically, the first hollow portion1301operates as a reactive element.

An electric field distribution in the cross section A-A′ at the time of the odd mode operation in which it is assumed that the cross section B-B′ shown inFIG. 5serves as an electric wall is shown inFIG. 15. It is determined that at the time of the odd mode operation in the directional coupler in accordance with Embodiment 1, the gap between the first signal conductor1001and the cross section B-B′ which serves as an electric wall is smaller than the gap between the first signal conductor1001and the first ground conductor1201, as shown inFIG. 15. Electric lines of force occurring from the first signal conductor1001exist only between the first signal conductor1001and the electric wall of the cross section B-B′. Therefore, a return current of the current flowing through the first signal conductor1001flows through the cross section B-B′ which serves as the electric wall regardless of the presence or absence of the first hollow portion1301disposed in the first ground conductor1201. More specifically, the passing phase at the time of the odd mode operation in the directional coupler in accordance with Embodiment 1 becomes equal to that at the time of the odd mode operation in the conventional directional coupler without the first hollow portion1106.

As a result, while the passing phase at the time of the even mode operation is delayed by the first hollow portion1301, there is no change in the passing phase at the time of the odd mode operation. Therefore, by determining the size of the first hollow portion1301in such a way that the passing phase at the time of the even mode operation matches that at the time of the odd mode operation even when the coupled line impedance is lower than the terminal impedance, the passages at the times of the even and odd mode operations can be made to cancel each other, and hence the isolation quantity can be decreased. Therefore, better directivity can be provided.

Further, although only one hollow portion is disposed as each of the first and second hollow portions1301and1302in the directional coupler in accordance with Embodiment 1, this embodiment is not limited to this example. As shown inFIG. 16, two or more first hollow portions1301can be arranged.FIG. 16is a top perspective view showing another directional coupler in accordance with Embodiment 1. In the figure, reference character strings1301a,1301b, and1301cdenote the first hollow portions formed in the first ground conductor1201. Further, although not shown inFIG. 16, second hollow portions are disposed at three positions in the second ground conductor1202which are symmetrical to those in the first hollow portions1301a,1301b, and1301crespectively. Because this structure makes it possible to further delay the passing phase at the time of the even mode operation as compared with the case in which only one hollow is disposed as each hollow portion, the passing phase at the time of the even mode operation can be easily made to match that at the time of the odd mode operation, and hence the design can be facilitated.

Although the first hollow portion1301in accordance with Embodiment 1 is shaped like a rectangle, this embodiment is not limited to this example. The shape of the first hollow portion1301should just be made to match that of the second hollow portion1302.

Further, although the first ground conductor1201and the second ground conductor1202are arranged in Embodiment 1, this embodiment is not limited to this example. The same advantages are provided as long as at least one of the ground conductors is arranged as shown inFIG. 17. In the case in which the number of ground conductors is reduced to one, a cost reduction can be accomplished because the number of layers can be reduced.

As mentioned above, the directional coupler in accordance with this Embodiment 1 includes the first hollow portion1301that is disposed in the first ground conductor1201and is arranged directly above the first signal conductor1001and the second signal conductor1002, and that is constructed of a discontinuous structure that has a function of delaying the phase and that is small with respect to the one-quarter wavelength of an operating frequency, and the second hollow portion1302that is disposed in the second ground conductor1202and is arranged directly below the first signal conductor1001and the second signal conductor1002, and that is constructed of a discontinuous structure that has a function of delaying the phase and that is small with respect to the one-quarter wavelength of the operating frequency. Therefore, even when the coupled line impedance is lower than the terminal impedance, the first hollow portion1301disposed directly above the first signal conductor1001and the second hollow portion1302disposed directly below the second signal conductor1002make it possible to make the passing phase at the time of the even mode operation match that at the time of the odd mode operation because the plane of symmetry between the first signal conductor1001and the second signal conductor1002serves as an electric wall at the time of the odd mode operation and hence the passing phases do not vary without being affected by the influence of the first and second hollow portions1301and1302, while the phase is delayed while being affected by the influence of the first and second hollow portions1301and1302at the time of the even mode operation as compared with a case in which the first and second hollow portions1301and1302are not formed. Therefore, the directivity of the directional coupler can be improved.

In accordance with this Embodiment 1, the first reactive element is constructed of the first hollow portion1301in which a part of the first ground conductor1201is removed, and the second reactive element is constructed of the second hollow portion1302in which a part of the second ground conductor1202is removed. Therefore, the first hollow portion1301in which a part of the first ground conductor1201is removed and the second hollow portion1302in which a part of the second ground conductor1202can easily construct the reactive elements.

In accordance with this Embodiment 1, two or more hollows are disposed as the first and second hollow portions1301and1302. Therefore, the passing phases can be easily made to match each other, and a directional coupler with good directivity can be designed easily.

FIG. 18is a perspective diagram showing a directional coupler in accordance with Embodiment 2.FIG. 19is a cross-sectional view showing a cross section A-A′ shown in FIG.18. InFIGS. 18 and 19, a reference numeral1000denotes a dielectric substrate, a reference numeral1001denotes a first signal conductor disposed in the dielectric substrate1000, and a reference numeral1002denotes a second signal conductor disposed in the dielectric substrate1000. A reference numeral1101denotes a first input output terminal, a reference numeral1102denotes a second input output terminal, a reference numeral1103denotes a third input output terminal, and a reference numeral1104denotes a fourth input output terminal.

Further, a reference numeral1401denotes a first floating conductor disposed in a first hollow portion1301, a reference numeral1402denotes a second floating conductor disposed in a second hollow portion1302, and a reference numeral1501denotes a connecting conductor that connects between the first floating conductor1401and the second floating conductor1402. The length of each side of the first and second hollow portions1301and1302is 1/10 or less of a free space wavelength at an operating frequency.

Because the directional coupler in accordance with this Embodiment 2 is symmetrical with respect to a cross section B-B′ shown inFIG. 19, an even/odd mode analysis can be applied. A cross-sectional view when the cross section B-B′ shown inFIG. 19serves as a magnetic wall/electric wall, that is, at the time of an even/odd mode operation is shown inFIG. 20. The cross section B-B′ shown inFIG. 20serves as a magnetic wall at the time of an even mode operation and serves as an electric wall at the time of an odd mode operation.

At the time of the even mode operation, the cross section B-B′ serves as a magnetic wall. More specifically, because the connecting conductor1501and the first floating conductor1401are connected to no conductors, no influence is exerted on an electric field propagating through the first signal conductor1001. Therefore, an electric field distribution as shown inFIG. 21is provided. Because the cross section B-B′ serves as a magnetic wall as shown inFIG. 21, electric lines of force occurring from the signal line are terminated at the first ground conductor1201. Therefore, as shown inFIG. 22, a current flowing through the first ground conductor1201flows in such away as to bypass the first hollow portion1301. More specifically, in the directional coupler in accordance with this Embodiment 2, the passing phase at the time of the even mode operation can be delayed as compared with that at the time of the even mode operation of a conventional directional coupler, like in the case of the directional coupler in accordance with above-mentioned Embodiment 1.

An electric field distribution at the time of the odd mode operation in which it is assumed that the cross section B-B′ shown inFIG. 20serves as an electric wall is shown inFIG. 23. Because the connecting conductor1501is connected to the electric wall at the time of the odd mode operation of the directional coupler in accordance with Embodiment 2, the first floating conductor1401and the connecting conductor1501operate as ground conductors. Therefore, electric lines of force occurring in the first signal conductor1001are terminated at the cross section B-B′ and at the first floating conductor1401. Therefore, a return current of the current flowing through the first signal conductor1001flows through the cross section B-B′ which serves as an electric wall, and also through the first ground conductor1201and the first floating conductor1401, as shown inFIG. 24. Therefore, the passing phase at the time of the odd mode operation in the directional coupler in accordance with Embodiment 2 becomes equal to that at the time of the odd mode operation in the conventional directional coupler.

As a result, while the passing phase at the time of the even mode operation is delayed, there is no change in the passing phase at the time of the odd mode operation. Therefore, by determining the sizes of the first hollow portion1301and the first floating conductor1401in such a way that the passing phase at the time of the even mode operation matches that at the time of the odd mode operation even when the coupled line impedance is lower than the terminal impedance, the passages at the times of the even and odd mode operations can be made to cancel each other, and hence the directivity of the directional coupler can be improved.

Although only one conductor is used as the connecting conductor1501in Embodiment 2, this embodiment is not limited to this example. As shown inFIG. 25, two or more connecting conductors can be used.FIG. 25is a top perspective view showing another directional coupler in accordance with Embodiment 2. In the figure, a reference numeral1601denotes a first connecting conductor that connects between the first floating conductor1401and the second floating conductor1402, and a reference numeral1602denotes a second connecting conductor that connects between the first floating conductor1401and the second floating conductor1402. Because each of the first and second floating conductors1401and1402at the time of the odd mode operation is connected to an electric wall at two or more points thereof in the case in which the two or more connecting conductors are used, each of the first and second floating conductors operates as a ground conductor more ideally than that in the case in which only one connecting conductor is used, and hence the design can be facilitated.

Further, although only one hollow is used as each of the first and second hollow portions1301and1302, only one conductor is used as each of the first and second floating conductors1401and1402, and only one conductor is used as the connecting conductor1501in Embodiment 2, this embodiment is not limited to this example. As shown inFIG. 26, two or more hollows can be used as each of the first and second hollow portions, two or more conductors can be used as each of the first and second floating conductors, and two or more connecting conductors can be used.FIG. 26is a top perspective view showing another directional coupler in accordance with Embodiment 2. In the figure, reference character strings1301a,1301b, and1301cdenote first hollow portions formed in the first ground conductor1201. Reference character strings1401a,1401b, and1401cdenote first floating conductors disposed in the first hollow portions1301a,1301b, and1301crespectively. In addition, although not shown inFIG. 26, second hollow portions are disposed at three positions in the second ground conductor1202which are symmetrical to those in the first hollow portions1301a,1301b, and1301crespectively, and second floating conductors are disposed at three positions in the second ground conductor1202which are symmetrical to those in the first floating conductors1401a,1401b, and1401crespectively. A reference character string1501cdenotes a first connecting conductor that connects between the first floating conductor1401cand the second floating conductor symmetrical to the first floating conductor, a reference character string1501bdenotes a first connecting conductor that connects between the first floating conductor1401band the second floating conductor symmetrical to the first floating conductor, and a reference character string1501cdenotes a first connecting conductor that connects between the first floating conductor1401cand the second floating conductor symmetrical to the first floating conductor. Because this structure makes it possible to further delay the passing phase at the time of the even mode operation as compared with the case in which only one hollow is disposed as each hollow portion, the passing phase at the time of the even mode operation can be easily made to match that at the time of the odd mode operation, and hence the design can be facilitated.

Further, although in Embodiment 2 the first signal line1001and the second signal line1002are arranged in such a way that they have the same width and can be seen overlapping each other with respect to a vertical direction, this embodiment is not limited to this example. The first signal line and the second signal line can be arranged in such away that they have different widths and are out of alignment with each other with respect to a vertical direction, as shown inFIGS. 27 and 28. In this case, the same advantages can be provided.FIG. 27is a top perspective view showing another directional coupler in accordance with Embodiment 2.FIG. 28is a cross-sectional view taken on a cross section A-A′ ofFIG. 27. In the figure, a reference numeral2001denotes a first signal line and a reference numeral2002denotes a second signal line.

Further, although in Embodiment 2 the first hollow portion1301is formed in such a way that a central part of the first hollow portion1301is aligned with both a central part of the first signal line1001and a central part of the second signal line1002, this embodiment is not limited to this example. As shown inFIG. 29, the first hollow portion can be formed in such a way that an end part of the first hollow portion is aligned with both the central part of the first signal line1001and the central part of the second signal line1002.FIG. 29is a top perspective view showing another directional coupler in accordance with Embodiment 2. In the figure, a reference numeral1701denotes a first hollow portion formed in the first ground conductor1201in such a way that an end portion thereof is aligned with the central part of the first signal conductor1001. A reference numeral1801denotes a first floating conductor disposed in the first hollow portion1701. In addition, although not shown inFIG. 29, a second hollow portion is disposed at a position in the second ground conductor1202which is symmetrical to that in the first hollow portion1701, and a second floating conductor is disposed at a position in the second ground conductor1202which is symmetrical to that in the first floating conductor1801. A reference numeral1901denotes a connecting conductor that connects between the first floating conductor1801and the second floating conductor which is symmetrical to the first floating conductor1801.

As mentioned above, in accordance with this Embodiment 2, the first reactive element is comprised of the first floating conductor1401that is disposed in the first hollow portion1301in such a way as to be in non-contact with the first ground conductor1201, in addition to the first hollow portion1301, and the second reactive element is comprised of the second floating conductor1402that is disposed in the second hollow portion1302in such a way as to be in non-contact with the second ground conductor1202, in addition to the second hollow portion1302, and the first floating conductor1401and the second floating conductor1402are connected to each other via the connecting conductor1501. Therefore, an adjustment of the sizes and shapes of the first floating conductor1401and the second floating conductor1402, in addition to an adjustment of the first hollow portion1301and the second hollow portion1302, can make the passing phases match each other and can easily design a directional coupler with good directivity. Further, the connecting conductor1501maintains the electric balance between the first floating conductor1401and the second floating conductor1402, thereby providing better characteristics.

While the invention has been described in its preferred embodiments, it is to be understood that an arbitrary combination of two or more of the above-mentioned embodiments can be made, various changes can be made in an arbitrary component in accordance with any one of the above-mentioned embodiments, and an arbitrary component in accordance with any one of the above-mentioned embodiments can be omitted within the scope of the invention.