Floating potential conductor coupled quarter-wavelength coupled line type directional coupler comprising cut portion formed in ground plane conductor

In a quarter-wavelength coupled line type directional coupler including a first dielectric layer having first and second surfaces parallel to each other, a ground plane conductor is formed on the first surface of the first dielectric layer, and two coupled microstrip conductors each having a quarter wavelength are formed on the second surface of the first dielectric layer, arranging close to each other so as to be electromagnetically coupled with each other. Further, a second dielectric layer is formed on the second surface of the first dielectric layer, on which the coupled microstrip conductors are formed, and a floating potential conductor is formed on the second dielectric layer, arranging close to the microstrip conductors so as to be electromagnetically coupled with the coupled microstrip conductors. Then a cut portion is formed in the ground plane conductor so that the ground plane conductor is separated apart from the coupled microstrip conductors by a predetermined distance.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a quarter-wavelength coupled line type 
directional coupler, and in particular, to a floating potential conductor 
coupled quarter-wavelength coupled transmission line type directional 
coupler comprising a cut portion formed in a ground plane conductor. 
2. Description of the Related Art 
Conventionally, directional couplers have been used when constituting a 
90-degree combiner or divider. In particular, in a microwave circuit, the 
directional couplers are applied to various kinds of microwave circuits 
such as a balanced amplifier or a balanced mixer. FIGS. 17 and 18 show a 
conventional quarter-wavelength coupled line type directional coupler 
employing two microstrip lines arranged so as to be electromagnetically 
coupled with each other. 
Referring to FIGS. 17 and 18, a ground plane conductor 12 is formed on a 
semiconductor substrate 11, and then, a dielectric layer 21 is formed on 
the ground plane conductor 12. On the dielectric layer 21, two coupled 
microstrip conductors 31 and 32 are formed as separated apart by a 
predetermined distance so as to be electromagnetically coupled with each 
other. In the above-mentioned structure, each of the microstrip conductors 
31 and 32 has a length of a quarter wavelength, i.e., (1/4) .lambda.g 
(where .lambda.g is a guide wavelength) in the longitudinal direction. 
When analyzing the above-mentioned conventional directional coupler by a 
quasi-TEM approximation method (See J. Reed, et al. "A method of analysis 
of symmetrical four-port network" IRE Trans., MTT-4, 1968) according to 
the even-odd mode excitation method which is known to those skilled in the 
art, the directional coupler is excited with in-phase in the even mode, 
while it is excited with out-of-phase excitation in the odd mode. 
Characteristic impedances Zodd and Zeven respectively in the odd mode and 
even mode of the respective coupled transmission lines of the directional 
coupler are expressed by the following equations (1) and (2). 
##EQU1## 
where .epsilon. represents a dielectric constant of the dielectric layer 
21, .mu. represents a permeability of the dielectric layer 21, C.sub.1 
represents an electrostatic capacity between the microstrip conductors 31 
and 32 and the ground plane conductor 12, and C.sub.12 represents an 
electrostatic capacity between the microstrip conductors 31 and 32. 
The coupling factor K between the two microstrip lines of the conventional 
directional coupler can be expressed with the above-mentioned 
characteristic impedances Zodd and Zeven by the following equation (3). 
##EQU2## 
However, since the coupling factor K expressed by the equation (3) can not 
be further increased in the conventional directional coupler, it is 
difficult to obtain specifications of the structure for achieving equal 
power dividing and power combining. Therefore, such directional couplers 
have not been often used conventionally in apparatuses which include a 
monolithic microwave integrated circuit (referred to as an MMIC 
hereinafter). 
For the above-mentioned reasons, a hybrid ring employing a transmission 
line such as a microstrip line or the like has been widely used upon 
constructing a microwave circuit. However, the hybrid ring requires a 
large circuit area, and this results in that the microwave circuit to be 
implemented becomes relatively large. 
In order to overcome the above-mentioned drawbacks, there has been tried to 
perform a method for decreasing the circuit area thereof by laminating 
metal conductors and thin film dielectric layers on a semiconductor 
substrate with a multi-layer structure and by using the resulting product 
as a microstrip line. However, due to use of the thin film electric 
insulating layer, the width of the conductor of the resulting microstrip 
line becomes narrow. In the case of a 90-degree hybrid ring, it is 
necessary to provide a transmission line having a line length of one guide 
wavelength .lambda.g of the frequency to be used, and therefore, the 
insertion loss of the transmission line increases. In other words, there 
have been such a drawback that neither desired power distribution nor 
desired synthetic or combining characteristics cannot be obtained and such 
a problem that the loss is increased in the MMIC employing such a hybrid 
ring. 
SUMMARY OF THE INVENTION 
An essential object of the present invention is to solve the 
above-mentioned problems and to provide a quarter-wavelength coupled line 
type directional coupler having a coupling factor larger than that of the 
conventional example. 
According to one aspect of the present invention, there is provided a 
quarter-wavelength coupled line type directional coupler comprising: 
a first dielectric layer having first and second surfaces parallel to each 
other; 
a ground plane conductor formed on the first surface of said first 
dielectric layer; 
two coupled microstrip conductors each having a quarter wavelength which 
are formed on said second surface of said first dielectric layer, said 
coupled microstrip conductors being arranged close to each other so as to 
be electromagnetically coupled with each other; 
a second dielectric layer formed on the second surface of said first 
dielectric layer, on which said coupled microstrip conductors are formed; 
a floating potential conductor formed on said second dielectric layer, said 
floating potential conductor being arranged close to said microstrip 
conductors so as to be electromagnetically coupled with said coupled 
microstrip conductors; and 
a cut portion formed in said ground plane conductor so that said ground 
plane conductor is separated apart from said coupled microstrip conductors 
by a predetermined distance. 
In the above-mentioned directional coupler, a space portion is preferably 
formed in a part of said first dielectric layer between said cut portion 
of said ground plane conductor and said coupled microstrip conductors. 
In the above-mentioned directional coupler, the dielectric constant of said 
first dielectric layer is preferably set so as to be lower than the 
dielectric constant of said second dielectric layer. 
According to a further aspect of the present invention, there is provided a 
quarter-wavelength coupled line type directional coupler comprising: 
a dielectric layer having first and second surfaces parallel to each other; 
a ground plane conductor formed on the first surface of said dielectric 
layer; 
a cut portion formed in said ground plane conductor; 
two coupled microstrip conductors each having a quarter wavelength which 
are formed in said cut portion on said first surface of said dielectric 
layer, said coupled microstrip conductor being arranged close to each 
other so as to be electromagnetically coupled with each other; and 
a floating potential conductor formed on the second surface of said 
dielectric layer, said floating potential conductor being arranged close 
to said microstrip conductors so as to be electromagnetically coupled with 
said coupled microstrip conductors. 
According to a still further aspect of the present invention, there is 
provided a quarter-wavelength coupled line type directional coupler 
comprising: 
a dielectric layer having first and second surfaces parallel to each other; 
a ground plane conductor formed on the first surface of said dielectric 
layer; 
two coupled microstrip conductors each having a quarter wavelength which 
are formed on the second surface of said dielectric layer, said coupled 
microstrip conductors being arranged close to each other so as to be 
electromagnetically coupled with each other; 
a floating potential conductor formed in a part of said dielectric layer 
which is located between said coupled microstrip conductors and said 
ground plane conductor; and 
a cut portion formed in said ground plane conductor so that the ground 
plane conductor is separated apart, respectively, from said floating 
potential conductor and said coupled microstrip conductors by 
predetermined distances. 
In the above-mentioned directional coupler, a space portion is preferably 
formed in a part of the dielectric layer which is located between said cut 
portion of said ground plane conductor and said floating potential 
conductor. 
The above-mentioned directional coupler preferably further comprises a 
further dielectric layer having a dielectric constant higher than the 
dielectric constant of said dielectric layer, said further dielectric 
layer being formed on the first surface of said dielectric layer on which 
said coupled microstrip conductors are formed. 
The above-mentioned directional coupler preferably further comprises 
further ground plane conductors respectively formed on both side surfaces 
of each of said dielectric layer and said further dielectric layer so as 
to be connected to said ground plane conductor. 
According to a still more further aspect of the present invention, there is 
provided a quarter-wavelength coupled line type directional coupler 
comprising: 
a dielectric layer having first and second surfaces parallel to each other; 
a ground plane conductor formed on the first surface of said dielectric 
layer; 
two coupled microstrip conductors each having a quarter wavelength which 
are formed on the second surface of said dielectric layer, said coupled 
microstrip conductors being arranged close to each other so as to be 
electromagnetically coupled with each other; 
a cut portion formed in said ground plane conductor so that the ground 
plane conductor is separated apart from said coupled microstrip conductors 
by a predetermined distance; and 
a floating potential conductor formed on a part of the first surface of 
said dielectric layer which is located in said cut portion of said ground 
plane conductor. 
The above-mentioned directional coupler preferably further comprises 
further ground plane conductors respectively formed on both side surfaces 
of each of said dielectric layer and said further dielectric layer so as 
to be connected to said ground plane conductor. 
When substituting above-mentioned equations (1) and (2) into the equation 
(3), the following equation (4) representing a coupling factor K is 
obtained. 
##EQU3## 
The present inventor paid attention to the above-mentioned equation (4), 
and then provided in the present invention, in order to obtain a tight 
coupling factor K, a quarter-wavelength coupled line type four-port 
directional coupler having a structure for reducing the electrostatic 
capacity C.sub.1 and for increasing the electrostatic capacity C.sub.12. 
In each of the directional couplers in accordance with the present 
invention having the above-mentioned construction, no line of electric 
force exists between the above-mentioned floating potential conductor and 
the two coupled microstrip conductors in the even mode, wherein the two 
coupled microstrip conductors have the same electric potential as each 
other. 
With the above-mentioned arrangement, the electrostatic capacity C.sub.1 
between the two coupled microstrip conductors and the above-mentioned 
ground plane conductor can be reduced. On the other hand, in the odd mode, 
the floating potential conductor and the ground potential conductor have 
the same electric potential as each other, and at the same time, the 
electric potential of the floating potential conductor becomes zero, then 
the floating potential conductor operates as a ground plane conductor. As 
a result, the ground plane conductor and the two coupled microstrip 
conductors are put extremely close to each other, and then this increases 
the electrostatic capacity C.sub.12 between the two coupled microstrip 
conductors. Eventually, the electrostatic capacity C.sub.1 is reduced, and 
the electrostatic capacity C.sub.12 is increased. This results in increase 
in the coupling factor K of the directional coupler as is apparent from 
the equation (4).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The following describes several preferred embodiments of quarter-wavelength 
coupled line type four-port directional couplers of the present invention, 
which are applicable to MMICs, with reference to the attached drawings. 
First preferred embodiment 
FIG. 1 is a top plan view of a quarter-wavelength coupled line type 
four-port directional coupler in accordance with a first preferred 
embodiment of the present invention, while FIG. 2 is a top plan view of 
the directional coupler shown in FIG. 1 when both of a floating potential 
conductor 50 and a dielectric layer 22 are removed. Further, FIG. 3 is a 
longitudinal cross-sectional view of the directional coupler shown in FIG. 
1 taken along a line A-A', while FIG. 4 is a longitudinal cross-sectional 
view of the directional coupler shown in FIG. 1 taken along a line B-B'. 
In FIGS. 1 through 4, the same components as those shown in FIGS. 17 and 
18 are denoted by the same numerals as those of FIGS. 17 and 18. In the 
top plan views of FIGS. 1 and 2, components which are invisible when 
viewed from the upper side are depicted by dotted lines. 
As compared with the conventional directional coupler as shown in FIGS. 17 
and 18, the features of the directional coupler of the first preferred 
embodiment are as follows. As shown in FIG. 3, the floating potential 
conductor 50 having a length of .lambda.g/4 in the longitudinal direction 
is formed just above the coupled microstrip conductors 31 and 32 through a 
dielectric layer 22 on a dielectric layer 21, and further, a 
rectangular-shaped cut portion 12c is formed in the center part of a 
ground plane conductor 12 which is located just below the microstrip 
conductors 31 and 32. Therefore, the directional coupler of the first 
preferred embodiment can be called a floating potential conductor coupled 
quarter-wavelength coupled line type four-port directional coupler. 
As shown in FIGS. 1 through 4, there is formed on a semiconductor substrate 
11 the ground plane conductor 12, on which the rectangular-shaped 
dielectric layer 21 made of an organic electrical insulating material such 
as polyimide resin is formed. Then coplanar waveguides 51, 52, 53 and 54 
for inputting and outputting microwave signals are formed at the four 
corners of the semiconductor substrate 11, wherein the coplanar waveguide 
51 is formed at the top left side corner, the coplanar waveguide 52 is 
formed at the bottom left side corner, the coplanar waveguide 53 is formed 
at the top right side corner, and the coplanar waveguide 54 is formed at 
the bottom right side corner. 
The coplanar waveguide 51 is composed of a center conductor 41 and ground 
plane conductors 12 formed on both sides of the center conductor 41 on the 
semiconductor substrate 11. The coplanar waveguide 52 is composed of a 
center conductor 42 and ground plane conductors 12 formed on both sides of 
the center conductor 42 on the semiconductor substrate 11. The coplanar 
waveguide 53 is composed of a center conductor 43 and the ground plane 
conductors 12 formed on both sides of the center conductor 43 on the 
semiconductor substrate 11. The coplanar waveguide 54 is composed of a 
center conductor 44 and the ground plane conductors 12 formed on both 
sides of the center conductor 44 on the semiconductor substrate 11. 
Further, in the center part of the ground plane conductor 12, the 
rectangular-shaped cut portion 12c is formed by, for example, the lift-off 
process which is well known to those skilled in the art, in an area or a 
part just below the two microstrip conductors 31 and 32 which are formed 
later. In this case, the etching process may be used instead of the 
lift-off process. 
Further, on the dielectric layer 21, the two microstrip conductors 31 and 
32 are formed so as to be separated apart by a predetermined distance, so 
that the longitudinal directions of the conductors are parallel to each 
other and the two conductors 31 and 32 are arranged close so as to be 
electromagnetically coupled with each other. In this case, each of the 
microstrip conductors 31 and 32 has a length of (1/4) .lambda.g in the 
longitudinal direction. In practice, since the guide wavelength in the 
even mode and the guide wavelength in the odd mode are different from each 
other, the lengths of the microstrip conductors 31 and 32 in the 
longitudinal direction are set at a guide wavelength obtained by averaging 
both the guide wavelengths in the even and odd modes. 
An end of the microstrip conductor 31 is electrically connected to the 
center conductor 42 through a through-hole conductor 62 provided in a 
first through-hole, which is formed so as to penetrate through the 
dielectric layer 21 in the direction of the thickness thereof as shown in 
FIG. 4. Another end of the microstrip conductor 31 is electrically 
connected to the center conductor 41 through a through-hole conductor 61 
(shown in FIG. 2) provided in a second through-hole, which is formed so as 
to penetrate through the dielectric layer 21 in the direction of the 
thickness thereof in the same manner as that as described above. Further, 
an end of the microstrip conductor 32 is electrically connected to the 
center conductor 44 through a through-hole conductor 64 provided in a 
third through-hole, which is formed so as to penetrate through the 
dielectric layer 21 in the direction of the thickness thereof as shown in 
FIG. 4. Another end of the microstrip conductor 32 is electrically 
connected to the center conductor 43 through a through-hole conductor 63 
(shown in FIG. 2) provided in a fourth through-hole, which is formed so as 
to penetrate through the dielectric layer 21 in the direction of the 
thickness thereof in the same manner as that as described above. 
Further, the rectangular-shaped dielectric layer 22 made of the same 
electric insulating material as that of the dielectric layer 21 is formed 
on the dielectric layer 21, on which the two microstrip conductors 31 and 
32 are formed as described above. On the dielectric layer 22, there is 
formed just above the two microstrip conductors 31 and 32, the 
rectangular-shaped floating potential conductor 50 which has not only two 
sides having a length of (1/4) .lambda.g in the longitudinal direction in 
parallel with the longitudinal direction of the microstrip conductors 31 
and 32 but also two sides having a predetermined width perpendicular to 
the longitudinal direction of the microstrip conductors 31 and 32. As a 
result, the directional coupler of the first preferred embodiment is 
obtained. 
FIG. 5 is a longitudinal cross-sectional-view of the directional coupler 
shown in FIG. 1 taken along the line A-A' showing an electric field 
distribution in the even mode, while FIG. 6 is a longitudinal 
cross-sectional view of the directional coupler shown in FIG. 1 taken 
along the line A-A' showing an electric field distribution in the odd 
mode. 
For the operation in the even mode as shown in FIG. 5, the cut portion 12c 
is formed in the ground plane conductor 12 just below the microstrip 
conductors 31 and 32, thereby reducing the electrostatic capacity C.sub.1 
between the ground plane conductor 12 and the microstrip conductors 31 and 
32. The above arrangement is adopted for such a reason that the ground 
plane conductor 12 is sufficiently separated apart from the floating 
potential conductor 50, and therefore the possible influence of the ground 
plane conductor 12 on the floating potential conductor 50 can be 
electromagnetically ignored. As shown by the electric field distribution 
in FIG. 5, there exists no line of electric force between the floating 
potential conductor 50 and the microstrip conductors 31 and 32, and then 
this means that both of the conductors 31, 32 and 50 have the same 
electric potential as each other. 
The above-mentioned fact can be easily explained from such consideration 
that the Kirchhoff's law does not hold since only a displacement current 
flows from the two microstrip conductors 31 and 32 into the floating 
potential conductor 50 and no current flows out because of the same 
electric potentials of these conductors 31, 32 and 50, if a potential 
difference took place between each of the two microstrip conductors 31 and 
32 and the floating potential conductor 50. Therefore, the floating 
potential conductor 50 comes to have the same electric potential as that 
of the microstrip conductors 31 and 32, thereby allowing the electrostatic 
capacity C.sub.1 to be reduced in the even mode. 
On the other hand, the floating potential conductor 50 is formed on the 
dielectric layer 22 just above the microstrip conductors 31 and 32 for the 
operation in the odd mode as shown in FIG. 6. As shown in FIG. 6, there 
exists no line of electric force between the floating potential conductor 
50 and the ground plane conductor 12, and this means that both the 
conductors 50 and 12 have the same electric potential as each other. 
Furthermore, in the same manner as the above-mentioned consideration, the 
electric potentials of the two microstrip conductors 31 and 32 have the 
same absolute value and opposite polarities in the odd mode, the electric 
potential of the floating potential conductor 50 is to be zero in order to 
satisfy the Kirchhoff's law. For the purpose to make the above-mentioned 
conditions hold, the floating potential conductor 50 is separated 
sufficiently apart from the ground plane conductor 12 so as to 
sufficiently suppress the influence of the ground plane conductor 12 on 
the floating potential conductor 50. Therefore, the electric potential of 
the floating potential conductor 50 is made so as to be zero, and then the 
floating potential conductor 50 operates as a ground plane conductor in 
the odd mode. As a result, the electrode distance between the ground plane 
conductor 12 and 50 and the microstrip conductors 31 and 32 is extremely 
reduced, thereby increasing the electrostatic capacity C.sub.12. 
Eventually, in the directional coupler of the first preferred embodiment, 
the electrostatic capacity C.sub.1 is reduced by forming the cut portion 
12c in the ground plane conductor 12, while the electrostatic capacity 
C.sub.12 is increased by forming the floating potential conductor 50 which 
operates as a ground plane conductor in the odd mode. With the 
above-mentioned arrangement, the coupling factor K can be increased as is 
apparent from the above-mentioned equation (4). 
In the directional coupler of the first preferred embodiment as constructed 
above, when, for example, the coplanar waveguide 54 is terminated with a 
resistive terminator (not shown) and a microwave signal is inputted to the 
coplanar waveguide 51, the microwave signal is outputted to the coplanar 
waveguide 52 through the transmission line of the microstrip conductor 31 
of the directional coupler and is also outputted to the transmission line 
of the microstrip conductor 32, which is coupled with the microstrip 
conductor 31 in a tight coupling. Therefore, with the above-mentioned 
operation, the above-mentioned microwave signal is outputted to the 
coplanar waveguide 53. 
It should be noted that, (a) the width of the cut portion 12c of the ground 
plane conductor 12 in the lateral direction in FIGS. 1 through 4, (b) the 
interval between the microstrip conductors 31 and 32, (c) the width of the 
microstrip conductors 31 and 32, (d) the conductor width of the floating 
potential conductor 50, and (e) the film thickness of the dielectric 
layers 21 and 22 are adjusted so as to obtain a desired coupling factor K. 
According to an experiment of trial production by the inventor of the 
present invention, when a semi-insulating GaAs substrate having a 
dielectric constant of 12.9 is used as the semiconductor substrate 11 and 
polyimide resin having a dielectric constant of 3.7 is used as the 
dielectric layers 21 and 22, a directional coupler having a coupling 
factor of 3 dB and input and output impedances of 50 .OMEGA. can be 
achieved by setting (a) the width of the cut portion 12c of the ground 
plane conductor 12, (b) the interval between the microstrip conductors 31 
and 32, (c) the width of the microstrip conductors 31 and 32, (d) the 
conductor width of the floating potential conductor 50, and (e) the film 
thickness of the dielectric layers 21 and 22, respectively, at (a) 112 
.mu.m, (b) 10 .mu.m, (c) 16 .mu.m, (d) 46 .mu.m, (e) 7.5 .mu.m and 2.5 
.mu.m. The above-mentioned specifications of the structure of the 
directional coupler can be determined by an analysis method such as finite 
element method or the like. 
According to a process of implementing the lamination or multi-layered 
structure of the present preferred embodiment, each conductor can be 
formed by the vacuum deposition method using the lift-off technique with 
photoresist, while the dielectric layers 21 and 22 can be formed by 
subjecting an organic electric insulating material to a spin coating 
method. As a result, the desired structure specifications can be obtained. 
The above-mentioned methods are generally used in the semiconductor 
processing technique, and are known to those skilled in the art. Since a 
production accuracy of about 1 micron in dimensional accuracy of each 
layer and about 0.1 micron in film thickness accuracy of each layer can be 
easily achieved, the design accuracy of the directional coupler can be 
improved. 
In the above-mentioned first preferred embodiment, it is preferably set so 
that the dielectric constant of the dielectric layer 21 is set so as to be 
lower than that of the dielectric layer 22. In this case, the dielectric 
layer 21 having a relatively low dielectric constant is interposed between 
the coupled microstrip conductors 31 and 32 and the floating potential 
conductor 50 having the same electric potential as that of the ground 
plane conductor 12 in the even mode, and therefore, the electrostatic 
capacity C.sub.1 between the ground plane conductor 12 and the coupled 
microstrip conductors 31 and 32 is reduced. On the other hand, in the odd 
mode, the electric field generated between the microstrip conductors 31 
and 32 and the floating potential conductor 50 is shut in or enclosed in 
between the dielectric layer 22 having a relatively high dielectric 
constant and the floating potential conductor 50, and therefore, the 
electrostatic capacity C.sub.12 between the microstrip conductors 31 and 
32 is increased. Therefore, the coupling factor K can be increased. 
Although the floating potential conductor 50 is formed on the dielectric 
layer 22 just above the two microstrip conductors 31 and 32, the present 
invention is not limited to this. The floating potential conductor 50 is 
at least required to be arranged close to the two microstrip conductors 31 
and 32 so that the floating potential conductor 50 is electromagnetically 
coupled with the microstrip conductors 31 and 32. Furthermore, the cut 
portion 12c of the ground plane conductor 12 is required to be formed so 
that the ground plane conductor 12 is separated apart from the microstrip 
conductors 31 and 32 by a predetermined distance in order to reduce the 
electrostatic capacity C.sub.1. 
FIG. 7 is a longitudinal cross-sectional view of a quarter-wavelength 
coupled line type four-port directional coupler in accordance with a first 
modification of the present invention, wherein FIG. 7 corresponds to the 
longitudinal cross-sectional view taken along the line A-A' in FIG. 1. 
As compared with the first preferred embodiment, referring to FIG. 7, a 
dielectric substrate 21a may be formed instead of the dielectric layer 21 
of the first preferred embodiment, and further, a space portion or slot 
21h may be formed in the dielectric substrate 21a just above the cut 
portion 12c of the ground plane conductor 12. The above-mentioned 
arrangement of the first modification can reduce the effective dielectric 
constant between the microstrip conductors 31 and 32 and the ground plane 
conductor 12, and further reduces the electrostatic capacity C.sub.1 as 
compared with that of the first preferred embodiment. This results in 
increase in the coupling factor K. 
FIG. 8 is a longitudinal cross-sectional view of a quarter-wavelength 
coupled line type four-port directional coupler in accordance with a 
second modification of the present invention, wherein FIG. 8 corresponds 
to the longitudinal cross-sectional view taken along the line A-A' in FIG. 
1. 
Referring to FIG. 8, as compared with the first preferred embodiment, the 
microstrip conductors 31 and 32 may be formed on the semiconductor 
substrate 11 in the center portion of the cut portion 12c of the ground 
plane conductor 12, and the floating potential conductor 50 may be formed 
on the dielectric layer 21 just above the microstrip conductors 31 and 32. 
In other words, in this case, a double coplanar waveguide, which is 
composed of the two microstrip conductors 31 and 32 and the ground plane 
conductors 12c and 12c located on the both sides of the two microstrip 
conductors 31 and 32, is formed in the line coupled portion of the second 
modification of the present invention. The above-mentioned arrangement, 
which does not include the dielectric layer 22, can simplify the 
production process, and then can achieve a dimensional reduction in the 
second modification as compared with the first preferred embodiment. 
In the above-mentioned second modification, the floating potential 
conductor 50 is at least required to be formed so that the floating 
potential conductor 50 is electromagnetically coupled with the two 
microstrip conductors 31 and 32. 
Second preferred embodiment 
FIG. 9 is a top plan view of a quarter-wavelength coupled line type 
four-port directional coupler in accordance with a second preferred 
embodiment of the present invention. FIG. 10 is a top plan view of the 
directional coupler shown in FIG. 2 when both of ground plane conductors 
13 and 14 and a dielectric layer 22 are removed. FIG. 11 is a longitudinal 
cross-sectional view of the directional coupler shown in FIG. 9 taken 
along a line C-C', while FIG. 12 is a longitudinal cross-sectional view of 
the directional coupler shown in FIG. 9 taken along a line D-D'. Referring 
to FIGS. 9 through 12, the same components as those shown in FIGS. 1 
through 8 and FIGS. 17 and 18 are denoted by the same reference numerals 
as those shown in the above Figures. In the top plan views of FIGS. 9 and 
10, components which are invisible when viewed from the upper side are 
depicted by dotted lines. 
According to the directional coupler of the second preferred embodiment, 
two coupled microstrip conductors 31 and 32 are formed on a semiconductor 
substrate 11. Further, on the microstrip conductors 31 and 32, a 
rectangular-shaped floating potential conductor 60 having a length of 
(1/4) .lambda.g in the longitudinal direction is formed just above the 
microstrip conductors 31 and 32 through the dielectric layer 21 formed 
thereon. Just above the floating potential conductor 60, a ground plane 
conductor 14 having a rectangular-shaped cut portion 14c located just 
above the floating potential conductor 60 is formed through the dielectric 
layer 22 formed thereon. 
In other words, when comparing FIGS. 9 and 12 which are viewed upside down, 
with the conventional directional coupler shown in FIGS. 17 and 18, the 
directional coupler of the second preferred embodiment is characterized in 
that, the floating potential conductor 60 which is not connected to the 
ground plane conductor 14 is provided in a boundary area located between 
the dielectric layers 21 and 22 which are interposed between the two 
coupled microstrip conductors 31 and 32 and the ground plane conductor 14, 
and the rectangular-shaped cut portion 14c is formed in the ground plane 
conductor 14 just above (or "just below" when FIGS. 9 and 12 are viewed 
upside down) the floating potential conductor 60. 
The manufacturing process for the second preferred embodiment of the 
present invention shown in FIGS. 9 through 12 will be described below. 
After a ground plane conductor 12 is formed on the semiconductor substrate 
11 in a manner as shown in FIGS. 9 through 12, a rectangular-shaped cut 
portion 12c having a relatively wide area is formed by the lift-off 
process in the center portion of the ground plane conductor 12, wherein 
the width of the cut portion 12c is slightly smaller than that of the 
dielectric layer 21. In the center portion of the cut portion 12c on the 
semiconductor substrate 11, the two microstrip conductors 31 and 32 are 
further formed so as to be separated apart by a predetermined distance, 
and to be arranged parallel in the longitudinal direction and 
electromagnetically coupled with each other in the same manner as that of 
the first preferred embodiment. In this case, coplanar waveguides 51, 52, 
53 and 54 for inputting and outputting microwave signals are formed in the 
four corners of the semiconductor substrate 11 in the same manner as that 
of the first preferred embodiment, and then, the coplanar waveguides 51, 
52, 53 and 54 are electrically connected to the microstrip conductors 31 
and 32, respectively, as follows. In a manner as shown in FIG. 10, one end 
of the microstrip conductor 31 is electrically connected to a center 
conductor 42 of the coplanar waveguide 52, while another end of the 
microstrip conductor 31 is electrically connected to a center conductor 41 
of the coplanar waveguide 51. On the other hand, one end of the microstrip 
conductor 32 is electrically connected to a center conductor 44 of the 
coplanar waveguide 54, while another end of the microstrip conductor 32 is 
electrically connected to a center conductor 43 of the coplanar waveguide 
53. 
Thereafter, a dielectric layer 21, which is made of an organic electric 
insulating material such as polyimide resin and has a rectangular surface, 
is formed in an area except for the input and output terminals area of the 
four coplanar waveguides 51 through 54 on the semiconductor substrate 11, 
on which the two microstrip conductors 31 and 32 are formed. Subsequently, 
a rectangular-shaped floating potential conductor 60 has not only two 
sides each having a length of (1/4) .lambda.g in the longitudinal 
direction as arranged in parallel with the longitudinal direction of the 
microstrip conductors 31 and 32 but also two sides having a predetermined 
width as arranged perpendicular to the longitudinal direction of the 
microstrip conductors 31 and 32, and the floating potential conductor 60 
is formed just above the two microstrip conductors 31 and 32 on the 
dielectric layer 21. 
Then a rectangular-shaped dielectric layer 22 made of the same electric 
insulating material as that of the dielectric layer 21 is formed on the 
dielectric layer 21 on which the floating potential conductor 60 is 
formed, and further, a top ground plane conductor 14 is formed on the 
entire surface of the dielectric layer 22. There is also formed on 
inclined side surfaces of the dielectric layers 21 and 22, a ground plane 
conductor 13 which electrically connects the top ground plane conductor 12 
with the ground plane conductor 14 is formed except for the input and 
output terminals area of the coplanar waveguides 51 through 54 in the same 
process as that used for forming the ground plane conductor 14. 
Furthermore, the rectangular-shaped cut portion 14c is formed in the 
ground plane conductor 14 in an area just above the above-mentioned two 
microstrip conductors 31 and 32 and the floating potential conductor 60 
by, for example, the lift-off process, and then the directional coupler of 
the second preferred embodiment is obtained. 
FIG. 13 is a longitudinal cross-sectional view of the directional coupler 
shown in FIG. 9 taken along the line C-C' showing an electric field 
distribution in the even mode, while FIG. 14 is a longitudinal 
cross-sectional view of the directional coupler shown in FIG. 9 taken 
along the line C-C' showing an electric field distribution in the odd 
mode. 
As is apparent from the electric field distribution in the even mode as 
shown in FIG. 13, there exists no line of electric force between the 
floating potential conductor 60 and each of the two microstrip conductors 
31 and 32, and this means that these conductors 60, 31 and 32 have the 
same electric potential as each other. Since the rectangular-shaped cut 
portion 14c is formed in the ground plane conductor 14 in the second 
preferred embodiment, the electrostatic capacity between the floating 
potential conductor 60, which has the same electric potential as that of 
the microstrip conductors 31 and 32, and the ground plane conductor 14 in 
the even mode can be reduced, and at the same time, the electrostatic 
capacity C.sub.1 between the microstrip conductors 31 and 32 and the 
ground plane conductors 12, 13 and 14 can be reduced. 
On the other hand, as is apparent from the electric field distribution in 
the odd mode as shown in FIG. 14, there exists no line of electric force 
between the floating potential conductor 60 and the ground plane conductor 
14, and this means that the conductors 60 and 14 have the same electric 
potential as each other. Therefore, since the electric potential of the 
floating potential conductor 60 becomes zero so that the floating 
potential conductor 60 operates as a ground plane conductor in the odd 
mode in the second preferred embodiment, this causes the electrode 
distance between the ground plane conductor and the microstrip conductors 
31 and 32 to be remarkably reduced, thereby increasing the electrostatic 
capacity C.sub.12. 
In other words, according to the second preferred embodiment, the 
electrostatic capacity C.sub.1 is reduced by forming the cut portion 14c 
in the ground plane conductor 14, and the electrostatic capacity C.sub.12 
is increased by forming the floating potential conductor 60 which operates 
as a ground plane conductor in the odd mode. With the above-mentioned 
arrangement of the second preferred embodiment, the coupling factor K can 
be increased as is apparent from the above-mentioned equation (4). 
In the thus constructed second preferred embodiment, when a microwave 
signal is inputted to the coplanar waveguide 51 while terminating, for 
example, the coplanar waveguide 54 with a resistive terminator (not 
shown), the microwave signal is outputted to the coplanar waveguide 52 
through the transmission line of the microstrip conductor 31 of the 
directional coupler, and also is outputted to the transmission line of the 
microstrip conductor 32 which is coupled with the microstrip conductor 31 
in a tight coupling. With the above-mentioned operation, the 
above-mentioned microwave signal is outputted to the coplanar waveguide 
53. 
The process for implementing the lamination or multi-layered structure of 
the second preferred embodiment can be the same as that of the first 
preferred embodiment. 
In the second preferred embodiment described as above, it is preferred to 
set the dielectric constant of the semiconductor substrate 11 so as to be 
higher than the dielectric constant of the dielectric layers 21 and 22. 
With the above-mentioned arrangement, the dielectric layers 21 and 22 
having a relatively low dielectric constant are arranged so as to be 
interposed between the two microstrip conductors 31 and 32, and each of 
the ground plane conductor 14 and the floating potential conductor 60 
which is made so as to have the same electric potential as that of the 
ground plane conductor 14 in the even mode, and therefore, the 
electrostatic capacity C.sub.1 between the ground plane conductor and the 
microstrip conductors 31 and 32 is further reduced. On the other hand, in 
the odd mode, since the electric field generated between the microstrip 
conductors 31 and 32 and the floating potential conductor 60 is shut in or 
enclosed in the space between the semiconductor substrate 11 having a 
relatively high dielectric constant and the floating potential conductor 
60, the electrostatic capacity C.sub.12 between the microstrip conductors 
31 and 32 is further increased. Therefore, the coupling factor K can be 
further increased. 
Although the floating potential conductor 60 is formed just above the two 
microstrip conductors 31 and 32 on the dielectric layer 21, the present 
invention is not limited to this. The floating potential conductor 60 is 
at least required to be formed close to the two microstrip conductors 31 
and 32 so that the conductors are electromagnetically coupled with each 
other. Furthermore, in order to reduce the electrostatic capacity C.sub.1, 
the cut portion 14c of the ground plane conductor 14 is at least required 
to be formed so that the ground plane conductor 14 is separated apart by a 
predetermined distance, respectively, from the floating potential 
conductor 60 and the two microstrip conductors 31 and 32. 
In order to further reduce the electrostatic capacity C.sub.1, for example, 
the dielectric constant of the dielectric layer 22 may be preferably set 
so as to be smaller than the dielectric constant of the dielectric layer 
21. 
FIG. 15 is a longitudinal cross-sectional view of a quarter-wavelength 
coupled line type four-port directional coupler in accordance with a third 
modification of the present invention, wherein FIG. 15 corresponds to the 
longitudinal cross-sectional view taken along the line C-C' in FIG. 9. 
Referring to FIG. 15, the dielectric layer 22 may be etched to a 
predetermined depth at a portion just beneath the cut portion 14c of the 
ground plane conductor 14, thereby forming a space portion or slot 22h 
which serves as a recess in contrast to the second preferred embodiment. 
With the above-mentioned arrangement of the third modification, the 
effective dielectric constant between the microstrip conductors 31 and 32 
and the ground plane conductor 14 can be reduced, the electrostatic 
capacity C.sub.1 is further reduced, and the coupling factor K can be 
increased as compared with the second preferred embodiment. 
FIG. 16 is a longitudinal cross-sectional view of a quarter-wavelength 
coupled line type four-port directional coupler in accordance with a 
fourth modification of the present invention, wherein FIG. 16 corresponds 
to the longitudinal cross-sectional view taken along the line C-C' in FIG. 
9. 
Referring to FIG. 16, the dielectric layer 22 is not formed, and instead of 
the dielectric layer 22, the ground plane conductor 14 having the cut 
portion 14c in the center portion thereof may be formed on the dielectric 
layer 21, and further the floating potential conductor 60 may be formed in 
the center portion of the cut portion 14c on the dielectric layer 21. With 
the above-mentioned arrangement, the directional coupler of the fourth 
modification, which is not provided with the dielectric layer 22, allows a 
simplified production process and dimensional reduction as compared with 
the second preferred embodiment. 
In the above-mentioned fourth modification, in order to reduce the 
electrostatic capacity C.sub.1, the cut portion 14c of the ground plane 
conductor 14 is at least required to be separated apart from the two 
microstrip conductors 31 and 32 by a predetermined distance. 
As described above, according to the first and second preferred embodiments 
and the first through fourth modifications, the electrostatic capacity 
C.sub.1 between the ground plane conductor and the microstrip conductors 
31 and 32 can be reduced, while the electrostatic capacity C.sub.12 
between the microstrip conductors 31 and 32 can be further increased. With 
the above-mentioned arrangement, the coupling factor K of the directional 
coupler can be increased. The directional couplers having the 
above-mentioned construction can be applied to MMICs. 
Other preferred embodiments 
Although the semiconductor substrate 11 is employed in each of the 
above-mentioned preferred embodiments, the present invention is not 
limited to this, and a dielectric substrate may be employed instead of the 
semiconductor substrate 11. In the first preferred embodiment, the 
dielectric layer 21 may be a dielectric substrate, and the ground plane 
conductor 12 may be formed on the rear surface of the layer without 
employing the semiconductor substrate 11. The same arrangement as above 
can be also applied to the first and second modifications. 
In the second preferred embodiment, the dielectric layer 21 may be a 
dielectric substrate, and the ground plane conductor 12 and the microstrip 
conductors 31 and 32 may be formed on the rear surface of the layer 
without employing the semiconductor substrate 11. In this case, the above 
directional coupler may have a vertically inverted construction, or the 
above directional coupler may have a construction which has been turned 
over. The same arrangement can be also applied to the third and fourth 
modifications. 
In each of the above-mentioned preferred embodiments, the floating 
potential conductors 50 and 60 are each required to have a length of at 
least (1/4) .lambda.g so that the floating potential conductors 50 and 60 
can operate as ground plane conductors, respectively, in the odd mode. 
Although the coplanar waveguides 51 through 54 are employed for inputting 
and outputting microwave signals in each of the above-mentioned preferred 
embodiments, the present invention is not limited to this. Instead of the 
coplanar waveguides 51 through 54, microwave transmission lines such as 
microstrip lines, strip lines, tri-plate lines or the like may be 
employed. 
According to the present invention described as above, a floating potential 
conductor is formed on the dielectric material or a floating potential 
conductor is provided in the dielectric material, and a cut portion is 
formed in the ground plane conductor in a conventional quarter-wavelength 
coupled line type four-port directional coupler. With the above-mentioned 
arrangement, the floating potential conductor and the two microstrip 
conductors are made so as to have the same electric potential in the even 
mode, the electrostatic capacity C.sub.1 between the above-mentioned two 
microstrip conductors and the ground plane conductor can be reduced. On 
the other hand, the above-mentioned floating potential conductor and the 
ground plane conductor are made so as to have the same electric potential 
as each other in the odd mode, and at the same time, the electric 
potential of the floating potential conductor becomes zero so that the 
floating potential conductor operates as a ground plane conductor. 
Therefore, the electrostatic capacity C.sub.12 between the above-mentioned 
two microstrip conductors is increased. Eventually, since the 
electrostatic capacity C.sub.1 is reduced while the electrostatic capacity 
C.sub.12 is increased, the coupling factor K is increased as is apparent 
from the above-mentioned equation (4). By virtue of the above-mentioned 
effects, a directional coupler having a coupling factor K higher than that 
of the conventional example can be provided according to the present 
invention. 
Although the present invention has been fully described by way of example 
with reference to the accompanying drawings, it is to be noted here that 
various changes and modifications will be apparent to those skilled in the 
art. Therefore, unless otherwise such changes and modifications depart 
from the scope of the present invention as defined by the appended claims, 
they should be construed as included therein.