JP2001345608A - Line converter - Google Patents

Abstract

(57) [Problem] To provide a line converter with low reflection and low loss. A first coplanar line and a microstrip line are formed on an upper surface of a Teflon substrate, and a second coplanar line composed of coplanar lines is formed therebetween. In the coplanar line 22, the center conductor is the strip conductor 11, and the second ground conductor 1
2b is formed in a fan shape having a radius of λ / 4 (λ: propagation wavelength). As a result, the central portion of the fan shape becomes a virtual ground, and a part of the electric field component of the signal goes to the second ground conductor 12b.
That is, the propagation mode is converted from the microstrip line type to the coplanar line type. Further, in the coplanar line 21, the center conductor 11 and the second ground conductor 12a are formed in a tapered shape to reduce a loss caused by rapid mode conversion. Also, the second ground conductors 12a, b and the center conductors 31a, b
Is adjusted to substantially match the characteristic impedance and reduce the reflection.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a line converter for converting different types of lines formed on a substrate. In particular, the present invention relates to a line converter that converts a coplanar line and a microstrip line without loss in a microwave circuit. INDUSTRIAL APPLICABILITY The present invention is applicable to, for example, a high-frequency on-wafer probing or a transmission line conversion unit of a millimeter wave communication module.

[0002]

2. Description of the Related Art Conventionally, there is a line converter for converting a microstrip line and a coplanar line. For example, there is a conversion line disclosed in JP-A-10-335910. FIG. 12 shows the conversion unit. The feature is that at the connection between the microstrip conductor 4 and the coplanar line composed of the first ground layer 2 and the center conductor 1, the tapered conductor part 5 that gradually widens the end of the center conductor 1 and connects to the strip conductor 4 And that a tapered ground 2 ′ is formed in the first ground layer 2 of the coplanar line toward the strip conductor 4. In particular, by setting the starting point of the tapered ground 2 ′ below the tapered conductor 5, it is experimentally shown that the reflection at the converter is reduced.

[0003] In addition, Gildas. Gauthier, Linda P. Katehi, e
tal., ”W-Band Finite Ground Coplanar Waveguide
(FGCPW) to Microstrip Line Transition, ”(IEEE
MTT-SDigest, pp. 107-109, 1998). This is characterized in that a converter is provided between a microstrip line formed on a silicon substrate and a coplanar line. In particular, the length of the center conductor and the ground conductor (ground layer) of the coplanar line in the converter is λ / 4,
It is characterized in that the loss in the converter is reduced by utilizing the resonance effect.

[0004]

However, the former shows its effectiveness experimentally at a frequency of 40 GHz, and is not specified for a high frequency range of 60 GHz to 70 GHz. Due to the imperfection of propagation mode conversion, deterioration is expected in the above configuration at higher frequencies. That is, the above configuration does not sufficiently reduce the reflection and the insertion loss with respect to the high frequency band required in recent years. Further, the former circuit is formed on an alumina substrate, and the latter circuit is formed on a silicon substrate. In order to make the characteristic impedance 50 Ω with the relative permittivity of both, the center conductor width of the coplanar line becomes several tens μm wide. This thinning increases the conductor loss, and is not applied to recent high-frequency high-frequency transmission requiring low conductor loss. The characteristic impedance of the transmission line may change depending on the connection method with a high-frequency element such as a monolithic microwave integrated circuit. In such a case, fine adjustment can be performed by trimming the center conductor or the ground conductor. However, since it is formed on an alumina substrate or a silicon substrate, an expensive trimming device such as laser trimming is required. or,
Since the center conductor is a thin wire, connection with a high-frequency element has not been easy.

An object of the present invention is to form the conversion section by a plurality of coplanar lines and perform mode conversion gently so as to reduce the loss accompanying mode conversion at a high frequency. Another object of the present invention is to achieve impedance matching by making the line impedance of the converter substantially equal to that of the microstrip line and the coplanar line, and to reduce the reflection at the connection at a high frequency band. Another object of the present invention is to increase the width of a transmission line by using a substrate having a small relative dielectric constant, thereby facilitating mounting of a high-frequency element.

[0006]

According to a first aspect of the present invention, there is provided a line converter for converting different types of transmission lines formed on a substrate, the line converter being formed on a back surface of the substrate. A first ground conductor, a microstrip line formed on the upper surface of the substrate, a first coplanar line formed facing the microstrip line, a center conductor formed between the microstrip line and the first coplanar line. A second coplanar line composed of a second ground conductor located on both sides of the second coplanar line.
At one end, the width matches the center conductor of the first coplanar line, widens in a tapered shape toward the microstrip line, and at the other end matches the strip conductor width of the microstrip line, and the second coplanar line has a second width. The ground conductor is formed such that the ground conductor is connected to the second ground conductor of the first coplanar line, the distance from the center conductor increases toward the microstrip line, and the width of the second ground conductor decreases in a tapered shape. It is characterized by the following.

According to the line converter of the second aspect, when the propagation wavelength of the signal is λ, the second ground conductor of the second coplanar line has a radius of λ / at the end on the microstrip line side. It is formed in a fan shape with a length of 4
The side is continuous with the second ground conductor of the adjacent coplanar line,
The other side is formed so as to form a predetermined taper angle with the strip conductor of the microstrip line.

According to the line converter of the third aspect, the second coplanar line is connected to the coplanar line having the sector-shaped second ground conductor, and the first coplanar line side with respect to the line. When the propagation wavelength is λ, λ
And a coplanar line having an effective length of / 4 and having a constant distance between the second ground conductor and the center conductor.

Further, according to the line converter of the present invention, the characteristic impedance of the second coplanar line is substantially matched to the characteristic impedance of the microstrip line and the first coplanar line, and the impedance is matched. It is characterized by the following.

According to the line converter of the fifth aspect, the substrate is made of a material having a relative dielectric constant of 3 or less. According to the line converter of the sixth aspect, a high-frequency element is mounted on the first coplanar line.

[0011]

The line converter according to the first aspect is formed between the first coplanar line with ground and the microstrip line with ground using the first ground conductor formed on the back surface of the substrate as ground. The second coplanar line with ground is provided. Hereinafter, the first coplanar line, the second coplanar line, and the microstrip line will be simply referred to with the grounding omitted. The second coplanar line is composed of a plurality of regions in a combination of shapes and intervals of the center conductor and the second ground conductor.

The center conductor of the second coplanar line coincides with the center conductor of the first coplanar line at one end, is widened in a tapered shape toward the microstrip line, and at the other end has the width of the strip conductor of the microstrip line. Match. Further, the second ground conductors formed on both sides of the center conductor are connected to the second ground conductor of the first coplanar line, and the distance from the center conductor increases toward the microstrip line. The width of the ground conductor is reduced in a tapered shape.

For example, the second coplanar line is formed from two coplanar line regions, one of which has a tapered center conductor, and is connected at one end to the first coplanar line. Another is a coplanar line having a microstrip line strip conductor as a center conductor.
These coplanar lines are configured to mediate the first coplanar line and the microstrip line. This configuration,
The configuration is such that the propagation mode of the coplanar line and the propagation mode of the microstrip line coexist with the two coplanar lines described above and are gradually converted. That is, the propagation mode does not change rapidly. Therefore, there is no loss due to abrupt mode conversion, for example, leakage loss. As a result, a line converter with lower loss than the conventional one is obtained. The tapered shape may be stepwise or continuous. Including both. The taper may be a straight line or a curve. Including both. When the center conductor width changes continuously, reflection and loss due to discontinuity are further reduced.

According to the line converter of the second aspect, the second ground conductor of the second coplanar line has a propagation wavelength of λ.
, The radius at the end on the microstrip line side is λ
It has a fan-shaped portion with a quarter length. One side of the fan shape is the second side of the adjacent coplanar line.
It is formed so as to be continuous with the ground conductor (the length of the side is equal to the width of the ground conductor) and the other side forms a predetermined taper angle with the strip conductor of the microstrip line. That is, the other side faces the center conductor and forms a tapered portion of the second ground conductor. A portion having a length of λ / 4 from the end of the second ground conductor becomes a virtual ground, and a part of the electric field component of the signal goes to the virtual ground. For example, when a signal travels from the microstrip line to the first coplanar line, a part of the electric field component is transferred from the first ground conductor to the virtual ground, that is, the second coplanar line.
Head to ground conductor. This means that a part of the propagation mode is converted from the microstrip line type to the coplanar line type. That is, it means that the mode conversion is not performed abruptly. Therefore, loss accompanying rapid mode conversion is reduced.

According to a third aspect of the present invention, when the propagation wavelength is λ, the distance between the second ground conductor and the center conductor is constant, and the effective length λ of the second coplanar line is λ. The / 4-length coplanar line portion is connected to the coplanar line portion having the fan-shaped second ground conductor. The former is arranged on the first coplanar line side with respect to the latter. With this configuration, there is no abrupt mode conversion for the same reason as in claim 2, and there is no loss due to this. In other words, the mode conversion can be stabilized and smoothened by interposing a coplanar line in which the center conductor and the ground conductor are parallel and have a constant width at one end. The λ / 4 length is used for impedance matching.

According to the line converter of the fourth aspect, the characteristic impedance of the second coplanar line is substantially equal to the characteristic impedance of the microstrip line and the first coplanar line, and the impedance is substantially matched. Have been. For example, the characteristic impedance of the second coplanar line can be adjusted by the distance between the center conductor and the ground conductors on both sides thereof, and impedance matching can be performed. Since the impedance is matched, reflection at both ends of the second coplanar line is reduced as much as possible. Therefore, a line converter without reflection is realized. Note that the impedance matching includes a characteristic impedance difference within several Ω.

According to the line converter of the fifth aspect, the substrate is made of a material having a relative dielectric constant of 3 or less. For example, it is composed of a fluororesin such as a tetrafluoroethylene resin having a relative dielectric constant of about 2.2. The line converter of the present invention is formed on such a substrate. This dielectric constant is about 4 to 5 times smaller than that of the alumina-based substrate and the silicon-based substrate. Since the dielectric constant is small, for example, if the characteristic impedance is unified to 50Ω, the center conductor width of the coplanar line on the substrate and the strip conductor width of the microstrip line can be widened, for example, about twice. Therefore, the line converter is excellent in workability such as bonding.

According to the line converter of the sixth aspect, a high-frequency element is mounted on the first coplanar line. According to the sixth aspect, since the center conductor width of the first coplanar line is widened, a high-frequency element such as an MMIC can be easily mounted directly on the line by, for example, wire bonding, flip chip bonding, ribbon bonding or the like. Since it is mounted directly on the track, the space can be saved, and the track converter can be downsized. Therefore, the line converter contributes to downsizing of products and the like.

[0019]

Embodiments of the present invention will be described below with reference to the drawings. Note that the present invention is not limited to the following examples. (First Embodiment) FIG. 1A shows an embodiment of a line converter according to the present invention. The figure is a block diagram. The line converter according to the present invention includes a first coplanar line 1 formed on a substrate.
0, the second coplanar line 20, and the microstrip line 30. These are a coplanar line with ground and a microstrip line with ground with the back surface of the substrate as ground. Here, for simplicity, they are simply referred to as first and second coplanar lines and microstrip lines.

This configuration is similar to that of the second coplanar line 20.
It is intended to gradually change the propagation mode from a coplanar line type to a microstrip line type or vice versa. Therefore, the first coplanar line 10 and the second coplanar line have different roles. More specifically, as shown in FIG. 1B, the second coplanar line 20 includes a plurality of coplanar lines 21, 22, and 23. At this time, the propagation mode of the coplanar line 21 near the first coplanar line 10 is set close to that of the coplanar line, and the propagation mode of the coplanar line 23 near the microstrip line 30 is set close to that of the microstrip line. The setting is obtained by making the center conductor width of the coplanar line 21 equal to that of the first coplanar line 10 and making the center conductor width of the coplanar line 23 equal to the width of the microstrip line 30. That is, it can be obtained by gradually changing the center conductor width.
In addition, it can be obtained by connecting the ground conductors on both sides of the center conductor and adjusting the shape of the ground conductor and the distance from the center conductor so that the impedance is matched. With such a configuration, the signal propagation mode is gradually converted from the microstrip line type to the coplanar line type or vice versa, as described later. That is, losses such as reflection and leakage due to a sudden change in mode are reduced.

FIG. 2 and FIG. 3 are perspective views of the line converter of the present embodiment. The line converter according to the present embodiment includes a Teflon (registered trademark) substrate 40, a center conductor 11 formed thereon, and a first conductor.
A first coplanar line 10 comprising a ground conductor 50, a second coplanar line 20 comprising a tapered center conductor 31a, 31b and a tapered second ground conductor 12a, 12b;
And it is comprised from the microstrip line 30. The Teflon substrate 40 is a glass cloth-based fluororesin double-sided copper-clad laminate having a thickness of 127 μm, and a first ground conductor 50 is formed on the back surface thereof. Also, Teflon substrate 4
0 is characterized by a relative dielectric constant of 2.2, a dielectric loss tangent of 1 × 10 ^{−4 at} 3 GHz and 2.1 × 10 ⁻³ at 12 GHz, and a small dielectric loss even in the millimeter wave band.
Each of the above components is formed by a lithography technique, a film formation technique, an etching technique, or the like, and has a thickness of about 18 μm.

FIG. 3 shows details of the second coplanar line 20.
You. The figure shows a circuit pattern. In addition, the second ground conductor 1 on one side
2 is omitted because it is vertically symmetrical in the drawing. 2nd
The planar line 20 has a tapered central conductor 31a and a tapered central conductor 31a.
Coplanar line 2 having par-shaped second ground conductor 12a
1 and a center conductor 31 whose line width is equal to the strip conductor 31
b and a coplanar wire having a fan-shaped second ground conductor 12b
It is composed of a road 22. An example of the size is shown below.
It is. The center conductor 31a of the coplanar line 21 is
Is about 132 μm on the first coplanar line 10 side,
390μm taper on the cross trip line 30 side
It spreads out like a letter. Its taper angle θ _TwoIs about 45 °
is there. Also, the second ground conductors 12a formed on both sides thereof
Is the width L on the first coplanar line 10 side.₁Is about 1090μ
m, width L on the microstrip line 30 side_TwoIs about 72
0 μm, and its width is tapered, contrary to the center conductor 31 a.
It is formed in a reduced shape. The taper angle θ_TwoIs about
67 °. As a result, the second ground conductor 12a and the center conductor
The distance from the body 31a is increased from 40 μm to 250 μm.
It is formed to be large.

The center conductor 31b of the coplanar line 22
Is formed to be continuous with the width of the strip conductor 31 as described above. The second ground conductor 12b has a side of about 720 μm and a central angle θ ₃ of about 5 μm.
It is formed in a 7 ° fan shape. One side x is continuous with the second ground conductor 12a of the adjacent coplanar line 21,
The other side y has a taper angle θ with the strip conductor 31.
₄ is formed to be about 33 °. Note that 720 μm of the fan shape corresponds to λ / 4 length when the propagation wavelength is λ. This is to form a virtual earth at point A as described later. The second coplanar line 20 is formed in this manner.

Next, the operation of the above configuration will be described with reference to FIG. The effect is indicated by a change in the propagation mode of the signal. For example, when a high frequency signal of several tens of GHz is input to the microstrip line 30, the propagation mode is as shown in FIG. The electric field component of the propagation signal is directed to the first ground conductor 50. Next, the signal is input to the coplanar line 22 of the second coplanar line 20. In the coplanar line 2,
The electric field is directed not only to the first ground conductor 50 but also partly to the second ground conductor 12. In particular, the second ground conductor 12b of the coplanar line 22 is formed in a fan shape having a radius of λ / 4. That is, the portion A of the second ground conductor 12b becomes a virtual ground, and more electric field components are generated by the second ground conductor 12b, 1
2a (FIG. 4B). Then, the mode conversion gradually progresses as the signal progresses, and is finally completely converted in the first coplanar line 10 (FIG. 4).
(C)). At this time, as described above, the width of the center conductor 31a of the coplanar line 21 is continuously reduced in a tapered shape, and the center conductor 31 of the first coplanar line 10 and the strip conductor 31 of the microstrip line 30 are smoothly connected. ing. Therefore, the propagation mode of the signal is gradually converted to that of the first coplanar line 10 and propagated.

Normally, when lines having different propagation modes are connected, reflection and leakage (loss) due to discontinuity of the mode are performed.
Occurs. The above configuration is a configuration in which the two different electric field modes are gradually and completely converted via the second coplanar line 20. This avoids rapid mode conversion. That is, signal reflection and leakage due to this are reduced. In FIG. 4, the arrows indicate the direction of the electric field.

At this time, the impedance of the second coplanar line 20 is matched with that of the first coplanar line 10 and that of the microstrip line 30. It is shown in FIG.
By adjusting the distance between the center conductor 31a of the coplanar line 21 and the second ground conductor 12a, the distance between the center conductor 31b of the coplanar line 22 and the second ground conductor 12b, and the shape of the second ground conductor 12b shown in FIG. can get. That is,
The characteristic impedance of the second coplanar line 20 is the first
The impedance is almost matched to the characteristic impedance (for example, 50Ω) of the coplanar line 10 and the microstrip line 3. This matching is performed, for example, on three lines with a difference of several Ω or less. If it is within several Ω, the second coplanar line 20 of the signal
The reflection at both ends is reduced as much as possible. As described above, if the second coplanar line 20 is formed between the first coplanar line 10 and the microstrip line 30 to realize gradual conversion of the propagation mode and impedance matching thereof, the line conversion with low reflection and low loss can be achieved. Container.

Next, the performance of the line converter having the above configuration will be described. Performance is shown by measuring transmission and reflection coefficients. The measurement was performed on a line in which two line converters as shown in FIG. A high-frequency signal is input from one of the left and right coplanar lines 10, and the output signal is measured by the other coplanar line 10. FIG. 6 shows the measurement results. FIG. 6 (a)
Represents transmission coefficient S21, and FIG. 5B represents reflection coefficients S11 and S22. The reflection coefficients S11 and S22 and the transmission coefficient S21 are components of the scattering matrix. As shown in FIG. 6A, the insertion loss of the line converter according to the present embodiment is -0.4 dB or more at 60 GHz or more.
-1 dB, which is an actual use level. 6B that the reflection coefficient S11 is about -1 at a frequency of 62 GHz.
8.1 dB, and that of the reflection coefficient S22 is about -22.9.
dB. That is, it was shown that a line converter with low reflection and low loss can be realized. At this time, the transmission loss of the microstrip line 30 is about -0.03 dB / mm,
In the main measurement with a length of 3 mm, the value was as extremely small as -0.15 dB. Therefore, according to the present invention, even in a higher frequency band, its reflection and insertion loss can be significantly reduced. That is, an excellent line converter in which reflection and insertion loss are significantly reduced can be realized.

(Second Embodiment) In the first embodiment, the end of the second grounding conductor is formed into a sector shape having a length of λ / 4, and a virtual earth is generated at the center of the sector shape to efficiently and gently operate the mode. Conversion was performed. In the second embodiment, as shown in FIG. 7, a coplanar line 23 having a constant separation distance between the second ground conductor 12c and the center conductor 31c and having an effective length of λ / 4 is formed. It is characterized by being inserted between the lines 22. That is, it is characterized in that the second coplanar line 20 is constituted by three coplanar lines 21, 22 and 23. According to this configuration, the coplanar line 23 in which the distance between the center conductor and the second ground conductor is constant.
Is formed, mode stability is achieved in this portion, and smoother mode conversion is possible.

(Modifications) The embodiment of the present invention has been described above, but various other modifications are conceivable. For example, although the Teflon substrate is used as the substrate in the first and second embodiments, a substrate of another material may be used. The dielectric substrate is not particularly limited as long as it has a dielectric constant equivalent to that of the above embodiment.

Although neither the first embodiment nor the second embodiment has been described with respect to the adjustment of the frequency to be propagated, the line converter according to the present invention uses a Teflon substrate for the substrate, so that the line converter can be easily connected to the second ground. The conductor 12 can be trimmed, and the frequency can be easily adjusted. FIG. 8 shows the adjustment portion C (shaded portion). By this adjustment, the frequency to be propagated can be shifted to a higher frequency. FIG. 9 shows a transmission coefficient S21 and a reflection coefficient S11 when the ground conductor 12 of the line converter (FIG. 5) used in the first embodiment is trimmed.
At 82 GHz, the transmission coefficient is 0.77 dB, and the reflection coefficient is -2.
Actual useability is maintained at 3 dB. The actual center frequency is shifted from 69 GHz (first embodiment) to 82 GHz. Thus, the propagation frequency may be adjusted by trimming the second ground conductor 12.

In the first and second embodiments, the line converter for converting an externally input signal has been described. The signal is not particularly limited to an externally input signal. It may be applied to a signal generated internally. FIGS. 10A and 10B show an example in which the high-frequency element 60 is flip-chip bonded onto the center conductor 11 of the first coplanar line 10. FIG. 10A is a top view thereof,
(B) is a side view of the mounting portion of the high-frequency element 60. Since the line converter of the present invention uses the Teflon substrate 40 having a small dielectric constant as described above, the width of the center conductor 11 of the first coplanar line 10 formed on the upper surface thereof is twice as large as that of the conventional one. I have. Therefore, the high-frequency element 60 can be directly mounted thereon. Therefore, for example, a miniaturized millimeter wave module with low loss can be provided.
The high-frequency element is, for example, a high-frequency active element such as an MMIC (monolithic micro integrated circuit) or an oscillator, or a passive element such as an antenna. Further, the connection of the high-frequency element may be wire bonding or ribbon bonding as well as the flip chip bonding.

In the first and second embodiments, a part of the center conductor 11 of the second coplanar line 20 is formed in a tapered shape and connected to the strip conductor 31, but as shown in FIG. Can be a curve D. For example, if the curve D is a quadratic curve or a cubic curve, no corner is formed. Therefore, the discontinuity of the reflection mode due to the corner and the propagation mode due to the corner is reduced, thereby reducing the loss. Although not shown, this curve may be applied to the second ground conductors formed on both sides thereof. Loss is further reduced.

[Brief description of the drawings]

FIG. 1 is a block diagram of a line converter according to a first embodiment of the present invention.

FIG. 2 is a perspective view of the line converter according to the first embodiment of the present invention.

FIG. 3 is an enlarged top view of a second coplanar line according to the first embodiment of the present invention.

FIG. 4 is an explanatory diagram of mode conversion in the line converter according to the first embodiment of the present invention.

FIG. 5 is a top view of a circuit pattern of the measurement line converter according to the first embodiment of the present invention.

FIG. 6 is a performance characteristic diagram of the line converter according to the first embodiment of the present invention.

FIG. 7 is a top view of a circuit pattern of a line converter according to a second embodiment of the present invention.

FIG. 8 is a top view of a circuit pattern of a line converter according to a modification of the present invention.

FIG. 9 is a performance characteristic diagram of a line converter according to a modification of the present invention.

FIG. 10 is an explanatory diagram of a line converter equipped with a high-frequency element according to a modification of the present invention.

FIG. 11 is a top view of a center conductor of a line converter according to a modification of the present invention.

FIG. 12 is a top view of a circuit pattern illustrating a conventional conversion line.

[Explanation of symbols]

 Reference Signs List 10 first coplanar line 11 center conductor 12 second ground conductor 20 second coplanar line 21, 22, 23 coplanar line 24 center conductor 30 microstrip line 31 strip conductor 40 Teflon substrate 50 first ground conductor 60 high frequency element

Claims (6)

[Claims] 1. A line converter for converting different types of transmission lines formed on a substrate, a first ground conductor formed on a back surface of the substrate, a microstrip line formed on an upper surface of the substrate, and the microstrip line. A first coplanar line formed opposite to the first line, a second coplanar line including a center conductor formed between the microstrip line and the first coplanar line and second ground conductors located on both sides of the center conductor. The width of the center conductor of the second coplanar line coincides with the center conductor of the first coplanar line at one end, and is widened in a tapered shape toward the microstrip line at the other end. The second ground conductor of the second coplanar line is connected to the second ground conductor of the first coplanar line, which corresponds to the strip conductor width. Both increased distance between the central conductors toward the microstrip line, line converter, wherein the second grounding conductor width is formed are reduced in a tapered shape. 2. The second ground conductor of the second coplanar line is formed in a fan shape having a radius of λ / 4 at an end on the microstrip line side when a propagation wavelength is λ, and the second ground conductor has a fan shape. 2. The one side of the micro-strip line is formed so that one side thereof is continuous with the second ground conductor of the adjacent coplanar line and another side forms a predetermined taper angle with the strip conductor of the microstrip line. Line converter. 3. The second coplanar line is connected to a coplanar line having the fan-shaped second ground conductor, is formed on the first coplanar line side with respect to the line, and has a propagation wavelength of λ. 3. The line converter according to claim 2, further comprising: a coplanar line having an effective length of λ / 4 and having a constant distance between the second ground conductor and the center conductor. 4. 4. The characteristic impedance of the second coplanar line is approximately equal to the characteristic impedance of the microstrip line and the first coplanar line, and impedance matching is performed. The line converter according to claim 3. 5. The line converter according to claim 1, wherein the substrate is made of a material having a relative dielectric constant of 3 or less. 6. The line converter according to claim 1, wherein a high-frequency element is mounted on the first coplanar line.