JP2005094314A - Transmission line - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve reflection loss in branching a signal line conductor in a transmission line. <P>SOLUTION: The transmission line is provided with a dielectric 11, a ground conductor 12 formed on the bottom surface of this dielectric 11, and a signal line conductor 13 formed on the top surface of the dielectric 11 so that one conductor is branched into two directions. The conductor 13 is branched into signal line conductors 13b, 13c after being branched, and these conductors 13b, 13c each have a bent portion so as to be guided into different directions. At least external end edges of the bent portion of both end edges in the widthwise directions of the branched conductors 13b, 13c are each formed into a curve. With this configuration, a current having high current density flowing along the end edge in the widthwise direction of the conductor before being branched can smoothly be guided to the conductor after being branched, and its discontinuity can be reduced. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a transmission line including a signal line conductor formed so as to branch one conductor in two directions.

  2. Description of the Related Art In recent years, various devices equipped with members that transmit and receive high-frequency signals such as antenna elements have become widespread. A so-called microstrip line is known as a high-frequency transmission line having a planar structure for transmitting such a high-frequency signal. A microstrip line is generally composed of a dielectric, a ground conductor formed on the lower surface of the dielectric, and a signal line conductor formed on the upper surface of the dielectric. Transmits high frequency signals. A so-called triplate line is also known as a high-frequency transmission line. Unlike a microstrip line, a triplate line has a signal line conductor inside a dielectric and transmits a high-frequency signal through the signal line conductor.

  By the way, in such a transmission line such as a microstrip line or a triplate line, for example, a signal line conductor is branched in two directions, such as a planar array antenna that effectively increases the area of an antenna element to improve sensitivity. In many cases, the form used is used. In this case, as the transmission line, as shown in FIG. 9, a signal line conductor 103 formed in a T shape on the upper surface of a dielectric 102 having a ground conductor 101 formed on the lower surface is used. As a result, in the transmission line, a current flows in the direction indicated by the arrow in the figure from the signal line conductor 103a before branching to the signal line conductors 103b and 103c after branching.

  Here, an important parameter for the transmission line is a characteristic impedance. In particular, in a high-frequency transmission line in which the frequency of a transmitted signal is high, it is important to keep the characteristic impedance stable in order to prevent signal reflection and avoid the influence of ambient noise and the like.

  In general, the characteristic impedance Z (Ω / m) is such that the resistance per unit of the signal line conductor is R (Ω / m), the inductance is L (H / m), the conductance is G (S / m), and the capacitance is C ( F / m), it is expressed by the following formula (1). The characteristic impedance Z can be expressed by a function such as a dielectric constant of a dielectric or a physical dimension of a signal line conductor, and many empirical formulas have been proposed.

In the transmission line in which the signal line conductor described above is formed in a T shape, the characteristic impedance in the signal line conductor before branching is Z _C1 and the characteristic impedance in the signal line conductor after branching is respectively shown in FIG. , Z _C2 and Z _C3 , the relationship shown in the following equation (2) is established between these characteristic impedances Z _C1 , Z _C2 and Z _C3 . Further, in the transmission line in the case of Z _C2 = Z _C3, the following equation (3) is satisfied, in the case of Z _C2 = αZ _C3, the following equation (4) is satisfied. As shown in FIG. 11, the transmission line including the signal line conductor shown in FIG. 11 has an effective resistance Z _C1 connected to a predetermined power source and two effective resistances Z _C2 and Z _C3 connected in parallel. Is equivalent to a circuit connecting the two.

Therefore, in designing a transmission line in which the signal line conductor is formed in a T shape, the physical properties of the signal line conductor before and after branching are obtained so that desired characteristic impedances Z _C1 , Z _C2 and Z _C3 can be obtained. It is necessary to determine the correct dimensions.

  In this type of transmission line, as a technique for facilitating impedance matching with an external circuit and mitigating the effects of reflection loss and insertion loss, for example, a strip line as described in Patent Document 1 has been proposed. ing.

JP-A-7-122901

  By the way, it is known that in the transmission line in which the signal line conductor described above is branched in a T shape, current discontinuity always occurs between the signal line conductor before branching and the signal line conductor after branching. There is a problem that reflection loss increases due to the discontinuity of the current. In particular, in a planar array antenna or the like in which a large number of antenna elements are arranged by using a plurality of such transmission lines, a reflection loss accumulates for each branch of the signal line conductor, resulting in a situation where the efficiency is extremely deteriorated.

  The present invention has been made in view of such a conventional situation, and an object of the present invention is to provide an efficient transmission line that can remarkably improve reflection loss when branching a signal line conductor. .

  In order to achieve the above-described object, a transmission line according to the present invention is a transmission line in which a ground conductor and a signal line conductor are formed across a dielectric, and the signal line conductor is branched in two directions. The branched branched signal line conductor has a bent portion immediately after branching, and at least an outer edge of the bent portion is formed in a curved shape.

  In the transmission line of the present invention, since at least the outer edge of the bent portion of the branched signal line conductor is curved, the current density flowing through the edge in the width direction of the pre-branch conductor constituting the signal line conductor is A high current is smoothly guided through the curved portion to the branched signal line conductor after branching. Therefore, the discontinuity of the current flowing through the edge in the width direction of the signal line conductor before branching is suppressed, and reflection loss due to this discontinuity is suppressed.

  In particular, not only the outer edge of the bent part but also the inner edge of the both ends in the width direction of the branched signal line conductor after branching are curved so that the shape of the entire branched signal line conductor is bent. A curved shape is formed at the portion, and the current discontinuity is reliably eliminated.

  In addition, the shape of the curve is a part of an arc having a predetermined radius of curvature from the viewpoint of ease of formation and the discontinuity of the current flowing through the edge in the width direction of the signal line conductor. Is desirable. Further, it is desirable that the conductor after branching is connected to the conductor before branching with a constant conductor width in order to prevent the characteristic impedance at the branching portion from becoming discontinuous.

  The structure of the transmission line to which the transmission line according to the present invention is applied is a microstrip line or a triplate line.

  According to the transmission line according to the present invention, since the discontinuity of the current flowing through the edge in the width direction of the signal line conductor can be reduced, reflection loss can be remarkably reduced, and an efficient branch transmission line can be obtained. Can be realized.

  Hereinafter, specific embodiments to which the present invention is applied will be described in detail with reference to the drawings.

  The transmission line of this embodiment is a transmission line having a planar structure as a so-called microstrip line or triplate line. This transmission line is formed so as to branch one signal line conductor in two directions on the assumption that it is used for a planar array antenna, for example.

  First, before explaining a specific transmission line, the cause of the reflection loss will be considered.

  Consider a microstrip transmission line in which a ground conductor 2 is formed on the lower surface of a dielectric 1 and a signal line conductor 3 is formed on the upper surface of the dielectric 1 as shown in FIG. In such a transmission line, when the current density distribution D of the high-frequency current in the signal line conductor 3 was obtained, the high-frequency current did not flow so much in the central portion of the signal line conductor 3, as indicated by the hatched portion in FIG. The tendency to concentrate on the edge in the width direction of the signal line conductor 3 was confirmed. It was also confirmed that this tendency becomes more prominent as the signal frequency increases. Similarly, in the triplate type transmission line, it was confirmed that the high-frequency current was concentrated on the edge in the width direction of the signal line conductor.

  Therefore, it is considered that the current discontinuity generated when the signal line conductor is branched is greatly influenced by the current flowing along the edge in the width direction of the signal line conductor. Therefore, in order to reduce the reflection loss in the transmission line, it can be said that the discontinuity of the current flowing along the edge in the width direction of the signal line conductor before branching may be reduced.

  The transmission line shown as the embodiment of the present invention is based on such knowledge obtained by the applicant of the present application, and the current flowing through the edge in the width direction of the signal line conductor before branching is changed to the signal line after branching. A branched signal line conductor is formed in a shape that smoothly leads to a conductor.

  Specifically, the transmission line includes a dielectric 11, a ground conductor 12 formed on the lower surface of the dielectric 11, and a dielectric as shown in a perspective view in FIG. 2 and a plan view and a side view in FIG. 3. The signal line conductor 13 is formed on the upper surface of the body 11. In the figure, a microstrip type transmission line is shown, but as the transmission line, as shown by a chain line in the figure, the upper surface of the signal line conductor 13 is a dielectric having a ground conductor formed on the upper surface. A triplate type transmission line can also be obtained by covering. Hereinafter, description will be made regardless of whether the microstrip type or the triplate type is distinguished.

  In the following, in order to clarify one signal line conductor before branching and two signal line conductors after branching, the former is referred to as pre-branch signal line conductor 13a, and one of the latter is a signal after branching. The other one of the latter is called a post-branch signal line conductor 13c. The branched signal line conductor 13b and the branched signal line conductor 13c are both bent 90 ° in opposite directions immediately after being branched from the unbranched signal line conductor 13a, and have bent portions.

  In such a transmission line, in the present embodiment, the signal line conductor 13 is not a conventional T-shape, and both end edges in the width direction of the post-branch signal line conductors 13b and 13c are formed in a curved shape at the branch portion. The signal line conductor 13a before branching and the signal line conductors 13b and 13c after branching are continuously connected. More specifically, the signal line conductor 13 is formed by forming the edges in the width direction of the post-branch signal line conductors 13b and 13c in an arc shape having a predetermined radius of curvature, and the pre-branch signal line conductor 13a and the post-branch signal The line conductors 13b and 13c are continuously connected. At this time, the post-branch signal line conductors 13b and 13c are connected to the pre-branch signal line conductor 13a with a constant conductor width in order to prevent the characteristic impedance at the branch portion from becoming discontinuous.

  In the transmission line, by forming the signal line conductor 13 in this way, the current flowing through the edge in the width direction of the signal line conductor 13a before branching can be smoothly guided to the signal line conductors 13b and 13c after branching. It is possible to eliminate the current discontinuity and reduce the reflection loss.

  Actually, for the purpose of verifying the significance of making the signal line conductors 13b and 13c after branching into a curved shape, a simulation was performed to obtain a reflection loss when a signal of a predetermined frequency was transmitted.

In the simulation, as shown in FIG. 4, a dielectric 11 having a ground conductor 12 formed on the lower surface thereof having a relative dielectric constant ε _r = 2.6 and a dielectric thickness d = 0.56 mm is used. As the signal line conductor 13 formed on the upper surface of No. 11, a conductor having a conductor thickness t = 35 μm was used. As shown in FIG. 5, the signal line conductor 13 is formed as a pre-branch signal line conductor 13a with a conductor width W ₁ = 1.6 mm and a characteristic impedance Z _C1 = 50Ω, and the post-branch signal line conductor 13b, 13c, conductor width W ₂ = W ₃ = 0.4 mm and characteristic impedance Z _C2 = Z _C3 = 100Ω, respectively.

In the simulation, the curvature radii of the edges in the width direction of the branched signal line conductors 13b and 13c are R = 0, 0.4 (= W ₂ = W ₃ ), 0.8 (= 2W ₂ = 2W ₃ ). , 1.2 (= 3W ₂ = 3W ₃ ). Here, the radius of curvature R is, as indicated by a chain line in FIG. 5, a signal line conductor before branching of two arcs that form edges in the width direction of the signal line conductors 13b and 13c after branching. It is defined as the radius of curvature of the arc inside the bent portion that is proximal to 13a. Accordingly, the curvature radius of the outer arc that is distal to the pre-branch signal line conductor 13a among the two arcs that form the edge in the width direction of each of the post-branch signal line conductors 13b and 13c is described above. As described above, since the branched signal line conductors 13b and 13c are formed with a constant conductor width, they are defined as R + W ₂ (= R + W ₃ ), respectively.

  Note that the case where the radius of curvature R = 0 is, as shown in FIG. 6, the edge in the width direction of the signal line conductor 13a before branching and the inner edge in the width direction of the signal line conductors 13b and 13c after branching. Are perpendicular to each other. Also in this case, the outer edges in the width direction of the branched signal line conductors 13b and 13c are formed in a curved shape.

  For comparison, the reflection loss was also obtained for a conventional transmission line in which the signal line conductor was formed in a T shape as shown in FIG.

The result is shown in FIG. In the figure, the vertical axis represents the reflection loss [dB], and the horizontal axis represents the frequency [GHz] of the high-frequency signal transmitted through the signal line conductor. Further, in the figure, a curve C ₀ is a result in a conventional transmission line in which a signal line conductor is formed in a T shape, and a curve C ₁ is a result in a case where the curvature radius R = 0, and a curve C ₂ Is the result when the radius of curvature R = 0.4, the curve C ₃ is the result when the radius of curvature R = 0.8, and the curve C ₄ is when the radius of curvature R = 1.2. It is a result.

  From the figure, it can be seen that the reflection loss is remarkably improved in the newly proposed transmission line compared to the conventional transmission line over all measured frequencies. In particular, the result was obtained that the reflection loss was reduced as the curvature radius R was increased. In addition, since the reflection coefficient is reduced even when the radius of curvature R = 0, at least the outer edge of each end in the width direction of the branched signal line conductors 13b and 13c is curved. It can be seen that the signal line conductor 13a before branching and the signal line conductors 13b and 13c after branching may be continuously connected.

In this transmission line, as described above, the reflection loss tends to decrease as the radius of curvature R increases. However, increasing the radius of curvature R is necessary to form the signal line conductor 13. This means that the area becomes larger. Therefore, in this transmission line, the radius of curvature R may be appropriately determined depending on the desired reflection loss and area. Actually, sufficient characteristics can be obtained if the radius of curvature is about R = 0.4 (= W ₂ = W ₃ ) to 0.8 (= 2W ₂ = 2W ₃ ). In other words, in the transmission line, the outer edge in the width direction of the post-branch signal line conductors 13b and 13c is 1 to 3 times the conductor widths W ₂ and W ₃ of the post-branch signal line conductors 13b and 13c. In order to correspond to this, the inner edge in the width direction of the branched signal line conductors 13b and 13c is set to the conductor width W _{2 of the} branched signal line conductors 13b and 13c. , W ₃ is preferably formed in an arc shape having a radius of curvature of 0 to 2 times W ₃ .

  As described above, in the transmission line newly proposed as an embodiment of the present invention, when one signal line conductor 13 is branched in two directions, the branched signal line conductor 13b, Of the both end edges in the width direction of 13c, at least the outer edge with respect to the signal line conductor 13a before branching is formed in a curved shape, and the signal line conductor 13a before branching and the signal line conductors 13b and 13c after branching are connected to each other. By making the pattern connected continuously, the current discontinuity between the signal line conductor 13a before branching and the signal line conductors 13b and 13c after branching can be eliminated, and the reflection loss can be remarkably improved.

  In particular, this transmission line is effective when the signal frequency is high, and is suitable for application to use a pencil beam in order to improve sensitivity. Specifically, the transmission line of the present invention is less affected by reflection loss by being applied to various devices such as a BS (Broadcasting Satellite) antenna that transmits a signal of 10 GHz or more and a vehicle-mounted radar equipped with a planar array antenna. High-efficiency signals can be exchanged.

  Note that the present invention is not limited to the above-described embodiment, and it is needless to say that modifications can be made as appropriate without departing from the spirit of the present invention.

It is sectional drawing of a microstrip type transmission line, Comprising: It is a figure for demonstrating the current density distribution of the high frequency current which flows through a signal line conductor. It is a perspective view which shows the structure of the transmission line shown as embodiment of this invention. It is the top view and side view which show the structure of the transmission line shown as embodiment of this invention. It is sectional drawing of a transmission line, Comprising: It is a figure for demonstrating the parameter of the dielectric material used for the simulation which calculates | requires the reflection loss at the time of transmitting the signal of a predetermined frequency, and a signal line conductor. It is a top view of a signal line conductor, Comprising: It is a figure for demonstrating the parameter of the signal line conductor used for the simulation which calculates | requires the reflection loss at the time of transmitting the signal of a predetermined frequency. It is a top view of a signal line conductor, Comprising: It is a figure for demonstrating the signal line conductor whose curvature radius is 0. It is a top view of a signal line conductor, Comprising: It is a figure for demonstrating the conventional signal line conductor used as a comparative example in the simulation which calculates | requires the reflection loss at the time of transmitting the signal of a predetermined frequency. It is a figure which shows the relationship of the reflection coefficient with respect to the frequency of the high frequency signal transmitted through the signal line conductor calculated | required as a simulation result. It is a perspective view which shows the structure of the conventional transmission line. It is a top view of the signal line conductor in the conventional transmission line, Comprising: It is a figure for demonstrating characteristic impedance. It is an equivalent circuit schematic of the transmission line provided with the signal line conductor shown in FIG.

Explanation of symbols

1,11 Dielectric 2,12 Ground conductor 3,13 Signal line conductor 13a Signal line conductor before branching 13b, 13c Signal line conductor after branching

Claims (7)

A transmission line in which a ground conductor and a signal line conductor are formed across a dielectric, and the signal line conductor is branched in two directions,
The branched branched signal line conductor has a bent portion immediately after branching, and at least an outer edge of the bent portion is formed in a curved shape.   2. The transmission line according to claim 1, wherein the branched branch signal line conductor has an inner edge at the bent portion formed in a curved shape.   The transmission line according to claim 1, wherein the curve is a part of an arc having a predetermined radius of curvature.   The transmission line according to claim 2, wherein the outer edge and the inner edge are formed concentrically in the bent portion.   The transmission line according to claim 1 or 2, wherein the branched signal line conductor after branching is connected to the conductor before branching with a substantially constant conductor width.   The transmission line according to any one of claims 1 to 5, wherein the transmission line is a microstrip line in which a ground conductor is formed on one side of the signal line conductor.   The transmission line according to claim 1, wherein the transmission line is a triplate line in which ground conductors are formed on both sides of the signal line conductor.

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