JP3484661B2 - Coaxial low-pass filter - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a coaxial type low pass filter, and more particularly to a coaxial type low pass filter used in a high power high frequency signal transmission line.

[0002]

2. Description of the Related Art Conventionally, for example, a coaxial low-pass filter inserted in a coaxial transmission (line) path, which is used for connecting a high-power transmitter for high-frequency signals and an antenna circuit, is shown in FIG.
The configuration is as shown in. In FIG. 2, both ends of the coaxial low-pass filter are connected to the coaxial line by connectors 7. Inside the coaxial type low-pass filter, a central conductor 5 having a structure in which a low impedance portion 3 and a high impedance portion 4 having different diameters are alternately connected in a coaxial cylinder made of metal is connected to a cylindrical outer conductor 6 made of metal. The structure is such that it is inserted inside and supported by the connector 7. When the center conductor 5 is long, a dielectric (layer) 9 such as Teflon may be provided to prevent contact between the center conductor 5 and the outer conductor 6.

The characteristics of the low-pass filter are determined by the outer diameter, length, number, etc. of the high impedance portion 4 and the low impedance portion 3.

The center conductor 5 is hollow, and when used in a high-power circuit, distortion occurs due to the heat generated in the center conductor 5, the electrical characteristics deteriorate due to the complicated structure of the center conductor 5, and the center conductor 5 and the outside. There is a possibility that a contact accident of the conductor 6 may occur.

When the dielectric 9 such as Teflon is inserted to prevent the contact between the central conductor 5 and the outer conductor 6, in addition to the generation of extra heat due to dielectric loss, the dielectric such as Teflon is not always necessary. Since it is not strong against heat, it does not help solve the problem.

Although it is conceivable to increase the diameter of the center conductor 5 to make a thermally advantageous design, the (inner) diameter of the outer conductor 6 is limited by the frequency band used and the characteristic impedance of the coaxial line. Therefore, it cannot be thicker than a certain diameter.

Japanese Utility Model Application Laid-Open No. 61-95102 suggests a method of providing a heat dissipation (through) hole in the dielectric 9. FIG. 3 shows a method of radiating heat from the central conductor 5 of the coaxial low-pass filter disclosed in Japanese Utility Model Publication No. 61-95102. In FIG. 3, the structure of the low-pass filter is basically the same as that of the conventional example shown in FIG. 2, and therefore a duplicate description will be omitted, but the difference is that the through holes 10 are provided in the dielectric 9. The through hole 10 is filled with an insulator having good heat conduction, and radiates the heat generated in the inner conductor 5 from the low impedance portion 3 to the outer conductor 6.

[0008]

Generally, there is a correlation between the electrical conductivity and the thermal conductivity, and a dielectric material having a large thermal conductivity usually has a large electrical conductivity, and thus a dielectric loss is large in many cases. . Beryllia porcelain is known as a special material having high thermal conductivity, high insulation, and low dielectric loss, but it is not suitable for filling through holes because it is expensive and has poor workability.

An object of the present invention is to provide a coaxial low-pass filter which can easily dissipate heat generated in the center conductor, reliably support the center conductor, and use no dielectric material.

[0010]

A coaxial low-pass filter used in a high-power high-frequency signal transmission line according to the present invention comprises:
A cylindrical coaxial outer conductor, first and second coaxial inner conductors in which a low impedance portion conductor and a high impedance portion conductor are alternately connected, and these first and second coaxial inner conductors. And a T-shaped member provided between
The first and second coaxial inner conductors are connected by the horizontal element portion of the character-shaped member, and the vertical element portion of the T-shaped member is set to have a length of ¼ wavelength of the used signal center frequency. The element part is grounded.

[0011]

The operation of the present invention is as follows.
A stub is provided at the center of the coaxial low-pass filter, and the heat generated by the center conductor is radiated by the vertical element of the stub, and the center conductor is mechanically supported, while the stub improves the termination characteristics of the coaxial low-pass filter. To do.

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a sectional view showing the structure of an embodiment according to the present invention, and the same parts as those in FIGS. 2 and 3 are designated by the same reference numerals. Note that duplicate description will be omitted.

In FIG. 1, a T-shaped stub portion 20 is provided at the center of the central conductor 5. This T-shaped stub portion 20
Is a metallic cylindrical horizontal element 1 having a length of 1/2 λ (a guide wavelength corresponding to the transmission center frequency of a signal transmitted through a coaxial line and a coaxial low-pass filter), and the center of the horizontal element 1. Similarly, a horizontal element 1 with one end grounded
It is a T-shaped element provided with a vertical element 2 made of metal and having a length of 1/4 λ of a cylindrical shape having the same diameter as.

The ground side of the vertical element 2 penetrates the outer conductor 6 without directly contacting the outer conductor 6 and is directly connected to a metal heat sink 8 having a heat radiation fin and having a high heat radiation efficiency for grounding. The heat dissipation plate 8 is also fixed to the outside of the outer conductor 6 so as to support the center conductor 5 including the horizontal element 1 at the center of the outer conductor 6 while also serving as the ground for the vertical element 2.

Since the horizontal element 1 has a length of ½λ, the stub portion 20 is impedance-matched in the used frequency band of the transmitted signal. Further, since the vertical element 2 has a length of 1 / 4λ, the impedance seen from the center conductor to the tip (ground end) of the vertical element 2 is open.

Furthermore, since the two low-pass filters are structurally connected in tandem via the stub section 20, it is possible to give each filter different characteristics, and the degree of freedom in designing the low-pass filter is high. Will increase.

[0018]

As described above, according to the present invention, since the metal vertical element of the stub directly connects the central conductor to the heat dissipation plate, the heat dissipation is extremely good, and the central conductor is connected to the central part of the outer conductor. Since it has a function of holding, there is no need for a dielectric, and there is no need to worry about dielectric loss or damage to the dielectric due to heat,
Further, since the stub portion is impedance-matched, there is an effect that the signal transmission characteristic is improved.

[Brief description of drawings]

FIG. 1 is a sectional view of an embodiment of the present invention.

FIG. 2 is a sectional view showing an example of a conventional coaxial low-pass filter.

FIG. 3 is a cross-sectional view showing another example of a conventional coaxial low-pass filter.

[Explanation of symbols]

1 Horizontal element 2 vertical elements 3 Low impedance part 4 High impedance part 5 center conductor 6 outer conductor 7 connector 8 heat sink

Claims (3)

(57) [Claims] 1. A cylindrical coaxial outer conductor, first and second coaxial inner conductors in which a conductor of a low impedance portion and a conductor of a high impedance portion are alternately connected, and these first and second coaxial inner conductors. A T-shaped member provided between the coaxial inner conductor and the T-shaped member, and the horizontal element portion of the T-shaped member connects the first and second coaxial inner conductors to each other, and A coaxial low-pass filter characterized in that the element portion is set to have a length of ¼ wavelength of a used signal center frequency, and the vertical element portion is grounded. 2. The coaxial low-pass filter according to claim 1, wherein the T-shaped member is configured to make impedance matching between the first and second coaxial inner conductors. 3. The coaxial low-pass filter according to claim 2, wherein the horizontal element portion of the T-shaped member is set to have a length of 1/2 wavelength of the used signal center frequency.