WO2003081628A1

WO2003081628A1 - An unsymmetrical-dielectric loaded helical structure with negative dispersion characteristics and a wideband travelling-wave tube using the same

Info

Publication number: WO2003081628A1
Application number: PCT/KR2002/000485
Authority: WO
Inventors: Sun-Shin Jung; Gun-Sik Park
Original assignee: Individual
Current assignee: Individual
Priority date: 2002-03-21
Filing date: 2002-03-21
Publication date: 2003-10-02
Anticipated expiration: 2004-09-21
Also published as: AU2002239144A1

Abstract

An unsymmetrical-dielectric loaded helical structure with negative dispersion characteristics and a wideband TWT using the same is disclosed. The helical structure has a helix disposed between the electron-beam-generating apparatus and the electron beam collector, dielectric supporting rods, and a metal envelope. The dielectric supporting rods is constructed of at least one first dielectric supporting rod with a first dielectric constant, and a plurality of second dielectric supporting rods with a second dielectric constant. The first dielectric constant is greater than the second dielectric constant, so that increasing capacitive impedances of the low-frequency waves are considerable with respect to those of the high-frequency waves are considerably with respect to those of the high-frequency waves, resulting in negative dispersion characteristics with larger phase velocities of the high-frequency waves than those of the low-frequency waves.

Description

AN UNSYMMETRICAL-DIELECTRIC LOADED HELICAL STRUCTURE WITH NEGATIVE DISPERSION CHARACTERISTICS AND A WIDEBAND TRAVELING-WAVE TUBE USING THE SAME

BACKGROUND OF THE INVENTION (a) Field of the Invention

The present invention relates to a traveling-wave tube (TWT) of a microwave generating system, and more particular to an unsymmetrical- dielectric-loaded helical structure with negative dispersion characteristics, and a wideband TWT using the same. (b) Description of the Related Art

A traveling-wave tube (TWT) that is used in a microwave generating system has a helical structure with a helix being supported by several dielectric rods extending parallel with the helix, the whole being enclosed in a metal envelope. Many proposals has been put forth for tailoring the design of non-resonant helical structures to render them to have negative dispersion characteristics, thereby making them suitable for over-octave wideband operation as well as for enhancing harmonic waves of low-frequency waves. A typical helical structure, however, gives positive dispersion characteristics, which can be at best flattened only if the metal envelope is brought very close to the helix. The close proximity of the envelope unfortunately reduces the interaction impedance of the structure and hence the gain and efficiency of the tube, and also entails the risk of arcing in the tube. Therefore, various shapes of the dielectric supporting rod or the metal envelope have been proposed in order to obtain negative dispersion characteristics of the helical structure. First, half-moon shaped dielectric rods have been developed to fill the space between the helix and the metal envelope along the radial direction. Accordingly, the variance of the capacitive impedance becomes larger mainly in low-frequency microwaves rather than in high-frequency microwaves in the frequency band, so that the phase velocities of the high-frequency microwaves are greater than those of the low-frequency microwaves.

In the proposal to vary the shape of the metal envelope, the second metals (so called as "metal vanes") are formed radially in an inward direction on the cylindrical metal tube, so that the metal vanes allow capacitive impedance in low-frequency microwaves to be greater. Therefore, the phase velocities of the high-frequency microwaves are also greater than those of the low-frequency microwaves.

However, when half-moon shaped rods are used, they occupy a large space between the helix and the metal envelope, increasing the loss of microwaves due to the dielectric rods and a reduction in the interaction between the microwaves and the electron beams, consequently resulting in poor performance of the TWT.

When the metal vanes are used in the metal envelope, the space between the metal vanes and the helix is remarkably smaller than that between the helix and the metal envelope. Therefore, when the power of the microwaves along the helical structure increases, it may cause arcing between the metal vane and the helix in the tube. Further, the dispersion of the tube is sharply changed according to the space between the metal vane and the helix, resulting in poor performance of the tube.

SUMMARY OF THE INVENTION

Therefore, the present invention has been made in an effort to solve the above-described drawbacks of the conventional helical structure in a traveling-wave tube.

It is an object of the present invention to provide an unsymmetrical- dielectric loaded helical structure with negative dispersion characteristics and a wideband traveling-wave tube using the same, in which at least one dielectric supporting rod is constructed of a material having a greater dielectric constant than the other rods, thereby increasing capacitive impedances of the low-frequency waves with respect to those of the high- frequency waves and resulting in larger phase velocities of the high- frequency waves than those of the low-frequency waves.

It is another object of the present invention to provide a helical structure with negative dispersion characteristics and a wideband traveling- wave tube using the same, capable of reducing losses of the microwaves due to the dielectric supporting rods, increasing the interactions between microwaves and electron beams, and diminishing the risk of arcing.

To achieve these objects, as embodied and broadly described herein, a helical structure according to the invention comprises: a helix disposed between the electron-beam-generating apparatus and the electron beam collector;

at least one first dielectric supporting rod with a first dielectric constant, for supporting the helix;

a plurality of second dielectric supporting rods with a second dielectric constant, for supporting the helix; and

a metal envelope for enclosing the helix and the first and second dielectric supporting rods,

wherein the first dielectric constant is greater than the second dielectric constant.

The difference between the first and second dielectric constants is preferably about 3.0 or more. The first dielectric rods are preferably made of alumina (AI₂O₃), and the second dielectric rods are preferably made of boron nitride (BN) or beryllium oxide (BeO).

According to another aspect of the present invention, a traveling- wave tube comprises: an electron-beam-generating apparatus;

an electron beam collector disposed facing the electron-beam- generating apparatus; and

a helical structure with negative dispersion characteristics, comprising: a helix disposed between the electron-beam-generating

apparatus and the electron beam collector;

at least one first dielectric supporting rod with a first dielectric

constant, for supporting the helix;

a plurality of second dielectric supporting rods with a second

dielectric constant, for supporting the helix; and

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings provide a further understanding of the invention, and together with the Detailed Description, explain the principles of the invention. In the drawings:

Fig. 1 shows a traveling-wave tube according to a preferred embodiment of the present invention; and

Fig. 2 shows a diagram illustrating relations between phase velocities

and frequencies in the traveling-wave tube of Fig. 1 .

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described in detail with reference to the accompanying drawings. Fig. 1 shows a longitudinal cross-section of a traveling-wave tube according to a preferred embodiment of the present invention, in which a helical structure 100 for a traveling-wave tube (TWT), disposed between an electron-beam generator and a collector which are not shown, has a helix 10 and three dielectric supporting rods 20, 30, 40 within a metal envelope 50 that is concentric with the helix 10. The helix 10 is constructed of a helically-wound wire or tape made of a material selected from a group consisting of molybdenum, tungsten, and so forth. The helix 10 is capable of reducing the phase velocity of microwaves with respect to the velocity of the electromagnetic waves.

The three dielectric supporting rods 20, 30, 40 support the helix 10 between the helix and the metal envelope 50, being capable of emitting heat generated at the helix 10. The dielectric supporting rod 20 is formed of a first dielectric material with a higher dielectric constant than that of the dielectric supporting rods 30, 40, which are formed of a second dielectric material. The first dielectric material is alumina AI₂O₃ with a dielectric constant ranging from 8.9 to 9.0, and the second dielectric material may be beryllium oxide (BeO) with a dielectric constant of about 6.5, or boron nitride (BN) with a dielectric constant of about 5.4. The sectional shape of the dielectric supporting rods 20, 30, 40 may be a rectangle, which is easily made. In the embodiment, the surface which contacts the helix 10 is a plane.

The metal envelope 50 encloses the helix 10 and dielectric supporting rods 20, 30, 40 not only to keep the inside of the helical structure in a vacuum state, but also to control the dispersion characteristics of the helical structure, such as capacitive and inductive impedances. The helical structure 100 is operated as follows. Electron beams generated by the electron-beam generator (not shown) are collected by the electron beam collector (not shown), and microwaves that propagate along the helical structure 100 displaced around the electron beams are amplified by interaction with the electron beams. Since microwaves with high frequencies (i.e., short wavelengths) are locally disposed around the helix 10, the electric fields thereof are considerably decreased along the radial direction from the center of the helix 10. Therefore, the high-frequency microwaves are affected by a part of the electric supporting rods 20, 30, 40 near the helix 10 to vary the capacitive loads. In contrast, since the low-frequency microwaves with long wavelengths have enough large electric fields near the metal envelope 50, the entire part of the dielectric supporting rod affects the capacitive loads, thereby increasing the variance of the capacitive loads of the low-frequency microwaves. Because the phase velocity of a microwave is inversely proportional to the square root of the capacitive impedance, the phase velocity varies considerably according to changes of the capacitive impedance. The inventors of the present invention noticed this feature, so they used different dielectric rods.

Since the first dielectric supporting rod has a higher dieiectric constant than that of the second dielectric supporting rods, the effective dielectric constant between the helix 10 and the metal envelope 59 become remarkably higher than the effective dielectric constant around the helix 10. Accordingly, the capacitive impedances of the low-frequency microwaves increase more than those of the high-frequency microwaves, and then the phase velocities of low-frequency microwaves decrease more than those of the high-frequency microwaves. Thus, the phase velocities of the high- frequency microwaves are larger than those of the low-frequency microwaves so that the helical structure 10 has negative dispersion characteristics.

Fig. 2 shows a diagram illustrating relations between phase velocities and frequencies when the first and second dielectric materials are alumina and beryllium oxide, respectively. Curve A shows ideal phase velocities that are calculated theoretically, while curve B shows phase velocities that are measured in the signals using a vector network analyzer (VNA). Curve C shows values that are measured in the conventional system using three beryllium oxide rods. As shown in Fig. 2 in curve C, when the dielectric supporting rods are formed of the same material, the helical structure has positive dispersion characteristics in the low frequencies, below 10 GHz. In contrast, it is noticed that the helical structure of the present invention (curve A or B) has negative dispersion characteristics in the low frequencies, even below 10 GHz.

In the above-described embodiment, it is explained that three dielectric supporting rods are used, with one having a higher dielectric constant than the other. However, it is possible to design helical structures and TWTs in which four or more dielectric supporting rods are used, with one or more rods have higher dielectric constants than the others.

Further, the explanation has been made to the supporting rod having a flat surface which is in contact with the helix 10, but it is possible to design the dielectric supporting rod in various shapes, not being limited to the above-described shape.

It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the spirit and scope of the invention. The present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

Claims

WHAT IS CLAIMED IS:

1. An unsymmetrical-dielectric loaded helical structure, comprising:

a helix disposed between an electron-beam-generating apparatus and an electron beam collector;

2. An unsymmetrical-dielectric loaded helical structure as recited in claim 1 , wherein the difference between the first and second dielectric constants is about 3.0 or more.

3. An unsymmetrical-dielectric loaded helical structure as recited in claim 1 , wherein the first dielectric rods are made of alumina (AI₂O₃), and the second dielectric rods are made of boron nitride (BN) or beryllium oxide (BeO).

4. An unsymmetrical-dielectric loaded helical structure as recited in claim 1 , wherein each supporting rod has a flat surface that is in contact with

the helix.

5. An unsymmetrical-dielectric loaded helical structure as recited in claim 1 , wherein the number of first supporting rods is one (1) while the number of second supporting rods is two (2).

6. An unsymmetrical-dielectric loaded helical structure as recited in claim 1 , wherein the number of first supporting rods is two (2) while the number of second supporting rods is two (2).

7. A traveling-wave tube comprising:

an electron-beam-generating apparatus;

a helical structure with negative dispersion characteristics, comprising:

a helix disposed between the electron-beam-generating apparatus and the electron beam collector;

8. A traveling-wave tube as recited in claim 7, wherein the difference between the first and second dielectric constants is about 3.0 or more.

9. A traveling-wave tube as recited in claim 7, wherein the first dielectric rods are made of alumina (AI₂O₃), and the second dielectric rods are made of boron nitride (BN) or beryllium oxide (BeO).