US2444206A - Transmission line for radio frequencies - Google Patents

Description

June 29, 1948. M. c. PEASE TRANSMISSION LINE FOR RADIO FREQUENCIES v IN VEN TOR.

MARSHALL C. PEASE A T TORNE Y Patented June 29, 1948 TRANSMISSION LINE FOR RADIO FREQUENCIES Marshall C. Pease, Boston, United States of Ameri Secretary of War Mass, assignor to the ca as represented by the Application September 14, 1945, Serial No. 616,386

2 Claims.

This invention relates generally to the transmission of radio frequency energy. More particularly its relates to low-loss radio frequency transmission lines.

In many installations of apparatus operating at radio frequencies it is desirable to transfer radio frequency energy over considerable distances by means of coaxial transmission lines.

Where these lines are of appreciable length it is necessary to provide supports to keep the innerconductor properly spaced within the outer conductor. Numerous ways of doing this have been proposed and used in the prior art but difficulties of one sort or another have been encountered in their use.

The main disadvantage is the introduction of energy losses of such magnitude that the operation of the apparatus is impaired. At relatively low radio frequencies, appreciable losses may be tolerated but at higher radio frequencies where the amount of energy available at the input end is limited and must be conserved if enough is to remain to accomplish a useful function at the output end, even a relatively small less becomes critical.

Resonant coaxial stubs of conductive material used as insulators have very low loss at the frequency for which they are designed and thus serve efficiently at that frequency and over a very narrow band about that frequency. However, the resulting line is of restricted usefulness because once the designed frequency is departed from, energy losses rise rapidly. The result is that such a line cannot be used for broad band operation. For broad band operation the art in general has used spaced dielectric discs for supporting the inner conductor. These have an advantage over the use of a solid dielectric as a support in that the volume of dielectric is reduced but they have a disadvantage because the characteristic impedance of the line changes at every point where a supporting disc is located. Thus, energy moving through such a line. encounters a discontinuity every time it passes from an air insulated section to a supporting disc and vice versa. This causes standing waves and the losses, even if the line is otherwise a low-loss line, are substantially increased. Hence a great part of the advantage of reducing the volume of dielectric gained in the spaced support construction is lost.

In striving to produce low-loss coaxial transmission lines, the art has been governed by the known fact that in general losses vary directly as the volume of dielectric material between conductors. That is to say, these methods have been based on keeping the volume of dielectric material between conductors at a minmum. It has been discovered that, although the above relationship is true, it is not the only determinative factor in reducing losses and that attainment of the absolute minimum volume of dielectric possible may be sacrificed in order to construct a line having no discontinuities whatsoever.

It is an object of the present invention to construct a supported coaxial transmission line having a minimum amount of attenuation of the energy passing through it in which the dielectric supports, while not of minimum overall volume per length of line, are so designed that no electrical discontinuity is present and the overall energy loss is at a minimum.

It is also an object of this invention to construct a coaxial transmission line that may be used over a very wide :band of operating frequencies.

Generally this invention comprises a coaxial transmission line having longitudinally continuous dielectric supports occupying only a small percentage of the volume of space between the inner and outer conductors.

Other objects, features and advantages of this invention will suggest themselves to those skilled in the art and will become apparent from the following description of the invention taken in connection with the accompanying drawings in which:

Fig. 1 is a sectional View of a bodying this invention; and

Fig. 2 is a longitudinal sectional view along line 22 of Fig. 1, showing an end section of such a coaxial line emline.

Referring now more particularly to Fig. 1, concentric transmission line i I) has an outer conductor II and an inner conductor I2 which are concentional parts of such a line. Inn-er conductor I2 is formed so as to have three longitudinal keyways I3, I3, I3, running along its full length. Each of three dielectric supporting fins I 4, I4, I4 is of proper width tp reach from the bottom of one of the keyways I3 to the inner surface of outer conductor II. Thus, when fully inserted into keyways I3, I3, I3, dielectric fins I4, I4, l4 support inner conductor I2 in the center of outer conductor II and prevent it from moving to any other position. Dielectric fins I4, I4, H- are of uniform thickness and run for the full length of transmission line I0.

At the end of the line the radial width of dielectric fins I4, I4, I 4 may be decreased, as shown at I 4a, Ida, Na, in Fig. 2, in a tapered manner until there is no portion above the respective keyways l3, l3, [3. The purpose of this is to provide a gradual change in impedance but the length of the tapered sections Ha, Ma, Ma of fins l4, l4, I4 should not be so long that inner conductor l2 lacks necessary support.

The novel principles of design of a coaxial line employing this invention result in several desirable features. As may be seen by referring to Fig. 1, the volume of dielectric material in the space between outcr conductor II and inner conductor I2 is but a small percentage of the total volume of space between said conductors and therefore the losses which would result from a solid dielectric support are greatly reduced. Although the volume of dielectric in a coaxial line employing this invention may be no less than that of a spaced support coaxial line, the losses are less for the reasons stated below. That is, all elements of the line are continuous from one end to the other so that any portion of the line is identical to all other portions and its characteristic impedance is uniform throughout its length.

The mechanical support provided in a line employing this invention is extremely rigid because the support is continuous for the whole length of the line thus avoiding any possible variation in characteristic impedance.

It will be obvious to those skilled in the art that alternate details of construction employing the principles of this invention may be used. For example, the number of dielectric fins may be increased or decreased or the shape of the dielectric fins may be changed so long as they are longitudinally continuous and uniform, occupy a relatively small volume, and provide adequate support.

It will be seen that the transmission line herein set forth will transfer radio frequency energy with low-loss, cause minimum reflections due to discontinuities within the line itself, and function over a very broad band of operating frequencies.

While there has been described what is at present considered to be the preferred embodiment of this invention it will be obvious to those skilled in the art that various changes and modifications 4 i may be made therein without departing from the scope of the invention as set forth in the appended claims.

What is claimed is:

1. In a coaxial line for transmitting energy at radio frequencies and having an inner conductor and an outer conductor, means for supporting said inner conductor within said outer conductor comprising longitudinally continuous dielectric fins radially fixed in, and parallel to, said inner conductor and having a portion at the end of said line of gradually decreasing radial width, said fins occupying a relatively small portion of the space between said inner conductor and said outer conductor.

2, A coaxial transmission line for radio frequencies comprising an outer conductor, an inner conductor coaxial therewith, the surface of said inner conductor being provided with a plurality of grooves formed parallel to the axis of said inner conductor and equiangularly disposed about said surface, a plurality of longitudinally continuous dielectric spacing fins disposed between said inner and outer conductors, one edge of each of said fins being fixed in a corresponding groove, said fins being thin relative to the cross sectional area of said coaxial transmission line, the terminal portions of said fins being radially tapered for gradually decreasing the radial width thereof whereby the terminal impedance of said line may be altered in accordance with said taper.

MARSHALL C. PEASE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,035,274 Mougey Mar. 24, 1936 2,149,223 Mason Feb. 28, 1939 2,396,871 Meyerhans Mar. 19, 1946 FOREIGN PATENTS Number Country Date Great Britain (1911) July 18, 1912

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