US2812501A - Transmission line - Google Patents

Description

Nov. 5, 1957 D. J. SOMMERS TRANSMISSION LINE Filed March 4. 1954 Fig.5

Donald J, Sommers INVTOR.

Attorney United States Patent TRANSMISSION LINE Donald J. Sommers, Brookline, N. H'., assignor, by mesne assignments, to Sanders Associates, Inc., Nashua, N. H., acorporation of Delaware Application March 4, 1954, Serial No. 414,165

6 Claims. (Cl; 333-95 The present invention relates to transmission lines. More particularly, the invention relates to transmission lines such as are used in association with. high frequency electronic devices.

In modern shortwave techniques, transmission lines in forms familiarly known in the art as wave: guides and coaxial lines are widely used. Because of the construction, these lines occupy a large amount of space, are heavy and are expensive to manufacture.

Theoretically, a transmission line composed of inner conductors of finite width and spaced between outer conductors, which are parallel and of infinite, width, would permit no radiation of high frequency energy from the line. By selecting the width of the outer conductors to be sufiiciently greater than that of the. inner conductors, a transmission line of relatively low loss due to radiation is obtained.

As an alternative to Wave guides, such configurations may be adapted to printed and etched circuit techniques. As is well known in the prior art, configurations. that are in fact adapted to etched-circuit-techniqucsi involve the use of relatively thin conductors, for example, copper foil .00135 or .0027 of an inch thick. The propensity of such alternative configurations toward the propagation of high frequency energy in undesirable modes has, however, been the source of much difiiculty.

It is therefore an object of the present invention to provide an improved transmission line utilizing printed and etched circuit techniques which avoids the undesirable propagation modes of high frequency energy.

Other and further objects of the invention will be apparent from the following description of typical embodiments thereof, taken in connection with the accompanying drawings.

in accordance with this invention there is provided a composite high frequency electric transmission line structure, comprising a pair of elongated parallel planar dielectric insulators with a thin, elongated, planar, inner conductor disposed in parallel with and between the insulators, the inner conductor being narrower than the'i'nsulators. The transmission line also comprises a pair of thin, elongated, parallel, planar, outer conductors afiixed to the outside surface of the insulators in insulated spaced relation to and in parallel with the inner conductors. The outer conductors are wider than the inner conductor. The transmission line also comprises conductive rods extending from the outer conductors through the dielectric insulators adjacent the narrower sides of the inner conductor for providing an electrical connection between the outer conductors. longitudinally and'transversely less than a half wave length apart at the operating frequency to provide a transmission line for propagating electromagnetic energy consisting substantially of apureTEM mode,

In the accompanying drawings: 7

Fig. 1 is a three-dimensional view of a preferred embodiment of this invention;

The rods are spaced Fig. 2 is a schematic diagram of a source of high frequency energy connected to the embodiment of Fig. 1, shown in section;

Fig. 3 is a cross-sectional view of a three-plate transmission line illustrating the disposition of the electric and magnetic fields;

Fig. 4 is a cross-sectional view of the line in Fig. 4, illustrating a distorted field pattern; and

Fig. 5 is a cross-sectional view of a modification of the line in Fig. 3.

Referring now in more detail to the drawings, a thin,

elongated, parallel, planar, etched-circuit-type conductor 1 is attached to the upper surface of the elongated planar, dielectric insulator 2 with a suitable cement. A thin, elongated, planar, etched-circuit-type, inner conductor 3 is attached to the under surface of the dielectric 2 and to the upper surface of a dielectric 4. The dielectric insulators are wider than and extend transversely beyond the limits of the inner conductor 3 as shown. An elongated, outer conductor 5, similar to conductor 1 above, is attached to the under surface of the dielectric 4 in-parallel with the insulators 2 and 4 and the conductors 1 and 3. The outer conductors 1' and 5 extend transversely beyond the limits of the inner conduct-or 3, the conductors 1 and 5 being wider than the inner conductor 3. Metallic rods 6 provide an electrical connection between the conductors l and 5. The rods may be positioned in pairs separated transversely by less than a half-wave length at the operating frequency, as shown, and in the direction of the length of the line at longitudinal intervals 8'which are preferably less than a quarter-wave length long. The rods 6 need not necessarily be spaced symmetrically as long as a suitable spacing therebetween is chosen.

In Fig. 2 the outer conductors 1 and 5 are connected to the ground side of a source 13 of high frequency energy. The other side of the source 13 is connected to the inner conductor 3.

The three-plate transmission line has similar characteristics tocoaxial transmission lines and supports the same mode of propagation, familiarly known in the art as TEM. The three-plate line comprises two parallel outer conductors 14 and a parallel inner conductor 16 of lesser width, as shown in Fig. 3. Electric'fields i7 and magnetic fields 18 are balanced if the inner conductor 16 is centrally located with respect to the outer conductors 14 and 15, that is, d, equals d,. A dielectric material separating the conductors must be mechanically and electrically homogeneous to maintain the balance of fields as shown. The term TEM is employed because the electric and magnetic fields are transverse with respect to the As long as the fields confineda net difference inpotential between conductors 1'4 and i5 permits part of the energy to be propagated in a different mode. Although this mode of propagation may a'c' count for only a small component of the total fiel'd it isa radiating component and, therefore, contributes to the losses of the transmission. line and permits undesirable coupling with other systems.

To overcome thediiference in potential between outer conductors 14: and 15, conductive members 19' may be added to short the outer conductors together as in Fig. 5. Since the conductors 14, 15 and 19 complete a boundary condition for wave guide propagation, for example TE mode, the parameters of this system must be so chosen that the cut-off frequency for wave guide operation is higher than the operating frequency of the line. The conductive elements 19 must be spaced less than a halfwave length-apart at the desired operating frequency. Since the parameter effects of the elements 19 are limited, they must also be spaced longitudinally 'less than a halfwave length apart to substantially confine the propagated energy Within the confines of the outer conductors.

The term half-wave length as employed herein refers to the wave length of the energy propagated by the line and varies with the dielectric constant of the insulating materials.

While there has been hereinbefore described What are at present considered preferred embodiments of the invention, it will be apparent that many and various changes and modifications may be made with respect to the embodiments illustrated without departing from the spirit of the invention. It will be understood, therefore, that all those changes and modifications as fall fairly within the scope of the present invention, as defined in the appended claims, are to be considered as a part of the present invention.

What is claimed is:

1. A composite, high-frequency, electrical, transmission line, comprising: a pair of thin, elongated, parallel, planar, outer conductors; a thin, elongated, planar, inner conductor in parallel with and between said outer conductors, said inner conductor being of lesser width than said outer conductors; solid dielectric material wider than said inner conductor and disposed between said inner and outer conductors for securing said inner conductor in insulated, spaced relation with respect to said outer conductors; and conductive rods adjacent the narrow sides of said inner conductor extending from said outer conductors through said dielectric material for providing an electrical connection between said outer conductors, said rods being spaced longitudinally and transversely less than a half wavelength apart at the operating frequency to provide, thereby, a transmission line for propagating high-frequency, electromagnetic energy consisting substantially of a pure TEM mode.

2. A composite, high-frequency, electrical, transmission line, comprising: a pair of thin, elongated, parallel, planar, outer conductors; a thin, elongated, planar, inner conductor in parallel with and between said outer conductors, said inner conductor being of lesser width than said outer conductors; solid dielectric material wider than said inner conductor and disposed between said inner and outer conductors for securing said inner conductor in insulated spaced relation with respect to said outer conductor; and conductive rods aligned adjacent the narrow sides of said inner conductor extending from said outer conductors through said dielectric material for providing an electrical connection between said outer conductors, said rods being spaced longitudinally and transversely less than a half wavelength apart at the operating frequency to provide, thereby, a transmission line for propagating high-frequency, electromagnetic energy consisting substantially of a pure TEM mode.

3. A composite, high-frequency, electrical, transmission line, comprising: a pair of elongated, parallel, planar, outer conductors; a thin, elongated, planar, inner conductor disposed in parallel with and in insulated, spaced relation between said outer conductors, said inner conductor being narrower than said outer conductors, with said outer conductors extending transversely beyond the limits of said inner conductor; and conductive rods adja' cent the narrow sides of said inner conductor and extending from said outer conductors for providing an electrical connection between said outer conductors, said rods being spaced longitudinally and transversely less than a half wavelength apart at the operating frequency to provide, thereby, a transmission line for propagating highfrequency, electromagnetic energy consisting substantially of a pure TEM mode.

4. A composite, high-frequency, electrical, transmission line, comprising: a pair of elongated, parallel, planar dielectric insulators; a thin, elongated, planar, inner con ductor disposed in parallel with and between said insulators, said inner conductor being narrower than said in sulators; a pair of thin, elongated, parallel, planar, outer conductors afiixed to the outside surfaces of said insulators in insulated, spaced relation to and in parallel with said inner conductor, said outer conductors being wider than said inner conductor; and conductive rods extending from said outer conductors through said dielectric insulators adjacent the narrower sides of said inner conductor for providing an electrical connection between said outer conductors, said rods being spaced longitudinally and transversely less than a half wavelength apart at the operating frequency to provide, thereby, a transmission line for propagating high-frequency, electromagnetic energy consisting substantially of a pure TEM mode.

5. A composite, high-frequency, electrical, transmission line, comprising: a pair of elongated, parallel, planar, dielectric insulators; a thin, elongated, planar, inner conductor affixed to said insulators in parallel with and between said insulators, said inner conductor being narrower than said insulators, with said insulators extending transversely beyond the limits of said inner conductor; a pair of thin, elongated, parallel, planar, outer conductors affixed to the outside surfaces of said insulators in insulated, spaced relation to and in parallel with said inner conductor, said outer conductors being wider than and extending transversely beyond the limits of said inner conductor;' and conductive rods extending from said outer conductors through said dielectric insulators adjacent the narrower sides of said inner conductor for providing an electrical connection between said outer conductors, said rods being spaced longitudinally and transversely less than a half wavelength apart at the operating frequency to provide, thereby, a transmission line for prop agating high-frequency, electromagnetic energy consisting substantially of a pure TEM mode.

6. A composite, high-frequency, electrical, transmission line, comprising: a pair of elongated, parallel, planar, dielectric insulators; a thin, elongated, planar, inner conductor disposed in parallel with and between said insulators, said inner conductor being narrower than said insulators; a pair of thin, elongated, parallel, planar, outer conductors aflixed to the outside surfaces of said insulators in insulated, spaced relationto and in parallel with said inner conductor, said outer conductors being wider than said inner conductor; and conductive rods extending from said outer conductors through said dielectric insulators and aligned adjacent the narrower sides of said inner conductor for providing an electrical connection between said outer conductors, said rods being spaced longitudinally and transversely less than a half wavelength apart at the operating frequency to provide, thereby, a transmission line for propagating high-frequency, electromagnetic energy consisting substantially of a pure TEM mode.

References Cited in the file of this patent UNITED STATES PATENTS 2,231,602 Southworth Feb. 11, 1941 2,531,777 Marshall Nov. 28, 1950 2,575,571 Wheeler M Nov. 20, 1951 2,603,749 Kock July 15, 1952 OTHER REFERENCES Barrett: Etched Sheets Serve as Microwave Components, Electronics, June 1952, pages 11448.

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