US2382217A - Concentric transmission line termination - Google Patents

Description

1945- H. B. FANCHER "2,382,217

GONCENTRIC TRANSMISSION LINE TERMINATION Filed March 16, 1943 In l3 7 I A 1- 4 CARRIER g f WAVE sol/Res. 8 I

Inventor Henry B. Fanchet,

His Attorney.

Patented Aug. 14, 1945, v

. UNl'i'ED 2,382,217 CONGENTBIO TRANSMISSION mm mama-non Henry B. Fancher, Sc'otia, N. v., assignor'to General Electric Company, a corporation of New York Application March is, 194:, Serial No. 419,841

4 Claims. (Cl. 178-44) My invention relates to concentric transmission lines, and it has for an object to provide an improved and simplified means for terminating such lines and connecting them with exterior circuits.

It is frequently desirable to provide, at the end conductor and a tubular outer conductor concen- Conductor I8 is mounted concentrically with th inner and outer conductors l6 and It oi the line of a concentric transmission line having an inner tric therewith, means for supporting the inner and outer conductors in fixed relation. It is also desirable in many cases that such means should act to seal the outer conductor against the passage of gas or water to prevent moisture from entering the transmission line, or to allow the line to be filled with gas under pressure. It is accordinglyan object of my invention to provide such supporting means at the end of a transmission line having an improved and simplified form and in which the lump capacity presented across the end of the line by the supporting the accompanying drawing in which Fig. 1 represents one embodiment of my invention and Fig.

2 is a modification thereof.

In Fig. 1 a carrier wave source Ill supplies carrier waves through a concentric transmission line H to an antenna 12 from which such wavesare radiated. Oscillations from the source III are impressed across the terminals of an output circuit I3, of which one terminal is grounded. The

major portion of the outer conductor ll of the on line I l is grounded at various points along its length, and one end it of the inner conductor I6 is connected to a point on the tuned circuit l3 between'the terminals thereof. At the other end of the ton line, a continuation of the inner conductor It forms the antenna II.

In order to present minimum lump capacitance means is caused to have substantially negligible acrosstheendoftheline ,aswellastosealthe line hermeticallyagainst the e of gas or liquid, each end of the transmission line H is terminated by a termination structure II. The

structure I! comprises a tubular conductor it having a diameter somewhat larger than that of the outer conductor H of transmission line H anda length equal to the length of a quarter wave at the operating frequency of the source ill.

' about by the tact that the capacitance through i I and. is so located that oneend oi the conductor I8 is adiacent the end of the conductor ll. The other end of conductor I8 is rigidly fastened to the outer conductor II by a conductive connection it having considerable strength and which may be formed of any suitable conducting material, such as metal. Conductor I8 is not grounded throughout its length except through the member I! which is conductively attached to the grounded outer conductor M. A cup-shaped insulating disk 20 is secured to the free end of conductor l8 and supports the end ii of inner conductor It in rigid relation with respect toouter conductor M of line i l. Disk 20 may be formed of any suitable insulating material, for example, a ceramic-bonded mica insulating material, such as Mycalex, and may be united to conductors l5 andit in any well known manner.

Conductor l8, member, is which supports it on the outer conductor l4, and insulator 20 are structurally rigid. enough to resist both lateral and longitudinal motions of the inner conductor it with respect to the outer conductor l4. Furthermore, these parts form a closure over the end of the outer conductor M, so that the transmission line II is hermetically sealed. By such sealthe entrance of moisture by condensation,

or the like, is prevented, and the characteristics of the line are maintained unchanged, If it is desired to fill the transmission line H with an inert gas under high pressure, the seal formed by the elements", It and Hot the structure I! is of a type capable of withstanding such gas pressure and preventing leakage of the gas.

When an insulator is connected directly between the inner conductor l6 and the end of the outer conductor l4, it presents a lumped capacitance at the end of the transmission line H, which, in conjunction with other portions of the tron line and the interconnected circuit,

makes the entire system have a narrow band pass characteristic. This is due to the discontinuity row band. Such a condition is highly undesirable in certain application such as, for example, in the transmission of television signals requiring very broad band pass characteristics. By the provision of the structure l1 substantially no eiiective I capacitanc is added between the outer conductor I4 and the inner conductor It. This is brought insulator 2| is effectively in series with the conductor l8, which, with the outer conductor I3, forms a short section of concentric transmission line having a length equal to a quarter wave length atthe operating frequency of waves from the source I0. This short section of line is shortcircuited at one end by member I! so that the impedance at its opposite end, by well known transmission line theory, is of extremely high value. The structure l1 therefore presents between the outer conductor l3 and the inner conductor I! an extremely highimpedance in series with the capacitance of the insulator 20, the effect of the capacitance of this combination on the line I I being rendered substantially negligible by the series resistance.

It should be noted that, in television applications where extremely wide band widths are utilized for signaling, the length of the conductor l8, whilehaving a length equal to a quarter wave length at carrier frequency, may be somewhat different from a quarter wave in length at side band freqencies widely removed from the carrier frequency. However, for band widths normally termination means 40 used in television practice, the impedance of the short section of line formed by conductors l3 and I3 is very much higher than the reactance of the insulator even at side band frequencies widely removed from the frequency of waves from the source III.- The eflect of the capacitance of the insulator 20 therefore remains negligible at all frequencies to be transmitted, even when such frequencies are considerably different from that frequency at which the conductor I 8 is exactly a quarter wave length. The effect of the reduced impedance of the above mentioned short section of transmission line at frequencies widely spaced from the carrier frequency may be minimized by choosing afrequency in the middle of the band to be transmitted, and making conductor l8 of such length that this section of transmission line has a length exactly equal to a quarter wave length at this chosen frequency. This is especially important in television applications where single sideband transmission is normally used; in such applications, the chosen frequency will be dinerent from thenominal carrier frequency.

At the end of transmission line H adjacent antenna I! a structure 2|, similar to the structure I1, is utilized to maintain the inner conductor II in fixed relation with outer conductor H at that end of the line I I. As shown in the drawing, the end of the line ll adjacent the antenna I2 is vertically disposed and, in some cases, may extend a considerable distance above the earth. In such cases outer conductor I may be additionally supported, as from a tower, and the structure 21 may be utilized as means to'support the inner conductor IS in a fixed position within the outer conductor H. x

In the equipment illustrated in Fig. 2 a carrier wave source having a balanced, or push-pull, output circuit 3|, grounded at a central point 32 transmits waves through a pair of transmission lines 33 and 34 respectively to the two halves of a dipole antenna 35 and 36. The inner conductor 31 of the tron line 33 is connected to the tuned output circuit 3| at one side of the ground point 32, while the inner conductor 38 of the transmission line 33 is'connected to the output circuit 3| at the other side of the ground point 32. The other end of the inner conductor 31 is extended toform the element 33 of the dipole antenna, and the inner conductor 33 is extended similarly in the opposite direction to form the other element 33 of the antenna.

The outer conductors of the lines 33 and 34 are grounded throughout the major portion of their length, and each line is terminated at each end by a structure 40, which is similar to the structure ll of Fig. l.

In a push-pull circuit of the type shown in Fig. it is particularly desirable that the surge impedance of the connecting lines 33 and 33 be maintained constant and that discontinuities which cause undesirable reflections be avoided. Such a result is obtained by use of my improved ich isolate the lump capacitances of the ins ators 23 from the lines 33 and 34 by inserting suitable short sections of a transmission line to present a high impedance between the ends of lines 33 and 34 and the insulators 20.

While I have shown particular embodiments of my invention, it will of course be understood that I do not wish to be limited thereto since various modifications maybe made, and I contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. In a concentric transmission line for high frequency oscillations having inner and outer conductors, means for supporting said conductors in spaced relation at an end of said line comprising, a substantially inflexible tubular conductive sheath surrounding said outer conductor and having its end remote from the end 0! said line rigidly attached and conductively connected to said outer conductor, said sheath throughout its inner and outer conductors comprising. a tubular metal sheath encircling said outer conductor at said end and extending along said outer conductor for a distance substantially equal to a quarter wave length at the operating frequency of said line, the end of said sheath remote from said end of said line being conductively connected to said outer conductor, and insulating means supporting said inner conductor in coaxial relationship with said sheath at its other end.

3. In combination, a concentric transmission line for high frequency oscillations having an inner conductor and a tubular outer conductor, and means for sealing an end of said line and for reducing reflection of oscillations at said end, said means comprising, a tubular conductive sheath surrounding said inner conductor at said end and having a length equal to a quarter wave length at the frequency of said oscillations, the end of said sheath remote from said end of said line being conductively connected to said outer conductor to form therewith a section of a tron line having a length equal to a quarter wave length at said frequency, and insulating means seallngly engaging both said inner conductor and the other end of said sheath and presenting an undesirable reactance therebetween, said section of transmission line presentinga high impedance in series with said reactance between said inner and outer aemon 3 4. In combination. a gas-iiiied concentric transmission line for high irequencr oscillations having an inner conductor and a tubular outer conductor,

and means for sealing an end 0! said line and 101' reducing reflection o! oscillations at said end. said means comprising, a tubular conductive sheath surrounding said inner conductor at said end and having r. length equal to'a quarter wave length at the frequency of said oscillations, the end of said sheath remote from said end of said line being 'conductiveiy connected to said outer HENRY B. FANC'HER.

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