US2387783A - Transmission line - Google Patents

Description

Oct. 30, 1945. e. L. TAWNEY TRANSMISSION LINE Filed Feb. 1, 1943 2 Sheets-Sheet 1 INVENTOR Gerald L.Ta/wne y ATTORNEY III Oct. 30, 1945. G. TAWNEY TRANSMISSION LINE Filed Feb. 1, 1943 2 Sheets-Sheet 2 [f y Mzb ATTORNEY INVENTOR dl'nTa/wn Y Gare Z B Patented Oct. 30, 1945 TRAN SMISSION LINE Gereld L. Tawney, Hempstead, N. Y., assignor to Sperry Gyroscope Company, Inc., Brooklyn, N. Y., a corporation of New York Application February 1, 1943, Serial No. 474,400

32 Claims.

My invention relates to electrical transmission lines, and is particularly concerned with high impedance transmission lines for use in high frequency apparatus.

The invention will be described in its preferred embodiments as employed in coaxial transmission lines which are now used widely for distributing high frequency energy in pulse forming lines, and in delay lines.

Prior to my invention, coaxial transmission lines generally comprised chiefly a tubular outer conductor and a coaxial inner solid or stranded wire conductor supported and spaced within the tubular conductor by suitable insulating means. Most present day coaxial lines employ as the insulating means suitably formed members made'of a low loss dielectric material such as polystyrene which is a polymerization product of styrene and is a thermoplastic adhesive resin having a fairly high dielectric constant.

These prior solid inner'conductor coaxial lines have low characteristic impedance. For example in a coaxial line having an outer conductor diameter of about one-half inch and minimum practical size of inner conductor, the calculated characteristic impedance is only about 195 ohms. These prior coaxial lines have proven reasonably satisfactory for transmission lines requiring such low characteristic impedance.

However, for many purposes, demand has arisen for relatively high characteristic impedance coaxial lines. For example, in some installations, it is preferable to transmit signals made up of periodic pulses each having a duration of only a fraction of a micro-second over as much as to 40 feet of coaxial line at impedance levels of about 1000 to 1500 ohms. Since the above described prior coaxial lines cannot by themselves attain such impedance levels, it has hitherto been necessary in such installations to supplement them with auxiliary equipment such, for example, as cathode follower apparatus requiring the use of vacuum tubes.

The invention contemplates coaxial transmission lines of revolutionary design for attaining desired high characteristic impedance levels, over a wide range of frequencies.

In general it may be stated that the characteristic impedance Zr of a transmission line equals where L and C are inductance and capacitance values per unit length. Hence, the characteristic impedance of any such line can theoreticall be increased either by decreasing its capacitance or increasing its inductance. I have studiedvarious arrangements for decreasing the capacitance in coaxial lines, but none seem to offer the improvements obtained by arrangements for increasing inductance which comprise thelpreferr'ed embodiments of my invention.

With the above in mind, it is a major object of my invention to provide a novel coaxial type transmission line having relatively high characteristic impedance.

A further object of the invention is to provide coaxial transmission lines of novel construction.

A further object of the invention is to provide a high characteristic impedance coaxial transmission line having novel arrangements providing high distributed inductance in the line. Preferably this high inductance is obtained by employing a coiled inner or outer conductor, or both, or cores of insulating material having high magnetic permeability within the coaxial line, or various combinations of coiled conductors and such cores.

A further object of the invention is to provide a novel coaxial type transmission line having one or both conductors disposed to provide relatively high distributed inductance.

A further object of my invention is to provide a novel manner of controlling impedance and time delay properties of a coaxial transmission line.

A further object of the invention is to provide a novel coaxial transmission line wherein at least one conductor, preferably the inner conductor, is a closely wound coil.

A further object of the invention is to provide a novel coaxial transmission line wherein a coiled inner conductor is disposed Within a tubular outer conductor having a diameter bearing predetermined relation to the coil diameter for obtaining maximum characteristic impedance in the line.

A further object of the invention is to provide a novel delay line comprising a coaxial transmission line having inductive and capacity loading arrangements.

Further objects of the invention will presently appear as the description proceeds in connection with the appended claims and the annexed drawings wherein:

Fig. 1 is mainly a section through the center of a coiled inner conductor flexible coaxial transmission line embodying the invention;

Fig. 2 is mainly a section through the center of a semi-rigid coaxial transmission line similar to v Fig. 1 wherein the inner conductor is a closely wound coil;

Fig. 2a illustrates, in an enlarged diagrammatic section, the overlapped winding of Fig. 2;

Fig.3 is mainly a section through the center of a coaxial transmission line having a coiled outer conductor;

Fig. 4 is mainly a section through the center of a coaxial transmission line wherein both inner and outer conductors are coiled;

Fig. 5 is mainly a section through the center of a coaxial transmission line wherein the inner nductor is a coil about a continuous core of relatively high magnetic permeability;

Fig. 6 is a section through the center of a coaxial transmission line according to another embodiment of the invention wherein the inner conductor is a coil closely wound about a sectional core of high magnetic permeability.

Fig. 7 is mainly a section through the center of a flexible coaxial transmission line having wound inner and outer conductors and special insulation and shielding arrangements.

Fig. 8 is mainly a section through the center of a flexible coaxial-transmission line wherein increased distributed inductance is obtained simply by the use of a sectional core of high magnetic permeability between the conductors;

Fig. 9 is mainly a section through the center of a delay line according to the invention having both inductance and capacity loading arrangements; and

Fig. 10 is a side elevation partly in section of a bead of the type used in Fig. 8.

In general, according to the invention, I preferably increase the characteristic impedance of a coaxial line by increasing the inductance of one or both of the conductors, as by coiling. Especially where one of the conductors is coiled, the inductance may be further appreciably increased by providing a core or cores of high magnetic permeability within the line. For example, the core may be within the coiled inner conductor, or between the outer conductor and the inner conductor. Any desired combination of coiled inner and/or outer conductors, and inner and/or outer cores may be employed.

Fig. 1 illustrates a flexible cable of circular cross-section made according to one embodiment of the invention. The outer conductor H, which may be a flexible metal tube or of braided copper construction as shown, is externally covered by successive braided cord and rubber sheaths l2 and i3 and any other desired insulating and protective sheathing.

The inner conductor M is a coil of relatively small diameter of fine insulated wire I5 closely wound upon a low loss dielectric core l6 which may be a bundle of twisted glass fibers as illustrated. Coil 14 provides a relatively high uniformly distributed inductance which insures that the characteristic impedance of the line is proportionately high. The absolute inductance value of coil it] may be obtained as desired, as by difierent winding and spacing arrangements of conductor l5, depending on the requirement of the purpose at hand. Coil I4 is relatively large in diameter as compared to the usual solid inner conductor of prior coaxial lines.

Coil i4 is supported coaxially within outer conductor H by means of a plurality of relatively short individually rigid insula or beads ll which may be of any low loss dielectric material such as polystyrene. Each bead I! is formed with substantially spherical convex nose and concave tail mating surfaces indicated at l9 and i8, 'respectively, whereby the beads are arranged in a universally jointed array within the cable. This permits the cable to be bent and manipulated as desired without injury to the insulation. Beads I! are of known construction and are shaped to provide a minimum of solid dielectric material between the conductors, thus illustrating some prior efforts to increase characteristic impedance by reducing the distributed capacity.

My invention enables high characteristic impedance transmission and delay lines to be accurately designed for most purposes. Considering a coiled conductor as a long, single layer closely wound solenoid, its characteristic inductance may be expressed as (1) L':[J.,U.0 1r(T7L') henrys per meter, and its characteristic resistance as -which, for a, closely wound coil, reduces to 8 r(n') 3 ohms per meter, where:

=the magnetic permeability of material adjacent the coil;

,u. =41r 10 henrys/meter, the magnetic permeability of free space in the meter-kilogramsecond system of units;

r=the radius of the coil;

n= number of turns per meter in the solenoid;

a=radius of the round wire in the solenoid (for a closely wound coil,

and =the resistivity of the wire in the solenoid.

The relation (1) above holds true for computing inductance of a coiled conductor of a. coaxial line, provided the other conductor is not too close to the coil.

In any concentric conductor line, wherein 211 is the inside diameter of the outer conductor and 2T2 is the outside diameter of the inner conductor, the characteristic capacitance is:

farads per meter, where k is the dielectric constant of the material between the conductors; and I :o=8.85 10- farads per meter, the dielectric constant of free space in the meter-kilogramsecond system of units.

Using now the known relations:

(5) p T (time constant) :VZF

and

(6) a (attenuation) =R/2Z,,.

It follows from (1) to (c) that Z ##0 go T2.

A comparison of computed and actually measured values for an exemplary coaxial line similar I to that shown in Fig. -1 will now be given. This overlap one or more previously wound turns, such as shown in Fig. 2a, the numerals I indicating "the successive windings of wir 24. Any suitable manner-oi winding wire 24 to formmulti-layer line has the following dimensions and characteristics: r,=.00394 meter (Q)=5 25' r =.00O75 ,meterJ T2 v a=No. 38 wire diameter 12 5600 turns/meter p= 1.7 X l0- ohms/meter (copper wire) .k: 1.35 (polystyrene outer beads) 1- (polystyrene beads and glass fibers) A comparison of calculated and measured characteristics give the following values:

mum) neper/meter.

The above measurements show clearly that the important properties of characteristic impedance [and time constant maybe closely designed in a transmission or delay line. The attenuation may be improved by closer winding of the coiled conductor. Equation 7 shows that Zk may be best improved by changes in a and 12, while Equation 8 shows that -T' may best be improved by changes in k and r2. cannot be increased very far because it may cause the attenuation to become too high. The value of T1 is limited by practical considerations, but T2 may be varied over wide limits.

Tests and computations have shown that a maximum characteristic impedance is obtainable in such a line when the outer diameter of the inner conductor coil is spaced at least a minimum distance from the inner periphery of the outer conductor. According to my invention, maximum characteristic impedance i obtainable when the ratio of the inner conductor coil external diameter to an outer non-slitted conductor internal diameter is approximately 0.6. Where the conductor is slit, as in Fig. 2, the ratio may be higher.

Fig. 2 illustrates the principles of Fig. 1 as applied to a relatively rigid or semi-rigid coaxial line having the outer conductor 22 formed as a longitudinally continuous tube of copper or the like. The inner conductor is a coil 23 of relatively fine insulated wire 24 closely wound about a continuous solid rod or-form 25. -I have found that high inductance may be obtained by winding wire 24 in such manner as to form a multi-layer coil, as by back winding wire 24 at intervals to Th number of turns in the coil coil 23 which has higher uniformly distributed inductance than a single layer coil is intended to be included herein.

fCoil 23 is supported within the line by a continuous insulator sleeve 26 which fits snugly within outer conductor 22 and has a central passage 21 through which the inner conductor extends. If

, desired, to reduce capacity along the line, dielectric sleeve 26 may be replaced by spaced beads capable of equivalent support andinsulating characteristics.

Rod 25 and sleeve 26, in this phase of the invention are preferably of a low loss dielectric material similar to heads I! of Fig. 1 where a rigid 'line is desired. Alternatively,.rod 25 and sleeve 26 may be of a flexible dielectric material where a flexible line is desired. Rod 25 and sleeve 26 function only as insulators or winding forms in this embodiment. Solid metal tube 22 provides a better conductor than braided conductor'll. When current is flowing through, coil 23, currents are induced in outer conductor 22. In order to reduce these induced currents and to increase the ease of obtaining high impedance, I preferably 1 able uniformly form conductor 22 with a continuous narrow longitudinal slot 28 which interrupts the induced current paths. Where such .a slitted outer conductor is employed, higher overall characteristic impedance of the line may be readily obtained.

Fig. 3 illustrates a coaxial transmission line wherein outer conductor 29 is. a coil of insulated wire 30 closely wound upon an insulating sleeve 26 through which passes a solid center conductor Wire 32. Sleeve 26 is preferably of the same material as sleeve 26 of Fig. 2 and may be rigid or flexible as desired. Coil 29 provides considerdistributed inductance which makes the illustrated line of relatively high characteristic impedance.

Fig. 4 illustrates a coaxial transmission line wherein both inner and outer conductors are coiled, thereby combining certain features of Figs. 2 and 4. Care should be taken that the respective coils are so wound as to be additive; in other words, so that their combined effect is to provide a higher distributed inductance than either alone.

In the above described embodiments of Figs. l-4, increased inductance is obtained by coiling one or both conductors, without other expedients. With regard to the remaining forms of the invention substitute and supplementary means for increasing the inductance will be disclosed.

Fig. 5 illustrates a coaxial line wherein the inner conductor wire I 5 is wound upon continuous core rod 3| and the coil l4 extends through insulator sleeve 26 within the outer conductor, similarly to Fig. 2. The coaxial line of Fig. 5 differs from that of Fig. 4 chiefly in that rod 3|, instead of being the usual low los dielectric, is composed of a material which has equivalent insulating properties but in addition has high magnetic permeability. Likerod 25, rod 3| may be rigid or flexible as desired.

This material of rod 3|, according to a very satisfactory reduction to practice of the invention, may comprise a suitable binder, such as Bakelite, containing powdered iron or other highly magnetic material liberally dispersed therein. Core 3|, more generally therefore, preferably comprises finely powdered or granular iron or its equivalent distributed uniformly through a binder having the required insulating strength. This material will hereinafter be described as carbonyl iron material.

In explanation of the improved result, apparently the powdered iron core provides effective paths of high permeability greater than 1 for magnetic lines of force within the cable, similar to the efiect of adding an iron core to an inductance coil, thereby increasing the overall inductance of the cable. Referring back to the above equations, it is seen that here I vary a, which was stated to be one of the best factors variable for improving Zr. The inductance may be increased by increasing the core diameter, but this consideration is limited by the practical conductor diameter ratio above given.

According to m invention the use of such high magnetic permeability cores enables the construction of a coaxial line having minimum losses for a given characteristic impedance during an appreciable range. Since the core increases the distributed inductance of the coil, I am enabled to use relatively large diameter wire for the coil winding in this embodiment, thereby keeping the ohmic losses low while obtaining a required high inductance,

As desired, sleeve 26 may be of the same low loss dielectric material as sleeve 26 in Fig. 2, or may be made of the same material as core 3|. This latter construction further increases the distributed inductance in the line. Another variation of the line in Fig. 5 is to replace core 3| with a polystyrene rod 25, while sleeve 26 only is of carbonyl iron material. This provides a high characteristic impedance cable available for obtaining certain impedance values.

A flexible coaxial line capable of very high characteristic impedance is shown in Fig. 6, where the outer conductor and insulating bead arrangements are the same as in Fig. 1, but wherein the inner conductor 33 comprises a coil of insulated fine wire 34 tightly and closely wound about a sectional core 35 of high magnetic permeability enclosed in a tubular sheath 3B of braided glass fibers. Conductor 33 extends through a series of beads |'I- similarly to Fig. 1.

Each section 31 of core 35 is a relatively short carbonyl iron bead of the same material as core 3| of Fig. 5. In coaxial lines successfully built and tested, beads 31 are slightly less than A; inch in diameter and about inch long. The ends of beads 31 are surfaced for universally jointed contact, similarly to beads l7, and the beads are threaded and held together in alignment within a flexible fiber glass braided tubing on which No. 40 insulated wire is closely wound. The inside diameter of outer conductor H is in the neighborhood of inch. With outer beads H of polystyrene, the characteristic impedance of this line has been measured to be about 3500 ohms.

Fig. 7 illustrates a further available form of coaxial line which the invention may take. The inner conductor 33 is wound about a sectional carbonyl iron core as in Fig. 6, and is located within continuous insulating sleeve 26 which may be of a low loss dielectric material as in Fig. 4, or of the same material as core 3| in Fig. 5, depending on the characteristic impedance value desired.

The outer conductor 38 is a coil of insulated wire 39, preferably wound in multi-layer arrangements similarly to Fig. 2a.

The coaxial line of Fig. '7 is enclosed within a relatively thick insulating sleeve M, which may be of cellular construction as shown, and the whole is enclosed within a continuous metal shielding tube 42 which protects the line both electrically and mechanically.

The coaxial line of Fig. 8 is similar in construction to the prior solid inner conductor coaxial lines above discussed, but differs therefrom in that universally contacting beads 40 of carbonyl iron material are employed. As shown also in Fig. 10, beads 40 are of such shape and crosssection as to provide maximum solid core material between the conductors. Simply by employing these special insulator beads in such a coaxial line, I am enabled with currently available carbonyl iron material to raise the characteristic impedance of the lineby a factor of about two. Tests on a coaxial line of this construction have shown the upper limit of characteristic impedance to be in the neighborhood of 500 ohms.

Other useful forms of the invention are in coaxial lines wherein the conventional polystyrene beads H in Figs. 1 and 6, and sleeve 26 in Figs. 2-4 and-7, are replaced by carbonyl iron beads 40 of the type shown in Fig. 10, so that these beads comprise effectively external or internal cores for the inner conductor, as the case may be. Using carbonyl iron outer beads in the cable of Fig. 6, for example, I am able to produce coaxial lines having characteristic impedance values as high as 20,000 ohms or more, with No. 46 Wire on the inner coil conductor.

In general any of the above arrangements described in Figs. l-8 may be used singly or in combination, depending on the desired inductive loading. Given the desired characteristic impedance of the required line, the above principles are employed in calculating and building a line of best practical construction. I have found that use of carbonyl iron cores and beads, besides increasing the inductance, increases the line capacity slightl which decreases the characteristic impedance.

By varying the powdered iron content in core 3| and in beads 3's or 30, it is possible to vary the effect of such material in the cable and thereby control the characteristic impedance values within desired limits. However, as a practical matter, with currently available carbonyl iron material it is usually desirable to employ cores and beads of maximum iron content, which accounts for the shape of bead 40. The above described coaxial transmission lines have proved satisfactory for intended purposes. Tests at frequencies up to 5 megacycles show that the lines transmit without noticeable distortion square wave pulses having about 0.1 microsecond rate of rise and about 0.5 microsecond duration. The attenuation losses up to 5 megacycles, at least, are not measurably different from direct current resistance losses, although for higher frequency they increase more rapidly. Eliminating the powdered iron cores for higher frequencies, it appears that satisfactory operation up to beyond 40 megacycles is possible.

Further, since the coaxial line construction of the invention comprises definite inductive and capacitance loading, the coaxial line may be employed as an accurate delay line having a uniform time constant per unit length. For example, a coaxial line similar to Fig. 3 was found to have a delay constant of 0:67 niicrosecond per foot.

Fig. 9 illustrates the invention as applied to construction of a coaxial conductor delay line having an appreciable time constant per unit length. Here the center conductor d3 is a coil of insulated fine wire wound about a solid rod ll of carbonyl .iron or the equivalent for inductive loading;- C'oil 43 is mounted snugly within 'the central passage of a solidsleeve 45 of some material having a very-high dielectric constant. For

example, I have found an oxide of titanium commonlyknown as rutile to be very satisfactory, and. this material has a dielectric constant of about 40 to 60 times as large as-polystyrene. The outer both inductively and capacitatively, The time constant T for such a line is equal to j W 417c? where l is the length of the line, so'that choice of a'suitable' length of line can be made to produce an appreciable desired delay in'.transmissi0n of pulses or the like along the line. In accord with the general mathematical discussion of the invention above, 'a-delay line of substantially anydesired value can be designed utilizing the principles of the invention.

As many changes could be made in the above formly along and within said line providing high characteristic impedance comprising insulating core means ofmagnetic permeability substantially greater than air disposed between said electrically insulated conductors.

2.'The transmission or delay line defined in claim 1, wherein said physically distributed means comprises also the formation of one of said conductors as a coil having-turns insulated from each other substantially coaxial with the other conductor.

3. In a coaxial type transmission line, an outer' conductor, an inner conductor extending through said outer conductor, and means distributed substantially uniformly along and within said line for providing high inductance for at least one of said conductors comprising insulating core means of magnetic permeability substantially greater than air disposed between said conductors.

4. In a coaxial type transmission line, an outer conductor and a spaced inner conductor, said conductors being electrically insulated and at least one of said conductors being formed to provide substantially uniformly distributed high in ductance along said line, and insulating core means having magnetic permeability substantially greater than air disposed between said electrically insulated conductors.

' construction and manyapparently widely diiTerent embodiments of this invention could be made without departing from the scope thereof, it is in-. '30

5. In a coaxialtype transmissionline, an outer conductor and winner conductor, at least one of said conductors being in the form of a substantiallyuniform diameter closely wound coil of insulated wire having relatively high inductance, and insulating means within said line spacing sa d conductors. V

6. A coaxial type high frequency transmission line section of high characteristic impedance comprising an outer conductor, .an inner conductor comprising a substantially uniform diameter 7. A coaxial type high frequency transmission line section of high characteristic impedance comprising an inner conductor, an'outer conductor comprising a substantially uniform diameter closely wound coil of insulated wire, and insul ating means spacing said conductors within said line.

'8. A coaxial type high frequency transmission line section of high characteristic impedance comprising an inner conductor and an outer conductor, each of said conductors comprising a substantially uniform diameter closely wound coil of insulated wire, an insulating means spacing said,

conductors within said line.

9. The transmission line section defined in claim 8, wherein said coiled conductors'are additively wound so that combined they produce a higher distributed inductance than either alone.

10. A transmission line comprising substantially coaxial inner and outer conductors, at least one of said conductors being in the form of a substantially'uniform diameter coil-of insulated wire having relatively high inductance, and insulating means within said line between said conductors, said insulating means comprising a material having higher magnetic permeability than air.

11. In a coaxial type transmission line, an outer conductor, and a coiled inner conductor, said conductors being substantially circular and the outer diameter of said coil being less than approximately 0.6 the inner diameter of said outer conductor.

12. In a coaxial type transmission line, an outer conductor,. an inner conductor of appreciably smaller diameter than said outer conductor extending through said outer conductor, and an insulating body having higher magnetic permeability than air within said line disposed adjacent at least one of said conductors.

13. The transmission line defined in claim 12,

wherein said body contains substantially uniformly dispersed powdered ferromagnetic material.

14. In the transmission line defined in claim 12, wherein said body comprises powdered iron dispersed in a binder of Bakelite or the like.

15. A transmission line comprising an outer conductor, an inner conductor comprising a coil of insulated wire closely wound about an insulating core, and a body of insulating material having a magnetic permeability substantially greater than air mounted within said line between said conductors.

16. The transmission line defined in claim 15, wherein said insulating core also comprises a body of insulating material having a magnetic permeability substantially greater than air.

17. In a transmission line, an outer conductor, an inner conductor comprising a coil having its turns insulated from each other substantially coaxial with said outer conductor, a core of a material having higher magnetic permeability than cessionof beads within said coil, said beads being of a material which has higher magnetic permeability than air.

19. The transmission line defined in claim 18, including a tubular sheath of insulating material in which said beads are mounted and about which said coil is wound.

20. For use in a high frequency coaxial conductor transmission line; a body of-paramagnetic insulating material, and coupling surfaces formed on opposite ends of said body for closely but universally relatively movably fitting with similarly formed bodies when arranged in close succession therewith in said line. said body being substantially solid and axially apertured in close conformity to the periphery of a conductor passing therethrough, so that said body provides a maximum cross-section of said material along the line.

21. A flexible high frequency transmissionline comprising substantially flexible inner and outer conductors and paramagnetic core means disposed between said conductors comprising an axial succession of individually rigid sections having adjacent ends universally mated and connected so as to permit flexing of said line. I

22. In a coaxial type transmission line having appreciable delay properties, a tubular outer conductor, a coiled inner conductor, an insulating core containing dispersed ferromagnetic material for said inner conductor, said cOre and coil providing a substantially uniformly distributed high inductance along said line, and a sleeve of a material having a high dielectric constant between said conductors, said sleeve providing a substantially uniformly distributed high capacity along said line.

23. A flexible high frequencytransmission line comprising substantially coaxial flexible inner and outerco'nductors, one of said conductors comprising a closely wound coil of insulated wire, and core means associated with said coil within said line comprising an axial succession of individually rigid sections of an insulating material having higher magnetic permeability than air, said sections having adjacent ends so connected as to permit flexing of the line.

24. The transmission line defined in claim 23, wherein said inner conductor is the coiled conductor, and said core is disposed within said inner conductor.

25. The transmission line defined in claim 23, wherein said inner conductor is the coiled conductor, and said core is disposed between said conductors.

26. The transmission line defined in claim 23, wherein said outer conductor'is the coiled conductor, and said core is disposed between said conductors.

27. A flexible high frequency transmission line comprising substantially coaxial flexible inner and outer conductors, each of said conductors being a closely wound coil of insulated wire, flexible insulating means spacing said conductors, and a flexible core within said inner conductor comprising a contacting succession of individually rigid sections of an insulating material having higher magnetic permeability than air.

28. The transmission line defined in claim 27, wherein said flexible insulating means comprises a succession of contacting individually rigid sections of an insulating material having higher magnetic permeability than air.

29. A flexible high frequency transmission line comprising substantially coaxial flexible inner and outer conductors, each of said conductors comprising a coil oiinsulated wire wound upon a flexible insulating core, the core between said conductors comprising a contacting succession of individually-rigid sections of an insulating material having higher magnetic permeability than air.

30. A high frequency transmission line comprising substantially coaxial inner and outer conductors, one of said conductors comprising a continuous coil of insulated wire wound with certain turns overlapping one or more previously wound turns so that said coil provides a high distributed inductance along said line, and insulating means spacing said conductors.

31. In a transmission line, an inner conductor comprising a coil of insulated wire, a longitudinally slotted outer conductor insulated from said inner conductor and supported substantially c0- axially with said inner conductor, and a core comprising a succession of paramagnetic beads Within said coil.

32. A flexible high frequency transmission line comprising substantially coaxial flexible inner and outer conductors insulated from each other, one of said conductors comprising a coil of insulated wire wound upon a flexible insulating core, a further core between said conductors comprising a contacting succession of individually rigid paramagnetic sections, and a cellular insulating sleeve enclosed within a metal shielding tube.

GERELD L. TAWNEY.

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