US1841473A - Arrangement for connecting or terminating coaxial conductors - Google Patents

Description

Jan. 19, 1932. I

ARRANGEMENT FOR E- GREEN 1,841,473

CONNECTING OR TERMINATING COAXIAL CONDUCTORS Fi led Jan. 50, 1930 bzsuldtzh 70 Spacer y (bpper wire or Braid.

INVENTOR EIG'weI/I/ ATTORNEY efiects. It is one of t ent invention to provide arrangements w ere-' Patented Jan. 19, 1932 UNITED STATES PATENT OFFICE ESTILL I. GREEN, ,OF EAST ORANGE, NEW JERSEY, ASSIGNOR TO AMERICAN TELEPHONE AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK ARRANGEMENT FOB CONNECTING OR TERMINATING CO'AXIAL CONDUCTOR-S Application filed January 30, 1930. Serial No. 424,677.

This invention relates to coaxial conductor systems, and more particularly to arrangements for connectin 'or terminatingcoaxial conductor systems of difierent sizes or dimensions without introducing impedance irregularities.

Where it is desired to transmit a wide range of frequencies as, for example, for television purposes or for providing a large number of carrier channels, it is advantageous to use a coaxial conductor system. Such a system may comprise an inner cylindrical conductor surrounded by a concentrically arranged hollow cylindrical conductor acting as a return for the inner conductor, the two conductors being separated preferably by a dielectric which is largely gaseous.

In a system of this type, if the ratio of the inner diameter of the outer conductor to the outer diameter of the inner conductor is fixed, the attenuation of the conducting system varies inversely with the diameter. It is, therefore, desirable in using such a system to make the main line circuit of as large a diameter as is economically and physically practicable in order that a low attenuation may be obtained, Occasions will frequently arise, however, in which it is desirable to use a part of the system a conductor arrangement of smaller diameter than that employed for the normal line construction. Under these conditions it is extremely advantageous to be able to match the impedance of the two 00- axial circuits, thereby eliminating reflection e purposes of the resby this result may be attained.

The invention will now be more fully understood from the following description, when read in connection with the accompanying drawings, Figures 1, 2 and 3 of which v illustrate difierent arrangements for interconnecting coaxial conductor ferent types and dimensions.

Let us consider concentric conductor arran ents comprising an inner cylindrical con uctor having an outer diameter b surrounded by a coaxial outer cylindrical conductor having an inner diameter 0, the two systems ofdifconductors bemg in proper concentric relation and out of electrical contact with each other by means of spaced dielectric washers or the like, preferably of small loss angle and low dielectric constant, so thatthe dielectric between the two conductors will be principally of air. Ordinarily in the design of such a system the diameter of the outer conductor will be determined by physical conditions and may he treated as fixed. The attenuation of the system for a given inner diameter of the outer conductor will then vary with different outer diameters of the inner conductor. By analogy tov formula page 109', of Calculation of alternating current problems, by L. Cohen, the capacity of the structure herein considered will have the value.

log

in which K is the dielectric constant of the insulation between the conductors. Likewise from formula (56) on page 72 of the. Cohen publication the high fre uenc inductance in abhenries per centimeter as t e value X 10' farads per mile (1) L=2log% 2 The high frequency impedance of the coaxial circuit is Substituting the values givenby Equations (1) and (3) in Equation (4) we have z.-e0x i -x log. 3 .(6)

too

From Equation (6) it is evident that the impedance at high frequencies is determined by the ratios of the diameters of the conductors for any given insulating material.

Now in an application of E. 1. Green, Serial No. 365,518, filed May 23, 1929, entitled Concentric conductor transmission system, it is shown that when the size of the outer conductor is fixed the attenuation will be a minimum when the ratio for interconnecting concentric conductor sections of different dimensions without introducing impedance irregularities. This is done by so designing the two sections which are to be oined that regardless of their actual diameters the ratio is the same for both where the same dielectric material is employed.

There are various conditions under which a change in diameter in a coaxial system may be necessary or desirable. For ordinary construction the diameter of the outer conductor may be determined by the fact that it must enter the opening of a conduit or pass through the bridle ring of an overhead cable system. When entering an ofiice, however, a concentric conductor system of smaller diameter may be very useful. In fact a flexible coaxial construction perhaps provided with solid insulation between conductors,-might be desirable. Also, for long spans over rivers a conductor of large diameter might be disadvantageous and a concentric conductor section of smaller diameter could be employed. For short submarine stretches under rivers, a form of construction somewhat diiferent from that normally employed would be necessary. Smaller conductors, probably stranded, and separated by solid insulation would doubtless be required in this instance. For certain types of duct constructions in cities or towns it might be necessary to introduce a concentric conductor section of smaller diameter. And finally,foremergency construction where a portion of the regular conductor system is out of order, a flexible form of con struction would be desirable for use in bridging around the line sections under repair.

In any of the foregoing cases a perfect impedance match at high frequencies can be obtained by using a proper diameter ratio for the two circuits which are to be joined. If air insulation is employed between conductors in both circuits or if the same solid insulation is employed in both circuits, the diameter ratios will be the same for both. The use of solid insulation between the conductors of one circuit and air insulation for the connecting circuit would necessitate dilferent diameter ratios for the two circuits. The correct diameter ratio when using solid insulation of dielectric constant K for one circuit and air insulation for the other would be I 1/]? %=[%l where is the diameter ratio'for the circuit employing the solid insulation, and

is the diameter ratio for the circuit with air insulation. "When the insulating medium is not uniform, the value of K should be the weighted average dielectric constant for the different insulating materials employed. When different types of solid insulation are employed for the two circuits the correct diameter ratios are given by the expression:

is the diameter ratio for the circuit employing insulation of the dielectric constant K and where ductors being separated by s aced dielectric washers 14. The concentric system thus formed is to be connected to a similar concentric system of smaller conductors comprising an inner conductor 12 surrounded by an outer conductor 10', these two conductors being separated by spaced insulating washers 14. Assuming that the diameter ratios are properly determined in accordance with Equation (7), the physical connections between the two sections may be made by employing unions .16 and 18 of shouldered construction, each union having its flanges secured to the corresponding conductors as shown.

Instead of employing shouldered flanges as in Fig. 1, the flanges may be tapered as illustrated in Fig. 2. I,

Fig. 3 shows an arrangement which may be employed where a concentric conductor'system employing substantially air dielectric is connected to aconducting system in which the insulation between the conductors is of solid material. For example, the latter conductor system is shown as comprising an inner conductor 12' surrounded by a braided copper conductor 10' which is insulated from the inner conductor by a solid mass of insulation 14'. Also the outer conductor may be covered by some solid insulating material such as 22. The outer dielectric doesnot affect the impedance of the conducting system, however, as the current, due to the skineflect, flows in the inner surface of the outer conductor and the outer surface of the inner conductor and there is substantially no electric or magnetic field extending into the space surrounding the outer conductor. To effect the junction the system comprising conductors 10 and 12 may have the endof the smaller cable 1012 inserted in the o ening of the conductor 10 and sealed thereto by means of a terminal plug or seal 20. The inner conductor 12 is then tied to the inner conductor 12 by means of wires 18, and the outer conductor 10' is connected to the outer conductor 10 by means of. wires 16. Here again the design of the conductor system 10-1214 may be readily determined from formula (7), assuming that K is the dielectric constant of the insulating material of the smallerconductor system while the dielectric constant of the space between the conductors 10 and 12 is not materially diflerent from that of air.

It will be obvious that the general principles herein disclosed may be embodied in many other organizations widely difi'erent from those illustrated without de arting from the s irit of the invention as def lned in the following claims.

What is claimed is:

1. In a transmission system, two concentric conductor sections each comprising inner and outer c lindrical conductors coaxisections being of different diameters but said sections being so designed as to have substantially the same characteristic impedance, and means for interconnecting said sections.

2. In a transmission system, two concentric conductor sections each comprising inner and outer cylindrical conductors coaxially arranged, the outer conductors of the sections being of different diameters but the ratio of the inner diameter of the outer conductor to the outer diameter of the inner con- .ductor of the one section being so related to the corresponding ratio for the other section that each section has substantially the same characteristic impedance, and means for interconnecting said sections.

3. In a transmission system, two concentric conductor sections each comprising inner and outer cylindrical conductors coaxially arranged, the outer conductors of the sections being of different diameters but the ratio ofthe inner diameter of the outer conductor to the outer diameter of the inner conductor being substantially the same for both sections, and means for interconnecting said sections.

4. In a transmission system, two concentric conductor sections each comprising inner and outer cylindrical conductors coaxially arranged, the outer conductors of the sections being of different diameters, the conductors of one section being separated by a substantially gaseous dielectric and the conductors of the other section being separated by insulation having a dielectric constant different from that of air, the diameters of the conductors of the two sections being related by the formula when 0 is the inner diameter of the outer conductor and Z) the outer diameter of the inner conductor of one section, 0 and b are January, 1930.

. ESTILL I. GREEN.

ally arranged, t e outer conductowhe

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