US2862293A - Method of erecting coaxial electric feeder - Google Patents

Description

Dec. 2, 1958 E. c. CORK ET AL METHOD OF ERECTING COAXIAL ELECTRIC FEEDER Original Filed- June 18, 1948 lKhH l1 1 United States Patent METHOD OF ERECTING COAXIAL ELECTRIC FEEDER Edward Cecil Cork, Ealing, London, and Michael Bowman-Manifold, Four Marks, Alton, England, assignors to Electric & Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain Original application June 18, 1948, Serial No. 33,866, now Patent No. 2,701,864, dated February 8, 1955. Divided and this application November 4, 1954, Serial No. 466,900

Claims priority, application Great Britain June 21, 1947 1 Claim. (Cl. 29-433) This invention pertains to radio frequency feeders, and more particularly to an improved method for erecting a vertical concentric line feeder for conveying radio frequency energy to an elevated point, such as the top of a transmitting tower, and is a divisional application from our co-pending application Serial No. 33,866, now Patent No. 2,701,864 of February 8, 1955.

A field in which such vertical feeders are of special utility is that of television transmission. In order to obtain a desirable range of coverage in the frequencies involved, the transmitting towers of television stations may well rise to heights of 800 feet or more above gnound level, the antenna itself being carried at the top of such a tower. Both the height of the tower and the relatively high values of the carrier frequency introduce problems not encountered in either horizontal or ver tical concentric line feeders of more modest length.

Conventional prior art feeders of the vertical type have heretofore been composed of a plurality of sections; that is to say, the inner conductor has been sectionalised and the outer conductor has also been sectionalised, since feeders of the type to which the invention relates cannot be constructed as composite units as is usual with the type of feeder employed in television receivers where the inner conductor is surrounded by solid insulation and the outer conductor is usually in the form of a braiding covering the insulation. Feeders of the kind to which the invention relates have frequently an outer conductor having an external diameter of 5 inches or more which is air spaced from the inner conductor and when these feeders have to be taken up a tower of 800 feet or more the mass of the feeder is considerable, since in general the inner and outer conductor sections are composed of copper. It will be appreciated, therefore, that whilst for comparatively small heights a concentric vertical feeder can be constructed, as is known in the prior art and the whole feeder supported at the bottom of the tower, when comparatively larger lengths of feeder are required then it is found that if the feeder is supported at its lower end the mechanical loading on the sections of the lower endof the feeder tend to cause buckling of the outer and centre conductors of the feeder, introducing electrical irregularities. Furthermore, the construction of a feeder even if possible in accordance with the prior art methods necessitates the use of considerable labour.

It is accordingly a principal object of the present invention to provide a method of erecting or assembling an improved vertical concentric line feeder of the above type which will be relatively free from electrical irregularities and losses which are characteristic of conventional feeders known to the art, and which can be assembled and installed with relative ease and at relatively low cost.

A further object of the invention is to provide a method of constructing a feeder of this general type in which the necessity for electrical testing and correction of individual sections, as the installation proceeds, is minimised or even completely eliminated, The necessity for such tests and corrections in the vertical assembly of long feeders of prior types constitutes a major objection to the use of such designs. The importance of the present invention in eliminating or reducing such operations can readily be appreciated when it is considered that the erection must often be accomplished under adverse conditions of weather and the like, and that any increase in erection time due to necessary electrical testing represents increased labour not only of the radiotechnicians but of the riggers handling the mechanical aspects of the installation. Still another object of the invention is to provide a method of erecting a vertical concentric line feeder in which both outer and inner conductors are assembled by a method which results in both the outer and inner conductors being only in tension; in other Words, one in which both the inner and outer conductors are suspended from the upper end of the transmitting tower. The satisfactory accomplishment of this object provides a feeder which is inherently free from mechanical buckling or loss of concentricity, regardless of the vertical length of such feeder. The above and other objects of the invention will best be understood by referring now to the following detailed specification of a preferred embodiment thereof, given by way of example and not of limitation, and taken in connection with the appended drawings, in which Figure 1 of the drawings illustrates the general construction of feeder which is assembled in accordance with the invention. The reference numeral 1 indicates the central conductor of a concentric line aerial feeder and the reference numeral 2 the outer conductor thereof. The central conductor of the feeder is suspended at its upper end from a tower or other supporting structure to which the aerial proper is mounted, a platform, girder or some other suitable support for the aerial being indicated at 3. The central conductor is attached at its upper end to a trunnion 4 carried by bearings 5 bolted or otherwise secured to the member 3 and depending from the trunnion 4 is a stirrup 6 to which the outer conductor 2 of the feeder is connected and suspended from its upper end. The central conductor of the feeder may, for example, be about 1'' in diameter and the outer conductor may be about 5" in diameter. The central conductor is in the form of a wire rope of the so-called locked-coil type or it may comprise an inner core of wire rope provided with a covering of insulating material such as polythene provided with an outer sheath of copper tape. A locked-coil type of rope is one in which strands of the rope have such cross-sectional shape as to interlock with one another so that when the rope is tensioned the interlocking strands serve to prevent any substantial twisting of the rope.

The lower end of the central conductor is preferably mechanically loaded either by the provision of a spring or weight which latter may be located in a dash-pot so as to maintain the central conductor taut. The mechanical loading is indicated by the reference numeral 7.

The outer conductor of the feeder comprises sections of copper tubing provided with annular end flanges as shown by means of which the sections are bolted or otherwise secured together end-to-end and at convenient positions along' the length of the outer conductor junction boxes 8 and 9 are provided housing an aerial lead 14 and a transmitter lead 11 respectively. At the upper end of the feeder a short-circuiting piston 12 is provided whilst a similar short-circuiting piston 13 is also provided at its lower end, these pistons being for tuning and matching purposes.

In assembling the feeder in accordance with the inven- Patented Dec. 2, 1958 ductor and junction boxes are passed successively over the lower end of the suspended'central conductor, hauled to appropriate positions-and'thenbolted toone another.

The central conductor is spaced inthe'junction box 8 by an insulating'sleevel l-and the lower end of the central conductor is similarly located in the junction box 9' by an insulating sleeve 15. The spacing of the central conductor along the major length of the outer conductor can be effected-by the provision of insulators disposed along the length of said'central-conductor and suitably spaced apart by a distanceof, for example, one-eighth of a wavelength at-the-operating frequency of the aerial. In the embodiment shownin Figure. 1, two of such insulators are shown,- such-insulators-comprising insulating arms 16 projecting radially from the central conductor and carried by conducting supports 17 of any suitable form. The insulating arms 16 may be inserted in the apertures in the supports 17 as the sections of the outer conductor are assembled. Preferably, as shown, the pairs of arms 16 are staggered rotationally along the length of the central conductor. If desired, insulators of the bead type may be used.

At the lower end of the outer conductor an expansion joint is provided which is of any suitable construction, and as shown in Figure 1 said joint comprises a metal bellows 18 secured to adjacent sections of the outer conductor. An expansion joint is also provided asstciated with the central conductor at its lower end, said joint comprising a metal bellows 19 attached at one end to the inner conductor and at its other end to a sleeve 20 to which the transmitter lead 11 is also connected, the sleeve having a tubular extension 21 through which the inner conductor passes in electrical contact therewith. The junction box 9 on the lower end-of the outer conductor of the feeder may be carried by a supporting platform or structure 22. The feeder is preferably hermetically closed'and may beprovided with a filling of a suitable gas such as nitrogen to reduce sparking and to avoid ingress of moisture.

The aerial lead 10and the transmitter lead 11 may be be tapped into the central conductor at suitable points along the length thereof, the points being suitably chosen so as to provide the required impedance matching between the aerial and transmitter leads. The outer conductor of the feeder will, of course, be provided with suitable branches to enclose the aerial and the transmitter leads in known manner, these branches being also suitably hermetically closed.

It will be appreciated that due to the construction described and the method of assembly the feeder can be assembled far more rapidly than the known prior art feeders and that buckling of the central conductor is avoided and, furthermore, due to the suspension of the outer conductor from its upper end buckling thereof is also avoided.

Figure 2 of the drawings illustrates diagrammatically a modified construction according to the invention in which the central conductor is suspended at its upper end from a transverse conducting support in the form of a trunnion 23 which is supported at its ends on girders 24, the trunnion 23 being enclosed by sections 25 of outer conductor which'enclose the trunnion on each side of the central conductor to an extent efiectively equal to one-quarter of the operating wavelength of the aerial so that said trunnion has negligible efiect on the electrical characteristics of the feeder. A similar construction is provided at the lowerend of the feeder and in this case the central conductor passes through a sleeve 26 provided in the supporting trunnion 27 and is in electrical contact therewith. The central conductor at its lower end issh'ort-circuited to the outer conductor by a piston 28.

What we claim'is:

The method of constructing in place, for a transmitting station or the like, a vertical coaxial electrical feeder whose vertical length ismany times its diameter, comprising the steps of hauling to the upper part of a tall supporting structure one end of a wire rope constituting the inner conductor of the feeder, suspending said inner conductor from the upper part of'said'structure with the remainder of said inner conductor depending therefrom, threading thelower end of said inner conductor through a tubular section ofouter conductor, hauling said tubular section to the upperpart'of saidsupporting structure and, suspending it therefrom, successively threading the lower end ofthe 11I1I16I C011d116t01 through additional tubular outer conductor sections, hauling them upward and attachingthemin'succession to the'first section and to one another to'complete the outer conductor, and inserting spacing means between the successive tubular sections and the inner conductor, to provide a coaxial feeder whose conductors are supported substantially entirely in tension from the upper part of the supporting structure.

References Cited in the file of this patent UNITED STATES PATENTS 1,463,015 Guay July 24, 1923 1,559,807 Thaden Nov. 3, 1925 2,355,833 Bertalan Aug. 15, 1944 2,637,895 Blaton May 12, 1953 2,680,901 Kaiser June 15, 1954 2,701,864 Cork Feb. 8, 1955

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