US3790950A

US3790950A - Antenna array for minimizing co-channel interference

Info

Publication number: US3790950A
Application number: US00180147A
Authority: US
Inventors: T Smith; J Weston
Original assignee: Scientific Atlanta LLC
Current assignee: Scientific Atlanta LLC
Priority date: 1971-09-13
Filing date: 1971-09-13
Publication date: 1974-02-05
Anticipated expiration: 1991-02-05

Abstract

An antenna system for reducing co-channel interference in a signal receiving network, said system comprising; at least four logarithmically periodic antenna elements having corresponding points disposed in a common plane, a first two of said antenna elements also disposed within a horizontal plane, the remaining two of said antenna elements also disposed within a vertical plane, said first two elements being substantially equal first distances from the line of intersection of said horizontal and vertical planes and said remaining two elements being substantially equal second distances from the said line of intersection; a support structure; and means for cantilever mounting said antenna elements on said structure with the maximum sensitivities of said antenna elements directed outwardly from said support structure.

Description

United States Patent [191 Smith et al.

[451 Feb.5,1974

[ ANTENNA ARRAY FOR MINIMIZING CO-CHANNEL INTERFERENCE [75] Inventors: Thomas D. Smith, Doraville; Jasper B. Weston, Jr., Tucker, both of Ga.

Related US. Application Data [63] Continuation of Ser. No. 18,365, March 12, 1970, abandoned, which is a continuation of Ser. No. 570,570, Aug, 5, 1966, abandoned.

OTHER PUBLICATIONS Andrew Catalog T; 2nd Edition; pp T1,T3 May 10,

Primary Examiner-Eli Lieberman Attorney, Agent, or Firm--C|ushman, Darby & Cushman [57] ABSTRACT An antenna system for reducing co-channel interference in a signal receiving network, said system comprising; at least four logarithmically periodic antenna elements having corresponding points disposed in a common plane, a first two of said antenna elements also disposed within a horizontal plane, the remaining two of said antenna elements also disposed within a vertical plane, said first two elements being substantially equal first distances from the line of intersection of said horizontal and vertical planes and said remaining two elements being substantially equal second distances from the said line of intersection; a support structure; and means for cantilever mounting said antenna elements on said structure with the maximum sensitivities of said antenna elements directed outwardly from said support structure.

3 Claims, 11 Drawing Figures PATENIEUFEB 5W swmoaso SHEU M M Q O O O O 9 O O O Q 0 O O O O 0 0 O O O O O O O O O O O O G O O O O O O O C) O INVENTO ANTENNA ARRAY FOR MINIMIZING (IO-CHANNEL INTERFERENCE This is a continuation of application Ser. No. 18,365 filed Mar. 12, 1970 which was a continuation of application Ser. No. 570,570 filed Aug. 5, 1966 both abandoned.

This invention relates to antenna arrays and in particular to antenna arrays for reducing co-channel interference.

Co-channel interference arises whenever two signals are received at a receiving station, the signals being of the same frequency but of different origin. For instance, in the important and growing field of community antenna television systems, two Channel signals may be respectively received from Philadelphia and Washington by a community television antenna located at Hagerstown, Md. If the Wahsington Channel 5 signal is desired, it has been experimentally determined that the Washington Channel 5 signal should be 48 db greater than the Philadelphia Channel 5 signal in order to obtain excellent picture quality. With lower signal-to-interference ratios, the picture quality deteriorates to fine and eventually to passable. Since the antenna location is not always an effective factor in minimizing co-channel interference, the largest part of interference rejection must be provided by the antenna array.

In the early days of community antenna television (CATV), co-channel interference was not a major problem; and CATV arrays and antennas were designed for maximum gain. However, CATV arrays must now be designed to operate with receiver systems where interference is present. Thus, maximum gain is desirable only insofar as it improves signal-to-noise ratio. An important factor now is the overall directivity of an array. Thus if the antenna pattern is so designed as to have a null directed toward the source of interference, a much higher signal-to-noise ratio is achieved even though some sacrifice in gain may be made.

It is well known that antenna directivity or radiation pattern shape may be adjusted in accordance with the spacing of the elements comprising the array, the relative current amplitude at each of the elements, and the relative phase of the current at each of the elements. Having calculated theoretically the desired antenna array configuration and energization desired for a particular location, the antenna array can be constricted in accordance with the theoretical calculations. However, it has been determined experimentally that the effect of the tower structure upon which an array is mounted cannot be predicted with any reasonable amount of accuracy. Thus, in those situations where antenna directivity is an important factor in determining overall system performance, it is necessary that some means be available for accurately theoretically predicting beforehand the required structure necessary to do the job.

Thus it is the primary purpose of this invention to provide an improved antenna array which can be mounted upon an antenna tower the antenna pattern of the array being predictable beforehand. In other words, the construction of the antenna array is such that the effect of the tower is negligible upon the radiation pattern predicted for the array.

Another purpose of this invention is to provide an improved antenna array for achieving the beforementioned signal-to-interference ratio necessary for CATV systems.

Another purpose of this invention is to provide an antenna array achieving very low sidelobe levels, thereby reducing the effect of co-channel interference.

Another purpose of this invention is to provide an improved antenna array achieving very high front-to-back ratios, thereby, once again, readily lending itself to the reduction of co-channel interference.

A further purpose of this invention is to provide an improved antenna array which achieves very narrow beam widths, the half power beam width not exceeding 20 X 22 for television Channels 7 through 13 and 25 X 30 for television Channels 2 through 6.

Other objects and advantages of this invention will become apparent upon reading the appended claims in conjunction with the following detailed description and the attached drawings, in which:

FIG. 1 is an illustrative embodiment of the invention showing a diagrammatic representation of an array mounted on a typical tower installation;

FIG. 2 is a diagrammatic illustration of the elements of an antenna array employed in the invention illustrating the location of the elements with respect to one another;

FIG. 3 is a front elevation view of an illustrative array employed in the invention;

FIG. 4 is a fragmentary side elevation view of the view shown in FIG. 3;

FIG. 5 is a fragmentary, longitudinal bottom view of the view shown in FIG. 3;

FIGS. 6 and 7 are top and side views respectively of a detail taken from section A of FIG. 4;

FIGS. 8 and 9 are side and top views respectively of the detail taken from section B shown in FIG. 4; and

FIGS. 10 and 11 are diagrammatic illustrations of more elaborate arrays employing the principles of this invention.

Referring to FIG. 1, there is shown an antenna array 10 comprising elements 12, l4, l6, and 18. The elements 12 through 18 are preferably logarithmically periodic structures that is, for example, a plurality of dipoles. the length of which varies logarithmically and the spacing between which also varies logarithmically. In the preferred embodiment of the invention the length of the dipoles and the spacing therebetween de' creases from the tower structure in accordance with a geometric progression mathematical relationship. Thus the radiation pattern is directed away from the tower. Such antennas are well known in the art.

The antenna array 10 is mounted onto the tower 20 which comprises three

support members

22, 24, and 26. The array 10 is attached to one of the support members, for example, member 24. The details of the attachment are described hereinafter with respect to FIGS. 3 through 9. The four elements 12 through 18 are respectively connected to the ends of

support arms

28 and 30, the support arms being arranged perpendicularly with respect to one another. The support arm 28 is attached to the tower 20. The antenna elements 12 through 18 are cantilever mounted to the

support arms

28 and 30.

The antenna array, shown in FIG. 1, combines many features which result in an array well suited for the elimination of co-channel interference in such applications as CATV. Thus, by arranging the elements 12 through 18 in the diamond-shaped pattern shown in FIG. 1, the sidelobe levels of the radiation pattern of the array are at least 26 db down on Channel 13 while they are at least 30 db down on the remaining VHF channels in both the vertical and horizontal planes. Since co-channel interference is mainly a horizontal plane phenomenon, the very low sidelobe level in the horizontal plane is particularly important.

The diamond-shaped array is effectively equivalent to a linear horizontal array of three anntennas, the two end antennas corresponding to

antennas

12 and 16 of FIG. 1, the center being imaginary and corresponding to one placed at the cross point of

arms

28 and 30 and having a current amplitude thereat twice that at the elements l2 and 16. This imaginary antenna, of course, results from

elements

14 and 18. Preferably, the currents at all antenna elements are in phase with each other. However, it would be obvious to one having ordinary skill in this art to adjust the relative phasing of the currents with respect to each other in accordance with the requirements of the particular situation.

Further, the array described in FIG. 1 is extremely suitable for use in the CATV situation because of the very high front-to-back ratio achieved in the horizontal plane, this being at least 25 db over the entire VHF television channel. This mainly results from the use of the log-periodic antenna structures 12 through 18.

Another important feature of the array shown in FIG. 1 is that the effect of the tower on the radiation pattern is negligible. Hence, the radiation pattern of the array can be predicted beforehand for a particular location and the array can be constructed in accordance with theoretical calculations without fear of having to make changes after the array is placed in operation upon a tower. In the past, arrays have been mounted on towers wherein each element of the array was a Yagi-type antenna, the Yagi antenna extending on both sides of the tower. As stated before, the effect of the tower on the antenna pattern of the array is undesirable. Thus, it is desirable that the antenna array elements extend only from one side of the tower array, thereby minimizing the effect of the tower on the array radiation pattern. This is achieved in the instant invention by cantilever mounting the elements 12 through 18 with respect to the tower 20. Further, the effect of the tower 20 on the radiation pattern of the array is reduced by the high front-to-back ratio of the log-periodic antenna elements employed.

Screen-back Yagi arrays have been experimented with in an effort to reduce the effect of the antenna tower on the radiation pattern, since these types of arrays also have a high front-to-back ratio and they also may be cantilever mounted with respect to the tower. However, it has been determined that this type of array is not satisfactory for television Channels 2 and 3 where large antenna elements are required, since the size of the structure makes cantilever mounting impractical. However, antenna arrays employing log-periodic antenna elements do not have this shortcoming at lower frequencies and, thus, the antenna array shown in FIG. 1 is suitable for the elimination of co-channel interference over the entire VHF television band. Of course, the principles are also applicable in the UHF television band or any other application where the elimination of co-channel interference is important.

applicatios in summary, the three features of the ansuitable for the elimination of co-channel interference in CATV application are:

l. The diamond-shaped configuration of the antenna array,

2. The cantilever mounting of the antenna array with respect to the tower installation, and

3. The use of log-periodic antenna elements in the array.

Reference should now be made to FIG. 2 which more precisely defines the spatial positioning of the antenna elements 12 through 18 shown in FIG. 2. Each of the elements 12 through 18 is substantially disposed in a common plane A. Further, the

elements

12 and 16 are disposed in a common horizontal plane B while the

elements

14 and 18 are disposed in a common vertical plane C. The

elements

12 and 14 are disposed substantially equal distances X from the line 0 of intersection of the planes B and C while the

antenna elements

14 and 18 are disposed substantially equal distances Y from the above-mentioned line of intersection. In FIG. 1, the distances X and Y are substantially equal. FIG. 1 illustrates the preferred spatial relation between the antenna elements of the array.

Reference should now be made to FIG. 3 which illustrates a front elevation of the array. As can be seen from FIG. 3, the vertical crossarm 28 is connected to the horizontal crossarm 30 by suitable means 32.

Reference should now be made to FIG. 4 which shows a fragmentary side elevation view of the array shown in FIG. 3. The vertical crossarm 28 is connected to the member 24 of tower 20 (see FIG. 1) at 34 and 36. Details of this connection will be described in more detail hereinafter with respect to FIGS. 8 and 9. The

antenna elements

14 and 18 are respectively connected to crossarm 28 by horizontally extending members 38 and 40, members 38 and 40 being integrally connected to crossarm 28. The details of the connection of the antenna elements to the vertically extending members 38 and 40 are described in more detail with respect to FIGS. 6 and 7.

Antenna elements

12 and 16 are connected to crossarm 30 in the same manner as

elements

14 and 16 are connected to crossarm 28.

A rod 42 is connected to crossarm 30 at point 44 (see FIG. 5). Rod 42 is connected at its other end to support member 22 of the tower structure 20. The purpose of this connection is to strengthen the support of the antenna array 10 when the requirements of the application so demand. A further rod (not shown) may be connected from point 46 to member 26 if so needed.

Reference should now be made to FIGS. 6 and 7 which respectively illustrate top and side views of the connection of antenna element 18 to the crossarm 28, see detail A. A mounting plate 48 is integrally connected to the vertically extending member 40. The dipoles are connected (not shown) to two

support members

50 and 52, the support members being connected to the mounting plate 48 by means of an L-shaped bracket 54. Bolts 56 and 58 secure the

support members

50 and 52 to the bracket 54 while

bolts

60 and 62 secure the L-shaped bracket 54 to the mounting plate 48. Thus, a cantilever mounting of the antenna elements is achieved. The antenna elements are high tensile strength, corrosion resistant aluminum, and

longitudinal support members

50 and 52 are tapered U- channels for maximum strength-to-weight ratio, with dipole elements of aluminum tubing, heli-arc'welded to the

members

50 and 52. The array support

structure comprising crossarms

28 and 30 is preferably steel, and all mounting hardware is preferably cadmium plated.

The antenna elements are fed by a coaxial line, which runs along the member 52, for example, the outer conductor being electrically connected thereto. At the outermost end of the element (the point most removed from the tower), the center conductor of the coaxial line is connected to the member 50. Thus the logperiodic structure is fed in a manner well known to those having ordinary skill in this art. The connection of the four antennae to a common feeder for the receiving circuitry is accomplished by impedance transformer techniques well known to those having ordinary skill in the art.

Reference should now be made to FIGS. 8 and 9 which respectively illustrate side and top views of the detailed section B shown in FIG. 4. The vertical crossarm 28 has integrally connected thereto an outwardly extending projection 64. Member 64 includes

parallel extensions

66 and 68 which enclose an open space 70. A bolt 72 or other suitable securing means connects the outward projection 64 to a bracket 74, as shown in FIG. 8. Bracket 74 is connected to a further bracket 76 by means of

bolts

78, 80, 82, and 84 (not shown). As can be seen from FIG. 8, the connection of the crossarm 28 to the tower support member 24 is achieved by the connection of projection 64 to bracket 74 by bolt 72 and the connection of bracket 74 to bracket 76 by bolts 78 through 84.

Brackets

74 and 76 include jaws portions 86 and 88 respectively for gripping the support member 24.

Reference should now be made to FIG. which diagrammatically illustrates one modification of the antenna array illustrated in FIG. 1, wherein 36 logperiodic elements are employed in a diamond-shaped array 90. Antenna array 90 achieves significantly a higher gain than that obtainable with the array shown in FIG. 1 while at the same time maintaining the high level of performance with respect to sidelobe level and front-to-back ratio. Array 90 is particularly effective over Channels 7 through 13 where smaller elements may be employed.

Referring to FIG. 11, there is shown another configuration similar to that shown in FIG. 10. However, the configuration shown in Fig. 11 is more suitable for television Channels 2 through 6 where larger size antenna elements must be employed.

Still numerous other modifications of the invention will become apparent to one of ordinary skill in the art upon reading the foregoing disclosure. During such a reading, it will be evident that this invention has provided a unique antenna system for accomplishing the objects and advantages herein stated. Still other objects and advantages, and even further modifications will be apparent from this disclosure. It is to be understood, however, that the foregoing disclosure is to be considered exemplary and not limitative, the scope of the invention being defined by the following claims.

We claim:

1. An antenna system for reducing co-channel inter ference in a signal receiving network, said system comprising:

at least four logarithmically periodic antenna elements having corresponding points disposed in a common plane, a first two of said antenna elements also disposed within a horizontal plane, the remaining two of said antenna elements also disposed within a vertical plane, said first two elements being substantially equal first distances from the line of intersection of said horizontal and vertical planes and said remaining two elements being substantially equal second distances from the said line of intersection, a support structure, and means for cantilever mounting said antenna elements on said structure with the maximum sensitivities of said antenna elements directed outwardly from said support structure, said support structure including a tower and at least two crossarm members, said crossarm members being connected to one another and disposed substantially perpendicularly with one another; said four antenna elements being cantilever connected to the respective four ends of said crossarm members; and at least one of said crossarm members being connected to said tower, said tower including at least three vertically ex tending support members, said one crossarm member being connected to one of said support members.

2. An antenna system for producing a signal without co-channel interference in a signal receiving network, said system comprising:

at least four logarithmically periodic antenna elements having corresponding points disposed in a common plane, a first two of said antenna elements also disposed within a horizontal plane perpendicular to said common plane, the remaining two of said antenna elements also disposed within a vertical plane perpendicular to said common plane, said first two elements being substantially equal first distances from the line of intersection of said horizontal and vertical planes and said remaining two elements being substantially equal second distances from the said line of intersection, a support structure, and means for cantilever mounting said antenna on said structure with; the maximum sensitivities of said antenna elements directed outwardly from said support structure, said antenna system including at least 16 of said elements including said four elements, with each of said 16 elements disposed in one of a plurality of parallel first planes which each have a plurality of elements and which are each transverse to said common plane with each element in each said first plane spaced equidistantly from the other elements in that firt plane, disposed in one of a plurality of parallel-second planes each having a plurality of elements and which are each transverse to said planes and to said common plane with each element in each said second plane spaced equidistantly from the other elements in that second plane, disposed in one of a plurality of parallel horizontal planes all but two of which have a plurality of elements and which are each transverse to said common plane with each element in each said horizontal plane spaced equi distantly from the other elements in that horizontal plane and disposed in one of a. plurality of vertical planes all but two of which have a plurality of elements and which are each transverse to said common plane with each element in each said vertical plane spaced equidistantly from the other elements in that vertical plane.

3. An antenna system as in claim 2 including at least 36 of said elements.

Claims

1. An antenna system for reducing co-channel interference in a signal receiving network, said system comprising: at least four logarithmically periodic antenna elements having corresponding points disposed in a common plane, a first two of said antenna elements also disposed within a horizontal plane, the remaining two of said antenna elements also disposed within a vertical plane, said first two elements being substantially equal first distances from the line of intersection of said horizontal and vertical planes and said remaining two elements being substantially equal second distances from the said line of intersection, a support structure, and means for cantilever mounting said antenna elements on said structure with the maximum sensitivities of said antenna elements directed outwardly from said support structure, said support structure including a tower and at least two crossarm members, said crossarm members being connected to one another and disposed substantially perpendicularly with one another; said four antenna elements being cantilever connected to the respective four ends of said crossarm members; and at least one of said crossarm members being connected to said tower, said tower including at least three vertically extending support members, said one crossarm member being connected to one of said support members.

2. An antenna system for producing a signal without co-channel interference in a signal receiving network, said system comprising: at least four logarithmically periodic antenna elements having corresponding points disposed in a common plane, a first two of said antenna elements also disposed within a horizOntal plane perpendicular to said common plane, the remaining two of said antenna elements also disposed within a vertical plane perpendicular to said common plane, said first two elements being substantially equal first distances from the line of intersection of said horizontal and vertical planes and said remaining two elements being substantially equal second distances from the said line of intersection, a support structure, and means for cantilever mounting said antenna on said structure with the maximum sensitivities of said antenna elements directed outwardly from said support structure, said antenna system including at least 16 of said elements including said four elements, with each of said 16 elements disposed in one of a plurality of parallel first planes which each have a plurality of elements and which are each transverse to said common plane with each element in each said first plane spaced equidistantly from the other elements in that firt plane, disposed in one of a plurality of parallel second planes each having a plurality of elements and which are each transverse to said planes and to said common plane with each element in each said second plane spaced equidistantly from the other elements in that second plane, disposed in one of a plurality of parallel horizontal planes all but two of which have a plurality of elements and which are each transverse to said common plane with each element in each said horizontal plane spaced equidistantly from the other elements in that horizontal plane and disposed in one of a plurality of vertical planes all but two of which have a plurality of elements and which are each transverse to said common plane with each element in each said vertical plane spaced equidistantly from the other elements in that vertical plane.

3. An antenna system as in claim 2 including at least 36 of said elements.