US2757369A - Antenna system - Google Patents

Description

July 31, 1956 w. DARLING ANTENNA SYSTEM Filed Dec. 10, 1952 Woodrow D L Y Ir 'ZIIy ATTORNEY ANTENNA SYSTEM Woodrow Darling, Merchantville, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application December 10, 1952, Serial No. 325,052 9 Claims. Cl. 343-800) Wayne Masters, filed August 14, 1950, for High Gain Very High Frequency Antenna Systems, Serial No. 179,- l81,'now Patent No. 2,691,102, issued on Oct. 5, 1954.

The above-identified Masters application utilizes a tower type of construction consisting of three or four corner legs with suitable bracing members for added rigidity. Some of the advantages of this type of construction are that stability under high wind velocities is obtained, transmission lines and junction boxes for the transmission line network to feed the antenna elements are located inside of the tower, and the vertical aperture of the antenna may be increased by simply making the tower higher and providing and feeding more layers of radiators, with the concomitant result that the vertical beam width is narrowed and a greater proportion ofthe. energy is radiated in the direction where it is most useful.-

The structure required to support an antenna having the required gain can be made to support other radiating systems at only a slight additional cost. of construction lends itself to multiple use, such as an amplitude modulation (AM) radiator and to support antennas for frequency modulation (FM) and other television (TV) stations. The use of a single tower by two or more TV stations is especially desirable because all receiving antennas are automatically oriented in the right direction for all channels so used. The advantages of multiple usage in many installations oifset the inherent" expensiveness of such structures.

The interior of the tower can be made available for access to the antennas. Thus, one antenna can be serviced or repaired without inconvenience to the schedules of other stations using the same tower. The interior of the tower allows more room for the multiplicity of feed lines and connections required in the use of a great number of radiators.

An object of this invention is to provide a television broadcast antenna system having improved pattern characteristics.

Another object of the invention is to provide an improved tower-supported antenna array which is especially adapted to be fed in phase rotational fashion, also termed turnstile feeding.

Briefly, in accordance with this invention there is pro vided a tower-supported antenna array consisting fundamentally of one-half wavelength dipoles mounted in front of conductive surface elements or reflecting screens mounted on the sides of the supporting tower. In this invention, the one-half wavelength dipole elements are slanted or tilted in the azimuthal plane at an acute angle with respect to the reflecting screens. As in the previously referred to Masters application, the reflecting screens are either electrically connected to the tower or to each other, or both, at the vertical edges thereof and serve to The tower type 2,757,369 Patented July 31, 1956 prevent coupling within the tower as well as to redirect energy radiated to the rear of the dipole.

The antenna array of this invention is especially adapted to be fed in phase rotational fashion, that is, turnstile fed, and the circularity of the horizontal or azimuthal pattern is considerably improved over prior tower-supported antenna systems. The antenna of this invention may be used for arrays which are directional in the azimuthal plane as well as omnidirectional arrangements, and in which phase rotational (that is, turnstile) feeding is employed.

A more detailed description follows in conjunction with the accompanying drawing, in which: 7

Fig. 1 is a perspective view showing the general arrangement of an antenna system according to this invention;

Fig. 2 is a top plan view of one layer of an omnidirectional array according to the invention; and I Figs. 3 and 4 are graphical representations of azimuthal field pattern characteristics for a directional antenna system and an omnidirectional antenna system incorporating the present invention.

Referring to Fig. 1, there is shown an antenna system according to this invention mounted around the sides of a tower 11. Although not shown on the drawing, the interior of the tower can be fitted with a ladder used for access and servicing the antenna,- and would be provided with hardware for mounting the necessary transmission line. The tower 11 itself is shown as straight sided for the portion of its length occupied by the antenna system of .this invention. The tower 11 may be used also as a low frequency radiator, or solely as a support for one or more very high frequency antenna systems such as that of this invention.

Arranged around the periphery of the tower 11 are conductive surface elements or current sheet reflectors 15. These elements 15 may be constructed of thin sheet metal or other conductive material, but from a practical standpoint are constituted by a plurality of conductors forming a network, grid, or screen approximating a sheet reflector surface. The screens forming the conductive surface elements 15 could be eflected by welding conductors directly to the tower 11; but to make the screens 15 more flexible in application to different tower structures, they are preferably made separate and self-supporting.

Half wave dipole elements 21 and 22 are mounted in front of the screen 15 by means of conductive supporting members 23, 24 and 25, 26. Reinforced mounting plates 27 are provided to insure the rigidity of the dipole elements and supporting members 23, 24' and 25, 26 with respect to the screen 15.

Broadband operation over the required 6 megacycle channel for television transmission is obtained by employing three conductors in pyramided configuration to make up the dipole elements 21 and 22 as disclosed in the above-identified Masters application. Adjustable shorting bars 31 and 32 may be provided between the conductive supporting members 23, 24 and 25, 26 respectively to adjust the impedance of the radiating system to the desired value since the transmission line sections formed by the conductive supporting members 23, .24 and 25, 26 are electrically in parallel with the dipole elements 21, 22.

Referring now to both Fig. 1 and to Fig. 2, the latter, of which is a top plan view of one layer of an omnidirectional array according ,to the invention, the dipole elements 21, 22 have their adjacent ends (the electrical center of the dipole) located between 0.3 and 025' Wavelength in front of the screens 15. The dipole elements 21, 22 are tilted or slanted in a plane normal to the axis of the tower 11 with respect to the rear reflecting screens 15 with which they are associated. The angle of this tilt or slant depends both upon the allowable variations in the impedance versus frequency characteristic and the resultant field pattern produced. Depending upon the considerations of the individual installation, the angle of tilt of the dipoles 21, 22 in an azimuthal plane (a plane normal to the axis of the tower 11) with respect to the screen 15 may vary between 10 and 30 degrees. It has been found that with the pyramidal construction of the dipole elements 21, 22 mentioned above, an angle of tilt of 22 with respect to the screen 15 produces excellent pattern circularity characteristics. In a particular installation, for example, with the broadband dipole elements 21, 22 parallel to the screens 15 on four sides of a square tower and fed in phase rotational (that is, turnstile) fashion, an effective radiated power variation in the azimuthal plane of 3 db was observed. Withthe dipole elements 21, 22 tilted according to this invention to an angle of 22 azimuthally with respect to the reflecting screens 15, the circularity was improved to the point that the variation in effective radiated power in the azimuthal pattern was reduced to less than 1 db.

With the arrangement just described utilizing tilted dipole elements 21, 22 according to this invention, it was found that the decoupling of the antenna elements on one pair of opposed faces (for example, north and south) from the elements on the other pair of opposed faces (for example, east and west) was approximately 10 db greater than a prior arrangement in which the broadband dipole elements 21, 22 were parallel to the screens 15. This improved decoupling between antenna elements in the same antenna radiating system is especially valuable when certain types of diplexing systems are used to combine radio frequency picture and sound signals so that they may be radiated from the same antenna system.

The reflecting screens 15 are preferably /8 wavelength high and /2 wavelength wide. The width of the screens 15 thus varies from about 9 feet for channel 2 to about 2% feet for channels 10, ll, 12 and 13. The screens 15 are used to keep radiation out of the tower and thus prevent changes in the impedance over the channel width due to coupling with objects inside the tower; hence, even if only one dipole were used on the tower, the tower would be screened on all sides. The spacing between the cross conductors should be less than 0.05 wavelength, preferably 0.03 wavelength.

The dipoles 21, 22 with the screens 15 are mounted in layers with a vertical separation of slightly less than one wavelength, approximately 0.9 to 1 wavelength in practice, as measured from center to center.

Electrical excitation of the dipole elements is usually accomplished in practice by using equal length feed lines joining each pair of dipoles 21, 22 to common junction boxes.

In Fig. 2, referring first to the dipole elements 21, 22 at the bottom of the drawing, a coaxial transmission line 33 is led through one of the supporting members, shown here as member 24, and the metallic sheath conductor is connected to one dipole element 22, while the inner conductor of the coaxial line 33 is connected to the other dipole element 21. Both connections are made at the adjacent ends of the dipole elements 21, 22 where they are mounted on the supporting members 23, 24 and 25, 26. The dipole elements associated with the opposite phase, that is, at the top of the drawing in Fig. 2, are connected through a coaxial transmission line 34 to a source of voltage of the same phase as that supplied to the transmission line 33. However, these last dipole elements 21, 22 are connected in reverse sense, that is, the coaxial transmission line 34 is led through the supporting member 23, and the metallic sheath conductor is connected to the dipole element 21, while the inner conductor of the transmission line 34 is connected to the other element 22. On the other two faces of the tower, the two sets of dipole elements 21, 22 are respectively connected in opposite sense through coaxial transmission lines 35, 36 to another source of voltage which is displaced in time tive to the first source of voltage.

.: structure and normal to the axis by from that to which the first two sets of dipole elements were connected.

The dipoles 21, 22 or one pair of opposite faces (for example, north and south) are excited from one source of voltage; and the dipoles on the other two faces (for example, east and west) are excited by another source of voltage which is displaced 90 in time relation rela- These two sets of voltages produce currents in the dipole elements which induce fields in the several elements arranged around the tower, and these fields combine to produce apparent rotation of the field at radio carrier frequency. This type of feed is termed phase rotational or turnstile feed.

Fig. 3 is a graphical representation of the azimuthal field pattern in relative field strength of a directional antenna according to the invention using one set of dipoles on one face of a screened tower. It will be noted that the lobe is somewhat broader in the azimuthal plane than that of prior arrangements and its main energy content is directed to one side of the normal to the tower face, that side toward which the dipoles are tilted.

The pattern of Fig. 4 is a graphical representation of the azimuthal field characteristic in relative field of an omnidirectional array in accordance with the invention. The pattern of Fig. 4 deviates from circularity by about 1 to 1.5 db, and has no deep nulls or low field strength points which cause poor reception of broadcast very high frequency television program services using frequencies of the order of 54 megacycles to 216 megacycles.

What is claimed is:

1. An antenna system including a supporting tower structure, conductive surface elements each having width and height substantially a half wavelength and five-eighths of a wavelength, respectively, at the desired operating frequency, said conductive surface elements being mounted about the circumference of said tower structure with adjacent edges in electrical contact, at least two of said conductive surface elements having a dipole element arranged thereon with its electrical center at a distance lying substantially between one-quarter and three-tenths wavelength from the respective surface element at said operating frequency, said dipole elements lying in a plane normal to the axis of said tower and in said plane making an acute angle between 10 and 30 with its respective conductive surface element, and means to couple a radio frequency transmission line to each said dipole element.

2. In an antenna system including a supporting tower a radiator system, a structural element serving both as screening element for said tower structure and reflector element for the radiator system, said structural element comprising a rectangular conductive screen substantially a half wavelength long by five-eighths of a wavelength wide at the frequency of operation of said radiator system, a dipole radiator element comprising elongated conductors arranged in pyramidal configuration with the bases thereof in back-to-back relationship, the axis of said pyramidal back-to-back dipole element lying in a plane of said tower and making an angle of 10 to 30 with respect to said conductive screen in said plane, further conductors arranged in parallel relationship and connected at one end to the adjacent bases of said pyramidal elements and connected at the other end to conductors of said screen, at least one of said further conductors being hollow, and means to couple a coaxial transstantially a half wavelength long by five-eighths of a wavelength wide at the frequency of operation of said radiator system, the conductive members of said screen in the direction of the five-eighths wavelength dimension being spaced apart substantially 0.03 Wavelength at said frequency, a dipole radiator element comprising elongated conductors arranged in pyramidal configuration with the bases thereof in back-to-back relationship, the axis of said pyramidal back-to-back dipole element lying in a plane normal to the axis of said tower and making an angle of to 30 with respect to said conductive screen in said plane, further conductors arranged in parallel relationship and connected at one end to the adjacent bases of said pyramidal elements and connected at the other end to conductors of said screen, at least one of said further conductors being hollow, and means to couple a coaxial transmission line to the adjacent ends of said pyramidal elements, the sheath conductor of said coaxial transmission line being coupled to the pyramidal element connected to the hollow further conductor, said coaxial transmission line being led through said hollow conductor to the rear of said structural element.

4. An antenna system comprising, ing tower having a width on each side of substantially of a half wavelength at the desired operating frequency, four conductive surface elements mounted about the circumference of said tower with adjacent edges in electrical contact at the four corners of the tower, and four halfwave dipoles mounted with electrical centers at a distance of substantially one-quarter to three-tenths wavelength at the operating frequency from the respective surface ele ments, said dipoles lying in a plane normal to the axis of the tower and in said plane each making an angle between 10 degrees and 30 degrees with the respective surface elements.

5. An antenna system comprising, a four-sided supporting tower having a width on each side of substantially a half wavelength at the desired operating frequency, four conductive surface elements mounted about the circumference of said tower with adjacent edges in electrical contact at the four corners of the tower, said surface elements having a height of substantially five-eighths wavelength at the operating frequency, and four half-wave dipoles mounted with electrical centers at a distance of substantially between one-fourtl1 and three-tenths wavelength at the operating frequency from the respective surface elements, said dipoles lying in a plane normal to the axis of the tower and in said plane each making an angle between 10 degrees and 30 degrees with the respective surface elements.

6. An antenna system as defined in claim 5, and in addition, four similar surface elements and associated dipoles mounted on an adjacent section of said tower axially spaced by a distance substantially between ninetenths and one wavelength at said operating frequency.

7. An antenna system comprising a four-sided supporta four-sided supporting tower having a width on each side of substantially a half wavelength at the desired operating frequency, four conductor surface elements mounted about the circumference of said tower with adjacent edges in electrical contact at the four corners of the tower, said surface elements having a height of substantially five-eighths Wavelength at the operating frequency, four half-wave dipoles mounted with electrical centers at a distance of substantially between one-fourth and three-eighths wavelength at the operating frequency from the respective surface elements, said dipoles lying in a plane normal to the axis of the tower and in said plane each making an angle between 10 degrees and 30 degrees with the respective surface elements, and means to energize said dipoles simultaneously in phase rotational fashion.

8. An antenna system comprising, a multi-sided supporting tower having a width on each side of substantially a half wavelength at the desired operating frequency, a plurality of conductive surface elements mounted about the circumference of said tower with adjacent edges in electrical contact at the corners of the tower, said surface elements having a height of substantially five-eighths wavelength at the operating frequency, and an equal plurality of half wave dipoles mounted with electrical centers at a distance of substantially between one-fourth and three-tenths wavelength at the operating frequency from the respective surface elements, said dipoles lying in a plane normal to the axis of the tower and in said plane each making an angle between 10 degrees and 30 degrees with the respective surface elements.

9. An antenna system comprising, a multi-sided supporting tower having a width on each side of substantially a half wavelength at the desired operating frequency, a plurality of conductive surface elements mounted about the circumference of said tower with adjacent edges in electrical contact at the corners of the tower, said surface elements having a height of substantially five-eighths wavelength at the operating frequency, and an equal plurality of half wave dipoles mounted with electrical centers at a distance of substantially between one-fourth and three-tenths wavelength at the operating frequency from the respective surface elements, said dipoles lying in a plane normal to the axis of the tower and in said plane each making an angle between 10 degrees and 30 degrees with the respective surface elements, said dipoles being tilted in the same direction when viewed going around the tower in a given direction.

References Cited in the file of this patent UNITED STATES PATENTS 2,292,342 Schelkunofi et al Aug. 4, 1942 2,489,287 Guarino et a1 Nov. 29, 1949 2,539,433 Kandoian Jan. 30, 1951 2,562,296 Christensen July 31, 1951 2,580,462 Ranger Jan. 1, 1952 2,691,102 Masters Oct. 5, 1954

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