US2706780A - Antenna array - Google Patents

Description

April 19, 1955 Filed July 31, 1952 R. H. Du HAMEL 2,706,780

ANTENNA ARRAY 2 Sheets-Sheet l i 5 -4/2 MWA/i mrs/5 fia/Mimi amm/W INI/ENTOR.

RaYMrJND H. Du HEMEL.

TTORNEY April 19, 1955 R. H. DU HAMEL 2,706,780

ANTENNA ARRAY IN IEN TOR.

RHYMUND H. DUHAMEL B, M AM ATTORNEY N. LA?

United States Patent O ANTENNA ARRAY Raymond Horace Du Hamel, Lawrenceville, N. J., as-

signor to Radio Corporation of America, a corporation of Delaware Application July 31, 1952, serial No. 301,891

The terminal fifteen years of the term of the patent to be granted has been disclaimed 12 Claims. (Cl. Z50-33.53)

This invention relates to antennas and particularly to antenna arrays in which the field pattern is shaped to provide a desired pattern characteristic.

In the development of broadcasting services, antennas which are directional, as well as antennas which are omnidirectional in the azimuthal plane, have been employed. The generation of large amounts of radio frequency power at ultrahigh frequencies is not, at the present state of the art, as efficient or economical as is desired. Antenna systems heretofore used at frequencies up to 300 megacycles are not sufliciently efficient themselves to recommend them for widespread use as broadcast antennas for ultrahigh frequency signals. The problem is therefore presented in ultrahigh frequency television broadcasting, for example, as to how most efficiently and economically to use radio frequency energy in the range from 470 to 890 megacycles.

For the most economical use of the avaliable radio frequency energy, the radiated field intensity throughout the primary service area of the broadcasting establishment should be as constant as possible. A specially shaped vertical radiation pattern may be used to obtain a substantially constant field strength over the primary service area of an ultrahigh frequency broadcasting installation.

The present invent-ion is directed to antenna arrays in which the field pattern characteristic can be shaped to provide a desired field pattern with extremely low radiation loss due to energy being wasted in undesired directions. Antenna arrays in accordance with the invention may be built to present a w-ide variety of field patterns, but it is expected that an important application thereof will be for ultrahigh frequency broadcasting services utilizing horizontal polarization to provide maximum coverage of the broadcast service area.

An object of this invention is to provide an antenna array of simple physical arrangement capable of producing specially shaped field pattern characteristics.

Another object of this invention is to provide an antenna array capable of producing a shaped beam radiation pattern characteristic which makes use of a straightforward and uncomplicated transmission line coupling arrangement.

A further object of this invention is to provide an antenna array for horizontally polarized waves in which the vertical radiation pattern may be simply controlled by the physical shape of the array.

Yet another object of this invention is to provide a new method of designing an antenna array to produce a specitied radiation pattern.

Still another object of this invention is to simplify design methods of antenna arrays to produce a specified field pattern.

In accordance with the present invention, these and other objects are attained by an antenna array which includes a plurality of antenna elements closely spaced to a current sheet reflector structure. All of the currents in the antenna elements are made equal and in phase by arranging the transmission line connections s`o that each element is coupled through a quarter-wave of line to a common voltage point. The amount that each of the antenna elements in the array contributes to the field pattern characteristic -is determined and controlled by the spacings of the individual antenna elements from the current sheet reector structure. The effective phase of the currents in the individual antenna elements relative to a far field point is controlled by dcviating or warping the sheet refiector from a linear reference base. The spacing "ice between the -individual antenna elements and the reflector sheet may vary from one-quarter wavelength to as little as 1/100 of a wavelength. In general, the warping of the sheet reflector at any point along the array relative to the linear base reference will be less than one-half wavelength, although greater variation in the reector sheet is contemplated under certain conditions. The resultant array produces a field pattern characteristic which is simply controlled by physical adjustments of the shape of the antenna array itself, and the antenna coupling networks are kept very simple.

A more detailed explanation follows in conjunction with the accompanying drawings, in which:

Fig. l is an elevation of an array of antenna elements in front of a current sheet reflector, embodying the principles of this invention;

Figs. 2 and 3 are curves explanatory of certain features of the invention;

Fig. 4 is an elevation of another antenna array in accordance with the invention;

Fig. 5 is an elevation of an antenna array, in accordance with this invention, which has an omnidirectional pattern in the horizontal plane and a shaped vertical pattern characteristic;

`gig. 6 is a cross section along the line 6 of Fig. 5; an

Fig. 7 is a cross section along the line 7 of Fig. 5.

Referring now to Fig. l, a plurality of antenna elements 11a through 11p are arranged in close proximity to a current sheet reiiector 13. For the purpose of this discussion, the antenna elements 11a through 11p are assumed to be identical elements in which the currents are parallel to the sheet reflector 13. Simple half-wave dipoles may be utilized as the antenna elements 11a through 11p, although it should be understood that simple dipoles of other lengths, folded dipoles, unipoles, full-wave loops parallel to the sheet reflector, and certain continuous wire arrays may also be employed. The current sheet reflector 13 may be of solid metallic or conductive material, or may consist of a conductive screen; or a grid or network of conductors may be used to approximate a solid sheet reflector.

Each of the plurality of antenna elements 11a through 11p Iis coupled to a transmission line network having a main transmission line 15 and a plurality of quarter-wave coupling lines 17a through 17p. The transmission lines 15 and 17 are indicated by a single line on the drawing. This representation is intended to be schematic, and includes as well transmission lines having either single or plural conductors, such as waveguides, coaxial lines and parallel wire lines.

All of the antenna elements 11a through 11p in the array are fed with equal and in-phase currents. The transmiss-ion line connection system shown in Fig. l is arranged to produce equal in-phase currents in all of the antenna elements 11a through 11p as follows: The current feed point of each antenna element 11a through 11p is coupled through an odd quarter-wavelength coupling line 17a through 17p to an equal and in-phase source of voltage. The length of the coupling lines 17 is selected as an odd multiple of a quarter-wavelength since the receiving-end current of a line having a length equal t0 is independent of terminating end impedances and is determined by the sending-end voltage.

By connecting the odd quarter-wavelength coupling lines 17 at multiples of a wavelength in electrical distance along the transmission line 15, equal and in-phase voltages are impressed across the quarter-wavelength coupling lines 17 yielding equal currents in the identical antenna elements 11a through 11p. It will be apparent to those skilled in the antenna art that the spacing of the antenna elements along the line of the array may be any integral number of half-wavelengths in electrical distance along the transmission line 15 for such a coupling arrangement. For odd multiples of a half-wavelength, such as one-half or three-halves wavelength spacing, the connection of the quarter-wave coupling line 17h associated with any one antenna element, 11h., for example, will be coupled in opposite sense across the transmission line to that of a next adjacent coupling line 17g or 171'. As shown in Fig. 1 of the drawing, however, each of the quarter-wave couat any point, the current sheet reflector 13 is shaped so that i/ Qitio 1) pling lines 17 will be connected in the saine sense 'across 5 the equal voltage points on the main transmission line 15. Wngre WX) is the required pnn-Se Variation Other simple coupling networksto feed all of the an- The antenna elements 11a through 11p are spaced eml'fl 1U1I1t5-11 through 111? Wlh equal CUFFEHS may from the current sheet reiiector 13 a distance such that readily be devised. An equal-length branching system like the one shown in connection with Fig. 4 may also be 10 RUIM) :sin (Zlsn) (2) employed. A

The field pattern in a plane perpendicular to the aritenna elements 11a through 11p is controlled by the where X is the distance measured from the center of physical shape of the array. Knowing the desired eld the aperture of the array to any element and Xp. i s pattern and the aperture of the array, it is relatively the distance to the position of the pth element, Sp is simple to calculate the current distribution across the the spacing of the element from the current sheet regiven aperture required to produce the desired pattern. ector 13 and R (Xn) is the normalized magnitude This required current distribution across the aperture ot of the current I as Xn. the array will have certain values of magntiudc and The curves of Figs 2 and 3 illustrate current magniphase. In this invention, the effective magnitude of the 20 tude and p hase distribution across an antenna array havfield from each antenna element 11a through 11p is proing a vertical aperture of ten wavelengths to secure a portional to the sine of the spacing in radians of the good approximation of a cosecant squared vertical patantenna element 11a through 11p from the current sheet tern characteristic. reiicctor 13. The effective phase of the field from each The following table is representative of the dimensions element is proportional to the displacement from a linear of a unidirectional antenna array having a linear aperbase reference line 19 of the current sheet reflector 13 ture of ten wavelengths with sixteen dipole antenna elenear the element in question. ments in front of a conductive current sheet reector. lt might appear to the Worker in the art that a plane The absolute values of current for theldipole element reflector can be utilized in which the current magnitude positions chosen represent those s hown in Fig. 2. The across the aperture of the array is controlled by the spacphase variation 1]/(X) for. the distance of the current ing of the antenna elements from a sheet reflector, and sheet reflector from the linear base reference is taken at the same timeasimple transmission line network used to from the .curve ot Fig. 3. The spacings of each of the produce the required phase distribution function. Such elements in the sixteen element array in centimeters is an arrangement gives rise to a serious design problem. given for a midband frequency of 940 megacycles.

W-ave' \Torrnal 3 i in Antenna lengths Absolute ed Clu ilpcn Phase Element from Current rent Ma` Spacing meter' Phase Deviation v0./2 center Magari mtudeg' s/,rx go fn i (X) in wave- Dipole) oi Ariay tudc R (Xu) s# lengths Y 41%@ .5s .31 .07s 2. 50 171 475 41m. 70 .32 .030 2. 55 155 431 3746 .75 .34 .035 2. 72 139 380 21` .s1 .37 .093 2. 97 123 342 2%, 92 .42 .105 3. 36 104 290 1916 1.0s .49 122 3. 97 30 233 UAG 1.4 .04 .i0 5.11 65 -l- 130 /i 2.2 1.0 .25 8 42 115 ai@ 2.2 1.0 .25 s 42 .113 15A@ 1.4 .04 .i0 5.11 55 .180 1%. 1. 03 .49 .122 3. 97 36 23s 2%; .92 .42 .105 3.30 104 .290 21%; .si .37 .093 2.97 123 .342 3%6 .75 .34 .085 2.72 130 .335 4145 .70 .32 .030 2.55 155 .431 411/15 .6s .3i .073 2, 50 171 .475

lf the transmission lines from points of equal voltage are varied in length to alter the phase of the current in the individual antenna elements, then the transmission line connection system Will no longer producel equal currents in all of the antenna elements and the effective current magnitude across the aperture will be altered. Such an arrangement therefore is subject to interaction between the adjustments of current magnitude and phase, and the design of sufliciently isolated transmission line coupling circuits capable of some degree of adjustment to duplicate the action of the preferred arrangement of the present invention becomes extremely complicated and tedious.

Referring now to Figs. 2 and 3 as well as to Fig. l, let it be assumed that the required phase and magnitude of the current distribution have been determined. Fig. 2 illustrates a representative desired current magnitude distribution across an aperture having a length L. The aperture of an antenna is used in this specification to denote the equivalent area over which the antenna interchanges energy with free space. Fig. 3 is illustrative of a desired phase ol' the current across the same aperture of length L relative to a far field point. lt is convenient for analytical purposes, although not necessary, to use the concept of a continuous Current distribution across the entire aperture length L.

The current sheet rellector 13 of the antenna array is shaped or warped with respect to the linear base reference i9 according to the phase curve shown in Fig. 3. Let Y represent the distance from the linear base reference 19 to the surface of the current sheet reflector i3 In Fig. 4 there is shown an alternative feeding arrangement for an antenna array in accordance with this invention. T o insure that all of the antenna elements 11a through 11p are supplied with equal current, they are coupled by means of odd quarter-wavelength coupling lines 17a through 17p to points of equal voltage 21 with respect to the remainder of the transmission line system.

The points of equal voltage 21 are formed by a transmission line branching system. A main transmission line 23 from the radio frequency apparatus 24 (such as a transmitter or receiver), to which the antenna system is to be coupled, has a iirst branching point 2S. From this first branching point 2S, a plurality of equallength transmission lines 27 extends to secondary branching points 29. From each of these secondary branching points 29 a plurality of also equal-length transmission lines 31 extend to the points of equal voltage 21.

For optimum operation, the transmission line coupling network should be arranged so that the equal length transmission lines 27 between the first branching point 25 and secondary branching points 29 are an integral multiple of half-wavelengths long, and the equal-length connection lines between the secondary branching points 29 and the points of equal voltage 21 are also selected to be some integral multiple of half-wavelengths long at the operating frequency. By selecting the first set of equal-length transmission lines 27 to be an integral number of half-wavelengths and the second set of equal-length transmission lines 31 to be also some integral number of half-wavelengths (of course, not necessarily the same number of half-wavelengths as that for the first set of equal-length transmission lines 27), the transmission line interconnection system presents equal voltage points at every branching point 25, 29 as well as the points of equal voltage 21.

With the arrangement just described, if the antenna is used as a transmitting antenna, whatever voltage is impressed across the first branching point 25 by the radio frequency apparatus 24 (in this case, a transmitter) will also appear across each of the points of equal voltage 21 to which the antenna elements 11a through 11p are coupled by the odd quarter-wavelength coupling lines 17a through 17p. The antenna elements 11a through 11p are therefore all excited with equal and in-phase currents.

The arrangement shown in Figs. 5, 6 and 7 has eight arrays similar to that described above in connection with Figs. 1, 2 and 3 spaced around a polygonal structure. This array has an omnidirectional pattern in the horizontal plane and a cosecant squared vertical pattern characteristic. The separate faces 13a through 13h perform the same function and are each similar to the single sheet reflector 13 of Fig. 1. Each of the several refiector faces 13a through 13h is deviated from a linear base reference in the same manner as the current sheet reflector 13 of Fig. 1 to control the effective phase of the field dueto each antenna element 11a through 11p. The amount of deviation from the linear base reference for each of the several reflector faces 13a through 13h is in accordance with the curves shown in Fig. 3 and described above. The linear base reference itself is referred to a vertical plane through the smallest dimension of the reector face, and in this instance is tilted upward by 5.7 degrees to bring the maximum radiation for the particular array shown to have a downward tilt of 1.3 degrees from the horizontal.

A comparison of the spacing Sa of the antenna elements 11 from the refiector sides 13a through 13h in Fig. 5 shows a representative variation in such spacing for an array to achieve a cosecant squared vertical pattern power distribution characteristic. The spacing of the individual elements 11 along each of the single reflector faces (for example, 13C) is in accordance with the table given above in the description of Figs. l, 2 and 3.

By referring to Fig. 6, it may be seen that the perpendicular distance between any two opposite reector sides (for example, 13C and 13g) may be of the order of Ak at the top of the array for an embodiment having an octagonal cross section; and near the bottom of the array, as shown in Fig. 7, the cross-sectional dimension between the same two reiiector sides is of the order of a wavelength. With such dimensions, the length of the sides of the polygon at the narrowest portion of the array becomes less than one-half wavelength. It is therefore necessary under certain conditions, like the one herein described, to make the individual antenna elements 11 shorter than one-half wavelength so that the sector occupied thereby will not overlap an adjacent sector. Normally, the spacing between the centers of adjacent dipole antenna elements 11 in a single layer like that shown in Fig. 6, will vary between 3/sx and SAM for an eight-sided array.

In the example shown, the spacing between the centers of adjacent antenna elements 11 in an upper layer as shown in Fig. 6 will be .55}\, while the spacing of similar elements in a lower layer like that of Fig. 7 will be .39)\. As was mentioned above, all of the antenna elements are preferably identical and will therefore be of the order of 1A to 3A; wavelengths in length.

It will, of course, be apparent that if the number of sides of the polygon is increased to l0, l2 or 14, there will be less variation in cross-sectional dimensions between upper and lower layers, and the individual antenna elements 11 may be made longer and still achieve an additive field relative to some far field point from two adjacent radiating elements in a single layer.

The cross-sectional configuration of the closed figure approaches circularity as the number of sides is increased. The sheet refiector structure may be in fact made circular in cross section without changing the vertical pattern characteristic and without detrimental effects on the horizontal circularity where the minimum diameter of the figure thus formed is of the order of a wavelength or more.

All of the antenna elements in a longitudinal sector, like those associated with a single refiector face, 13c for example, in Fig. 5 are coupled by a simple transmission line network to a common voltage point as explained in conjunction with Figs. 1 and 4. Such networks are omitted from the drawing in Figs. 5, 6 and 7 for the sake of a clearer showing. All of the elements in the several sectors may be coupled to a common transmission line network, if the entire omnidirectional array is coupled in phase to the associated radio frequency apparatus, or a network individual to each sector (or oppositely disposed sectors) is necessary for phase rotational coupling, commonly called turnstile feeding.

Unidirectional antenna arrays having a cosine distribution pattern in the horizontal plane like those shown in Figs. l and 4 have further application in laboratory or experimental uses. Such directional arrays are especially convenient for pattern synthesis or analysis of broadside arrays. Since the antenna array of this invention provides separate and non-interacting physical adjustments of both the current magnitude and phase relative to a far field point, a desired pattern characteristic may be obtained by a trial and error method. Also, a calculated current and phase distribution across an aperture to produce a certain pattern may be verified experimentally by actual pattern measurements very rapidly and with extremely simple physical equipment. The number and complexity of the required physical adjustments for such pattern synthesis and verification are materially reduced over prior systems. Furthermore, the results obtained from an experimental verification using an antenna in accordance with this invention more nearly represent the actual pattern characteristic conditions than can be obtained by mathematical solution either manually or by pattern calculating devices.

What is claimed is:

l. An antenna array comprising a conductive sheet reflector and a plurality of antenna elements closely spaced to said sheet reflector, said sheet reflector being warped from a straight linear reference base extending along the length of the aperture of said antenna to control the effective phase of the field pattern characteristic due to each of said antenna elements relative to a far field point, the spacing of said elements from said reflector surface varying across the aperture of said array in. accordance with the relative magnitude of the field due to the individ ual elements in the array with respect to a far field point.

2. An antenna array comprising a plurality of antenna elements distributed across an aperture, a current sheet reflector structure in the near zone of said plurality of elements, said current sheet reiiector structure having a warped spatial deviation from a straight base reference line extending along the length of said aperture, said deviation varying therealong according to a required phase characteristic, said antenna elements being spaced from said current sheet reflector by different amounts in direct proportion to a desired current distribution across said aperture.

3. An antenna as defined in claim 2 wherein said spaced antenna elements are dipoles.

4. An antenna array comprising a plurality of antenna elements distributed across an aperture, a current sheet reflector structure in the near zone of said plurality of elements, said current sheet reflector structure having a spatial deviation from a straight base reference line extending along the length of said aperture, varying according to a required phase characteristic, said antenna elements being spaced from said current sheet reflector by different amounts in direct proportion to a desired current distribution across said aperture, and a transmission line coupling network connected to said antenna elements to couple in equal degree to each of said elements whereby equal instantaneous currents are produced in all of said antenna elements.

5. An antenna array comprising a plurality of antenna elements distributed across an aperture, a current Vsheet reflector structure in the near zone of said plural- 6. An antenna array comprising a plurality of antenna elements distributed across an aperture, a current sheet reflector structure in the near zone of said plurality of elements, said current sheet reflector structure having a spatial deviation from a straight base reference line along the length of said aperture varying according to a required phase characteristic, said antenna elements being spaced from said current sheet reflector by different amounts in direct proportion to a desired current distribution across said aperture, and transmission line coupling means for feeding each of said elements from a common voltage reference point through a transmission line having a length of an odd multiple including unity of one-quarter wavelength at the operating frequency.

7. An antenna array comprising a plurality of antenna elements distributed along an aperture of several wavelengths, a continuous surface conduction current sheet reflector in the near zone of said plurality of elements, said current sheet reflector being spaced from a straight base reference along the length of said aperture by an amount which varies directly in free space wavelengths as a required phase characteristic of current in said elements across said aperture, said antenna elements being spaced from said current sheet reflector by different amounts each in direct proportion to a desired current magnitude distribution characteristic along the length of said aperture.

8. An antenna array comprising a plurality of antenna elements distributed along an aperture of several wavelengths, a continuous surface conduction current sheet reflector in the near zone of said plurality of elements, said current sheet reflector being spaced from a straight base reference line along the length of said aperture by an amount which varies directly in free space wavelengths as a required phase characteristic of current in said elements across said aperture, said antenna elements being spaced from said current sheet reflector by different amounts each in direct proportion to a desired current magnitude distribution characteristic along the length of said aperture, and transmission line coupling means for feeding each of said elements from a common voltage reference point through a transmission line having a length of an odd multiple including unity of onequarter wavelength at the operating frequency.

9. An omnidirectional antenna array comprising a plurality of conductive sheet reflector structures forming the sides of a closed polygon, each of said plurality of conductive sheet reilector structures having a plurality of antenna elements closely spaced thereto, each of said plurality of conductive sheet reflectors being deviated in free space wavelengths from a linear reference base eX- tending along the length of the aperture of said antenna in direct proportionto a predetermined phase characteristic, the antenna elements associated with each of said plurality of reflector structures being spaced therefrom by different amounts along the length thereof in direct proportion to a predetermined current distribution across said aperture.

10. An omnidirectional antenna array comprising a plurality of conductive sheet reflector structures forming the sides of a closed polygon, each of said plurality of conductive sheet reflector structures having a plurality of antenna elements closely spaced thereto, each of said plurality of conductive sheet reflectors being deviated in free space wavelengths from a linear reference base extending along the length of the aperture of said antenna in direct proportion to a predetermined phase characteristic, the antenna elements associated with each of said plurality of reflector structures being spaced therefrom by different amounts along the length thereof in direct proportion to a predetermined current distribution across said aperture, and a plurality of transmission line coupling networks, each of said networks being individual to all of said elements associated with one of said reflector structures and feeding each of said elements associated with said reflector structure from a common voltage reference point through a section of transmission line having a length of an odd multiple including unity of one-quarter wavelength at the operating frequency.

1l. An omnidirectional antenna array comprising a plurality of conductive sheet reflector structures forming the sides of a closed polygon, each of said plurality of conductive sheet reflector structures having a plurality of antenna elements closely spaced thereto, each of said plurality of conductive sheet reflectors being deviated in free space wavelengths from a linear reference base eX- tending along the length of the aperture of said antenna in direct proportion to a predetermined phase characteristic, the antenna elements associated with each of said plurality of reflector structures by different amounts along the length thereof in direct proportion to a predetermined current distribution across said aperture, and a transmission line coupling means common to all of said antenna elements for feeding each of said antenna elements with equal instantaneous currents.

12. An omnidirectional antenna array comprising a conductive sheet reector forming a closed gure in cross section and having a length of several wavelengths at the operating frequency and having a longitudinal axis, a plurality of antenna elements arranged along each of a number of longitudinal sectors, said sheet reector being deviated in distance from the longitudinal axis along the length thereof in direct proportion to a predetermined phase characteristic, said antenna elements in each of said sectors being spaced from said reflector by different amounts along the length thereof in direct proportion to a predetermined current distribution, and transmission line means for feeding each of said antenna elements in each sector with equal and in phase currents with respect to the remaining elements in said sector.

llberg May 2, 1939 Cork et al. Dec. 26, 1939

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