US2744249A - Antenna feed systems - Google Patents

Description

May 1, 195e E. H. SHIVELY ET AL ANTENNA FEED SYSTEMS Filed Jan. 50. ,1953

5 Sheets-Sheet l I N VE N TORJ` fan/ARD bf 5H/Vari@ WML/AM 5,?,4 Afp/95H6 BY MW M ATTORNEY May l, 1956 Filed Jan. 30, 1953 E. H. sHlvELY ET Ax. 2,744,249

ANTENNA FEED SYSTEMS 3 Sheets-Sheet 5 KY ll m BY Mw' d TTORNE Y ANTENNA FEED SYSTEMS Edward H. Shively, Haddonlield, and William S. Brandberg, Delaware Township, Camden County, N. I., assignors to Radio Corporation of America, a corporation of Delaware Application January 30, 1953, Serial No. 334,142 7 claims. (cl. 343-853) This invention relates to antennas, and particularly pertains to feeding systems for antenna arrays having an aperture several wavelengths long at the operating frequency.

The desire for uniform coverage in the primary service area of a television broadcasting station may be satisfied by the use of an antenna having a vertical radiation pattern which varies as the consecant of the angle of depression. Although this relation is not strictly true because of ground reflection and intervening obstacles, it provides a useful approximation in practice. An antenna aperture which is uniformly illuminated along its length by the radio frequency energy has a pattern characteristic which is generally similar to this consecant distribution, except that deep nulls occur wherein the signal strength is theoretically zero.

The term aperture in this specication and the appended claims is used to denote the area in space over which the antenna interchanges radio frequency energy between itself and surrounding space. The term length of aperture denotes one dimension of the area of aperture, customarily considered the longest dimension of the area of aperture over which the antenna radiates energy into surrounding space or extracts a passing radio wave therefrom. f

It should be borne in mind that the absolute value of signal strength necessary to produce a useable signal at the home receiver is the same at all points in the primary service area. However, with a given radiated field in a predetermined direction, the signal strength available at receivers at varying distances from the transmitting antenna decreases in proportion to the distance from the antenna. A vertical radiation pattern characteristic may be calculated for any given antenna height to take account of this variation in field strength for different distances and produce equal signal strength at all locations in the primary service area. The cosecant eld strength distribution mentioned above provides the desired uniform signal strength in the service area.

It has been found from propagation measurements that interfering signals beyond the useful service area of an ultrahigh frequency broadcasting station can be greatly reduced by tilting the vertical pattern of a high-gain antenna downward a suitable amount. This feature of beam tilting enables a high-gain tilted beam antenna to give greater elfective radiated power within its service area than at the horizon or boundary of the service area. A consequent better coverage eiiciency of the broadcast transmitter power can be gained in many instances by utilizing a tilted beam. However, even with this tilted beam, it has been found that troublesome nulls on areas of low signal strength may occur in the useful area near the antenna.

This invention has as an object the elimination of nulls in the vertical pattern of ultrahigh frequency broadcast antenna systems.

States Patent() Another object of this invention is to provide an antenna array which has a vertical pattern characteristic wherein the signal strength does not fall to deep nulls and also wherein the vertical pattern can be tilted electrically by a simple mechanical adjustment.

A further object of this invention is to provide an antenna feed system for ultrahigh frequency broadcasting antenna arrays which produces a shaped vertical pattern characteristic to yield more uniform coverage of the primary service area and in which the beam tilting of the vertical pattern can be changed electrically by a simple mechanical adjustment.

These and other objects are achieved, in accordance with the present invention, by providing an improved antenna feed system for antenna arrays which include a plurality of radiating elements arranged over an aperture a plurality of wavelengths long. In this invention, the antenna elements are divided into a number of groups of elements occupying different lengths of aperture, and each group is fed with the same amount of power. An asymmetrical power distribution along the aperture of the array results from this feed system which produces a vertical radiation pattern (for a vertical array and horizontal polarization) having no nulls at all wherein the iield strength drops to zero either theoretically or actually.

An embodiment of the invention is described in connection with a slotted cylinder antenna having from 14 to 13 layers of slots and suitable for ultrahigh frequency television broadcasting. lt should be understood, however, that the principles of the invention are applicable as well to other antenna arrays.

In the specific embodiment, two groups of antenna elements are fed from a single feed point, but are adjusted to present the same impedance, and therefore receive equal proportions of the total power. There are fewer elements in one group than in another, and consequently the power radiated per element is different in the one group from that in the other. A representation of the power distribution per antenna element along the length of the antenna array is therefore asymmetrical.

A more detailed description follows in conjunction with the drawings, wherein:

Figure l is a graphical representation of power distribution across an antenna array aperture used in explaining the invention;

Figure 2 is a graphical showing of lield strength patterns used in explaining the invention;

Figures 3 `and 4 are lield pattern curves of center-fed arrays;

Figure 5 is a field pattern curve of an antenna array fed in accordance with the invention; and

. Figure 6 is an elevation, partly in section, of a slotted cylinder antenna embodying the invention.

Referring to Figures l and 2, Figure l represents the power distribution along an antenna aperture having a length of 2a with the center of total power removed from the electrical center of the antenna by the quantity A and Figure 2 is a graphical showing of the relative eld strength of an antenna fed according to the power division of Figure l represented as two separate antennas electrically removed by the'distance D. l

In Figure l let it be assumed that the feed point is located a distance A from the center line of the antenna array, and that the total power fed to the elements from (f) to (-I-a) is the same to that fed to the greater nurnber of elements from (f) to (-a), Stated in another way, the area shown hatched under the power curve for each of the two groups is equal, but the length of aperture occupied by one group of elements is greater than that occupied by the other. The following relation is If it be assumed that the types of elements used in the two groups of elements in the antenna array are similar, radiation patterns of the two groups will vary in sharpness as a function of the length of aperture occupied by the single group of elements. Figure 2 illustrates representative radiation patterns or" two groups of elements containing different numbers of elements and occupying different linear dimensions of aperture. it will be noted that the upper section has a narrower main beam, while the lower section, due to its shorter aperture, has a somewhat broader main beam. lf equal total power is fed to each of the groups, the areas included iu the graphical representation of both of the radiation patterns of the 4two groups will be the same.

Since the two groups of antenna elements have different radiation patterns as represented in Figure 2, the nulls in the individual patterns do not occur at the same depression angle, and thus do not occur at all in the overall radiation pattern characteristic (which is the vector summation of the two separately represented pattern characteristics shown in Figure 2). Furthermore, nulls in the over-all pattern which would normally be due to cancellation of radiation from the two sections in known antennas do not occur in the antenna of the invention because the amplitudes of the radio frequency field radiated by each section of the antenna are unequal at those depression angles where they are in phase opposition.

The amount that the feed point may be removed from the physical and electrical center of the antenna aperture and still obtain the beneficial result of this invention may vary from 5% to 25% of the total length of the apen ture 2a. This represents offsetting the feed point about l1/2 wavelengths in a 30 wavelength aperture up to olfsetting the feed point 4*/2 wavelengths in an i3 wavelength aperture. An offset of less than 5% 4ces not create sutiicient power asymmetry in a long aperture antenna to ll in the nulls properly, while displacing the feed point more than 25% from the center of the aperture may create bandwidth problems as well as pattern scanning with frequency.

As an illustration of the improvement in the vertical radiation pattern characteristic of a high gain ultrahigh frequency antenna array resulting from employing the principles of this invention, reference is made to Figures 2, Ll, and 5. Figure 3 shows the calculated and measured field patterns of a slotted cylinder -antenna having 18 layers of slots spaced 1.5 wavelengths at the operating frequency, three longitudinal slots per layer equally spaced around the cylinder, each slot having a length of approximately 1.3 wavelengths at the operating frequency, with the slots in adjacent layers staggered 60 to give an om nidirectional horizontal pattern. The antenna was fed at its electrical center with equal power input to the two halves of the antenna array.

The solid line 21 indicates an actual measured field strength pattern of the antenna array, while the dot-dash line 23 indicates the theoretical eld pattern of the array described. The part of the graph to the left of the zero mark on the abscissa and showing positive (l) values of elevation represents power directed above vthe horizon and hence wasted power for most applications. The part of the graph to the right of the zero mark on the abscissa and showing negative values of elevation represents power directed below the horizon and hence useful power in the primary service area of the broadcasting station. The largest negative values of angle on the abscissa denote points closer to the antenna location, while the lower negative values of angle denote points closer to the horizon.

It will be noted that the theoretical or calculated vertical pattern falls to deep nulls wherein the radiated field strength is zero at a number of points along the abscissa of the curve. These nulls occur where the differences in the radiated iield from the upper and lower halves of the antenna are in phase opposition at various elevation or depression angles, or where the nulls in the individual patterns of the upper and lower halves occur at `the same depression angle.

As a practical matter, these deep nulls are filled in to some degree by slight departures from the uniform inphase current distribution assumed for the theoretical pattern calculations. in the practical antenna, small dimensional errors and variations in impedance of the dit'- ferent layers of elements (due to the different mutual impedance between layers) cause some nonimifcrrnity in amplitude or phase of the current distribution across the aperture. The amount of null lill-in which can be expected from these departures from the theoretical patterns is shown by the measured pattern indicated by the solid line 21 in Figure 3,

Figure 4 shows a typical measured pattern 2.4 of an antenna like that described to obtain the pattern of Figure 3 in which the antenna was fed near thc electrical center, but with the feed point removed a small amount (less than 2 percent of the total length of the aperture) to eiiiect a difference in phase in the radio frequency energy fed to the upper and lower halves. This dilfcrenc in phase produces the electrical beam tilt of the antenna and in the case illustrated by the pattern shown in Figure 4, the main beam was tilted downward (192.

The beam tilting produced by the ditlerence in phase between the two halves of the antenna has a concomitant effect of providing a certain small amount of null till-in. lt may be observed that there are still points of low signal strength occurring at angles of depression of about Ll-l/z", 9, and l3l/z" for the particular antenna array measured. ln an actual commercial installation in a populated area, the eld intensity probably would not fall to such low levels for every available receiving antenna location at each home installation. Scattering ef- `fects, ground reliections, and possible propagation by other than the direct path combine to produce useable signals in what would be expected to be a low signal strength area. The feeding system of the present invention may be utilized to lessen the likelihood of the lield intensity in the primary service area falling below a desired value, to achieve better null fill-in, and to provide a vertical radiation pattern characteristic more closely approaching a cosecant function of angular depression from the horizon.

Figure 5 is a measured lield pattern of a slotted cylinder antenna like those utilized to obtain the patterns of Figures 3 and 4 but fed in accordance with this invention. The 18 layers of slots were divided into two groups, ten slots in the upper layer and eight slots in the lower layer, with equal power being fed to the two groups. T his resulted in each layer of slots in the lower group being fed with more radio frequency energy than each layer in the upper group. In addition to the asymmetrical power distribution across the aperture of the antenna, a one degree beam tilt was obtained by displacing the feed point from its normal position midway between Athe eighth and ninth layers of slots to produce the necessary difference in phase of the two currents.

In Figure 5, the solid line 2S indicates the measured field pattern of the antenna described, and the dot-dash curve 27 is a representative cosecant field strength distribution indicated above as being highly desirable for uniform coverage of the service area. An inspection of the two curves of Figure 5 will show that the measured field pattern of an antenna fed in accordance with the principles of this invention having similar radiating elements occupying different amounts of aperture and excited with equal amounts of radio frequency energy, very closely follows the desired uniform coverage curve.

Points on the actual measured eld strength curve 25 in Figure 5 which are above the dot-dash curve 27 represent more signal strength at the receiver location than is necessary for good reception, and in one sense, therefore, represent wasted power. By providing a feeding arrangement for an antenna array which so closely follows the desired uniform coverage curve, and which restricts the range of variation of the signal strength from the desired coverage curve, greater signal strength is obtained over large portions of the primary service area without wasting radiated power by supplying more signal than is necessary for excellent operation of the receivers being served.

Figure 6 is a view, partly in section, of a slotted cylinder antenna embodying the invention like that described to obtain the field pattern characteristic shown in Figure 5 The slotted cylinder antenna has an outer conductive cylinder 31 with a plurality of slots 33 arranged in layers around the periphery thereof. There are three slots 33 in each layer, equally spaced around the conductive tubular member 31. Each layer of slots 33 is staggered 60 with respect to a next adjacent layer. There are 18 layers of slots in all, each slot having a length of 1.3 wavelengths and being spaced center to center 11/2 wavelengths at the operating frequency.

Radio frequency energy is distributed to the 18 layers of slots by a coaxial feed system in which the outer conductive tubular member 31 serves as the outer conductor of a coaxial transmission line, and an intermediate tubular member 35 within and coaxial with the tubular member 31 serves as the inner conductor for feeding one portion (the lower portion) of the antenna array. This intermediate conductive tubular member 35 also serves as the enclosing side wall of another transmission line for coupling radio frequency energy between the feed point and the associated radio frequency apparatus (not shown). This additional transmission line may be coaxial, in which case and inner conductor 37 is surrounded by and is coaxial with the intermediate tubular member 35.

The transmission line for coupling the radio frequency energy to the slots in the other portion the upper portion) of the antenna is formed by the outer slotted cylinder 31 as the outer conductor and a tube 39 coaxial therewith as the inner conductor. Coupling of the two portions of the antenna array to the energy feed transmission line 35-37 is accomplished by connecting the inner conductor 37 of the feeder transmission line 3:5*37 to the tubular inner conductor 39 of the upper portion of the antenna. Since the intermediate tubular member 35 forms the other conductor of the energy feed transmission line 35-37, wave energy is introduced (or taken from) the adjacent ends of the intermediate tubular member 35 and the tube 39. The structure may, therefore, be considered as two lengths of energy distribution transmission line 31-39 and 31-35 connected in series with respect to the feed point. The ends of the energy distribution transmission line 331-39 and 31-35 remote from the feed point were electrically connected together by means of metallic shorting blocks 43. While these shorting blocks 43 may be in the form of annular discs or the like, they are preferably made of a plurality of spoke members between the inner conductor 39 or 3S and the outer conductor 31 to allow the antenna to'be ventilated.

The feed point is displaced from the electrical center of the antenna array aperture so that the two portions of the antenna aperture are unequal in length. Each slot 33 is coupled to the interior of the energy distribution transmission line 31-35 and 31--39 by means of a coupling loop 41 having a reactance which is an integral part thereof. Such coupling loops for coupling radio frequency energy from a transmission line to a load are described and claimed in an application of Owen A. Fiet and Charles Polk, Serial No. 279,138, tiled March 28, 1952. Such coupling loops may include a capacitive reactance or an inductive reactance arranged in series with the loop and forming an integral part thereof.

The coupling loop 41 associated with each individual slot is used to make the coupling device and the slot to which it is coupled appear as a load impedance of a desired magnitude and phase as considered from the transmission line 31-35 or 31-39. y

In the specific embodiment, the coupling loops in the longer portion (the upper portion) were adjusted so that each one individually presented an impedance which, when taken together with the impedance presented by each of the other loops in the upper group, constituted one-half the impedance seen by the energy feed line 31, 37 at the feed point between the adjacent ends of the tubular members 35-39. Each coupling loop 41 associated with the slots 33 in the shorter portion of the aperture (the lower portion) was also adjustedy so that the total impedance presented by the slots 33 in the lower portion was the same as that of the upper portion. When radio frequency excitation was applied, since the impedances of the two ends of the energy distribution trans'- mission line 31-39 and 31-35 were equal and effectively in series across the feed point, equal amounts of power were distributed to the unequal numbers of radiating elements, that is, to the upper and lower portions. This asymmetrical power distribution acts to achieve the elimination of nulls in the vertical pattern characteristic and provides vertical radiation pattern closely approaching the described cosecant function of angular depression from the horizon.

While the specific embodiment above has been described as having a different number of elements placed either side of the feed point of the aperture, the invention may be practiced by having equal numbers of elements arranged over different lengths of aperture. ln this case, the same result applies: equal amounts of power are fed to different lengths of aperture so that when the two portions of the aperture are considered separately, the eld pattern characteristic of the portion with the longer aperture produces a sharper beam than that of the shorter portion. A caveat to be observed in this last arrangement is that the spacing of the elements in the longer of the two portions should still be close enough together that substantially uniform radio frequency illumination of the aperture length is obtained.

What is claimed is:

l. An antenna feed system for an antenna array having a plurality of antenna elements occupying an aperture a plurality of wavelengths long comprising: a feed point for Asaid antenna elements displaced from the center of said aperture whereby the length of aperture on one side of said feed point is greater by more than 5% of the total aperture length than the length of aperture on the other side of said feed point, transmission line means coupling all of the elements on said one side of said feed point to said feed point, further transmission line means coupling all of the elements on the other side of said feed point to said feed point in equal degree, whereby equal amounts of power are fed to the two groups of elements either side of said feed point.

2. An antenna feed system for an antenna array having a plurality of radiating elements occupying an aperture a plurality of wavelengths long comprising: a feed point for said radiating elements displaced from the electrical center of said aperture whereby the number of radiating elements on one side of said feed point is greater than the number of antenna elements on the other side of said feed point, and means coupling equal amounts of power to the two groups of elements either side of said feed point whereby an asymmetrical power distribution along the aperture of said antenna array is obtained.

3. An antenna feed system for an antenna array having a plurality of horizontally polarized antenna elements occupying a vertical aperture a plurality of wavelengths long comprising: a feed point for said antenna elements displaced from the physical center of said aperture, the number of antenna elements above said feed point being greater than the number of elements below said feed point, rst transmission line means coupling said feed point and all of the elements above said feed point, further transmission line means coupling said feed point coupling all of the elements below said feed point in equal degree to that of said first nsmission line means, whereby equal amounts of power are fed to the two groups of elements above and below said feed point.

4. An antenna feed system for antenna arrays which include a plurality of radiating elements occupying an aperture a plurality of wavelengths long comprising: a feed point for said antenna array displaced by more than 5% of the total aperture length from the physical and electrical center of said aperture, a first transmission line means from said feed point coupled to all of said plurality of antenna elements on one side of said feed point, said transmission line and said antenna elements coupled thereto having a predetermined impedance, second transmission line means from said feed point coupled to the remaining antenna elements on the other side of said feed point, said second transmission line means together with said antenna elements coupled thereto having the same said predetermined impedance, and means for coupling radio frequency apparatus to said first and second transmission lines at said feed point.

5. An antenna feed system for a slotted cylinder antenna having a plurality of longitudinal slots therein having a length a plurality of wavelengths long comprising: a tubular conductor coaxial with said slotted cylinder having each end connected to said slotted cylinder, said tubular conductor having an interruption therein intermediate the ends thereof to constitute a feed point for said antenna, said interruption being physically displaced from the center of said antenna whereby the length of said antenna on one side of said feed point is greater by more than 5% 0f the total aperture length than 'the length on the other side of said feed point, said slotted cylinder and said tubular conductor constituting transmission line means between said feed point and said slots, said transmission line means together with said slots coupled thereto having the same predetermined impedance to said feed point on either side of said feed point.

6. An antenna feed system for a slotted cylinder antenna having a plurality of longitudinal slots therein having a length a plurality of wavelengths long comprising: a tubular conductor coaxial with said slotted cylinder having each end connected to said slotted cylinder, said tubular conductor having an interruption therein intermediate the ends thereof to constitute a feed point for said antenna, said interruption being physically displaced by more than 5% of the total aperture length from the center of said antenna whereby the length of said antenna on one side of said feed point is greater than the length on the other side of said feed point, said slotted cylinder and said tubular conductor constituting transmission line means between said feed point and said slots, said transmission line means together with said slots coupled thereto having the same predetermined impedance to said feed point on either side of said feed point, said tubular conductor being hollow for at least the length of one of the portions formed by said interruption to effect the enclosing side wall of another transmission line, an additional conductor coaxial within and extending entirely through the hollow portion of said tubular conductor and having one end connected to the other of the portions formed by said interruption.

7. An antenna feed system for antenna arrays which include a plurality of horizontally polarized radiating elements occupying a vertical aperture a plurality of wavelengths long comprising: a feed point for said antenna. array displaced from the physical center of said aperture by a distance from 5% to 25% of the length of said aperture, a first transmission line means from said feed point coupled to all of said plurality of antenna elements on one side of said feed point, said transmission line and said antenna elements coupled thereto having a predetermined impedance, second transmission line means from said feed References Cited in the tile of this patent UNlTED STATES PATENTS 2,196,187 Blair Apr. 9, 1940 2,464,276 Varian Mar. l5, 1949 2,502,155 1950 2,658,143 Fiet et al. Nov. 3, 1953 OTHER REi-ERENCES Riblet: Microwave Omnidirectional Antennas, in Proc. IRE, vol. 35, No, 5, May 1947.

Fiet: New UFH-TV antenna in FM-TV, the Journal of Radio Communication, July 1952.

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