US2928087A - Omnidirectional beacon antenna - Google Patents

Description

March 8, 1960 E. G. PARKER OMNIDIRECTIONAL BEACON ANTENNA 4 Sheets-Sheet 1 Filed Aug. 19, 1957 March 8, 1960 EQ G. PARKER oNNrnIREcTIoNAL BEACON ANTENNA 4 Sheets-Sheet 2 Filed Aug. 19, 1957 Illvlllll March 8, 1960 Filed Aug. 19, 1957 E. G. PARKER 2,928,087

OMNIDIRECTIONAL BEACON ANTENNA y 4 Sheets-Sheet 3 aCos Cas@ o 1b' a'o s'o o s'o so v'o ao s? Inventor 25a/Ano 4A/gu', aim-fs Filed Aug. 19, 1957 E. G. PARKER 2,928,087

OMNIDIRECTIONAL BEACON ANTENNA March 8, 1960 4 Sheets-Sheet 4 6. 5 @Ao/,ws

7. .msn/A /O/v 4625, fwff's Inventor national Telephone and Telegraph Corporation, Nutley, NJ., a corporation of Maryland Application August 19, 1957, Serin No. 678,938 19 claims'. (Cian- 2106) IThis invention relatesto omnidirectional beacon antennas and more particularly toomnidirectional beacon antennas for use in producing ay multilobed radiation pattern having afundamental modulation frequency and one or more Aadditional harmonicsv of the fundamental frequency for use in radio navigation systems suchas that commonly known as TACAN, j Y Y Omnidirectional beacon systems such as in TACAN have a high order of directional accuracy which is dependent upon the :use of a directive antenna pattern rotatedat a, fundamental#frequencyV and modulated by a harmonic of fundamental frequency so. as to produce a generally; multilobedrotatingdirective radiation pattern. Due to the rotation of .thezlr'rrultiple-modulation,antenna pattern, areceiverlocated remotely from the transmitter receivesv energy which appearsl,asman amplitude-modulated wave having azfundlamental modulation component and a modulation componnttat `a,harmonic frequency of the fundamental. Both fundamenta- 1 and harmonic frequency. e refe"rencev signals" .transmitted lfor, comparison with the receiv.dcompo` lnt'sffof therotating pattern so that the, receiverrnay `dtermine its azimuth relative tothe beaconslan'teniia' system.`

gainat low angles iii ,the vertical plane, as compared to ja simple radiator. Sufficient modulation is easily obtained at lov., angleshut at hi lrangles, especially for the harf1 acrry level is. dimens is or car'e'rstrei'igth, can be reduced odlation strength, therela'tive Y VVb-yQtlievertical vstacking ke v `:e` i1,t'e 1, Howeven thisrr'iaylead o y, 7 `t,` thevertical,sta l:ir1glof,elements to form a Icarrier' p` only for the ,group prohibits the stacking of complte'groups at anloptimu'm spacing to obtain additional gain., S econd,"itfis diiiicult to keep the effective center of radiationgfrom1:shifting vertically which in tur may c'aus afd'et 'oratio ofver'tical coverag'e by; causing undesirable relative, phase `shifts to occur. lnjgantennafsfhaving" vertically staokedelements for the central Lgradiator, 1 thevrotatin'guparasite elements are mounted ondielectric cylinders with a 'tot-al vertical height equivalent to o the eentral radiator'. In'addition most previous antennas haveV been mad 'lativelyglarg'einf diameter to obtainthegdesired harmoiii'c-Qmodulatioii pattern. T hereforemsiichj antennas being'large iiiV height and diameter4 are' relatively bulkymand heavy, 'and therefore introduce complications inthe driving mechanism for rotating the SifuCllewr Y, f

e The` principal object ofthis invention isto provide an omnidirectional' beacon antenna,lhayin'g, af small height and diameter' while Qbtiliniug high carrier` vgain in the vertical pattern` and good; modulation characteristics.

One possible 'solution Lto" tlie'problemisfto use! a' horizointal array of 'eler'i'erits^-` to form" carrier! pattern. However, impossibility of iifrdiiaiigsxed'phase comnted States Patent 2,928,087 'atented Mar. 8, 1960 of the antenna system for producing thevcarrier,patternl comprises `a central radiating element and rotatingele-` ments, with coupling means including a transmission line to each rotating element for coupling energy to the rotating elements. The rotating elements i, are .preferably spaced approximately one wavelength from the cente and fedinphase with the centralradiator. L

According to a further aspect ofthe invention parasitic modulating elements are spaced less than three `half Wave-f lengths from the center, either along the same radiusas the carrier elements or equally spaced between the carrier elements. When the harmonic modulation component is the ninth harmonic, it is preferred to use nine parasitic modulating elements and nine rotating carrier elements. The elements may be quarter-wavelength elements onV a counterpoise or they may be half-wavelength center fed elements. ,Y ,i A According to a further aspect of the invention the means for coupling energy to the rotating carrier elementsrincludes vertical coupling elements spaced near the. center element. In the quarter-wavelength embodiment these coupling elements are connected at the .upper endvby .a coupling ring and the lower end is adjacent the counterpoise and connected to the transmissionline. The rotating carrier elements may be folded monopoles. In, the half-wave embodiment the transmission line is coupled between the centers of the' coupling elements and the centers of the rotating carrier elements, and the upper and lower edsgof the, coupling elements are connected by coupling rings. The rotating carrier elements may be folded dipoles. tu

The foregoing Aand other objects and features of this invention and the manner of attaining themrwill,becor'nel of Fig. l, with the portion below the counterpoise shown diagrammatically; i

Fig. 3 is a plan view of an embodiment using hlf- Wavele'ngth elements;

Fig. 4 is a cross-section View taken along lines 3 3 of'Fig. 4;

Fig. 5 is a diagram in perspective useful in explaining the formation of the radiation pattern; g

Fig. 6 is a vector diagram of the radiation components;

Fig 7 is a graph of two Bessel coeicients of the radiation components used in the analysis; and

Figs. 8 and 9 are computed and measured graphs respectively of the variation of carrier signal strength with elevation angle. Y

Referring to Figs. 1 and 2, an Vomnidiiectional beacon antenna is shown having a central quarter-wavelength radiating element 1 in the form of a cup, rotating carrier elements 2, a fundamental parasitic element 4a, and harmonic parasitic elements 4. The center element is supported by insulation 5 above a counterpoise 6, and the rotating elements 2, 3, and 4 are mounted on a counterpoise 7. The center stationary portion of the counterpoise has a skirt portion 8 and the rotating portion has a skirt portion 9, the skirts being a quarter-wavelength long forming' an R-F choke joint. The rotating'counte'rpoise 7 is mounted on a supporting structure shown diagrammatically at 10 which is rotated by a motor 11. A coaxial feedline 12 passes through the hollow shaft of the motor. Line 12 has its center conductor 13 connected to the center radiating element 1 and its outer conductor 14 connected to the stationary counterpoise 6.

The elements 2 are yfed by a transmission line 3 which is spaced from the counterpoise 7 by a spacer 15, which may be balsa wood. These transmission lines are connected, respectively, at their inner ends to vertical coupling elements 16. The upper ends of these elements 16 are connected by a ring 17 which is supported by a dielectric cylinder 18. The elements 2 are folded monopoles to match the impedance of the transmission lines 3.

In one embodiment designed for use in the band of 1150 to 1215 megacycles, the center radiator has a radius of 1%/4 inches, the cupling elements 16 are at a radius of 2%; inches, the rotating carrier elements 2 have their inner wire at a radius of 91%6 inches and their outer wire at a radius dk of l inches. The parasitic elements 4 are at a radius dm of 12 inches. The rotating elements and transmission line are formed from #18 A.W.G. wire.

An embodiment using printed half-wavelength elements, shown in Figs. 3 and 4, includes a stationary central radiator in the form of two cups 19 and 2t) fed by a transmission line 21. The inner conductor 22 of line 21 is connected to the upper portion 19 of the dipole and the outer conductor 22 is connected to the lower portion 23. The rotating portion 20 of the antenna includes carrier elements 24 and parasitic elements 25. The carrier elements 24 are in the form of folded dipoles fed by respective transmission lines 26, which are connected at their inner ends to the centers of respective vertical coupling elements 27. These coupling elements 27 are connected at their upper ends by a coupling ring 28 and at their lower ends by a coupling ring 29. The elements 24, 25 and 27 and lines 26 are printed on respective supports 30, which are mounted on a dielectric cylinder 31. A fundamental parasitic element 32 is mounted on an inner surface of cylinder 31. Cylinder 31 may be mounted on a rotating counterpoise 33. This counterpoise 33 has an inner skirt 34 and an outer skirt 35 which are a quarter wavelength and form R-F choke joints. The rotating assembly may be supported and driven by a structure such as that shown diagrammatically in Fig. 2.

In an embodiment designed for use in a band of 1150 to 1215 megacycles, the coupling elements 27 have a radius of 21/2 inches, the outer conductor of elements 24 is at a radius of inches, and the parasitic elements 25 are at a radius of 13 inches. The transmission lines 26 have the conductors spaced 1%6 inch and are mounted 5 inches above the counterpoise. The inner and outer conductors of the elements 24 are spaced 3A; inches. The vertical length of the elements 24 and 25 may be 4 inches.

A description of TACAN may be found in Electrical Communications, published by International Telephone and Telegraph Corporation, New York, New York, vol. 33, No. 1, March 1956, with the principles of antenna design on pages 35-59, the mathematical analysis of the derivation of the antenna pattern being found on pages 55-59. The theory of operation and the results obtained with the antenna of this invention may be explained by reference to Figs. 5 to 9.

Fig. 5 is a diagram in perspective of one rotating element at a distance d from a center element. For a distant receiver at zero bearing and an elevation angle B from the center element, the rotating element being at an angle 0 from zero bearing, the received signal will include a component C from the center element and a component R from the rotating element.

Fig. 6 is a simple vector diagram of the radiated signals. The center element radiates a carrier signel KC. The rotating element will produce a signal having two components KR and MR which are always in quadrature. The relative radiation phase if of KR with respect to KC depends upon the factors including the relative excitation phase and the phase angle of self-impedance of the clements. Analysis using the Bessel function expansion shows that the component KR includes a carrier component and all even harmonic modulation components of 0, and the component MR contains all the odd harmonic terms. With a carrier radio frequency equal to WK/Znthe resulting eld pattern at the distant point may be given by the following equation:

When nine rotating elements are used, the analysis shows that the odd harmonic terms other than the ninth and the even harmonic terms other than the carrier term are negligible for the values of radius d to be considered. Therefore, the coefcients in the equation given above may be expressed as follows:

MR=18Q RnB) Jgd es B) cos 9 o KR=9a fR(B) J0(d cos B) Kc is a function fc(B) of the vertical angle The term J0 (d cos B) is the Bessel term of order zero and argument (d cos B), and the term I9(d cos 0) is the Bessel term of order nine and argument (d cos B). The constant a depends on the relative signal strength from. the rotating and center elements. The expressions fR(B) and fC(B) are the space factors, or vertical patterns, of the individual rotating and center elements, respectively. If the angular rate of rotation elements about the central elements is wM, and p is the bearing angle to the receiver, then 0 is equal to (wMt-).

To obtain maximum amplitude modulation, it may be seen that the relative radiation phase \If should be centered around an odd integral number of 90, that is mr/ 2 with n an integer. The excitation phase with parasitic rotating elements is the spacing d in electrical distance. If the self-impedance phase is set equal to zero, the proper radiation phase for maximum modulation occurs at the odd numbers of quarter wavelength for the spacing d. The values of these odd integers may be referred to as modes.

A graph of the J0 and J9 Bessel coecient terms, according to wavelength and radians, is given in Fig. 7. The variation of the Bessel coefficient with vertical angle (d cos B) for any value of d may be obtained as indicated on the graph for eleven radians.

A maximum in the J9 curve occurs near eleven radians, which is in the seventh mode, and this spacing has often been chosen for parasitic modulating elements in previous antenna designs. At this spacing, the J9 term decreases as the vertical angle increases, the value at 35 being shown on the graph.

An inspection of Fig. 7 shows that ninth harmonie modulation may be obtained in the fifth mode, and that the carrier term J0 could be used to obtain gain if the correct phasing were used. Further, thes lope of I0 which occurs from about four to seven radians has a desirable slope such that for spacings of about seven radians, the value decreases as the vertical angle increases.

Fig. 8 is a calculated pattern using half-wave elements in free space at a radius of 61/2 radians. The curve Kc is the pattern, or space factor, for a half-wave center element. KR is the variation vof the Bessel term Io. The sum of these terms if added in phase is shown by the curve Kyi-KR. This curve is seen to predict high gain at low angles, dropping to a very low value at about 50. Since the curve KR does not take into consideration the space factor of the rotating element, the second lobe predicted by the curve Kc-l-KR should be very minor. Experimental results from a model using a spacing of 7.5 radians are shown by the curve in Fig. 9.

Reference to Fig. 6 shows that for carrier gain the relative radiation gb' should be near zero. With parasitic excitation the phase'rever'sal in conductive elements would produce the opposite radiation phase from that desired, and the amplitude would not be s uliicient. i Therefore,

some positive feed is necessary. Any coupling arrangement for obtaining this feed is considered to be within the scope of the invention. One arrangement found to be very satisfactory is that using rotating transmission lines between coupling elements and the rotating carrier elements'with coupling rings connecting the coupling elements, as shown in Figs. 1 to 4.

A counterpoise or ground plane will modify the patterns shown in Figs. 8 and9 by producing uptilt, so that the maximum carrier radiation occurs above Zero degrecs, or the horizon. v

This is especially true of embodiments using quarterwavelength elements on a counterpoise, as inFigs. l and 2; but also occurs to some extent with' half-wave elements above a' counterpoise, asin Figs. 3 and 4.Y The choice of embodiment will depend upon the particular location and conditions of use. It may be noted that groups of halfwave elements may be stacked above a single'counterpoise to increase the gain.

The embodiments of Figs. l to 4 use a radius of one wavelength at a frequency near the centerrof the band for the carrier elements. Y Y

The quadrature radiation component from the carrier elements will produce a small amount of ninth harmonic modulation. vIt is-necessary to supplement this by using parasitic elements at a spacing at which the .T9 term is greater, and which.. have the proper radiation phase. These elements maybe placed at about 8.5 radians, and either in line with the carrier elements or on radii midway between them. The .T9 curve in the fifth mode has a slope which produces a vertical pattern which approximates the slope ofthe resulting carrier pattern, thereby' producing relative modulation within the desired limits at vertical angles up to a high value.

Thus, according to the invention, an antenna is designed for operation in the fifth mode, with rotating carrier elements having positive feed in phase with the center radiator. Such an antenna has a small diameter and small vertical height, and the desirable characteristics of large diameter antennas with a vertically stacked central array. By using the fifth mode, an additional advantage is Obtained; the antenna operates satisfactorily over a broader band of frequencies, since the variation of phase with frequency isa smaller percentage at smaller diameters.

It should be noted that additional carrier elements may be used at greater radii to obtain still more gain, if proper points on the L, curve for the desired slope ane chosen, and the phasing is correct.

While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation tothe scope of my invention as set forth in the objects thereof and in the accompanying claims.

I claim:

1. An omnirange beacon antenna system comprising a vertically disposed vertically polarized central radiator, a verticallyI polarized carrier element vertically disposed for rotation about said central radiator, means for supporting said carrier element, means to rotate said supporting means, a source of input energy, rst Itransmission line means coupling radio-frequency energy from said source to said central radiator, and means for coupling energy from said source to said carrier element to obtain a radient-energy pattern having gain in the average value at low angles in the vertical planes.

2. An antenna system according to claim l, further including a parasitic element mounted on said supporting means for rotation about said central radiator to produce a modulation component in said radiant-energy pattern.

3. An antenna system according to claim l, including a plurality of said carrier elements, and further including a plurality of parasitic elements mounted on said supporting means for rotation about said central radiator to produce modulation in said radiant-energy pattern.

4. An antenna system according to claim 3, wherein said energy may be at any frequency within a given band, and said rotating carrier and parasitic elements are located at a radius of less than three half wavelengths at a frequency within said band.

5. An antenna system comprising a vertically disposed vertically polarized central radiator, a vertically polarized carrier element vertically disposed for rotation about said cen-tral'radiator, means for supporting said carrierelement, and means to rotate said supporting means, a source of input energy, rst transmission line means coupling energy from said source to said central radiator, and means including second transmission line means forcoupling energy from said source to said carrier'eler'nent to obtain a radiant-energy pattern havingrgain in the average value at low angles inthe vertical planes.

6. An antenna system according to claim 5, wherein said means f or coupling energy to said carrier element in cludes a vertically disposed coupling element mounted on said supporting means at a small radius compared to the radius to said carrier element, and said second transmission line couples said coupling element to said carrier element.

7. An antenna system according to claim 5, further including a parasitic element mounted on said supporting means for rotation about said central radiator to produce a modulation component in said radiant-energy pattern.

8. An antenna system according to claim 5, including .la plurality of said carrier elements, and further including a plurality of parasitic elements mounted on said supporting means for rotation about said central radiator to produce modulation in said radiant-energy pattern.

9. An antenna system according to claim 8, wherein said energy may be at any frequency within a Agiven band, and said rotating carrier and parasitic elements are located at a radius of less than three half wavelengths at a frequency within said band.

l0. An antenna system according to claim 9, wherein said supporting means and the elements mounted thereon are rotated at a given angular velocity, there being n said carrier elements equally spaced around a circumference at a radius of approximately one wavelength, said plurality of parasitic elements comprising one element for modulation at said angular velocity and a group of n. elements equally spaced around a circumference at a radius greater than one wavelength for producing modulation at the nth harmonic of said angular velocity.

l1. An antenna system according to claim l0, wherein iz is equal to nine.

12. An omnirange beacon antenna system comprising a vertically disposed vertically polarized central radiator, an outer vertically polarized element vertically disposed for rotation about said central radiator, means for supporting said outer element, means to rotate said supporting means, a source of input energy, first'transmission line means coupling radio-frequency energy from said source to said central radiator, and means including second transmission line means for coupling energy from said source to said outer element.

13. An antenna system comprising a vertically disposed vertically polaiized central radiator, supporting means, means to rotate said supporting means, a source of radio-frequency energy within a given frequency band, means coupling energy from said source to said central radiator, n carrier elements mounted on said supporting means equally spaced around a circumference at a radius from the center of the central radiator of less than three half wavelengths at a frequency within said band, n vertically disposed coupling elements mounted on said supporting means around a circumference at a small radius compared to the radius to said outer elements; a coupling ring connected to an end of each of said coupling elements, n transmission lines, each coupling one of said coupling elements to a corresponding one of said carrier elements, said antenna system having a radiantenergy pattern such that at a remote point at a low vertical angle it has substantial gain in the average value with respect to the central radiator pattern.

14. An antenna system comprising a vertically disposed vertically polarized central radiator, supporting means, means to rotate said supporting means at a given angular velocity, a source of radio-frequency energy within a given frequency band, means coupling energy from said source to said central radiator, n carrier elements mounted on said supporting means equally spaced around a circumference at a radius from the center of the central radiator of less than three half wavelengths at a frequency Within said band, n. vertically disposed coupling elements mounted on said supporting means around a circumference at a small radius compared to the radius to said outer elements, a coupling ring connected to an end of each of said coupling elements, n transmission lines, each coupling one of said coupling elements to a corresponding one of said carrier elements, and n parasitic elements equally spaced around a circumference at a radius of less than three half wavelengths at a frequency within said band, said antenna system having a radiant-energy pattern such that at a remote point at a low vertical angle it has substantial gain in the including a counterpoise, said central radiator, carrierelements and parasitic elements being quarter-Wavelength elements disposed above said counterpoise, and said transmission lines comprise respective conductors parallel to said counterpoise, each connected to a corresponding coupling element and carrier element.

18. An antenna system according to claim 14, wherein-4 said central radiator, carrier, and parasitic elements are each approximately a half-Wavelength long, and said transmission lines each comprise a pair of parallel conductors coupling the center of a coupling element to the center of a carrier element.

19. An antenna system according to claim 18, further including a counterpoise, with said central radiator, coupling elements, carrier elements, and parasitic elements spaced above said counter-poise.

References Cited in the file of this patent FOREIGN PATENTS 797,676 France Dec. 13, 1950 new", v-

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