US 2979719 A
Description (OCR text may contain errors)
April 11, 1961 H. AVERY ErAL OMNIDIRECTIONAL BEACON ANTENNA 6 Sheets-Sheet 1 Filed Oct. l0, 1957 Inventors Hou/Ana AVERY ann/a ff. k/A/sM/vo, JR, ERNEST q. PAR/ 5@ Byf/ 62% Attorney April l1, 1961 H. AVERY Erm. 2,979,719
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OMNIDIRECTIONAL BEACON ANTENNA Filed Oct. 10, 1957 6 Sheets-Sheet 4 PCos 0 "I o Q@ .9 QCos 35 .O I1 l C kL! O s 5 0.5 I I I I Si a 3^ a s a 4 4 e 4 4 2 fF/cfc/Vf SPAC//VG /IV WAVfZ-/VGTHS A ttorn e y April 11, 1961 H. AVERY ET AL 2,979,719
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A Harney April ll, 1961 H. AVERY ETAL 2,979,719
oMNIDIREcTIoNAL BEACON ANTENNA Filed oct. 1o, 1957 e sheets-sheet e I 0 lIl ."l' I) l O MW/N N se, a e o S2 Nou V 7n 0o 1^/ Inventors A Harney 2,979,719 `OMNIDIREC'I'IONAL BEACON ANTENNA Howard Avery, Paramus, and Daniel H. Kingsland, Jr.,
land Ernest G. Parker, Morristown, NJ., assignors to International Telephone and Telegraph Corporation, Nutley, NJ., a corporation of Maryland Filed oci. 1o, 19,57, ser. No. seo-,isi 6 claim. (ci. 343-761) Vharmonic of this fundamental frequency so as to produce a generally multilobed rotating directive radiation pattern. The antennas usually consist of av central omnidirectional radiator surrounded by radiation pattern moditying elements 'adapted to revolve around the central radiator. Dueto the rotation of the multiple-modulation antenna pattern, a receiver located remotely from the transmitter receives energy which appears as an amplitudemodulated wave having a fundamental modulation component' and a modulation component at a harmonic frequency of the fundamental. Both fundamental and harmonic frequency reference signals are transmitted for comparison with the received components of the rotating pattern so that the receiver may determine its azimuth relative to the beacons antenna system.
Antennas of this type are usually designed to achieve their. greatest gains at low angles in the vertical plane. Suicent modulation is easily obtained at these low angles, but at high angles, especially for the harmonic modulation, a satisfactory level is `diiiicult to obtain. If at high angles the carrier strength canl be reduced while main'- tainingl the modulation strength, the relative modulation is increased. -Such a carrierpatternl may be obtained by `the lvertical stacking of elements to form an array for'tl'ie., central radiator. With this type of central*Y radia't'or,'it has been-they practice to use vvlong resistive wires 'as parasitic modu-latingelements. TheseY wires are` vertically disposed on A a rotating dielectric cylinder, and extend substantially thelengtlr off thecentralarray. However, these long parasitic elementsz have some disadvantages in obtaining optimum" modulation characteristics over the desired frequency' banjd, andV at high vertical angles.` For-example, the spacing from the centerlfor proper excitation does not vcoincide with the best spacing for the .vertical pattern. jAlso the standingwave of cur- Irentrset up 4near the ends of the longv wire parasite is notfinsignicant, effecting a; cons-iderable change incurrentdi's=tribution along the length'of the parasite; There'- fore, the RF. phase and radiation pattern frornthe parasites changes rapidly: with'ffrequ'ency, producinga large 'change' iiitniodulation level'.
principalobject"ofv1 this invention isto'provide an improvedntenna foragbeaconsystem, of theftype with a verticallystackedj array asacentral` radiator, having goodmodulation.l levelsrfoverz a broad bandl of frequencies and, in vertical coverage..-v j v v Y. Y
L, According@ to the principal aspect of the invention, an
2,979,719 `Patented Apr. l1, i961' ICCv antenna having a vertically stacked central array is provided with a plurality'of parasitic modulating elements, at least some of which are limited to approximately a half wavelength long to permit adjustment of the parasites to obtain greatly improved characteristics.
Further, the exact length of some of the parasites may differ from a half wavelength by a substantial portion of a quarter wavelength, that is, the total length of each individual element may be between one and three quarter wavelengths, therebyaffecting its current phase and permitting the radiation phase of the total modulation component to be adjusted as desired. Using short elements also permits the elements to be located vertically to place the radiation centers at the best points. With short elements the radiation center does not shift appreciably for changes in frequency, as it does with long elements.
In addition to length and vertical location, the radius from center, resistance and number of the elements may be adjusted as in previous designs. j
The foregoing and other objects and features of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to tlhe following description of embodiments of the invention taken in conjunction with the accompanying drawings comprising Figs. l to 1l, wherein:
Fig. l is a fragmentary perspective view, partly diagrammatic, of a beacon antenna system embodying the principles o-f this invention;
Fig. 2 is a cross-section view, partly diagrammatic taken along lines 2 2 of Fig. 1; i
Figs. 3 and 4 are views of the harmonic and funda'- mental parasite arrangements, respectively, of the antenna system of Figs. l and 2;
Fig. 5 is a view of a harmonic parasite arrangement for an alternative antenna system;
Figs. 6 and 7 are diagrams in horizontal and vertical planes, respectively, useful in explaining the formation of the radiation pattern;
Fig. 8 is a vector diagram of the radiation components;
Fig. 9 is a graph of two Bessel coeflicients of the radiation components; and
Figs. 10` and 11 are graphs showing radiation patterns obtained with the antenna system of Figs. 1 to 4.
A description of TACAN may be found in Electrical Communications published by International Telephone and Telegraph Corporation, New York, New York, vol'- urne 33, No. 1, March 1956, with the principles of antenna design on pages 35-59.
Referring to Figs. 1 and 2, a beacon antenna system is shown comprising a stationary central array 1 enclosed in a dielectric cylinder 2, and surrounded by a rotating structure which includes a group of fundamental para'- sitic modulating elements 3 on a dielectric cylinder 4 and nine groups of harmonic parasitic modulating elements 5 on Aa dielectric cylinder 6.
The central array 1 comprises four vertically disposed and vertically aligned center-fed half wavelength radiating elements 7 to 10, each consisting of two tubular quarter Vwavelength sections. The feed system is generally similar to that described on pages 42 and 43 of the Electrical Communications publication. A coaxial line having an inner conductor 11 and an outer conductor 12 is coupled at its lower end -to a source of R.F. energy not shown.r The upper end 13 of inner conductor 11,-reduced in cross-section, is`inserted into and connected to anv upper rod conductor 14. The rod 14 and tubular con# ductor 12Y are equal in outside diameter and separated to form a main feed point 15. Rod 14 and the outside surface of conductor 1'2 then form the innerconductors of coaxial transmission'lines, the outer conductors being formed by tubular sections 1'5, 16 and 17. The respectivefends of` tubular sections 15, 16 and 17 are'connec'ted 3 by flange portions 1S to respective sections vof the radiating elements as shown. The upper section of radiating element 7 has a ange portion 19 connecting it to conductor 14, and` the lower section of element 10 has a flange portion A20 connecting it to conductor 12. A conductive sleeve 21 surrounding conductor `14 y and a conductive sleeve 22 surrounding conductor 12 serve as impedance transformers in the transmission lines. The R.F. power flows in transmission line 11-12 to feed point 15 and divides between transmission lines 14 16 and 12-16. At the center feed point of element 8 the energy divides between radiating element 8 andby transformer 21 to transmission line 14 15 and thence to radiating element 7. At the center feed point of element 9 the energy divides between radiating element 9- andk by transformer 22 to transmission line 17a-17v and thence to radiating A element 10. The energy to the radiating elements 7 to 10, respectively, divides approximately in the proportion 1, 2, 2, 1 and phase relation 45, 30, +30, 45, respectively.
'Ihe rotating dielectric cylinders 4 and 6 have an upper dielectric cap 23 and are supported on a conductive counterpoise surface 24. The rota-ting assembly is mounted on a supporting structure 25, is supported on the inner stationary structure by bearings not shown, and is driven at 900 r.p.m. by an arrangement shown as a motor 26 and a gear arrangement 27. The entire antenna unit is covered by a stationary dielectric radome 28 on a support 29, and is mounted on a base, not shown.
The inner parasitic elements 3 produce 15 c.p.s. modulation, and the outer parasitic elements 5 produce the ninth harmonic modulation at 135 c.p.s. Fig. 3 shows one of the nine groups of 13S-cycle parasites 5 on a porwith a beacon antenna in the form having a central array of seven vertically aligned biconical elements as disclosed in the Electrical Communications publication. The structure will be similar in all respects as disclosed there except for the -arrangement of the parasites on the rotating cylinders. The upper counterpoise below the top radiating element is also removed. Fig. 5 shows an arrangement for one of the nine groups of 135 c.p.s. parasites mounted on the outer rotating cylinder. Three elements having their bottom ends 31/2 inches above the bottom edge of the cylinder include a long 341/z-inch center element 40 and two 41/2 inch elements 41 and 42, 1/2 inch either side of element 40. Seven elements having their bottom ends 431/2 inches above the bottom edge of the cylinder include a 7inch center element 43 in vertical alignment with element 40, and six 41Ainch elements including two elements 44 spaced 1/2 inch from element 43 and four elements 45 each spaced 3%: inch from the adjacent elements. All of the elements are of Nichrome wire, element 43 having resistance of 450 ohms per foot, and the remaining elements having a resistance of 700 ohms per foot.
The mathematical analysis of the derivation of the antenna pattern for TACAN may be found on pages 55-59 of the Electrical Communications publication.
- The theory of operation and the results obtained with Ation of the dielectric cylinder- 6, and Fig. 4 shows the ,l5-cycle parasites 3 on a portion of dielectric cylinder 4.-
In an embodiment of the antenna system for operation in the frequency bandfof 960 mc. to 1025 mc., the outer cylinder 7 may have an outer diameter of 40% inches. Each group of the parasites 5 (Fig. 3) includes elements 30 to 32 with their bottom edges 17V; inches from the bottom of the cylinder, with element 30, 7 inches long and elements 31 and 32, each 5 inches long and spaced .1/2 inch from element 30. An element 33 in vertical alignment with element 3l]l is 4% inches long with its center spaced38% inches from the bottom of the cylinder. An element 34 is spaced 20 degrees from elements 30 and 33, is 7 inches long and is spaced with its bottom edge 41/2 inches below the center of element 33. Elements 30 and 33 are of #30 A.W.G. copper wire, element 34 is of #24 A.W.G. copper wire and elements 31 and 32 are of Nichrome wire having a resistance of 450 ohms per foot.
The inner rotating cylinder 4 has an outside diameter of 5 inches. The fundamental parasite group 3 (Fig. 4) includes elements 35 and 36 in vertical alignment spaced 2% inches apart, each 61/2 inches long with the point midway between them 2113716 inches above the counterpoise surface. V-shaped elements 37 and 38 have legs 31A inches long forming an angle of 60 degrees, with the center points each inch from the point midway between elements 35 and 36. Elements 37 and 38 serve to increase the coupling between elements 35 and 36. Element 39 in vertical alignment with elements 35 and 36 is 4% inches long with its bottom end 35% inches above the counterpoise surface. All of the elements 35 to 39 are of #36 A.W.G. copper wire.
The approximate distance between the centers of the half wave elements 7 and 8, is 11 inches or about one wavelength; between elements 8 and 9, 7 inches or about 0.6 wavelength; and between elements 9 and 10, 1l inches. The bottom of element 10' is 213/16 inches above the counterpoise surface, and 5%@ inches above the bottom edge of the outer rotating cylinder 6.
The principles of this invention may also be employed the antenna of this invention may be explained by reference to Figs. 6 to 1l. i
Fig. 6 is a diagram of the projection in a horizontal plane of one'central radiating element and one rotating parasitic element ata horizontal distance p from the center. A distant receiver is at an angle from a point due south of the antenna. The parasite has an angular displacement 0 from aline drawn to the receiver from the central radiator. The received signal includes a component C from the central element and a component P from the parasite.k s
' Fig. 7 is a diagram in the vertical plane of the two elements lof Fig.Y 6, with 0 equal to zero. Considering the centers of the elements, the parasite is displaced a vertical distance h above the central element. The receiver is at a vertical angle B from the horizontal plane of the antenna. .The direct distance d between the elements is equal to \/h 2+p2.
Fig. 8 is a simple vector diagram of the radiated signals. The center element radiates a carrier signal Kc. The rotating `element will produce a signal having two componentsKp and Mp which are always in quadrature. There is a relative current phase a between the parasites and the central element which depends upon factors including the relative excitation phase and the phase angle of self-impedance of the elements. If the radiation centers xof the center and parasitic elements are in a single plane, the relative radiation phase 1p of Kp with respect to Kc is equal to the relative current phase a. However, by displacing the parasites a vertical distance h, the radiation phase of the parasites becomes Analysis using the Bessel function expansion shows that the component Kp includes a carrier component and all even harmonic modulation components of 0, and the component Mp contains all the odd harmonic terms. With a carrier radio frequency equal to wk/21r, the resulting field pattern at the distance point may be given by the following equation:
E= (Mp sin mp4-KP cos :p4-Kc) cos wkt -l-(MP cos 111+Kp sin 1p) sin wki The radiation pattern for each element is assumed to be circular in the horizontal plane, and in the vertical plane to bey functions F(B) and f(B) of the angle B for the center element and parasite respectively. Assuming one center element and nine parasites equally spaced around a cylinder at a radius p and angle B, the analysis shows that the oddharmoni'c terms other than the ninth and the even harmonic terms other than the carrier term are negligible for the values of radius p to be considered,y and therefore the coefficients in the equation given above may be expressed as follows:
MP =18af(B)J9(p cos B) cos 90 Kp=9af(B)]0(p cos B) Kc is a function F (B) of the vertical angle The term J0(p cos B) is the Bessel term of order zero and argument (p cos B), and the term .19(p cos B) is the Bessel term of order nine and argument (p cos B). The constanta depends on the relative signal strength from the rotating and center elements. If the angular rate of rotation about the central element is wN, then 0 is equal to (WMI-rp).
From Fig. 8 it may be seen that the carrier components KC and KP will combine to produce a resultant KR. For most effective amplitude modulation, the modulation component MP should be at 0 or 180 to KR. This occurs when the relative radiation phase gb is centered around an odd integral number of 90, that is, mr/Z with n an odd integer, with a slight adjustment because of the effect of KP. The radiation phase of an element depends on its current phase, the determining factors of which include the excitation phase and the self-impedance of the element. The excitation phase withparasitic 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.
A graph of the J0 and I9 Bessel coeflicient terms, according to wavelength, is given in Fig. 9. A maximum in the J9 curve and Aa point of proper excitation phase occurs near seven quarter wavelengths. However, the effective value of spacing for radiated modulation signals decreases in proportion to the cosine of the elevation angle B. With h equal to zero and equal to M wavelengths, the J9 term drops rapidly as the vertical angle is increased. The graph shows the determination of the value for an angle of 35 at point X on the J9 curve. For good high angle coverage, it is desirable to use a spacing which is well past the maximum on the J9 curve. In the prior art antennas with long parasites, a compromise spacing was chosen so that the point of operation at 0 was between the two optimum values. The result was that for 0 elevation the excitation is late, causing reduction in modulation at the high frequency end of the band and increased modulation at the low end. Further, due to the shift along the Bessel curve, there is a more rapid decrease in modulation with elevation angle than would ideally occur.
However, by using short parasites with their radiation centers displaced vertically by a distance h, the radiation phase it becomes a function of the vertical angle B, as explained above. This factor may be designed to compensate for the drop in the Bessel function with vertical angle, and thereby greatly improve the vertical coverage.
Also, with long resistive parasites, the standing wave of current set up near the ends of the parasites is not insignificant. Although the use of high resistance wire for parasites tends to minimize this effect, it is now apparent that the standing wave is suicient to effect considerable change in the current distribution along the length of the parasite. The result is that ythe R.F. phase and the pattern of the radiation from the parasites changes rapidly with frequency. Due to the large slope of the sideband and carrier lobes around zero, this slight shift in pattern produces a large change in modulation level.
At higher angles, the effect of radio frequency (R.F.) phase shift is more important. A relative shift of 90 for the R.F. phase of radiation from the parasites will result in complete obliteration of the ninth harmonic modulation. This means that the equiphase surfaces for the ,radiation from the central array and frornthe parasites must conform within an inch or `two .for all points in space where modulation is to be received. For large vertical apertures, this becomes difficult to achieve at high angles, due to the vertical shifts in the effective center of radiation produced by the changing current distribution in the parasites.
According to the invention short parasites approxi mately a half wavelength long are used topermit adjustment to obtain the optimum spacing and radiation phase for good vertical coverage and broadband modulation characteristics. The short parasites have definite centers of radiation which do not shift appreciably as the frequency is changed.
The current IP in a parasite having an impedance ZP and excited by a voltage Ep is given by IP=Ep/ZP. The strength and phase of Ep depend on the distance d and the current in the central radiator. The factors determining ZP include the resistance and length of the parasite.
It is to be noted that in an antenna system as disclosed in this application, each of the parasites is in the radiation lield of each of the central array elements.
The radiation pattern for the antenna system is the resultant of all of the central and parasitic radiating elements. The diagram of Fig. 8 may be used to represent the total radiation components from the central array and the parasites for one modulation frequency. The total central array component is the KC, and the total parasite components are KP and Mp. It is desirable that MP be nearly at 0 or 180 with respect to KR, and that the depth of amplitude modulation, determined by the ratio of the properly phased component 0f Mp to KR, be between l2. and 30 percent, at all frequencies within the band and up to high vertical angles.
The factors which determine the value and phase of MP include the number, resistance, and length of the wires used for the parasites, their spacing from the center, and their vertical location on the cylinder. yIt is usually desirable to keep KP to a minimum. The J0 curve shows the effect of the spacing on this quadrature carrier term. The value of KC varies with vertical angles, and is determined by factors such as the spacing of the central array elements, the feed system, and the driving power.
The antenna system of Figs. l to 4 has a carrier radiation pattern which varies with vertical angle as shown by the curve of Fig. 10. The percentage of modulation is shown in Fig. 11 with the l5-cycle modulation at 990 mc. shown by curve 46, and the 13S-cycle modulation at 960, 990, and 1025mc. shown by curves 47, 48, and 49, respectively.
While we have described above the principles of this invention in connection with specic apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of our invention as set forth in the objects thereof and in the accompanying claims.
l. An antenna system comprising the combination of a central array having a plurality of radiating elements disposed in vertical alignment coupled to a source of input energy feeding a greater amount of energy at different phases to the middle ones of said radiating elements than is coupled to the upper a-nd lower ones of said radiating elements, the vertical height of said array being substantially greater than a wavelength and a plurality of parasitic elements disposed around said array at a given radial distance therefrom, each parasitic element being located within the radiation field of each radiation element, the majority of said parasitic elements being located radially of said middle radiating elements with at least some of said parasitic elements being approximately a half wavelength long, said parasitic elements being distributed in vertically disposed arrangements along the length of said central array, the distance 7 between any two adjacent parasitic elements in each of said -arrangements notrexceeding one wavelength.
V2. A4 system asin claim l, in which said radiating elements are vertically disposed half wavelength elements.
3. A system as in claim 1, in which said parasitic elements are disposed for rotation about said central array to produce a space modulated radiation pattern and in which at least some Vo said parasitic elements are between one quarter and three quarter wavelengths long.,`
4. A system as in claim 3, further including a second plurality of parasitic elements at a second radial distance from said array and disposed for rotation thereabout, said second group of parasitic elements being distributed inf vertically disposed arrangements along the length of said central array, the distance between any two adjacent parasitic elements in each of said arrangements not exceeding one wavelength, the majority of said second group of parasitic elements being located radially of said middle radiating elements and at least some of said second group of parasitic elements being between one quarter and three quarter wavelengths long. 5. A system as in claim 3, further including a second plurality of parasitic elements at a second radial distance from said array and disposed for rotation thereabout, said second group of parasitic elements being distributed in vertically disposed arrangements along the length of said central array, the distance between any two adjacent parasitic elements in each of said arrangements not exceeding one wavelength, the majority of said second group of parasitic elements being located radially of the upper radiating elements of said central array, the greater number of said majority being less than a half wavelength long and the remaining elements of said second group including some elements approximately three wavelengths long,
6. A beacon antenna system producing a multilobed rotating pattern of radiation having a fundamental lobe and harmonic lobes comprising the combination of a central array having a plurality of half wavelength elements vertically disposed with relation to each other, 4
coupled to a source of carrier frequency energy feeding a greater amountof said energy at different phases to the middle ones of said radiating elements than is coupled to the upper and lower ones of said radiating elements, a first plurality of parasitic elements at a given radialdistance from said central array disposed for rotation around said array to produce said fundamental lobe, said first plurality of elements consisting of vertically disposed members and V-shaped members, said V-shaped members and the majority of said vertically disposed members of said first group being located radially of said middle radiating elements, a second group of parasitic elements at a second radial distance from said array disposed for rotation around said array to produce said harmonic lobes, said second group consisting of closely spaced vertically disposed elements located radially of said middle radiating elements and widely spaced vertically disposed elements located radially of the upper radiating elements, at least some of the elements of said iirst and second groups being between one quarter and three quarter wavelengths long, and means for rotating said tirst and second groups about said central array to produce said rotating multilobed pattern, each of said first and second groups of parasitic elements being distributed in vertically disposed arrangements along the length of said central array, the distance between any two adjacent parasitic elements in each of said arrangements not exceeding one wavelength.
References Cited in the le of this patent UNITED STATES PATENTS 2,486,597 Greene Nov. 1, 1949 2,726,389 Taylor Dec. 6, 1955 2,767,397 Byrne Oct. 16, 1956 2,836,820 Pickles May 27, 1958 2,889,552 Thomas et al. June 2, 1959 OTHER REFERENCES Pub, I: Antenna for the AN/URN-3 TACAN Beacon, Electrical Communications, vol. 33, March 1956, pp. 35-59, pp. 37, 38, and 42-46.