Circularly polarized omnidirectional cone mounted spiral antenna



June s, 1965 J D. DYSON ETAL 3,188,643 v CIRCULARLY P'OLARIZED OMNIDIRECTIONAL CONE Filed Dec. 29, 1960 MOUNTED SPIRAL ANTENNA 2 Sheets-Sheet 2 CIRCULARLY PGLARIZED UMNIDIRECTIGNAL f CNE MUN'IED SPL ANTENNA John D. Dyson and Paul E. Mayes, Champaign, Iii., y assignors to The University of Illinois Foundation, a non-profit organization of Iliinois Filed Dec. 29, 196i), Ser. No. '79,432 It) Claims. (Cl. 343-895) This invention relates to antennas and, more particularly, it relates to antennas having conical beam or omnidirectional radiation patterns that are essentially independent of frequency over wide bandwidths. ' It is known that if the shape of an antenna were such that it could be specified entirely by angles, the antenna would make an ideal broadband radiator, since its operation would. be theoretically independent of frequency. All antennas which meet this criterion, however, are infinite in extent, so that it is necessary to specify at least one length for an antenna of finite size. By making this one length very large compared with the wavelength of operation of a given antenna, it is possible in some cases to achieve antenna performance which is practically independent of Wavelength over wide bandwidths. An antenna which can be made to have a very wide band of operations in which performance is independent of wavelength is the balanced equiangular spiral antenna. The one specified length for this antenna, the arm length, need not be large compared to a wavelength, kpendicularly to the plane of the antenna. As described in Dyson U.S. Patent 2,958,081, issued on February 11, 1960, the balanced equiangular spiral antennas described above can be modified to exhibit unidirectional radiation patterns while maintaining the broad bandwidths which such antennas possess. The unidirectional radiation patterns are achieved by wrapping a planar balanced equiangular spiral antenna on the surface of a cone and feeding the antenna at the apex thereof by a feed cable carried along one arm. Such antennas exhibit substantially symmetric radiation patf terns having a maximum on the antenna axis off the apex of the cone. In accordance with the invention it has been found that it is possible to obtain a conical beam mode of operation from the balanced conical antenna by constructing the antenna with an even number of arms more than 'two and connecting these arms to provide a suppression of the radiated fields on the axis of the antenna. The construction of the antennas of the invention will be better understood from the following detailed description thereof taken in conjunction with the accompanying diagrams in which: FIGURE 1 is a plan View of an antenna embodying the features of the invention; 'f FIGURE 2 is an enlarged detail of the apex of the antenna shown in FIGURE 1; FIGURE 3 is an enlarged detail looking down on the apex of the antenna of FIGURE 1; FIGURE 4 is a sketch of the coordinate system used inr obtaining radiation patterns for the antennas of the invention; and g'Jnitecl States Patent i. Btlli Patented June 8, 1965 FIGURES 5(51), 5(b), and 5 (c) are typical radiation patterns of the electric fields in the planes indicated. FIGURE 1 depicts a typical embodiment of the invention comprising four spiraling arms 11, 12, 13, and 14 which are wound on the surface of an electrically non-conductive cone 16. As shown in FIGURE 2, a small portion of the cone near the apex is truncated, since it would be physically impossible to construct this portion of the antenna because of the extremely small size of the arms in this region. FIGURE 3, a top view of a portion of the antenna of FIGURE 1, shows a projection thereof on a plane surface. In such a projection each of the arms of the antenna is defined by an equiangular (or logarithmic) spiral. An equiangular spiral is a plane curve which may be defined by the equation: where p and g5 are the conventional polar coordinates and a and k are positive constants. The equiangular spiral curve derives its name from one of its properties, namely, the fact that the angle (designated a) formed by a radius vector and a tangent to the curve at the point of intersection with the radius vector, is always constant. The value of a can be determined from the relationship The constant a thus determines the rate of spiral of the curve and the constant k determines the physical starting point of the curve when 95:0, as will be apparent to those skilled in the art. Considering the plane curves partially shown in FIG- URE 3 (that is, the projection of a portion of the outlines of the arms of the actual antenna), the outer and inner edges of every arm are defined by the same curve which is rotated about the origin or central point of the figure. Thus, for example, consider the outer edge of arm Il. This curve can be represented by an equation of the form The inner edge of arm Il is actually the same curve as that defining the outer edge, except that it has been d1splaced about the originy through an angle The equation of the inner edge of arm 11 may therefore be represented by pz-:k-lzK/)l where In a similar manner the curves defining arms I2, 13, and I4 are identical with those defining arm 11 but have been rotated through consecutive angles of (1r/2 radians) and thus have the equations ap-- p3= ke 2 for the outer edge of arm 12 and ane-a for the inner edge of arm I2, ansi-'t is the ilusieione angle. The. inner @des ,Qiarm ll'isderiefibya Y Y F [gebot-) Kip! where Y K/:e-bt n v f The yedges of the otherrarms (eg. 12) can be defined by the equations of the same type: ` #sme-'bz2 (outer :edge of arm 1 2), A and (inner ,edgeof arm 12). etc-i Y It can be .Ser-i1 that, for an included Cone angle tlf, .of 180 (Leitha planar' ferm Qf the antenna) the above equations reduced to the form j 91:19, 'P2=KP1 etc. as previously described.,` , "In contrastA to the balanced two-arm version-in which the arms arfe fed 180 out of phase, with the antennas of the invention the number of choices ofV feeding systems increases. lIn general, l-theantenna can be-'fed in anyA symmetrical mannerV which provides a supression Yo f the radiatedriields on the axis .of thefantenna.y VFor fexample, with a four-arm antenna this' can be achieved by connecting 'opposite arms together and feedingone pair againsttheother, i.e., 1180 Yout of phase. Such a sys- Vtern is shown in FIGURES 2 and 3, which `show a coaxial 'Hatteras a'rfefdsiredf Forthis reason .each Qfthe arms ris provided with a VdummyY coaxialjcablel forpreserving .A physical symmetry. As an alternative to thisr method ofY feeding, the antenna can befedbyi-ar balanced feed' line, or a coaxial-line and balun, placed on the axis of sym- The four-armantenna can valso be fed in accordance with the invention by exciting thentw'o pairs of arms with a 9.0 phase shiftl b'etweenthemi. Thus, forV example, if arm 11 of FIGURE 3 isiused as a datum, suppression of radiation along the antenna axis will be obtained if the excitations to the arms have the following phase relationships: Y Aim 11 0? Arm 14 90 Arm 13 180v Arm 12 v 270 Antennas having more than 4, eg., 6 arms, ca nibe'excited in methods similar to those described above. In oneme'thod a uniform delay Y is introducedV progressively between adjacent-armsf'amounting to 60 for a six-arm antenna. (In the general case of anvantenna having n arms, the phase change between successive arms would be where n is the number of arms.) Thus if one arm is taken as the datum andfas'suming that the arms spiral outwardly in a clockwise direction when viewed from the top or apex of the cone, successive arms proceeding inra counand isV usedzonlyto support the arms, it can ibe eliminated ter clockwise directionl would have the phase relationships: 0,'-"60, '-120", -.180, -240, -300. If thearms spiraled in the opposite direction to that assumed, lthe relative phaseV shift would alsoA be vreversed and appliedtorthe arms in a clockwise direction.y In another suitable lmethod' forfeeding an antenna of the invention, the phase shifts V1.80,? ebetweenfarms, In this c ase, for a six-armL antenna successive arms would have the phase relationships; 0"', -k-l'80, 0, -180, 0, V-'l80. This manner of feeding also can be applied generallyto an `antenna `having any even number of arms. Although in general antennas of the invention with Yonlyfour arms/will rbe'satistactory-.for many practical purposes, a greater number ofa-rms'can'be used-to provide more uniform pattern Vand far `iield'phase characteristics as a function ofthe azimuthalanglee (FIGURE 4). As shown in FIGURE lythe antennas of the-,invention do not radiate inan axial direction, so thatthe radiation patternis conical in shape. By suitable selectionlof the Yantenna parameters, particularly the angle or governing the rate of spiral, the angle of orientationof the maxi- VVV'mum radiation off the axis of `the antennaY can be made to assume values over a widerangel. Thus, for example, the angle of orientation 0 lofi the beam maximum can be varied through the range from about 40 to more than 90 -by yvarying or from about 75" to about 45', for cones having a cone angle ,gl/,of about 15720". When the beam maximum ris located at 0:90", the antenna lconstitutes a simple, very broad band, eircularly polarized, omnidirectionalradiation source. f e A Thearms of the antenna are made of anelectrically conducting material, suitably sheet copper, aluminum, or the like. The antennas canbe `constructed in any suit- Yablemanner.` A preferred method is by drawing the outline of the arms on the development of the leone, which drawingis thentransferred by a vsilk screen lprocess to a thin copper-clad Teflon impregnated` glass cloth. After 'forming-the Yarms by etching away theiurndesiredr portions of the copper cladding, the base material is formed into a cone and Ithe arms are soldered along the joint. The cone thus formed can be supported by any ofthewellknown materials which vare eicient insulators Aat high frequencies, suchasn polystyrene.- Since the coneA of insulating material is not anv essential partof the antenna if if the arms are madelof a'rigid material, such as a sheet @of copper stnongfeno'ugh to support its ownuweight. " The` vupper. frequency limitrof Athe 4band/ofoperation for the. antennasoof the` invention is determined by the '.nenessofl the construction of the yspiral atathe feedpoint, .i.e.,'atthefvertex of the cone. .i Since equiangular lspiral curves, converge toa point as a limit-atthe origin, it is necessary in a .practical structure yto terminate the central portion ina small straight or tapered section'. The 4upper -cut-off frequency ofV the antenna .is kthe frequency at which ',the'truncated apex of thecone.ibecomes'approximately 1A wave-length in diameter. As an example, for a termination` vof the Yapex at a I-inChdameter the upper frequency limit is approximately-30004000mc.. ` The operatingbandwidth is at the control of the designer. The low frequency limitsf'are effected by the length and thefwid'th of thearms of the antennas and the low frequency limit can be lowered by increasing the arm ylength and/ or by increasing the width of the arms as in the case of the balanced two-arm conical antennas. ' Although in the preferred form the antennas of the invention have arms each edge of which is defined by an equiangular spiral, so that the width of each arm constantly increases at increasing distances from the apex of the cone, it is also possible to construct practical antennas having advantageously wide `bandwidths in which the width of the arm is constant or essentially constant. Thus, for example, antennas in which the arms consist of y coaxial cables alone, arranged to follow equiangular spiral paths, may have acceptable patterns, particularly if a is large, e.g., 60 or more. These wire versions of the antenna can most conveniently be constructed from rigid wall coaxial cable and are advantageous for use at the UHF and VHF frequencies. It is also possible to construct practical antennas having advantageously wide bandwidths in which the Iarms are constructed to follow an Archimedes spiral, which is defined by the equation Although these antennas are not frequency independent, they also produce conical beam or omnidirectional patterns with the excitations previously described. For antennas based on the Archimedes spiral the angle between the axis of the antenna and the maximum of radiation varies with the frequency of operation. The practical results which are obtainable with the kantennas of the invention are demonstrated by a four arm antenna constructed on a 15 coney with a diameter of 31 cm. at its base and further defined by the parameters a=45 and K=0.925. This antenna was etched from flexible, copper-clad, Teflon-impregnated, fiberglass material and then formed into a cone. The feed cable was RGl4l/U. The energized cable was carried along one arm; dummy cables were on the other arms to maintain structural symmetry. Typical radiation patterns of this antenna are shown in FIGURE 5 for a frequency of 1200 mc. The patterns shown are for E, and E, polarized fields. FIGURES 5(a) and 5 (b) are pattern cuts through the axis of the antenna at right angles to each other; FIGURE 5 (c) is for a cut perpendicular to the axis, on the 0=90 plane. Antennas in accordance with the invention can be made to have bandwidths of -t-o 2O or more to 1 over which the radiation patterns and input impedance are essentially constant. In order to obtain the maximum bandwidth, however, it should be noted that these are balanced antennas and a balanced feed is necessary for optimum performance. The feed may be brought in along the axis of the antenna by using a balanced feed line or by an unbalanced line and balancing transformer or balun. The bandwidth of this latter method, of course, depends upon the bandwidth of the balun. The rapid decay of the current along the arms, however, makes possible the previously mentioned highly useful method of feeding the balanced antenna with an unbalanced transmission line. Since the ends of the antenna arms do not carry appreciable antenna currents except at the very lowest frequency of operation, the arms themselves act as an infinite balun, the feed terminals are isolated from ground in a balanced manner and the outside of the feed cable beyond the antenna arms does not carry a significant amount of antenna current. However, as the frequency Iof operation is decreased a point will be reached where the presence and location of this cable alters the radiation pattern.. This frequency, however, is below that at which the antenna should be expected to operate satisfactorily. In order to compensate for the presence of the feed cable insofar as possible, a dummy cable may be placed on the other arm to maintain physical symmetry. The input impedance ofthe antennas of the invention remains relatively constant over a wider frequency range than the usable pattern bandwidth. The foregoing detailed description has been given for clearness of understanding only, and no unnecessary limitations should be understood therefrom, as modificattions will be obvious to those skilled in the art. What is claimed is: 1. A broadband antenna comprising an even number greater than 2 of substantially identical electrically conducting elements wound so as to lie on a conical surface having an included cone apex angle in the range between approximately 15 and 20, at least one of said elements being electrically insulated from the remainder, each of said elements having at least one edge lwhich projected on a plane perpendicular to the axis of said conical sur-` face is substantially in the form of a spiral, said elements being symmetrically placed about the axis of said conical surface. 2. The antenna of claim 1 wherein said spiral is an equiangular spiral. 3. The antenna of claim l wherein the number of said arms is 4. 4. The antenna of claim 1 which is fed at the apex by a feed cable which is carried on 4one of said elements. 5. The antenna of claim 4 in which a dummy feed cable is carried by each of said other elements. 6. A broadband antenna comprising an even number greater than 2 of substantially identical electrically conducting elements wound so as to lie on a conical surface having an included cone apex angle in the range between approximately 15 and 20, at least one of said elements being electrically insulated from the remainder, each of said elements being dened by a pair of curves which when projected on a plane perpendicular to the axis of said conical surface have the form of equiangular spirals, said elements being symmetrically placed about the axis of said conical surface. 7. The antenna of claim 6 which is formed of thin electrically conducting sheet material. 8. The antenna of claim 6 wherein the number of said arms is 4. 9. The antenna of claim 6 which is fed at the apex by a feed cable which is carried on one of said elements. 10. The antenna of claim 9 in which a dummy :feed cable is carried by each of said other elements. References Cited by the Examiner UNITED STATES PATENTS 2,640,928 6/53 Kandoian 343-908 2,958,081 10/ 60 Dyson 343--895 2,990,548 6/61 Wheeler 343--895 3,019,439 6/62 Reis et al 343-895 X OTHER REFERENCES Research Studies on Problems Relating to E.C.M. Antennas, AF33 (616), 3220, Report No. 9 published- Wright Field, Feb. 20, 1958. HERMAN KARL SAALBACH, Primary Examiner. GEORGE N. WESTBY, ELI LIEBERMAN, Examiners. UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,188,643 June 8, 1965 John D. Dyson et al. It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below. Column 2, lines 32 and 33, the equation should appear as shown below instead of as in the patent: t -i an Ct-a same column 2, lines 67 and 68, the equation should appear as shown below instead of as in the patent: Signed and sealed this 2nd tiny of November 1965. (SEAL) Attest: ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents



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    US-2958081-AOctober 25, 1960Univ IllinoisUnidirectional broadband antenna comprising modified balanced equiangular spiral
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