US 3618114 A Description (OCR text may contain errors) SHEET 1 [IF 3 ANTENNA SPIRAL ARM TERMINATING ACTIVE REGION ANTENNA SPIRAL ARM FIG. 2 INVENTOR. FREDERICK J. DIETRICH ATTORNEY United States Patent 1111 3,618,114 [72] Inventor Frederick J. Dietrich [56] References Cited Columbus, Ohio UNITED STATES PATENTS [21] P 968 2,958,081 10/1960 Dyson 343/895 [221 ,1 2,964,748 12/1960 Radford 343/895 [45] Patemed 3188643 6/1965 Dsonetal 343/895 The OhioStateUniversity Research [73] Asslgnee 3,381,297 4/1968 2181 8118131. 343/895X Foundatlon Columbus, Ohio Primary Examiner-Herman Karl Saalbach Assistant Examiner-Marvin Nussbaum Attorney-Anthony D. Cennamo ABSTRACT: The invention is for an antenna of the conical logarithmic-spiral class. Specifically there is disclosed a means of utilizing a terminating active region to reduce the overall [54] CONICAL LOGARXTHMlC-SPIRAL ANTENNA 11 Claims, 12 Drawing Figs. [52] US. Cl 343/895 size of such an antenna by a factor oftwo without significantly [51] Int. Cl HOlg 1/36 modifying the antenna characteristics. Reference is made to [50] Field of Search 343/895 the claims for a legal definition ofthe invention. TERMINATING ACTIVE REGION FFEE END PATENTED wnvz i9?! PITCH ANGLE (DEGREE?) J w m @105 0 SHEET 2 UF 3 1.2 V LO FREQUENCY (MHz) FIG. 4 FIG. 5 INVIL'N'I'UR. FREDERICK J. DIETRICH ATTORNEY PATENTED rmvz IQTI SHEET 3 UF 3 IE 0 0m UV -12 0 0m mmgOm Ev -12 Q3 QON MJOZM mZOU Wm H mJOZ IUFE INVEN'I'UR. FREDERICK J. DIETRICH ATTORNEY CONI'CAL LOGARITI'IMIC-SPIRAIL ANTENNA CROSS-REFERENCES This is an improvement of a conical logarithmic-spiral antenna; such an antenna is shown in US Pat. No. 3,296,536,- assigned to The Ohio State University, issued on Jan. 3, 1967, to J. R. Copeland et al. Similar antennas are also shown in US. Pat. No. 2,958,081, by J. D. Dyson, issued on Oct. 25, 1960, and US. Pat. No. 3,188,643, by J. D. Dyson and P. E. Mayes, issued on June 8, I965. BACKGROUND Ionospheric sounding is a process in which radiofrequency (RF) energy, usually in the form of pulses, is transmitted toward and received from the ionosphere. Various properties of the ionosphere may be deduced from the character of the returned pulses. When the sounder antenna system is arranged to direct the energy vertically, the process is known as vertical-incidence sounding. A critical frequency of the ionosphere is a frequency above which a wave entering one of the dense regions (layers) of the ionosphere at normal incidence is not returned to the earth by that layer. This frequency is directly related to the highest frequency useful for a given ionospheric communication circuit. Since this property of the ionosphere has not only daily, seasonal, and year-to-year systematic variations, but also significant short time (order of 1 hour) variations, it is desirable to monitor the changes in the ionosphere on a virtually continuous basis. Monitoring the range of critical frequencies possible under various circumstances requires a sounder (including the antenna system) which may be scanned over a frequency range of at least 2 to Ml-Iz., with even more widely separated limits being desirable for some situations. The antenna structure required for the 2 to 20 MHz. frequency range can be quite large and costly due to the long wavelengths at the lower frequencies. In addition, for most temperate latitude locations there is an attenuation effect in the ionosphere which varies approximately inversely with the square of the frequency for frequencies above a few MI-Iz. This has the effect of reducing the received power quite rapidly as the frequency decreases, thus requiring good antenna performance at the low frequencies. Improved performance usually implies greater size. To have good performance over a wide bandwidth requires an antenna whose characteristics are virtually independent of frequency. The concept of frequency-independent antennas consists essentially of two principles. Flrst, any radiating structure which may be described completely by angles will have no frequency sensitivity in any of its properties. Second, such an antenna may be truncated beyond the region where currents have become negligible with no change in its properties. Such a truncation defines the lower frequency limit of the antenna with the high-frequency limit ultimately fixed by the finite dimensions of the feed region and transmission line. Although these principles imply the possibility of virtually infinite bandwidths, the requirements of extreme accuracy for very high frequencies and very large dimensions for low frequencies have limited practical structures to approximately 40:1 maximum bandwidths. The most common antenna used for vertical-incidence ionospheric sounding is the vertical delta, which is essentially one-half of a resistively terminated rhombic and does not possess the frequency independent characteristics. For the dimensions usually chosen, the vertical delta demonstrates poor efficiency and a highly variable radiation pattern especially at the low-frequency end of its operating range. The logperiodic dipole array is much more efficient and uniform in its performance because it possesses the frequency-independent properties. However, it possesses these properties only if constructed for good performance down to the required lowfrequency limit. This means that for operation at 2 MHz. a typical log-periodic antenna would have dimensions approaching 300 feet in length and feet in height. The logperiodic antenna has a linearly polarized radiation field and also exhibits a gain which is highly dependent on the geometrical constants chosen. SUMMARY OF THE INVENTION The invention relates to a means for reiiucing the size of antennas in the conical logarithmic-spiral class. The antenna comprises a conical surface and two substantially identical electrically conductive elements. The two elements are spiraled down the conical surface at a constant pitch angle 5 which is measured on the conical surface with respect to the intersection of the cone with a plane parallel to the base of the cone. The two elements form the antenna terminals at the apex of the cone, and are positioned with respect to each other on the surface such that a diameter drawn through the cone which intersects one element will always intersect the other one. There is constructed at the base of the conical spiral a region comprising several turns of the electrically conductive elements immediately adjacent to but electrically insulated from each other with zero pitch and constant circumference. There must be sufficient turns in this region so that the current would be reduced to zero before the end of the element is reached. This takes place in a region of constant circumference equal to one wavelength, therefore, the low frequency limit of the composite structure would correspond to a base circumference of one wavelength instead of the two wavelength criterion generally deemed required for maintaining the midband value of axial ratio. It is a geometrical property of a cone that reduction of the base diameter by a factor of two also reduces the height by a factor of two for a given cone angle, therefor, the present invention reduces both the height and diameter by a factor of 2, or equivalently, reduces the volume occupied by the structure by a factor of 8. Design equations are, also, disclosed for the construction of conical spiral antennas which have a wide cone angle. By wide it is meant cone angles greater than 30, which in the prior art was considered to be the maximum cone angle with which one could obtain a uniform, unidirectional beam. OBJECTS Accordingly, it is a principal object of the present invention to provide an improved conical logarithmic-spiral antenna. Another object of the invention is to provide a conical spiral antenna which is substantially reduced in size without any comprise of electrical characteristics. Another object of the invention is to provide a conical spiral antenna which possesses circularly polarized radiation capabilities; A further object of the invention is to provide a conical spiral antenna which permits high efficiency at all frequencies including the low frequency limit. Still a further object of the invention is to provide a conical spiral antenna which has a uniform pattern shape and consistently high front-to-back ratios at all frequencies within the design bandwidth. BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a diagrammatic representation of the general configuration and the significant parameters of a conical spiral antenna (only one spiral arm is shown for purposed of clarity); FIG. 2a is a diagrammatic representation of the side view of a truncated cone showing the significant parameters; FIG. 2b is a diagrammatic representation of the top view of a truncated cone showing the significant parameters; FIG. 3 is a diagrammatic representation of the preferred embodiment of the invention including a radiating ring termination; FIG. 4 is a graphical representation of the axial ratio of the antenna of the preferred embodiment, shown in FIG. 3, as a function of frequency; FIG. 5 is a graphical representation of the relationship between the maximum cone angle, ill, and spiral arm pitch angle, 5, which allow preservation of the desired radiation properties; FIG. 6a is a graphical representation of the radiation pattern of the preferred embodiment shown in FIG. 3 constructed with a 70 cone angle and a 3 pitch angle under linear excitation power at a frequency of 1.9 Ml-lz.; FIG. 6b is a graphical representation of the radiation pattern of the preferred embodiment shown in FIG. 3 constructed with a 70 cone angle and a 3 pitch a angle under linear excitation power at a frequency of 14.0 MHL; FIG. 60 is a graphical representation of the radiation pattern of the preferred embodiment shown in FIG. 3 constructed with a 70 cone angle and a 3 pitch angle under linear excitation power at a frequency of 20.0 MHz.; FIG. 6d is a graphical representation of the radiation pattern of the preferred embodiment shown in FIG. 3 constructed with a 70 cone angle and a 5 pitch angle under linear excitation power at a frequency of 1.9 MI-Iz.; FIG. 62 is a graphical representation of the radiation pattern of the preferred embodiment shown in FIG. 3 constructed with a 70 cone angle and a 5 pitch angle under linear excitation power at a frequency of 14.0 MHz.; and FIG. 6f is a graphical representation of the radiation pattern of the preferred embodiment shown in FIG. 3 constructed with a 70 cone angle and a 5 pitch angle under linear excitation power at a frequency of 20.0 MHz. DETAILED DESCRIPTION OF THE DRAWINGS Referring now to FIG. 1, there is illustrated the general configuration and the significant parameters of a conical spiral antenna. The antenna comprises two electrically conductive elements wound so as to lie on a nonconductive conical surface in the form of an equiangular spiral. The electromagnetic excitation point of the elements is the apex of the cone, wherein the apex of the cone points vertically upward. The parameters H, D, and 41 are interrelated in such a way that specification of any two of them determines the third. Thus, selection of a maximum height, H, and a base diameter, D, will automatically determine the cone angle, III. The parameters usually specified for conical antennas are cone angle, ill, and base diameter, D. The pitch angle, g, of the antennas spiraling elements is independent of the geometrical properties of the cone and may, therefore, be-chosen separately. The cone angle, Ill, height, H, and pitch angle, all interrelate to affect the radiation pattern and axial ratio within the operation region of the antenna. The directivity of the conical spiral antenna generally increases with decreasing cone angle, 41, and pitch angle, f. The axial ratio is generally improved under the same conditions, also. However, a decreasing cone angle, 41, means a rapidly increasing height, H, for a given base diameter, D, which is fixed by the low-frequency operating limit. Also, decreasing the pitch angle, 5, increases the total length of the spiral arm, and this can increase the antenna losses. The explanation of the operation of the conical spiral antennas is based on the concept of an active region, that is, the portion of the antenna structure which radiates at a given frequency. It is characterized by high current amplitudes and a particular, uniform, phase progression. The portion of the total structure included in the active region at any frequency depends on the cone angle, ill, and pitch angle, 5, of the structure under consideration. For many combinations of these angles representing useful structures the active region will extend over half of the entire structure at the lower frequencies of its operation region. The lower limit of the active region is the point on the cone nearest the base where the current amplitudes have been reduced to a negligible value. It is essential to the uniform performance of the antenna that this take place before the end of the spiral arm is reached. This condition defines the low-frequency limit of a given structure. It is known that a reasonable value of the longest operating wavelength (lowest frequency) at which all radiation properties and notably axial ratio, maintain mid-band valves may be found from the relationship ma.r |nn.r q' l where A max longest wavelength of operation; R base radius of the cone. It may be seen from equation (I) that the circumference of the base (circumference =21rR,,,,,,) is two wavelengths long at the lowest operating frequency. In an actual physical embodiment of the conical spiral antenna it is necessary to truncate a small portion of the cone near the apex to permit the support structures and feedlines. This truncation is shown in FIGS. 2a and 2b. The locus of an antenna spiral arm on the surface of a cone may be determined by utilizing the designations shown in the profile and top view of a quarter-cone illustrated in FIGS. 2a and 2b, respectively, where: H height of the truncated cone, R radius of the top of the truncated cone, R radius of the cone base, 5 spiral arm pitch angle b (r) slant height to a generalized point on the spiral arm, 'r= rotation angle about the cone axis, and p=radius to the spiral arm at any point. Eq. (2 where k maximum radius of the spiral, and a parameter determining the rate of expansion of the spiral. Satisfying the criterion of equation (I), letting r 0 at p R and r 0 elsewhere, the following relationship is obtained, =R ef Eq. (3) For a conical spi i alfh, {Tar id, iii, are related through the geometrical relation a=tansin (41/2) Eq. (4) As the spiral becomes more tightly wrapped (i.e., pitch angle, g, is decreased) there are more turns which have a circumference near one wavelength for any given operating frequency. If the desired frequency range of a conical antenna were very narrow, and there was a requirement for high circularity of polarization, the design would call for a small value of a in equation (3) which, in turn, would require a small value of f for most values of ill in equation (4). Also, since the upper and lower frequency limits are very close to the same value, R and R would be substantially the same. It is evident that as the frequency range approaches zero (single-frequency operation), the conical spiral reduces to a ring with adjacent turns lying next to or on top of each other. An antenna comprising a ring of adjacent turns of conducting elements will generate very good circular polarization and have a circumference of one electrical wavelength. This could be somewhat different from the free-space wavelength because the close spacing of the elements might alter the phase velocity of the traveling wave which they carry. The turns of adjacent conducting elements must be insulated from each other so that each turn carries its own current. As the currents are known to be rapidly attenuated in a well-defined active region, it is necessary to provide only enough turns so that the current in the elements has decreased to a negligible value before the end of the element is reached. This condition of negligible currents at the end of the conducting elements is essential to true frequency-independent operation of any frequency-scaling structure. The fields must be attenuated before any truncation occurs in order that the structure appear infinite, and consequently frequency-insensitive, to the traveling waves. A preferred embodiment of the invention utilizing the radiating ring is illustrated in FIG. 3. The ring had the effect of extending the low-frequency limit by a factor of 2 or, equivalently, reducing the required antenna radius by a factor of 2 for a given low-frequency limit. That is to say, the required radius is reduced from 8tl l1r, as shown in equation (1) to M /2w. For example, the axial ratio measured on the preferred embodiment shown in FIG. 3 at 2.12 MHz. was 1 1:1 without the radiating ring in place. When the radiating ring was added the axial ratio at the same frequency was measured to be l.3:l as shown in FIG. 4. It is a geometrical fact that reducing the base radius of a cone of fixed cone angle by a factor of 2 also reduces the cone height by the same factor. This is a significant reduction when tower costs and air traffic interference are considered. The conical spiral antennas in use today generally have cone angles of 3030" or less. This choice of cone angles is believed due in a large part to the fact that J. D. Dyson, Research on the Conical Log Spiral Antennas, Proceedings of the Applications Forum on Antenna Research, University of Illinois Urbana, Ill. p. 223, Jan. 27-30, 1964, observed general deterioration of radiation properties for cone angles wider than this. A small 30") cone angle would be entirely unsuitable for the present application even with the incorporation of the radiating ring just described. Utilizing equation 1) to determine the required base radius for operation at 2 MHz., a radius of78 feet is obtained. Ifa 30 cone angle half-angle) were chosen the height of the antenna would be 320 feet, which is a wholly unsuitable figure. To reduce the height of the antenna, relationships between the cone angle, (11, and the spiral arm pitch angle, 5, were derived. The relationship between the two angles when the cone angle, d, is less than 45 is Log 55)=O.O2(tl1) Eq. (5) If the cone angle, r11, is greater than 45 the relationship is FIG. 5 is a graphical representation of equations (5) and (6) when plotted on semilogarithmic graph paper. As can be seen in FIG. 5 a decrease in pitch angle is required as the cone angle is increased. If the relationships in equations (5) and (6) are followed the resulting antenna will demonstrate good radiation properties. By this it is meant a radiation pattern which is symmetrical with small back radiation. The full size antenna which was constructed to experimentally verify the teachings of the present invention utilized a 70 cone angle. The development of a 70 cone angle conical spiral with the desired radiation properties effected an even larger reduction in the height requirement than did the radiating ring (320 to 125 feet, a factor of2.56). From FIG. 5 it can be seen that for a cone angle of 70 a pitch angle of 3 isrequired. To illustrate the effect a slight change in pitch angle can have on the radiated pattern, FIGS. 611,612, and 60 show the pattern with a 3 pitch angle and FIGS. 6d, 6e, and 6f show the pattern with a 5 pitch angle at the same frequencies. As can be seen the back radiation is substantially reduced with a 3 pitch angle and the main beam is consistently uniform with typically 85-90 percent of the radiated energy (which results in a high front-to-back ratio). The size of the main beam relative to the back lobe is a measure of the effectiveness of the antenna in directing the energy vertically upward. The antenna of the present invention was developed for use on an antenna system for ionospheric sounding but the same structure has wide application for ionospheric communication. Other areas where this size reduction may be significant are antennas for spacecraft and feed antennas for parabolic reflectors. Although a certain and specific embodiment has been illustrated, it is to be understood that modifications may be made without departing from the true spirit and scope of the invention. What is claimed is: 1. A unidirectional broadband antenna comprising an electrically nonconductive conical surface, two substantially identical electrically conductive elements, and a terminating active region; said conductive elements wound on said conical surface in the form of constant pitch angle spirals; and elements positioned with respect to one and the other on said surface such that a diameter drawn through said conical surface intersecting the first of said elements will always intersect the second of said elements; said terminating active region including said conductive elements with a reduction in said spiral to a ring positioned at the base of said conical surface. 2. A unidirectional broadband antenna as set forth in claim ll wherein said elements are excited by electromagnetic energy at the apex of said conical surface. 3. A unidirectional broadband antenna as set forth in claim 1 wherein said terminating active region comprises several turns of said electrically conductive elements positioned immediately adjacent to but electrically insulated from each other. 4. A unidirectional broadband antenna as set forth in claim 3 wherein said turns of said electrically conductive elements are at zero pitch angle and constant circumference. 5. A unidirectional broadband antenna as set forth in claim 2 wherein said terminating active region permits utilization of a conical surface with a base radius, R,,,,,,, related to the f, wavelength, of said exciting electromagnetic energy by the relation R,,,,,,.=l\,,,,,,/rr. 6. A unidirectional broadband antenna comprising an electrically nonconductive conical surface and two substantially identical electrically conductive elements; said conductive elements wound on said conical surface in the form of constant pitch angle, 5, spirals; said elements positioned with respect to one and the other on said surface such that a diameter drawn through said conical surface intersecting the first of said elements will always intersect the second of said elements; said conical surface having a cone angle, :11, related to said pitch angle, 86, by the relation where said pitch angle, 5, is the maximum pitch angle allowable when said cone angle, :11, is less than 45. 7. A unidirectional broadband antenna as set forth in claim 6 wherein said elements are excited by electromagnetic energy at the apex of said conical surface. ii. A unidirectional broadband antenna as set forth in claim 6 wherein said antenna further comprises several turns of said electrically conductive elements positioned immediately adjacent to but electrically insulated from each other. 9. A unidirectional broadband antenna comprising an electrically nonconductive conical surface and two substantially identical electrically conductive elements; said conductive elements wound on said conical surface in the form of constant pitch angle, spirals; said elements positioned with respect to one and the other on said surface such that a diameter drawn through said conical surface intersecting the first of said elements will always intersect the second of said elements; said conical surface having a cone angle, r11, related to said pitch angle, 5, by the relation where said pitch angle, 5, is the maximum pitch angle allowable when said cone angle, ill, is greater than 45. 10. A unidirectional broadband antenna as set forth in claim 7 wherein said antenna further comprises several turns of said electrically conductive elements positioned immediately adjacent to but electrically insulated from each other. 11. A unidirectional broadband antenna as set forth in claim 7 wherein said elements are excited by electromagnetic energy at the apex of said conical surface. a a =i t a Patent Citations
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