US 3808599 A
A periodic antenna includes a plurality of quarter wavelength resonant, radiating elements spaced along a feed line positioned above a ground plane. Adjacent ones of the resonant elements are separated from each other by a distance substantially less than a quarter wavelength of the operating frequency of the elements and are positioned to extend from the feed line in a direction away from the ground plane. Substantially 180 DEG phase shift is provided between adjacent ones of the elements by including quarter wavelength, shunt distributed parameter slow-wave delay lines between adjacent ones of the elements. The delay lines are located inside of an electrically conducting sleeve that comprises the feed line.
Description (OCR text may contain errors)
Brunner [4 1 Apr. 30, .1974
[ PERIODIC ANTENNA ADAPTED FOR HANDLING. HIGH POWER John E. Brunner, Hamilton, Ohio  Assignee: Cincinnati Electronics Corporation,
Evendale, Ohio  Filed: Nov. 29, 1972  Appl. No.: 310,543
 US. Cl 343/792.5, 343/846, 343/853 [5l] Int. Cl. .Q H0lq 11/10  Field of Search 343/792.5, 816, 846, 853
 0 References Cited UNITED STATES PATENTS 3,389,396 6/1968 Minerva et al 343/792.5
Primary Examiner-Eli Lieberman Attorney, Agent, or Firm-Lowe, King and Price  ABSTRACT A periodic antenna includes a plurality of quarter wavelength resonant, radiating elements spaced along a feed line positioned above a ground plane. Adjacent ones of the resonant elements are separated from each other by a distance substantially less than a quarter wavelength of the operating frequency of the elements and are positioned to extend from the feed line in a direction away from the ground plane. Substantially l80phase shift is provided between adjacent ones of the elements by including quarter wavelength, shunt distributed parameter slow-wave delay lines between adjacent ones of the elements. The delay lines are located inside of an electrically conducting sleeve that comprises the feed line.
10 Claims, 2 Drawing Figures MTENTEmmo m4 sum 1 or 2 PERIODIC ANTENNA ADAPTED FOR HANDLING HIGH POWER FIELD OF INVENTION The present invention relates generally to periodic antennas and more particularly to a periodic antenna including improved means for providing 180 phase shift between adjacent resonant radiating elements of the antenna.
BACKGROUND OF THE INVENTION One type of periodic antenna includes a feed line and a plurality of distinct resonant radiating elements that are connected to and spaced along the feed line. To provide a directive, relatively high ga'in'pattern, rather than a substantially omnidirectional low gain pattern, it is necessary to provide 180 phase shift between adjacent radiating elements along the feed line. A 180 phase shift between adjacent radiating elements is required for a directional pattern regardless of whether the periodic antenna is narrow band, i.e., uniformly periodic, or wide band, i.e., log-periodic. In a uniform periodic antenna, all of the radiating elements have the same length and are equally spaced from each other, while in a log-periodic antenna adjacent elements along the feed line have differing lengths and spacings. In a log-periodic antenna, the ratio of the length of adjacent elements equals the ratio of the spacing of adjacent elements from an apex of the antenna, which ratio is referred to as the structural scaling parameter of the antenna and denoted by 1'.
Many systems have been developed in the prior art for providing the 180 phase shift between adjacent resonant radiating elements. Perhaps themost well known involves periodic antennas including a plurality of half wavelength dipole radiating elements which are-interconnected by a two-conductor common feed line, transposed between connections. The elements and feed are interconnected in such a manner that adjacent quarter wavelength elements on the same side of the line are connected to opposite leads of the feed. However, for many applications the bulk required by a half wavelength dipole is not practical and the half wave dipole configuration is replaced by monopole, quarter wavelength resonant radiating elements and a ground plane conductor.
Several different structures for providing 180 phase shift between adjacent monopole radiating elements have been proposed; namely one-to-one phase reversing transformers, coaxial capacitors, grounded parasitic elements located between adjacent active monopoles, lumped parameter resonant circuits between adjacent monopoles and quarter wavelength, resonant lines between adjacent radiating monopoles. Each suggested prior art device of these classifications has had problems, particularly in applications requiring high power (approximately 1 kilowatt), low frequency over a wide bandwidth (such as 4-30 MHz), and transportability. Transformers are cumbersome, expensive and not particularly well suited for high power applications. Parasitic elements are relatively narrow bandwidth structures and, therefore, function for only a small range of array scaling parameters.
While coaxial capacitors combine simplicity and high power capability, they are not practical if relatively small spacing between adjacent elements is required as is usually the case for an easily transported antenna capable of operating over a wide bandwidth and with a broad beamwidth. For these requirements, it is neces' sary to select a spacing between adjacent elements that is a relatively small fraction (0.06 or less) of a wavelength. For spacings between adjacent elements equal to or less than 0.06 wavelength, the tolerance of coaxial capacitors is usually excessively critical.
It has been proposed to provide spacings between adjacent elements of 0.06 wavelengths or less by including series resonant elements or quarter wave resonant lines. However, the proposed structures have not been adequate to meet the high power and transportability requirements. A series tuned circuit includes lumped parameter inductors having very high Q. These inductors are required to conduct significant amounts of current when operated near resonance and, therefore, have a significant 1 R power loss which is heat that must be dissipated. Heat dissipation of the inductors has generally been attained by packaging them in relatively large housings which detract from the ability of the antenna to be moved from one place to another.
To avoid the heat dissipation problem of the lumped parameter inductor, it has been suggested to use distributed parameter quarter wavelength transmission lines which extend from and are connected to either side of the feed in a plane generally parallel to the ground plane. The amount of space required for such an array is relatively great because of the long elements extending outwardly from the feed. In certain regions, sufficient area cannot be cleared to enable the antenna to function properly. In the specific frequency range mentioned, approximately feet to the sides of the feed element are required for the longest quarter wavelength lines.
BRIEF DESCRIPTION OF THE INVENTION In accordance with the present invention, the phase shift between adjacent resonant radiating elements of a periodic antenna is provided by forming the feed transmission-line from a hollow electrically conductive sleeve in which distributed parameter, quarter wavelength slow-wave structures are provided. Each of the slow-wave structures is preferably a helical transmission line having distributed capacity between adjacent turns and an inner wall of the hollow conductive sleeve. One end of the transmission line is connected, via a small trimer capacitor, to the conducting sleeve, while the other end is connected to the ground plane against which the feed line is driven, between adjacent pairs of radiating elements for which the slow-wave delay line is tuned. The slow-wave delay line does not require a relatively massive container for dissipating heat generated therein since the heat is distributed over the entire length of the line, rather than being concentrated in a small volume, as is the case with an inductor of a lumped parameter shunt resonant circuit. Hence, the slow-waver, distributed parameter delay line is particularly suited for relatively high power applications. Because the slow-wave distributed parameter quarter wavelength line is located within the interior of the electrically conductive sleeve, the unit is relatively transportable and does not require a significant area on the terrain in which it is located. For the high power applications of the present invention, it is not feasible to utilize a lumped parameter inductance inside the conducting sleeve because eddy currents of the sleeve reduce the Q of the coil and increase losses, and may cause excess heat dissipation.
To provide vemier tuning for the helical delay line, a distributed parameter capacitor is provided in series with the helix. The distributed parameter capacitor comprises a solid dielectric having one electrode formed by the conductive sleeve and a second electrode connected to one end of the helical line.
It is, accordingly, an object of the present invention to provide a new and improved periodic antenna which is capable of operating at relatively high power, at a relatively low frequency, is relatively transportable, can easily be erected without requiring excessive amounts of ground area, and does not exhibit excessive heating.
An additional object of the invention is to provide a periodic antenna wherein quarter wavelength elements are located inside of a feed line for a number of resonant radiating elements.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of one specific embodiment thereof, especially when taken in conjunction with the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic diagram of one embodiment of the present invention; and
FIG. 2 is a cross-sectional view of a portion of the feed line illustrated in FIG. I, particularly illustrating one quarter wavelength slow-wave structure.
DETAILED DESCRIPTION OF THE DRAWING Reference is now made to FIG. 1 of the drawing wherein there is illustrated an antenna array comprising a plurality of monopole, quarter wavelength radiating elements 11-12 which have differing lengths and spacings between adjacent elements in accordance with the well known log-periodic criteria, to satisfy the antenna scaling factor R =distance from apex point 23 to one of the elements (n l as measured along boresight axis 24,
R distance from apex point 23 to an element (n) adjacent the element (n l) as measured along boresight axis 24, L l length of element (n l) as it extends from boresight axis 24, and L length of element (n) as it extends from boresight axis 24. Elements 11-21 are fed from a common feed line 25, having a longitudinal axis coincident with boresight axis 24. The spacing between adjacent ones of elements ll-21 is considerably less than a quarter wavelength of the resonant frequency of each element of the pair, and is typically between 0.02 and 0.06 of a wavelength to provide a relatively short length for feed element 25. Elements 11-21 are positioned to extend from feed line 25 in a direction at right angles to the plane of ground plane 26, above which the feed line is positioned.
Feed line 25 is situated slightly above ground plane 26 so that there is a radiation image of each of the radiating element 11-21 in the ground plane, whereby the array of monopole elements appears to include an array of half wavelength dipole elements.
Connected to the end of common feed line 25 adjacent the shortest monopole radiator 11 is an active, generally wide band, such as 4-30 MHz, source 27. Substantial energy from source 27 is transmitted along feed line 25 until it is coupled to the radiator with which it is most nearly resonant. The monopole elements having a length less than the wavelength of the energy from source 27 present a relatively high impedance to the source energy and do not, therefore, absorb substantial energy from the source. The monopole elements which have a length enabling them to be resonant with the frequency of source 27 radiate considerable energy from the source, and considerably attenuate the source energy to cause a reversal in slope of the propagation constant along the feed line 25. Energy radiated from the element most nearly resonant with source 27 normally has a tendency to be radiated in both directions relative to boresight axis 24, i.e., there is a tendency to generate lobes that are respectively directed toward the end of feed 25 to which source 27 is connected and toward the end of the feed where the longest radiating monopole 21 is located. To suppress the lobe that has a tendency to radiate toward monopole 21 and provide a substantial backfire radiation pattern, a 180 phase shift is provided between adjacent ones of radiating elements 11-21.
The description and method of operation of the structure illustrated in FIG. 1 is well known to those skilled in the art. The present invention is concerned with a new and improved apparatus for achieving the 180 phase shift between adjacent ones of radiating elements 11-21. I
To this end, common feed line 25 is formed of a hollow electrically conductive, non-magnetic sleeve, which is preferably fabricated of aluminum. Within sleeve 25 are located a plurality of quarter wavelength, slow-wave shunt distributed delay lines 31-40, one of which is provided for each pair of adjacent radiating elements 11-21. Hence, slow-wave delay line 31 is provided for radiating elements 11 and 12, slow-wave delay line 32 is provided for radiating elements 12 and 13, etc. and slow-wave delay line 40 is provided for radiating elements 21 and 20. Each of delay lines 31-40 is designed so that the resonant frequency thereof, the frequency at which a phase shift is provided, is equal to the resonant frequency of the longest radiator of the radiator of its associated pair divided by VT For example, slow-wave delay line 40 is designed to provide a 90 phase shift for the resonant frequency of radiator 21 divided by [T One end of each of quarter wavelength delay line 31-40 is connected to any point on common feed line 25, while the other end of each quarter wavelength delay line is connected to ground plane 26 at a point that is at the geometric means between the two adjacent radiating elements of each radiating pair, as referenced to apex point 23. Hence, for radiating pairs having lengths L and L spaced from apex point 23 by the distances R, I and R, the quarter wavelength, slow-wave structure has one terminal connected to ground plane 26 at a point spaced from a'pex point 23 along boresight axis 24 by a distance equal to V l R,,l
(R, If radiating elements 11-21 are not logarithmically periodic but are uniformly periodic, whereby the spacings between adjacent elements and the lengths of all elements are the same, the connection point of each quarter wavelength slow-wave structure to the ground plane is equidistant from adjacent radiating elements. While the one end of the quarter wavelength, delay line structures 31-40 can be connected at any point to feed line 25, it is usually convenient to connect the slowwave structures to the feed line in proximity to the longest radiating element of a particular radiating element pair.
One particular configuration for each of the slowwave, quarter wavelength, shunt distributed parameter delay lines located within feed line 25 is illustrated in P16. 2 and broadly comprises a helical coil 41 and a vemier capacitor 42. Each of the distributed parameter quarter wavelength delay lines 31-40 is essentially the same, except for the length of the helical conductor and the value of the vernier capacitor, which enables the capacitance of the helical conductor to change by approximately fl percent. I a
As illustrated in FIG. 2, helical conductor 41 is mounted inside of aluminum sleeve 43, which comprises feed line 25, and to which are soldered radiating element s 1l-21. Helical conductor 41 is supported by dielectric tube 44 which is coaxial with sleeve 43. Tube 44 is supported and maintained in situ by spaced dielectric rings 45 and 46 which are positioned in proximity to the opposite ends of helical conductor 41.
The end of helical conductor 41 proximate ring 46 is capacitively coupled to metal sleeve 43 by capacitor 42 which comprises annular, solid dielectric cylinder 47, having inner and outer peripheries respectively in contact with tube 44 and sleeve 43. Electrodes of capacitor 42 are respectively formed by sleeve 43 and metal tube 48 which is nested within tube 44 so that the outer surface of the former abuts against the inner surface of the latter. Tube 48 and dielectric cylinder 42 have approximately equal lengths to provide the greatest variation of the vemier capacitor, the value of which is controlled by the longitudinal position of tube 48 relative to cylinder 42. The electrode of capacitor 42 formed by metallic tube 48 is connected to one end of helical conductor 41 by metallic plate 49 that is fixedly bonded to the end of tube 48 closest to ring 46.
The other end of helical conductor 41 is connected to ground plane 26 which comprises a metallic threshold strip 51 having a length equal to the length of tube 43 and which is adapted to be placed on the terrain on which the antenna is erected. To this end, helical conductor 41 includes, at its end remote from plate 49, a straight segment 52 which extends through dielectric grommet 53 that is provided in an aperture of tube 43. To support the antenna, a dielectric leg 54 surrounds grommet 53 and straight segment 52 of helical coil 41 and is fixedly connected between the exterior of sleeve 43 and metallic threshold 51. Leg 54 spaces sleeve 43 from threshold 51 by a distance which affects the characteristic impedance of the feed line and, therefore, the entire array. For a typical situation where it is desired for the antenna to exhibit an input resistance of 50 ohms, the spacing of the bottom of sleeve 43 from threshold 51 is on the order of two and one-half inches.
It is to be understood that in certain instances the distributed capacitance between the helical coil 41 and hollow conductive tube 43 and the coil length are such that the distributed parameter slow-wave structure is resonant to exactly the correct frequency, whereby the need for a vernier capacitor is obviated. Precise control of the phase shift introduced by the slow-wave structure can be achieved by accurately controlling the length, pitch and diameter of helical conductor 41. Precise control of these parameters can be achieved by providing dielectric tube 44 with an accurately machined helical groove in which the helical coil is wound.
One particular configuration of the antenna of the present invention was designed to cover a bandwidth from 4 to 30 MHz and included the distributed parameter helical delay line configuration illustrated in FIG. 2. To provide a relatively short structure, the value of 'r was selected to be 0.900, and the spacing, generally denoted by 8, between adjacent radiators 11-21 was selected to be 8 0.045 A, where A the wavelength of the resonant frequency of the longest element of a pair of the adjacent elements. The length of sleeve 43 in this configuration was 98.5 feet, approximately a 50 percent reduction relative to standard log-periodic antennas covering the same frequency band. Aluminum sleeve 43 had a 1.75 inch diameter and a wall thickness of 0.049 inches, while dielectric tube 44 was fabricated from fiberglas having a 1.00 inch outer diameter and a 0.050 inch wall thickness. Helical center conductor was fabricated from number eight copper wire and wound to have 4.5 turns per inch, a spacing that was found to provide the highest Q per unit length for the relatively small 1.129 inch diameter between the centers of the wire coil on opposite sides of tube 44. The length of helical coil 41 was between 5 inches and 4 feet, depending upon which of the pair of resonant monopoles with which it was associated. The described antenna exhibited a gain of between 8 and 9 db relative to a tuned quarter wavelength monopole and a voltage standing wave ratio of less than two to one over the entire band. Testing of the antenna at power levels up to one kilowatt indicated no evidence of arcing or excessive heating of the distributed parameter delay lines.
While a helical distributed parameter delay line is most advantageous for relatively low frequency bands, the principles of the invention are applicable to distributed parameter delay lines comprising a straight-line conductor in a high dielectric medium. ln such an instance, the helical center conductor 41 is replaced by a straight-line conductor that is coaxial with tube 44 and extends longitudinally thereof. The straight-line conductor is surrounded by a solid dielectric having a relatively high dielectric constant, such as certain ceramics.
The principles of the invention are also applicable to any distributed parameter slow-wave delay line elements which introduce an integral multiple of a quarter wavelength delay between common feed element 25 and ground plane conductor 26. The quarter wavelength structure, however, is preferable to higher multiples of a quarter wavelength because of its shorter length and lower losses.
While there has been described and illustrated one specific embodiment of the invention, it will be clear that variations in the details of the embodiment specifically illustrated and described may be made without departing from the true spirit and scope of the invention as defined in the appended claims.
1. A periodic'antenna particularly adapted for han-' dling high power comprising at least three resonant radiating elements, a common feed line for said elements, said feed line comprising a hollow electrically conducting, metallic sleeve extending along substantially the entire length of the line, adjacent ones of said elements being spaced along said feed line by a distance substantially less than a quarter wavelength of the operating frequency of the elements, means for providing substantially 180 phase shift between adjacent ones of the elements, each of said phase shift means including a shunt distributed parameter slow-wave delay line connected between adjacent ones of said elements and located inside of the sleeve, said line including a conductor having distributed inductance for heat distribution over its entire length and distributed capacitance as a dielectric between the conductor and sleeve, each of said shunt lines being spaced longitudinally in the sleeve from the adjacent line and tuned so it has a length substantially equal to an integral multiple of a quarter wavelength of the resonant frequency of a radiating element adjacent thereto.
2. The periodic antenna of claim 1 wherein each of the shunt lines includes a helical coil coaxial with the sleeve and having a length substantially equal to a quarter wavelength.
3. The periodic antenna of claim 2 wherein the shunt line further includes a tuning capacitor for precisely determining the resonant frequency of the shunt line.
4. The periodic antenna of claim 3 wherein the capacitor includes first and second electrodes respectively formed by the hollow, electrically conducting sleeve and an electrically conducting tube coaxial with the conducting sleeve, and an annular, solid dielectric cylinder positioned between said electrodes.
5. A periodic antenna particularly adapted for handling high power comprising a ground plane, a feed line positioned above the ground plane, at least three monopole resonant radiating elements spaced along the ground plane conductor and positioned to extend from the feed line in a direction away from the ground plane, said feed line and elements being positioned above the ground plane so that there is a radiation image of each radiating element in the ground plane, said feed line comprising a hollow, electrically conducting, metallic sleeve extending along substantially the entire length of the line, adjacent ones of said elements being spaced along said feed line by a'distance substantially less than a quarter wavelength of the operating frequency of the elements, means for providing substantially phase shift between adjacent ones of the elements, each of said phase shift means including a shunt distributed parameter slow-wave delay line connected between adjacent ones of said elements and located inside of the sleeve, said line including a conductor having distributed inductance for heat distribution over its entire length and distributed capacitance as a dielectric between the conductor and sleeve, each of said shunt lines being spaced longitudinally in the sleeve from the adjacent line and tuned so it has a length substantially equal to an integral multiple of a quarter wavelength of the resonant frequency of a radiating element adjacent thereto.
6. The periodic antenna of claim 5 wherein the ground plane includes a metallic threshold strip, and the shunt line includes first and second terminals of the conductor respectively connected to the feed line and the ground plane conductor, said second terminal being connected by a dc. connection to the threshold strip between the resonant elements for which the delay line is tuned.
7. The periodic antenna of claim 6 wherein each of the shunt lines includes a helical coil coaxial with the sleeve and having a length substantially equal to a quarter wavelength.
8. The periodic antenna of claim 7 wherein the shunt line further includes a tuning capacitor for precisely determining the length of the shunt line.
9. The periodic antenna of claim 8 wherein the capacitor includes first and second electrodes respectively formed by the hollow, electrically conducting sleeve and an electrically conducting tube coaxial with the conducting sleeve, and an annular, solid dielectric cylinder positioned between said electrodes.
10. The periodic antenna of claim 5 wherein the lengths of said elements differ from each other and the spacing relative to a common apex point between adjacent ones of said elements differs in accordance with the log periodic criteria, said ground plane includes a metallic threshold strip, said shunt line including first and second terminals of the conductor respectively connected to the feed line and the ground plane conductor, said second terminal being connected by a dc. connection to the threshold strip between the resonant elements for which the delay line is tuned at a point spaced from the apex by substantially the geometric mean of the distance from the apex of the adjacent elements to which the shunt line is tuned.