FREQUENCY-INDEPENDENT IcR ANTENNA

Abstract

The invention provides interrupted coaxial-line radiators upon a ground plane and arranged in a frequency-independent manner for broadband radiation from a flush-mounted structure. The radiators may be arranged in a log-periodic array or in a spiral array for frequency-independent operation.

Description

BACKGROUND OF INVENTION

Requirements for broadband radiation from antennas has led to the development of what are generally termed "frequency-independent" antennas. In general, characteristics of an antenna of a given shape are dependent upon antenna dimensions measured in wavelengths; thus antennas with several characteristic dimensions are normally limited to operation within a fairly narrow band of frequencies. In order to overcome this limitation, it is possible to formulate antenna shapes described by angles rather than linear dimensions, and, in practical applications, these antennas are essentially independent of frequency for all frequencies above a certain lower limit. This general class of antennas is herein termed "frequency independent," and it is noted that this type of antenna may be defined in terms of polar coordinates with input terminals at the origin thereof wherein the antenna is bounded by curves which are equiangular spirals. The form of the bounding surfaces of such an antenna is fixed by the requirement that a change in scale should be equivalent to a rotation in azimuth angle, and this applies both to planar configuration and three-dimensional shapes.

An alternative frequency-independent antenna configuration comprises plane shapes that are essentially cross sections of the general three-dimensional shape identified above. Although the electrical properties of such antennas are not strictly frequency independent, they do repeat periodically with the logarithm of the frequency; consequently are generally termed "log-periodic" antennas. For this type of antenna the frequencies for which the structure has the same electrical properties are arranged in geometrical progression. Many different log-periodic antenna shapes are known, and various different log-periodic dipole arrays and log-periodic slot arrays have been developed. Reference is made, for example, to U.S. Pat. No. 2,989,749 setting forth one type of antenna in accordance with this general proposition.

Further with regard to frequency-independent antennas, it is noted that log-periodic structures formed as dipole arrays are basically free-space antennas so as to be incapable of operation efficiently against a ground plane. Log-periodic slot antennas, on the other hand, require cavity backing, as set forth, for example, in U.S. Pat. No. 3,218,644. While this general classification of antennas will be seen to be highly advantageous, it is yet limited insofar as low-profile or flush-mounted antenna structures are concerned. The present invention does employ the principles of frequency-independent antennas which may incorporate log-periodic configuration but without the limitations noted above.

Further with regard to the propagation of electromagnetic waves, there has been developed a class of radiators commonly denominated as interrupted coaxial-line radiators or slotted TEM-line radiators. In this respect, it is noted that the term "IcR" is oftentimes employed as an identification of this type of radiator. A slotted TEM-line antenna is basically a series-fed array of electrically small radiating loop elements, and in practice this is normally embodied as a coaxial line having the sheath thereof transversely slotted along the length of the line to form the radiating elements. The length of transmission line between slots or radiating loops, as they may be termed, provides the requisite phase delay between elements of the antenna. Characteristics of this type of antenna have been developed by prior workers in the field, and a brief description thereof is to be found, for example, in IEEE transactions on antennas and propagation, Mar., 1969, page 260, as written by Mr. John R. Copeland. Radiation from this type of antenna may occur from the slot and/or from the center conductor at the slot but for convenience such is herein termed slot radiation.

The present invention combines interrupted coaxial-line radiators in frequency-independent antenna configuration to achieve results unavailable with either alone. Although various antenna configurations employed in the present invention are actually approximations of two frequency independent shapes they are herein denominated as frequency independent for convenience of nomenclature.

SUMMARY OF INVENTION

In brief, the present invention provides a plurality of interrupted coaxial-line radiators upon a ground plane and energized from input coaxial lines with the radiators being oriented in frequency-independent array upon the ground plane. Inasmuch as the ground plane is an integral part of the IcR, it is not necessary to back the structure with cavities; consequently, there is achieved a low-profile frequency-independent antenna, particularly suited for utilization upon aircraft, spacecraft and the like. In accordance with one embodiment of the present invention, the radiators of this antenna are arranged in a log-periodic fashion in two sections energized with like voltages of opposite polarity to produce a linear principle polarization action of electromagnetic radiation in space. Another embodiment of this invention provides interrupted coaxial-line radiators disposed in an equiangular or logarithmic spiral or an Archimedean spiral with arms being energized at the center of the spiral with like voltages of either the same or opposite phase relationship to produce a conical circularly polarized beam with a null along a central axis normal to the ground plane, or a broad circularly polarized beam with a peak along such axis.

The present invention is particularly directed to the provision of a frequency-independent antenna structure of low-profile requiring no cavity backing for unidirectional radiation. The invention is applicable for mounting upon metallic nonplanar surfaces, such as those of aircraft, missiles, and the antenna structure is particularly useful in the VHF region.

DESCRIPTION OF FIGURES

The present invention is illustrated as to particular preferred embodiments thereof in the accompanying drawings, wherein:

FIG. 1 is a partial side elevational view of an interrupted coaxial-line radiator;

FIG. 2 is a transverse sectional view of an interrupted coaxial-line radiator taken in the plane 2--2 of FIG. 1;

FIG. 3 is a plan view of a log-periodic antenna in accordance with the present invention;

FIG. 4 is a schematic illustration of one section of the antenna of FIG. 3; and

FIG. 5 is a perspective view of an Archimedean spiral antenna in accordance with the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Considering first the general aspects of an interrupted coaxial-line radiator, reference is made to FIGS. 1 and 2 of the drawings. There is illustrated in these figures a conventional coaxial transmission line 11 having a central conductor 12, and a coaxial electrically conducting sheath 13. Many coaxial lines employ a solid dielectric 14 between the central and outer conductors thereof, and this may, for example, comprise solid polyethylene dielectric. In accordance with the present invention, the transmission line 11 has spaced slots 16 formed transversely through the outer sheath of conductor 13, with the central conductor 12 being continuous throughout the line. In addition to the transmission line, as briefly described above, there is provided an electrically conducting ground plane 21, upon which the line is disposed, in electrical contact therewith. The slots 16 and the center conductor 12 at these slots act as radiating elements, and the slots may extend partially or entirely about the outer conductor 13. In the illustrated embodiment, the slots are shown to extend about an angle which is somewhat less than the entire circumference of the outer conductor. Radiation from the line is, in part, dependent upon the circumferential extent of the slots therein, and, in addition, by forming the line as illustrated, it is possible to readily attach the line to the ground plane, as by solder or welds 22, because a part of the sheath extends continuously along the ground plane.

Electrical energization is provided to the transmission line 11, and electromagnetic radiation emanates from the slots 16 therealong and from the center conductor exposed at these slots. A plurality of factors are involved in the characteristics of such radiation, as for example, the radii of the conductors, the slot width and circumferential extent, as well as the voltage existing across the slots and the distance between slots. Without entering into a detailed discussion of this type of radiator, it is noted that the radiation characteristics thereof are known in the art, and for particular physical configuration can be calculated by conventional theory. The slotted transmission line 11 is termed a slotted TEM-line or an interrupted coaxial line radiator (IcR).

Referring now to FIGS. 3 and 4 of the drawing, there is illustrated one preferred embodiment of the present invention, wherein interrupted coaxial line radiators are employed, in accordance with the present invention, as radiators in a log-periodic antenna structure. The antenna is formed of two portions 31 and 32, with these being fed by coaxial lines 33 and 34, respectively. The coaxial lines 33 and 34 are mounted upon an electrically conducting ground plane 36, and are fed or energized by signals of equal amplitude but with 180.degree. phase relationship. Energization of the coaxial lines may be accomplished from an appropriate transmitter, or the like, 37, including means such as a hybrid circuit for achieving the above-noted opposite phase relationship of signals applied to the separate lines. The lines 33 and 34 are appropriately terminated as by a matched load therebetween.

Each of the coaxial feedlines 31 and 32 is provided with branch-slotted TEM lines. It is to be appreciated that inasmuch as each portion may have a substantial number of branches, the illustration of FIG. 3 is broken, so that only initial and terminal branches are illustrated for compactness of illustration.

Referring to FIG. 4, there will be seen to be schematically illustrated the orientation of branches of a single portion 31 of the antenna. No attempt is made in FIG. 4 to show details of structure, but instead, the line diagram thereof is intended only to clarify the relative location and extent of the branches of the section. In accordance with established theory of log-periodic antennas, the portion 31 illustrated in FIG. 4 will be seen to have a number of branches 41, 42, 43, 44, etc., extending from one side of the feedline 34, and a number of intermediate branches 51, 52, 53, etc., extending from the other side of the feedline 34. Energy is fed into the coaxial line 34 at the left thereof in the drawing and propagates along the line as well as out the branch lines noted above. As this energy passes through the branch lines, electromagnetic energy thereof is radiated into space through the gaps in the lines as described above.

The embodiment of the present invention illustrated in FIG. 3 has the two portions 31 and 32 thereof formed as mirror images so that the first branch lines of each extend in opposite directions from each other and the second branch lines extend toward and across each other. It is particularly noted that the branch lines of each section alternate in their direction of extension from the central feedline or coaxial cable thereof. It is also to be noted that in accordance with the theory of log-periodic antennas the length of successive branch lines 41 to 44, for example, increases successfully from the feed end of the section, in somewhat the manner as illustrated in FIG. 4. In certain respects this may be likened to a log-periodic dipole array except that interrupted coaxial radiators are employed in place of dipoles. Furthermore, the sections 31 and 32 of the present invention are mounted upon a conducting ground plane 36 in marked distinction to log-periodic dipole antennas which operate as free-space antennas. In circumstances where the antenna is not operated at too high a frequency the crossing branches may be slightly offset to both pass over the other coaxial line.

The theory of "frequency independency" of log-periodic antennas is applicable to the present invention and consequently the relationship of the branch lines of radiators of the present invention is not further developed herein. It is, however, noted that the nature of the radiation patterns and the radiation intensity from individual branch lines depend upon the termination of the end of the lines and the parameters of the gaps in the radiators. The branches may be terminated as short circuits, as illustrated, or alternatively may be open circuited or loaded. It is also to be appreciated that the center conductor of the interrupted coaxial line radiator need not be straight at the gaps therein. The conductor may in fact be formed as a single or multiple loop if desired in order to achieve additional control of radiation properties. With regard to radiation pattern it is noted that the principal polarization of the antenna of FIG. 3 is linear and the direction thereof in space is parallel to the branches or radiator lines. The orientation of principle polarization of the antenna may be changed by changing the orientation of lines and in this respect, it is noted that these lines or branches need not be straight but may take other forms such as, for example, T-shaped or L-shaped. It will be appreciated that once the desired radiation characteristics of individual branches or radiator lines have been established the overall antenna pattern may be synthesized by well-known methods.

An alternative embodiment of the present invention is illustrated in FIG. 5 wherein there is shown what may be termed an IcR Archimedean spiral antenna. The antenna comprises a pair of parallel spiral coaxial lines or radiators 61 and 62 disposed upon a ground plane 63 in the form of an Archimedean spiral. The lines 61 and 62 are fed at the center of the spiral as by means of input coaxial cables 64 and 66 from a suitable transmitter unit, not shown. The two arms 61 and 62 of this spiral are formed as interrupted coaxial line radiators, as indicated, with the input portions 67 and 68 comprising impedance-matching sections. The arms are energized from the center of the spiral, with energy then being propagated outwardly along the two arms 61 and 62 to the outer ends of the arms where they are appropriately terminated at a desired impedance to to prevent reflection. The maximum spiral diameter is limited, in accordance with established theory to the free-space wavelength at lowest operating frequency divided by twice the relative dielectric constant in the loaded coaxial lines. It will be appreciated that Archimedean spirals are essentially frequency independent and it is noted that while these types of antennas are known in the art they have previously required cavity backing for unidirectional radiation.

The two arms of the antenna of FIG. 5 may be fed with input signals of the same or opposite phases depending upon the desired radiation pattern from the antenna. Desired signal amplitude and phase relationships may be achieved by utilization of a hybrid circuit at the input, for example. In the instance wherein the two spiral arms 61 and 62 are fed in 180.degree. phase relationship the antenna radiates a broad circularly polarized beam with its peak directed along the axis normal to the ground plane. Alternatively, when the two spiral arms 61 and 62 are energized with signals of the same phase relationship the antenna radiates a conical circularly polarized beam with a null thereof directed along the axis normal to the ground plane.

In common with the embodiment of the invention illustrated in FIG. 4 and described above, the radiation patterns of the antenna of FIG. 5 are essentially independent of frequency provided that the gaps in the interrupted coaxial radiators radiate a proper amount of power, as discussed above. This embodiment of the present invention does provide unidirectional radiation through the use of interrupted coaxial line radiators in a frequency-independent array. In common with the above-described embodiment of the present invention the present embodiment has the advantages of a low profile and ability to conform to nonplanar surfaces and is suitable for application in the VHF region.

Claims

I claim:

1. An improved antenna comprising an electrically conducting ground plane, a plurality of interrupted coaxial-line radiators disposed on said ground plane with the exterior of each radiator electrically contacting said ground plane, said radiators being disposed in frequency-independent array, and a pair of coaxial cables disposed in adjacent parallel relation on said ground plane and connected to separate radiators extending as stub branches from said cables in log-periodic array on said ground plane for supplying electrical energization to said radiators whereby electromagnetic energy is unidirectionally radiated into space from said radiators.

2. The antenna of claim 1 further defined by said cables carrying electrical energization of opposite phase for supply to said radiators.

3. The antenna of claim 1 further defined by each of said cables having said stub branches extending from both sides thereof and alternate branches of each extending across the adjacent cable as a coaxial cable with the interrupted coaxial line commencing on the opposite side of the cable crossed.

4. The antenna of claim 1 further defined by said antenna having two mirror image portions with each including one of said cables and each having branches extending alternately from opposite cable sides, said branches comprising said interrupted coaxial line radiators, and said portions being disposed with the cables thereof in adjacent parallel relation so alternate branches from the combination of portions extend outwardly on each side thereof from different cables.

5. An improved antenna comprising an electrically conducting ground plane, a plurality of interrupted coaxial-line radiators disposed upon said ground plane in equiangular spirals with the exterior of each radiator electrically contacting said ground plane, said radiators being disposed in frequency-independent array, a pair of coaxial cables connected to separate radiators at the center of said spirals for supplying electrical energization thereto whereby electromagnetic energy is unidirectionally radiated into space from said radiators.

6. The antenna of claim 5 further defined by there being two spiral arms of radiators and said arms being energized with signals of like phase by said cables.

7. The antenna of claim 5 further defined by there being two spiral arms of radiators and said arms being energized with signals of 180.degree. phase relationship by said cables.