Sheet array antenna structure

[57] ABSTRACT

Arrays wherein the radiator elements act also as shield conductors of feed lines arranged to provide cophasal interconnection of the radiators without physical transpositions, particularly adaptable to fabrication by etched circuit techniques, as microstrip on the surfaces of a single dielectric substrate.

Claims, 3 Drawing Figures

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PATENTED» 41973

3,757,342

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FIGURE 2

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3,757,342

SHEET ARRAY ANTENNA STRUCTURE

BACKGROUND

This invention relates to improvements in antenna structures of the type that comprise arrays of collinear cophasally connected radiator elements and sheet-like arrays of such collinear arrays, and more specifically to such structures with thin foil-like conductors on plates of dielectric material. In general, the individual radiators of an array must be interconnected through a network of feed lines to a common feed port. The lines must be arranged to minimize leakage radiation and parasitic excitation.

Prior art solutions to the feed line problem have usually involved mechanically complex structures not well suited to economical mass fabrication by modern printed circuit methods. An apparent exception is illustrated in U. S. Pat. No. 3,643,262. That arrangement, although no doubt useful, is subject to certain inherent design limitations, some of which are disclosed in the patent specification.

Another pertinent prior art antenna is a collinear array of coaxial line sections with inner and outer conductors transposed at half wavelength intervals, as illustrated in U. S. Pat. No. 2,158,376. An analogous structure could be made amenable to printed or etched circuit fabrication, although applicants are not presently aware that this has been done, by substituting microstrip transmission line sections for the coaxial line sections. However, a sheet array of such structures would have the serious practical disadvantage of requiring numerous connections through the substrate, to accommodate the necessary transpositions.

SUMMARY

According to this invention, the need for external phasing devices or crossover transposition connections is eliminated by forming the collinear arrays, or subarrays of sheet arrays, of parallel two-conductor transmission lines with each conductor alternating between wide and narrow regions over successive intervals along the line, the wide regions of each conductor being adjacent the narrow regions of the other. The intervals are preferably one half line wavelength long, taking into account the dielectric constant of the medium between the conductors. The wide conductor in each interval acts as a grounnd plane or shield for the adjacent narrow conductor, in the manner of a microstrip line operating in the TEM mode, and also as a radiator element which is coupled to the others of such elements for exCitation of cophasal, codirected current flow.

DRAWING

FIG. 1 is an isometric view of an embodiment of the invention in a simple collinear array.

FIG. 2 is a plan view of a modified embodiment of the invention in a collinear array.

FIG. 3 is a plan view of a sheet array of collinear arrays similar to that of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a thin electrically conductive strip 1 is supported on the upper surface of a plate 2 of dielectric material such as polyolefin. The strip 1 consists of relatively wide regions 3,4 and 5 alternately dis2 posed with respect to relatively narrow regions 6 and 7. Another conductive strip 8 is supported on the lower surface of the plate 2, with narrow regions 9, 10 and 11 directly under and adjacent to the wide regions 3,4 and 5 of the upper strip, and with wide regions 12 and 13 directly under and adjacent to the narrow regions 6 and 7 of the upper strip.

A gap 14 is provided at the center of the lower narrow region 10, which is connected on opposite sides of the gap to the conductors 15 and 16 of a parallel conductor transmission line extending downwardly perpendicular to the plane of the plate 2 through a conductive ground plane 17. The plate 2 is supported upon and separated from the ground plane by a body 18 of foamed dielectric material.

Each of the narrow regions of the conductive strips 1 and 8, together with the adjacent wide region of the other strip, forms a microstrip transmission line one half wavelength long in the TEM mode at the design center frequency. Wavelength in the line is shorter than the free space wavelength in accordance with the dielectric constant of the material of plate 2.

The series of half wavelength lines, cascaded end to end, constitutes in effect a single transmission line of resonant length, in this example 5/2 wavelength, open circuited at both ends.

Although external connections to the composite line 1, 8 may be made in any of various known ways, it is preferred in this example to do so by means of the balanced parallel conductor feed line 15, 16 across the central gap 14 in region 10. Equal, oppositely flowing currents in the feed conductors 15 and 16 excite longitudinally codirected currents in the two halves of the narrow region 10, accompanied by an oppositely directed current in the opposed wide region 4. Because of its high impedance terminations (its ends being an integral number of half-wavelengths from the open circuited ends of the overall structure) the elementary line section 4,10 is excited as a half wavelength resonator with a current maximum at its center and voltage maxima, 180° out of phase with each other, at its ends.

The voltage across the right hand end of line section 6, 12 is necessarily the same as that across the adjacent end of section 4, 10. Accordingly, section 6, 12 is excited as a half wave resonator. Section 3, 9 is similarly excited from the adjacent end of section 6,12, and sections 7, 13 and 5, 11 are excited from the right hand end of the central section 4, 10. At the resonant frequency, (except for the effects of transmission line losses, which can be made negligible), the component microstrip line sections are all effectively connected electrically in parallel with each other across the feed line 15, 16.

In the TEM microstrip transmission line mode, the transverse electric field is mostly confined to the space between the narrow and wide conductors. The wide conductor acts as a shield for the narrow conductor, in a manner analagous to the outer conductor of a coaxial line, and there is very little radiation. The individual half wavelength microstrip line sections operate in this usual mode to provide the electrically parallel connections to the feed line as described.

Owing to the arrangement of alternate wide and narrow conductors as in FIG. 1, an additional, highly efficiently radiating mode is excited. There is a voltage maximum between the adjacent ends of wide regions 3 and 12, another voltage maximum of opposite phase

3,757,342 between the right end of region 12 and the left end of wide region 4, and so on. The wide regions constitute a collinear array of radiators, excited by alternately opposite voltages between their adjacent ends. These voltages produce codirected cophasal currents along the length of the array, causing radiation in the normal manner of such an array. The wide regions 3 and 5 at the ends are slightly longer than the half wavelength narrow regions 8 and 11 to compensate end effects of the radiating array.

The array of FIG. 1 radiates equally in both directions from the plane of the wide regions. The ground plane 17 acts as a reflector to direct the radiation upwardly in FIG. 1, in a pattern that is relatively narrow in the E plane and wide in the H plane.

Referring to FIG. 2, two thin conductive strips 21 and 22 are supported on a plate 23 of dielectric material. The strips consist of alternate wide and narrow regions extending generally parallel to each other with each wide region adjacent to a respective narrow region of the other strip, separated from it by a small gap 24.

The transitions between wide and narrow regions may be tapered as shown, with the gap 24 extending obliquely to the longitudinal axis of the structure, and are spaced at half line wavelength intervals. The conductors 25 and 26 of a balanced transmission line, extending perpendicular to the plate 23, are connected to the strips 21 and 22 respectively at the central transition region.

The conductive strips 21 and 22 cooperate as an open wire line of resonant length, two wavelengths in this example, open circuited at both ends.

Voltage at the resonant frequency, applied by the feed line 25, 26 across the gap 24 at the central transition, excites the line 21, 22 in a TEM mode, producing cophasal voltage maxima at its ends and maxima of the opposite phase at the two intermediate transistions between wide and narrow regions. The transverse electric field of the TEM mode is largely confined to the gap 24 between the edges of strips 21 and 22 and produces negligible direct radiation.

The wide regions of strips 21 and 22 act as radiators of a collinear array, excited by the alternately opposite voltages across the gaps between their adjacent ends. These voltages produce codirected cophasal currents longitudinally of the array, with resulting radiation as in the array of FIG. 1.

Referring to FIG. 3, four collinear arrays 31, 32, 33 and 34, each similar to that of FIG. 1, are placed side by side on a single dielectric substrate 35 to form a sheet array. The substrate 35 is illustrated as translucent to show parts of the conductive pattern on its lower surface, and as partially broken away to show certain details.

The lower pattern includes a wide strip 36 extending the width of the array and forming a ground plane for two narrow microstrip conductors 37 and 38 on the upper surface, which act as parallel feed lines for the entire array. An external coaxial feed line 39 is coupled in opposite phases to lines 37 and 38 by way of a balun 40, also of microstrip construction. The component collinear arrays 31-34 are connected to the feed lines 37, 38 at one wavelength intervals along the feed lines to provide cophasal excitation and to maintain fixed relationships between the voltage amplitudes at the respective feed points.

In this example, the upper half of each collinear array is fed from line 37 through a narrow strip 41 formed in a meandering pattern to provide an electrical path of one half line wavelength in conjunction with the substantially shorter wide conductor forming the upper part of the central radiator of the collinear array. The lower halves of the collinear arrays are similarly fed from line 38. This arrangement ensures that the voltage at the gap between a central radiator and the endwise adjacent radiator will be determined only by the characteristics of this meandering line and that voltage between the respective feed line and its ground plane at the point of connection.

The widths of the feed line conductors 37 and 38 are stepped as at points 43 and 44 for impedance transformations in known manner to taper or reduce the excitation of the outer arrays 31 and 34 with respect to that of the inner arrays 32 and 33. Similarly, the excitations of the outer elements of the collinear arrays are tapered by means of steps 45 in the widths of the narrower strips. The steps at points 42 and 48 provide transformation for impedance matching the antenna structure to the characteristic impedance of the balun structure.

In the operation of the structure of FIG. 3, the collinear arrays 31-34 are cophasally excited through lines 37 and 38, and all radiators carry codirected longitudinal currents for radiation as a current sheet. An important feature of this invention is that the illustrated arrays, as well as similar but larger ones, can be excited from a single feed port without any mechanical transpositions or feed line crossovers. The term “untransposed” as used in the claims is intended to mean the absence of such transpositions. The term “open transmission line” means a line wherein neither conductor encloses the other. Although the line conductors are illustrated as imperforate films, it is to be understood that other field defining structures such as screens, meshes or multiple parallel-connected conductors may be used instead.