Broadband Antenna Polarizer

Abstract

In a meanderline array random-polarizer comprising a plurality of stacked substantially identical arrays of laterally spaced square-wave shaped conductive strips or meanderlines arranged with parallel extending axes with each such axis spaced one array-period from its nearest counterparts, the improvement consisting of offsetting or staggering the meanderline axes of adjacent arrays by a distance preferably equal to one-half of the array-period. The meanderline axes in one set of alternate arrays are thus aligned in parallel planes spaced apart by an array-period; the meanderline axes in the remaining or second set of alternate arrays are also aligned in parallel planes spaced apart by one array-period; and the second set of parallel planes are offset or staggered by a distance equal to one-half of an array-period from the first set of parallel planes. A polarizer comprising a plurality of such staggered arrays has utility when placed in front of the aperture of pyramidal horn antenna for converting the linearly polarized wave of the horn to a circularly polarized wave.

Description

BACKGROUND OF THE INVENTION

This invention relates to polarizers and more particularly to a broadband polarizer for use in converting a linearly polarized wave to one having circular polarization.

A polarizer of the type to which this invention relates may, for example, be constructed as a radome or cover placed in front of the aperture of an antenna such as a pyramidal horn for the purpose of converting the linearly polarized wave in the horn to a circularly polarized wave on the other side of the polarizer. Such a polarizer consists of a plurality of arrays of metallic meanderline strips extending across the horn aperture at an angle of 45.degree. to the plane of polarization of the horn. The effect is to change by substantially 90.degree. the relative phase of the two orthogonally related componets of the linearly polarized signal of the horn antenna as both components propogate through the polarizer, thus achieving circular or near circular polarization on the side of the polarizer opposite from the antenna. The effectiveness and value of such a polarizer is often determined primarily by the frequency bandwidth over which the radiation pattern remains circularly or nearly circularly polarized, where the quality of the prevailing polarization relative to the ideal case (exact circular polarization) is described by the "axial ratio" of the radiated field. An electric field axial ratio of unity or 0.0 db corresponds exactly to circular polarization. A typical limit for the axial ratio of satisfactory "circularly polarized" antenna systems is .sqroot.2 in field strength variation, or 3 db.

One prior art construction of a polarizer of this type is called a meanderline array radome-polarizer comprising a plurality of meanderline arrays stacked in the direction of the horn axis. Each array comprises a plurality of meanderline conductors preferably formed by photoetching a thin, copper-clad dielectric board. Each line has a rectangular serpentine shape resembling a square wave with the conductive strip formed symmetrically about the longitudinal axis of the line so as to appear to "meander" about that axis. The axes of the meanderlines of the several arrays in the polarizer are parallel and adjacent lines in each array have the same lateral spacing and are aligned with each other in the direction of propagation of the electromagnetic waves through the polarizer. In other words, the axes of all the lines in the several arrays lie in parallel planes which are perpendicular to the planes of the arrays. When mounted in the operative position on the horn antenna, the axes of the meanderlines extend diagonally of the horn aperture at an angle of 45 degrees to the plane of polarization of the horn.

An advantageous and inherent characteristic of the meanderline array polarizer as compared to other prior art polarizers is that the former has no intrinsic or theoretical low-frequency operating limit. In any particular practical polarizer, however, a low-frequency operating limit always prevails, being set primarily by the physical dimensions of the meanderlines, the array-period and the total number of arrays employed. Decreasing such a low-frequency operating limit by any significant or useful amount cannot be achieved without employing a larger number of meanderline arrays of significantly different physical dimensions from those which set the original low-frequency operating limit above implied.

Attempts to extend the prevailing high-frequency operating limit already set as described above have not met with success. Antenna systems in which such polarizers are used are therefore bandwidth limited.

An object of this invention is the provision of meanderline array radome-polarizers which have no deterioration in low-frequency operating limits compared to the corresponding prior art polarizers but which are operative with substantial improvement or decrease in axial ratios to extended higher frequency limits.

SUMMARY OF THE INVENTION

In accordance with this invention, the upper-frequency operating limits of meanderline array radome-polarizers are significantly increased by staggering or laterally offsetting the meanderlines of adjacent arrays and such improvement is achieved with no deterioration or increase in low-frequency operating limits.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic elevational view of a prior art horn antenna and polarizer combination;

FIG. 2 is a front view of the polarizer taken on line 2--2 of FIG. 1;

FIG. 3 is greatly enlarged fragmentary view of the polarizer of FIG. 2 showing details of the meanderline array construction;

FIG. 4 is a section of the prior art polarizer taken on line 4--4 of FIG. 3;

FIG. 5 is a fragmentary plan view similar to FIG. 3 showing a polarizer with the positions of lines in adjacent arrays staggered in accordance with the invention;

FIG. 6 is a transverse section taken on line 6--6 of FIG. 5; and

FIGS. 7(a) and 7(b) illustrate comparative performance curves of polarizers embodying the invention and those of the prior art for five-layer and six-layer polarizers.

DESCRIPTION OF PREFERRED EMBODIMENT

Referring now to the drawings, FIGS. 1 and 2 illustrate a pyramidal horn antenna 10 connected at one end to a waveguide 11 and at the other or aperture end to a polarizer 12. Horn 10 is linearly polarized in the direction of the electric vector shown by the arrow E.sub.R. The purpose of polarizer 12 is to convert the signal passing through it from linear to circular polarization by delaying one of the orthogonal components E.sub.N of the vector E.sub.R by 90 degrees with respect to the other orthogonal component E.sub.P.

The aperture of horn 10 is rectangular as shown and polarizer 12 comprises a plurality of meanderline arrays 14, see FIGS. 3 and 4, which extend transversely of the horn axis A. Each of the arrays 14 comprises a plurality of conductive meanderlines 15, described in detail below, formed on a thin, low-loss dielectric sheet 16 preferably by photoetching a conductive covering on the sheet, and the arrays are separated and supported by low-loss spacers 17 such as polyfoam or thin-walled dielectric honeycomb material. The lines in each array have parallel axes M and the arrays are stacked to maintain this parallel relationship through-out the polarizer. As shown in FIG. 2, the polarizer is oriented with line axes M at an angle of 45.degree. with respect to the polarization E.sub.R of the horn. As a consequence, the meanderlines produce a phase differential for the components E.sub.P and E.sub.N of the vector E.sub.R such that the component E.sub.N normal to the axes M is delayed with respect to the orthogonal component E.sub.P. Selection of the number of arrays for polarizer 12 is made to provide the desired 90 degree phase differential between the components E.sub.P and E.sub.N to produce circular polarization.

Each meanderline 15 comprises a conductive strip which has a sepentine preferably rectangular shape and which extends longitudinally along and transversely of axis M in the manner of a square wave. Thus the conductor "meanders" from one side to the other of Axis M, which accounts for its name. Each meanderline is physically defined by its longitudinal period b, transverse length l, width w of the longitudinally extending legs, width s of the transversely extending legs, and the thickness t of the strip. The array is further defined by the distance a between axes of adjacent meanderlines, called the period of the array. The spacing between adjacent arrays is denoted as H. The dimensions of each meanderline, the array-period a and the inter-array spacing H are selected in accordance with the number of arrays in the polarizer so as to provide the approximately 90.degree. phase shift between orthogonal components needed to produce the nearly-circular polarization at all usable signal frequencies.

Another structural feature of prior art polarizers is the stacking of the substantially identical arrays 14 in such a manner that the meanderlines 15 on each array are aligned respectively with the meanderlines of all the other arrays. This is shown in FIG. 4 wherein the laterally spaced axes M of the meanderlines in the four arrays 14 are aligned so as to lie in planes P.sub.1, P.sub.2 and P.sub.3 which are parallel to each other and extend generally in the direction of propagation of the signal through the polarizer.

The foregoing description of the meanderline array radome-polarizer relates to a prior art construction and does not per se constitute this invention.

In accordance with this invention, a substantial improvement in the operating bandwidth of the meanderline array radome-polarizer is achieved with polarizer 18, see FIGS. 5 and 6. Polarizer 18 comrises a plurality of meanderline arrays 19a - 19d, inclusive, substantially identical to the foregoing arrays 14 and stacked in such a manner that the axes M and M' of meanderlines 15 and 15', respectively, in adjacent arrays are parallel to but offset from each other by a distance corresponding to one-half of the array-period a. Thus the axes of the lines in alternate arrays of the polarizer are aligned with each other, i.e., lie in the same plane, but are offset from the planes containing the axes of the lines in the other arrays by a distance preferably equal to a/2 or one-half the period of the array. Planes P.sub.1, P.sub.2 and P.sub.3 containing the axes of the meanderlines 15 of arrays 19a and 19c, see FIG. 6, are therefore displaced by a/2 from planes P.sub.1 ', P.sub.2 ' and P.sub.3 ', respectively, containing the axes of the meanderlines 15' of arrays 19b and 19d.

Polarizers embodying this invention were constructed, tested and compared with otherwise identical prior art polarizers shown in FIGS. 3 and 4 for each of a five-array and a six-array polarizer. These polarizers had an inter-array spacer thickness H equal to 0.220 inch and 0.180 inch for the five-array and six-array units, respectively, and the conductors were of thickness t=0.0015 inch and were photo-etched on 0.003 inch thick teflon-fiberglass dielectric sheet.

These polarizers were then tested in the following manner:

a. A pyramidal horn antenna with either of the polarizers placed on its aperture was employed as a receiving antenna with the peak of its radiation pattern pointing directly at a linearly polarized transmitting antenna. The radiation pattern of this latter antenna was boresighted precisely in the direction of the receiving antenna.

b. The transmission line connected to the transmitting antenna contained a high-quality rotary joint which permitted continuous rotation of the transmitted radiation pattern about its boresight axes. Such rotation causes a variation in the received signal level as a function of time, the variation being identically equal to the axial ratio of the polarizer. The signal variation (or axial ratio) described above was recorded employing conventional received-signal-strength recording equipment.

Results of these tests are shown in FIG. 7 wherein FIG. 7(a) shows a comparison of performance curves of the five-array polarizers and FIG. 7(b) shows similar curves for the six-array polarizer. The broken line curves show performance of the prior art polarizers of FIGS. 3 and 4, the solid line curves for the polarizers of FIGS. 5 and 6 embodying the invention. Using an axial ratio of 3 db as the tolerable limit for acceptable performance, FIG. 7(a) shows that five-array polarizer of the prior art operated satisfactorily over a frequency range of 6.9 to 14.7 GHz whereas the five-array polarizer embodying this invention with staggered arrays operated satisfactorily over the frequency range of 6.9 - 15.8 GHz, for a bandwidth increase of approximately 7.5 percent. It will be noted from FIG. 7(a) that the performance curves are substantially identical over most of the lower two-thirds of the frequency band and that the polarizer embodying the invention has a substantially improved axial ratio as well as an extended frequency range over the remaining upper portion of the band.

The performance curve shown in FIG. 7(b) for the six-array polarizer indicates that the polarizer embodying this invention produced an increase in the operating bandwidth of 12.5 percent over the otherwise identical polarizer of the prior art. As in the case of the five-array polarizer, the performance curves of the polarizers embodying the invention and of the prior art were substantially the same over the lower 65 percent of the operating frequency range with the polarizer embodying the invention likewise having a substantially improved axial ratio over the prior art polarizer for the remaining upper portion of the band.

It is believed that the axial ratio performance improvement provided by polarizers embodying this invention as demonstrated by the performance curves of FIG. 7 is due to a significant reduction in certain deleterious or highly unfavorable effects of higher-order mode coupling between various arrays in the meanderline array radome-polarizer.

Claims

What is claimed is:

1. A polarizer comprising

a plurality of meanderline arrays arranged in stacked spaced relation with the planes of the arrays parallel,

each array comprising a plurality of conductors formed in the configuration of meanderlines having parallel axes,

the axes of meanderlines of adjacent arrays being parallel to and offset from each other by a predetermined distance while axes of such lines in alternate arrays are aligned with each other.

2. The polarizer according to claim 1 wherein said predetermined distance is substantially equal to one-half the distance between axes of adjacent lines in each array.

3. In the combination of a pyramidal horn antenna and a meanderline array radome-polarizer, said antenna having a longitudinal axis and an aperture and adapted to be linearly polarized, said polarizer being disposed in said horn aperture transversely of the horn axis and having a plurality of arrays of substantially identical conductive meanderlines stacked in the direction of said antenna axis, each array having a plurality of laterally spaced meanderlines with parallel axes extending at an angle of 45.degree. to the plane of polarization of said horn, the improvement consisting of a polarizer in which adjacent meanderlines in adjacent arrays are laterally offset by a predetermined distance.

4. The combination according to claim 3 in which said distance is substantially the same as one-half the spacing between the axes of adjacnt lines in the same array.