Broadband antenna polarizer

[57] ABSTRACT

In a meanderline array random-polarizer comprising a 4 Claims, 8 Drawing Figures

<img file="US3754271A_D0001.tif" />

PATENTE0MI621 1973

3.754.271

SHEET 1 OF 3

<img file="US3754271A_D0002.tif" />

<img file="US3754271A_D0003.tif" />

<img file="US3754271A_D0004.tif" />

rnivn hr i

FIG. 4

PATENTED AUG 211973

SHEET 2 OF 3

3,754,271

<img file="US3754271A_D0005.tif" />

FIG. 5

19q —

I9d —©*»

19b —©»»

<img file="US3754271A_D0006.tif" />

FIG. 6

PATENTED AUS 21 1873

3,754,271

SHEET 3 OF 3

RATIO (DB)

<img file="US3754271A_D0007.tif" />

FREQUENCY (f) - GH_Z

FIG. 7 (b)

3,754,271

BROADBAND ANTENNA POLARIZER

BACKGROUND OF THE INVENTION

This invention relates to polarizers and more particularly to a broadband polarizer for use in converting a 5 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 10 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° to the plane of polarization of the horn. The effect is to change by substantially 90° 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 V2 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 arrayperiod 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 upperfrequency 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 ³θ 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 adja³⁵ cent 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 ⁴⁰ 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 illus⁵ trate a pyramidal horn antenna 10 connected at one end to a waveguide 11 and at the other or aperture end to a polarizer 12. Hom 10 is linearly polarized in the direction of the electric vector shown by the arrow E_s.

₅θ The purpose of polarizer 12 is to convert the signal ^υ passing through it from linear to circular polarization by delaying one of the orthogonal components E_w of the vector E_R by 90 degrees with respect to the other orthogonal component E_P.

₅₅ 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 _6Q 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 thinwalled dielectric honeycomb material. The lines in ₆₅ each array have parallel axes M and the arrays are stacked to maintain this parallel relationship throughout the polarizer. As shown in FIG. 2, the polarizer is oriented with line axes M at an angle of 45° with re3,754,271 spect to the polarization E_K of the horn. As a consequence, the meanderlines produce a phase differential for the components E_P and E_N of the vector Ε_β such that the component Ε_Λ. normal to the axes M is delayed with respect to the orthogonal component E_P. Selec- 5 tion of the number of arrays for polarizer 12 is made to provide the desired 90 degree phase differential between the components E_Pand E_lV to produce circular polarization.

Each meanderline 15 comprises a conductive strip 10 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 15 physically defined by its longitudinal period b, transverse length I, width w of the longitudinally extending legs, width 5 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 interarray spacing H are selected in accordance with the number of arrays in the polarizer so as to provide the approximately 90° 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_I( P₂ and P₃which 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 ⁴⁵of 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„ P₂and P₃ containing the axes of the meanderlines 15 of arrays 19α and 19c, see FIG. 6, are therefore displaced by a/2 from planes P/, P₂' and P₃', respectively, containing the axes of the meanderlines 15' of arrays 196 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 25 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 ³⁰ 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 op³⁵ erated 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 fivearray 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.