U.S. patent number 3,754,271 [Application Number 05/268,479] was granted by the patent office on 1973-08-21 for broadband antenna polarizer.
This patent grant is currently assigned to GTE Sylvania Incorporated. Invention is credited to James J. Epis.
United States Patent |
3,754,271 |
Epis |
August 21, 1973 |
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.
Inventors: |
Epis; James J. (Sunnyvale) |
Assignee: |
GTE Sylvania Incorporated
(Mountain View, CA)
|
Family
ID: |
23023183 |
Appl.
No.: |
05/268,479 |
Filed: |
July 3, 1972 |
Current U.S.
Class: |
343/756; 343/786;
343/909 |
Current CPC
Class: |
H01Q
15/244 (20130101) |
Current International
Class: |
H01Q
15/00 (20060101); H01Q 15/24 (20060101); H01q
019/00 () |
Field of
Search: |
;343/756,786,909 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lieberman; Eli
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.
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.
* * * * *