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"'34 3 " 'Z5 6
June 4, 1963
J. G. MocANN ETAL
3,092,834
SPLIT FARABOLIC RADAR ANTENNA UTILIZING MEANS T0
DISCRIMINATE AGAINST CROSS-POLARIZED ENERGY
Filed Deo. 23, 1958
2 Sheets-Sheet 1
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June 4, 1963
J. G. MGCANN ETAL
3,092,834
SPLIT PARABOLIC RADAR ANTENNA UTILIZING MEANS TO
DISCRIMINATE AGAINST cRoss-PoLARïzED ENERGY
Filed DGO. 23, 1958
2 Sheets-Sheet 2
MG. zz.
INVENToRs
Jog 9. Mcm/VN
United States Patent Oil ice
3,092,834
Patented June 4, 1963
1
3,092,834
SPLH‘ PARABOLEC RADAR ANTENNA UTILIZHNG
MEANS T0 DISCRIMINATE AGAI YST CROSS
POLARIZED ENERGY
2
and other disadvantages of the prior art by providing in
combination therewith, means associated with a split
radar dish to discriminate against cross-polarized electro
magnetic energy. A deiinition of cross-polarized electro
magnetic energy will be given hereinafter. However,
even the general manner in which the invention performs
its desired function, i.e., the manner in which it eliminates
Joe G. McCann, Encino, Robert I. Stegen, Van Nuys, and
William llalstrom, Encino, Calif., assignors, by mesne
assignments, to Canoga Electronics Corporation, Van
Nuys, Calif., a corporation of Nevada
the above-described phase shift error in the modulation
Filed Dec. 23, 1958, Ser. No. 782,599
envelope produced by off-boresight targets during track
10 Claims. (Cl. 343-756)
10 ing, cannot be understood without resorting to at least
This invention relates to airborne radars and, more
particularly, to an antenna system for a somewhat un
symmetrical generally dish-shaped or parabolic reflector
used in radar tracking systems.
some basic antenna theory.
Firstly, portions of the prior art parabolic reilector
may be defined as being made up of four identical quad
rants divided by perpendicular E- and H-planes through
In the past, it has been the practice to use a dish-shaped 15 the symmetrical axis thereof. When a plane wave of a
given polarization is intercepted by a quadrant, currents
reflector or “radar dish” split in half by a plane through
flow in planes parallel to both of the E- and H-planes.
its symmetrical axis. The halves are generally kept to
This may be understood by recognizing that the inter
gether for searching operation; however, it has been the
ception of a plane through a line perpendicular to the
practice to rotate these halves through a small angle
about a line tangent to the symmetrical center of the 20 axis of symmetry of a parabola of revolution, intercept
ing the axis at the point of feed or illumination, and the
parabolic surface in the plane of the split away from each
parabola itself delines a predetermined curve. The pro
other to a position in which they are maintained station
jection of this curve on a plane perpendicular to the E
ary with respect to each other for tracking operations.
and H-planes is then circular. Orthogonal vector cur
In this case, the halves are illuminated with electromag
netic energy having a plane of polarization or E-plane 25 rents combining to make for “circular” current tlow thus
explain why currents actually do flow in planes parallel
through the symmetrical axis of the antenna system par
to the H-plane. Electromagnetic energy transmission
allel to the split.
caused by current flowing in planes parallel to the H
During tracking, both halves are rotated in a conven
plane on the radar dish conductive surface is called
tional manner although their symmetrical axes are main
tained at a lìxed angle with respect to each other. The 30 “cross-polarized.” This phenomenon is not unknown in
the prior art and is explained in vol. l2, Radiation Lab
split arrangement is employed to broaden the main lobe
oratory Series, page 419. Reflection from a radar-dish
of the antenna in a plane perpendicular to the E-plane,
of cross-polarized energy exists in the above-mentioned
i.e., the H-plane.
four quadrants but radiation from adjacent quadrants
To the present time, a serious disadvantage has ac
companied the use of the above-described split reflector 35 are out of phase with respect to each other although,
in phase transmission, the energy at 'the common point
during tracking. As is well known, an approximately
of feed from which ‘they were derived was in-phase
sinusoidal modulation envelope is normally imposed on
energy. Thus, to the present time, cross-polarized lobes
received electromagnetic energy by reflection from ay
were thought unimportant, firstly, because it was thought
target which is spaced from the axis of rotation during
rota-tion in tracking. The angular position of the target 40 they would cancel each other out to a certain extent and,
secondly, because they would simply add a second har
monic signal to the fundamental frequency of signal of
relative to the radar transmitter is thus conventionally
determined for tracking or other purposes by measuring
the magnitude of this modulation- and comparing its
the modulating envelope produced by an oE-boresight
target during the antenna rotation of tracking. In the
phase with an alternating analog signal representative of
the instantaneous rotating position of the reflector. The 45 prior art, the second harmonic signal was theorized be
magnitude of the modulation envelope, thus, is a func
tion of the angle between the rotational axis and a line
cause antenna rotation would carry the two nearest-to
target cross-polarized lobes alternately nearer and far
ther away from a target while the main lobe would be
through the target and through the point of radiation,
carried toward and away from the target only once dur
and the phase of the modulating envelope is a function
of the angular position of a plane through the same line 50 ing a single cycle of antenna rotation. According to this
theory, a selective filter to pass only the fundamental fre
and the axis of rotation and another fixed reference
quency signal of the modulating envelope and to dis
plane through the axis of rotation.
criminate against the second harmonic signal produced
The parabolic split radar dish operates satisfactorily
by the cross-polarized lobes should have worked al
generally when a target exists at a relatively large angle
from the axis of rotation, but a phase shift of the modu 55 though it did not. Thus, to the present time it has al
ways been assumed that electromagnetic energy received
lating envelope occurs at relatively small angles which
in two adjacent cross-polarized lobes would both add to
introduces error. IIn fact, at a certain small angle for
energy received in the principal plane of polarization in
a split radar dish of a given geometry, the phase shift
the main lobe.
error appears to be approximately 90 electrical degrees.
The split radar dish thus has become practically useless 60 In accordance with the invention, a theory was worked
out, which 4as yet cannot be proved analytically, but
for accurately tracking targets located close to the axis
which has been proved experimentally, lthat a fundamen
of rotation of the dish or close to “boresight.”
tal frequency phase shift error could be produced by
The present invention overcomes the above-described
3,092,834
4
row 49 indicates the direction of the E-plane while ar
row 50 indicates the direction of the H-plane.
As can be seen in FIGS. 5 and 6, each half 21 and
cross-polarized lobes if one added and the other sub
tracted from energy of the principal polarization that was
received in the main lobe. A‘lthough there is still no ade
quate analysis which bears out this theory, the desired
result is obtained because when radiation of cross-polar
ized energy is substantially prevented in accordance with
22 of reflector 23 is parabolic in shape having an inner
lamination 31 and two outer laminations 32 and 33 all
of which may be made of any convenient dielectric fab
ric material. A plurality of conductors 34 are embedded
in a dielectric material 35 to maintain a fixed spaced
position. It is to be noted that conductors 34 conform t0
the parabolic shape of reilector halves 21 and 22 but
the invention, the split dish operates satisfactorily with
out introducing noticeable error in the position indica
tion of any oif-boresight targets. It could be argued
that reflected or received cross-polarized energy is gener
they are aligned in parallel planes substantially parallel
to the plane of polarization of energy directed toward
main lobe and when received in an adjacent cross-polar
halves 21 and 22 by feed 30.
Another embodiment of the invention is shown in
ized lobe is subtracted from the energy received in the
main lobe; however, there is no physical basis even to 15 FIGS. 7, 8 and 9 where conductive parabolic halves 36
and 37 are made of a sheet metal material having elon
support this hypothesis. However, in spite of the mis
gated slots 38 therein best illustrated in FIG. 8. It is
leading presumptions made in the prior art, the inven
to be noted that slots 38 are ranged in two sets of rows,
tion has proved successful. In accordance with the in
the first set being shifted lengthwise from the second set
vention, cross-polarization is completely eliminated and,
with it, phase shift error which was produced by the split 20 a distance equal to one-half the length of slots 38 plus
one-half the length of the distance between slots. This
radar-dish of the prior art.
means that cross-polarized energy tending to cause cul'
The invention will be better understood when consid
rents to ilow in the direction of arrow 39 will be inter
ered in connection with the following description.
ally only of one phase and that when this is received in
one lobe it is actually added to energy received in the
rupted by the slots, whereas current flowing in planes
parallel to the direction of polarization and thereby paral
In the accompanying drawings, which are to be re
garded as merely illustrative:
lel to a symmetrical plane through the center of the re
flector halves 36 and 37 indicated at a split 40 will not
be interrupted because current induced by such radiation
FIG. 2 is a schematic diagram illustrating the shape
may flow continuously in the direction of arrow 41’ be
of the reliector shown in FIG. 1;
FIG. 3 is a front perspective view of the apparatus of 30 tween the rows of slots in an uninterrupted fashion.
As shown in FIGS. 10, 1l and 12, a parabolic solid
the invention shown in FIG. 1;
conductive reilector 41 may also be employed when split
FIG. 4 is a front elevational view of a split radar
on the line 42, and semi-.circular dielectric sheets 43 may
dish made in accordance with the invention;
FIG. 5 is a side elevational view of a ilat edge of one
be mounted by suitable means 44 on halves 45 and 46 of
of the halves of the dish shown in FIG. 4;
35 reflector 41. Conductors 47 may be embedded in the
FIG. 6 is an enlarged sectional view of half the radar
material of a dielectric sheet 43 as shown in FIG. 12.
It is to be noted that conductors 47 are perpendicular
reflector taken on the line 6-6 shown in FIG. 5;
FIG. 1 is a rear perspective view of the apparatus of
the invention;
FIG. 7 is a front elevational view of a radar-dish
to line 42 whereas this is not the case in the other em
made in accordance with an alternative embodiment of
the invention;
bodiments of the invention. This is true for the reason
40 that reflected cross-polarized energy is again freilected
FIG. 8 is -an enlarged view taken at area 8 shown in
FIG. 7;
FIG. 9 is a sectional view taken on the line 9_9
back away from reflector 41 by conductors 47 whereas,
for example, conductors 34 act as a “sieve” and cross
polarized energy simply passes through it.
shown in FIG. 8;
Thus, by use of the conductors of FIG. 5, the slots
FIG. 10 is a front elevational view of still another 45 38 and reflector halves 36 and 37 shown in FIG. 7, 0r
embodiment of the invention;
the use of dielectric sheet 43 with conductors 47, cross
FIG. 11 is a side elevational view of one of the halves
polarization can be prevented since current flow in a `di
of the radar-dish shown in FIG. 10; and
rection perpendicular to the principal plane of polar
FIG. 12 is an enlarged sectional view of half of the
ization may be substantially eliminated by the use of
antenna system shown in FIG. 11.
50 parallel spaced conductors. As stated previously, this
In FIG. 1, a conventional airborne radar is indicated
prevents unwanted phase shift error of a radio frequency
at 10 having special means 20 to open two halves 21 and
signal modulation envelope which accompanies the use
22 of dish-shaped reflector 23 to a position shown at
of split reliectors such as those indicated in the drawings
23 in FIG. 2. Halves 21 and 22 may be hinged as in
during nutation.
dicated at 48. In the position shown in FIG. l, halves 55
21 and 22 are disposed at an angle indicated at A in
FIG. 2, the positions of halves 21 yand 22 being indi
Although only a few specific embodiments of the in
vention have been `shown and described, it is to be un
derstood that this invention is by no means «limited there
cated at 21’ and 22’. The means 20 to open halves 21
to. Many changes and modifications will, of course, sug
and 22 may simply include projections 25 and 26 having
gest themselves to those skilled in the art. Thus, the in
`cables 27 and 28 iixed to them, adapted to be pulled to 60 vention is not to be limited by the above disclosure, the
gether by any motive power means or mechanical con
true scope of the invention being delìned only in the ap
nection therewith. Halves 21 and 22 may even be opened
manually since halves 21 and 22 will not be moved with
pended claims.
‘ What is claimed is:
respect to each other during tracking or searching but
1. An aircraft radar antenna system comprising: a
only at a switching time when it is desired to change 65 generally dish-shaped reñector split into halves at its
from a searching to a tracking operation, or vice Versa.
center by a plane through its symmetrical axis with each
When no force ‘is applied to cables 27 and 28, halves 21
half being rotated in opposite directions away from the
and 22 may be kept normally closed by a spring 29.
other so that the intersecting obtuse angle formed by
Dish 23 is illuminated by a feed 30 shown in FIG. 3,
chords in a common plane subtending the arcs of each
which may be conventional, to »direct electromagnetic
of said reflector halves in said common plane is increased
energy toward halves 21 and 22, having a plane of polar
so as to produce an antenna pattern having a main lobe
ization through the axis of feed 30 extending symmetri
broader in its ~H-plane than a perfect parabolic reflector
cally through the split between halves 21 and 22. The
of the same size; and means associated with said reflector
front elevational view of FIG. 4 shows halves 21 and 22
to discriminate against electromagnetic energy received
when »they are closed for the searching operation and ar 75 by said antenna system having a predetermined plane of
3,092,834
5
6
and through the symmetrical axis of said reflector.
2. An aircraft radar antenna system comprising: a
dish-shaped reflector split at the center to provide two
halves, each of said halves being rotated in opposite di
to said dielectric support means in parallel planes perpen
dicular to said plane of reference.
7. A radar antenna system comprising: a reflector in
cluding two individual substantially identical halves, said
rellector having a generally parabolic shape when said
polarization perpendicular to a plane through said split
halves are maintained in a first position contiguous to
rections away from the other so that the intersecting ob
tuse angle formed by chords in a common plane sub
each other, each of said reflector halves having coincident
symmetrical axes 'and a predetermined plane of separa
tending the arcs of each of said reflector halves in said
tion when maintained in said first position, said reflector
common plane is increased, said reflector having con
ductor means extending substantially only in one direc 10 halves being rot-atable from said first pois-tion to a second
tion parallel to the direction of the split of said reflector.
position such that each of Isaid reflector halves is rotated
at an angle in a direction opposite to and away from the
3. An aircraft radar antenna system :comprising: a
other so that the intersecting obtuse angle `formed by
dish-shaped reflector split in the center for providing two
halves, each half having its respective split edge disposed
chords in -a common plane subtending the -arcs of each of
away from the split edge of the other halt` at an angle 15 said reflector halves in said common plane is increased,
located in a predetermined plane perpendicular to the di
said rellector halves being rotatable about an axis perpen
rection of said split so that the intersecting obtuse angle
dicular to said coincident symmetrical axes, said rotatable
formed by chords in a common plane subtending the arcs
axis being in said plane of separation; and means associ
of each of said reflector halves in said «common plane is
ated with said reflector to discriminate against electro
increased so as to produce an antenna pattern having a
magnetic wave energy having a plane of polarization per
main lobe broader in said perpendicular plane than a
pendicular to said plane of separation.
_ perfect parabolic reflector of a similar size, said reflector
8. A radar lantenna system comprising: a rellector in
having conductor means extending substantially only in
cluding two individual substantially identical halves, said
one direction parallel Ito the direction of the split of said
reflector h-aving »a generally parabolic shape when said
reflector.
25 halves `are maintained «in a lirst position contiguous to each
4. An aircraft radar antenna system comprising: first
other, each of said reflector halves having coincident
and second reflector halves, each of said reflector halves
symmetrical axes and a predetermined plane off separa
having a flat edge and being in »the shape of one-half of
tion when maintained in said first position, said reflector
a parabola of revolution about its axis of symmetry,
halves being rotatable from said first position to a second
each said reflector half having its respective flat edge dis 30 position such that each of said reflector halves is rotated
posed at an angle away from the flat edge of the other
at an an-gle in ra `direction opposite to and away from the
said reflector half in a predetermined plane passing
other so that the chords in a common plane subtending
through said axes of symmetry so that the intersecting
the arcs of each of said rellector halves in said common
obtuse angle formed by chords in a common plane sub
plane is increased, said reflector halves being rotatable
tending the arcs of each of said reflector halves in said 35 about an axis perpendicular to said coincident symmetrical
common plane is greater than if said halves were in a
axes, said rotatable axis being in said plane of separation,
perfect parabolic position, said reflector halves compris
said rellector halves comprises a dielectric material and
ing a dielectric material and a plurality of spaced linear
a plurality of spaced linear conductors having shapes
conductors having shapes similar to the intersection with
similar to the intersection of planes parallel to said plane
a parabola of revolution of planes parallel to said plane 40 of separation and a parabola of revolution.
of symmetry.
9. A rad-ar antenna system comprising: a reflector in
5. An aircraft radar antenna system comprising: first
cluding two individual substantially identical halves, said
and second reflector halves, each of said reflector halves
reflector having Ia generally parabolic shape when said
having a flat edge and being in the shape of one-half of
halves are maintained in -a lirst position contiguous to
a parabola of revolution about its »axis of symmetry, each 45 each other, each of said reflector halves having coincident
said reflector half having its respective llat edge disposed
symmetrical axes and a predetermined plane of separa
at an angle away from the flat edge of the other said re
tion when maintained in said first position, said reflector
flector half in a predetermined plane passing through said
halves being rotatable »from said first position toa second
axes of symmetry so that the intersecting obtuse langle
position such that each of said reflector halves is rotated
formed by chords in a common plane subtending the arcs 50 at an angle in a direction opposite to «and -avvay vfrom the
of each of said reflector halves in said common plane is
other so that the intersecting obtuse angle formed by
greater than if said halves were in a perfect parabolic
chords in a common plane subtending the arcs of each of
position, said reflectors being made entirely of a oon
said reflector halves in said common plane is` increased,
ductive material with a plurality of first :and second alter
said reflector halves ‘being rotatable about «an axis perpen
55
nate rows or' elongated slots therethrough, the axis of each
dicular to said coincident symmetrical axes, said rotat
of said rows of slots extending in parallel planes parallel
able 4axis being in said pla-ne of separation, said reflector
to said plane of symmetry, said first rows being spaced
halves being made entirely of «a conductive material with
lengthwise from said second rows a distance approxi
a plurality of first and second alternate rows of elongated
mately equal to one-half of the length of the slot plus one
slots therethrough, the axes of said rows of slots extend
60 ing in planes parallel Ito said plane fof separation when
h-alf the distance between slots.
6. An aircraft radar antenna system comprising: first
said reflector halves are maintained in said first position,
and second reflector halves, Ieach of said reflector halves
said first rows being spaced lengthwise from said second
having a flat edge and being in the shape of one-half of
rows a distance approximately equal to one-half the length
a parabola of revolution about its yaxis of symmetry, each
of a slot plus one-half the longitudinal distance between
said reflector half having its respective llat edge disposed 65 slots.
at yan angle away from the llat edge 4of the other said
l0. A radar antenna system comprising: «a reflector in
reflector half in a predetermined plane passing through
said axes of symmetry so that the intersecting obtuse
cluding two indivi-dual substantially identical halves, said
rellect-or having a generally parabolic shape when said
angle formed by chords in a common plane subtending
halves are maintained in a first position contiguous to
the arcs of each of said reflector halves in said common 70 each other, each of said reflector halves having coincident
plane «is greater than if said halves were in a perfect para
symmetrical axes and a predetermined plane of sepa- f
bolic position, said rellectors being made entirely of a
ration when maintained in said first position, said reflector
halves being rotatable from said first position to a second
position such that each of said reflector halves is rotated
cover it, and a plurality of linear spaced conductors lixed 75 a-t an angle in a direction opposite to and away from
conductive material with a dielectric supporting means
affixed to the peripheral edge of said reflector so as to
3,092,834
7
8
the other so that the intersecting obtuse angle formed
in parallel planes perpendicular to said plane of separa
by chords in a common plane subtending the arcs of each
tion.
of said reñector halves in said common plane is increased,
References Cited in the fue of this patent
said reflector halves -being rotatable about `an taxis perpen
dicular to said coincident symmetrical axes, said rotatable 5
UNITED STATES PATENTS
axis ‘being in said plane of separation, s'aid reflector halves
2,408,373
Chu _________________ __ Oct. 1, 1946
being made entirely of =a conductive material, -a dielectric
2,522,562
Blitz _______________ __ Sept. 19, 1950
supporting means fixed fto said reflector halves in 'a po'sition to cover said reñe'ctor halves, anda plurality of linear
spaced conductors fixed to said ydielectric support means 10
2,597,339
2,790,169
2,870,440
Krutter _____________ __ May 20, 1952
Sichak _____________ __ Apr. 23, 1957
Butler _______________ __ Jan. 20, 1959
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