Radar antenna with rotating linear polarization designed to reduce jamming

A radar antenna of the Cassegrain type comprises a paraboloidal principal reflector confronting a hyperboloidal or flat auxiliary reflector of smaller radius directing linearly polarized radiation from a source on their common axis toward the principal reflector along lines passing through the focal point thereof to produce an axially extending outgoing beam whose central part is intercepted by the auxiliary reflector. The latter consists of an array of linear conductors generally parallel to the direction of polarization while the principal reflector is formed by another conductor array generally inclined at 45.degree. to that direction, the latter array being backed by a solid mirror separated therefrom by a distance smaller than a quarter wavelength at the midfrequency of the emitted radiation. The composite wave leaving the principal reflector has an elliptical polarization which it retains in an annular portion of the outgoing beam bypassing the auxiliary reflector; only a linearly polarized component of that wave rotated through 90.degree. from the original direction of polarization, however, passes the array of the auxiliary reflector in the central beam portion intercepted thereby.

FIELD OF THE INVENTION 
Our present invention relates to a radar antenna of the Cassegrain type 
rotating linear polarization, associated with means for tracking and 
angular-deviation measurement establishing an off-boresight curve which 
always has a notch in the same direction whatever the direction of the 
rectilinear polarization of the echo. 
BACKGROUND OF THE INVENTION 
A radar of this type used for tracking and for angular-deviation 
measurement, i.e. determining the offset in the position of the target 
with respect to the boresight axis of the antenna, presents a radiation 
pattern with linear polarization which is perpendicular to the direction 
of polarization of the wave emitted by the source illuminating the 
antenna. Since the skin echo reflected by the target, just like the signal 
sent by a jammer with which the target may be equipped, may have any 
direction of polarization whatever, the tracking radar can no longer 
accurately perceive the target. This is particularly true when the 
direction of polarization of the signal received is close to the normal 
operating polarization of the antenna, as may be the case with a rotating 
rectilinear polarization jammer. 
OBJECT OF THE INVENTION 
The object of our invention is to provide means in such a radar for 
reducing jamming with elimination of the above-mentioned disadvantages. 
SUMMARY OF THE INVENTION 
We realize this object, pursuant to our present invention, by providing a 
Cassegrain-type antenna with a paraboloidal principal reflector and an 
auxiliary reflector or smaller radius coaxially confronting each other, 
the auxiliary reflector being illuminated by a source of linearly 
polarized outgoing radiation centered on the reflector axis. The principal 
reflector includes a solid concave mirror and a first array of linear 
conductors disposed in parallel planes which include an angle of 
45.degree. with the direction of polarization of the radiation emitted by 
the source, this array being spaced from the mirror by a distance that is 
substantially different from and preferably less than a quarter wavelength 
at the midfrequency of that radiation. The auxiliary reflector is formed 
from a second array of linear conductors disposed in planes which are 
parallel to one another and to the direction of polarization of the source 
and which therefore direct the outgoing radiation from that source toward 
the principal reflector whence the radiation is sent out with elliptical 
polarization in an axially extending beam. Owing to the difference between 
the radii of the two reflectors, only a central part of the outgoing beam 
is intercepted by the auxiliary reflector whose conductors pass only a 
certain component of its radiation whose polarization is linear and 
perpendicular to the original direction; the radiation in an 
nonintercepted annular part of the beam retains its elliptical 
polarization. 
When the mirror and the conductor array of the paraboloidal principal 
reflector are homofocal, their common focal point may coincide with a 
focus of a hyperboloidally curved conductor array of the auxiliary 
reflector whose other focus coincides with the phase center of the source. 
Our improved radar antenna thus radiates, in addition to the pattern with 
linear polarization perpendicular to the direction of polarization of the 
wave emitted by the source, an elliptical polarization pattern; this 
property enables an off-boresight curve to be obtained having the same 
direction as that obtained with normal polarization, whatever the angle of 
the linear polarization emitted by the jammer, and is used in accordance 
with our invention to reduce the effect of a jammer with rotating linear 
polarization.

SPECIFIC DESCRIPTION 
The embodiment of our invention shown in the drawing comprises a Cassegrain 
type antenna with polarization rotation including a paraboloidal principal 
reflector 2 illuminated by a source 1 of electromagnetic waves with linear 
polarization. The antenna further includes a secondary reflector 3, formed 
by a hyperboloid, one of whose focal points is at the phase center of 
source 7 while the other is at the focal point F of reflector 2. The 
secondary reflector 3 may also be planar and located midway between point 
7 and the phase center of source 1 so that the image of the phase center 
coincides with point F. In any event, the diameter 2r.sub.2 of reflector 3 
is less than the diameter 2r.sub.1 of the principal reflector 2. The 
direction of the polarization of source 1 is assumed horizontal in the 
illustrated embodiment. The principal reflector 2 is formed by a 
reflecting parabolic mirror 4 and by a homofocal network or array 5 of 
parallel metal wires 6 placed at a given distance L therefrom which is 
substantially less than a quarter of a wavelength at the central frequency 
of the operating band. The wires 6 lie in axial planes including an angle 
of 45.degree. with the direction of polarization of source 1. The 
auxiliary reflector 3 is formed by a network or array of metal wires which 
lies in axial plane parallel to the direction of polarization of the wave 
emitted by source 1. The diameter and the spacing of wires 7 are suitably 
chosen so that the network is reflecting for the polarization of the wave 
emitted by source 1 and transparent for perpendicular polarization. Wires 
6 and 7 may be replaced by glass fibers enveloped by metal wires, they may 
also be metal strips. 
In FIG. 2 an annular portion of reflector 2 external to wires 7 can be 
seen. 
As shown in FIG. 3, which represents a substantially planar area of the 
network 5 of wires 6, the incident wave E.sub.i coming from source 1 and 
reflected by the network of wires 7 may be broken down in two orthogonal 
directions, a first component E.sub.1 being parallel to wires 6 and a 
second component E.sub.2 being perpendicular thereto. Component E.sub.1 is 
reflected on wires 6 with a phase change of 180.degree. giving a component 
E'.sub.1. Component E.sub.2 passes through network 5 and is reflected with 
a phase change of 180.degree. by mirror 4. The total phase shift undergone 
by E.sub.2 is equal to a phase shift of 180.degree. due to reflection at 
mirror 4 added to the phase shift corresponding to the round trip between 
mirror 4 and network 5. 
In a conventional Cassegrain antenna with polarization rotation, the 
round-trip path is equal to .lambda./2 causing a total phase shift of 
360.degree.. In the antenna according to our invention, this total phase 
shift is substantially different from 360.degree., preferably less with 
L&lt;.lambda./4; at E'.sub.2 we have indicated the projection in the plane of 
FIG. 3 of the vector E".sub.2 of the wave reflected by mirror 4 upon the 
direction perpendicular to wires 6, i.e. the component of that reflected 
wave clearing the network 5. Vectorial addition of this component E'.sub.2 
to component E'.sub.1 yields a resultant E.sub.3. This vector E.sub.3 may 
in its turn be broken down into a vector E'.sub.3 perpendicular to E.sub.i 
and a vector E".sub.3 parallel to E.sub.i. 
After this reflection at principal reflector 2, the composite wave whose 
direction of polarization is that of vector E.sub.3 forms an axially 
oriented beam whose central part, of radius r.sub.2, is intercepted by the 
conductor array of reflector 3. Component E".sub.3 parallel to wires 7 is 
reflected but the component E'.sub.3 perpendicular to wires 7 passes 
through the network. An outgoing wave W.sub.1 traversing reflector 3 is 
polarized linearly in a direction perpendicular to that of the wave 
emitted by source 1, whereas an outgoing wave W.sub.2 bypassing the 
reflector 3 remains polarized elliptically. It can be shown that the power 
loss due to the foreshortening of vector E'.sub.2 relative to vector 
E.sub.2 is equal to: 
##EQU1## 
where T=tan .phi./2 is the ellipticity coefficient with .phi. representing 
the difference between the phase shift undergone by the reflected 
component E.sub.2 in a conventional Cassegrain antenna with polarization 
rotation (i.e. 360.degree.) and the corresponding phase shift undergone by 
the corresponding component E".sub.2 in the present instance. 
The distance L and the ratio r.sub.1 /r.sub.2 are so chosen that the energy 
of the wave radiated by the antenna with elliptical polarization is lower, 
by 15 to 20 db, than the energy of the wave radiated with linear 
polarization. 
The elliptical-polarization pattern radiated by the annular bean portion 
bypassing the wires 7 of reflector 3 has then an aperture at half power 
which is less by about 20% than that of the principal pattern generated by 
the linearly polarized central beam portion. 
In the case of a monopulse tracking radar, whatever the polarization of the 
signal received, the sum channel will always have a nonzero output to be 
used as a reference for demodulation.