Common antenna for primary and secondary radar

A bifunctional antenna of a primary/secondary radar system comprises a reflector having a concave front surface illuminated by a source of primary radiation and formed with a row of circular ports along a horizontal generatrix, the ports lying forwardly of respective cavities or waveguides of cylindrical configuration excitable to emit circularly polarized secondary radiation. In some embodiments the polarization may be changed to a linear or an elliptical mode under the control of inserts rotatable about the cavity axis.

CROSS-REFERENCE TO RELATED APPLICATION 
This application contains subject matter first disclosed in my copending 
application Ser. No. 105,733 filed Dec. 20, 1979, now U.S. Pat. No. 
4,284,991. 
FIELD AND BACKGROUND OF THE INVENTION 
My present invention relates to a common antenna for primary and secondary 
radar systems. 
Generally, when the antenna of the secondary radar is integrated with the 
antenna of the primary radar, thus forming a combined primary/secondary 
antenna structure, this bifunctional antenna comprises a single reflector 
illuminated by an associated source in such a way as to be capable of 
radiating outgoing energy into space for the purpose of detecting a target 
such as an aircraft, this being called the primary radar function, and 
also of emitting an interrogation signal in the direction of this aircraft 
which is assumed to be equipped with an onboard automatic responder termed 
a transponder, this being called the secondary radar function. 
The radiated beam conveying the interrogation signal is effective in the 
direction where the aircraft has been detected; however, it has been 
noticed that the transponder of the interrogated aircraft could be 
triggered by the secondary lobes of the interrogation pattern whose level 
is liable to be relatively high with respect to that of the principal 
lobe. To remedy this disadvantage, as noted in my above-identified prior 
patent, the single antenna here considered can be provided with 
supplemental radiating elements affecting the reception of the 
interrogation signal by the remote transponder as well as the reception of 
the response from the latter by the local receiver; these elements radiate 
in a substantially omnidirectional control pattern whose level is such as 
to blank the secondary lobes of the interrogation pattern. 
By means of a comparison in the associated circuits of the amplitude of the 
pulses received from the transponder with those received from the control 
system, this arrangement facilitates a detection of the pulse received in 
response to the interrogation by the principal lobe. The means for 
establishing the control pattern must be such that the gain of the 
associated control channel is greater than that of the interrogation and 
reception channel in the angular zones comprising secondary lobes of the 
directional interrogation pattern but much smaller in the direction of the 
principal lobe. 
According to the disclosure of my above-identified copending application 
and patent, the secondary radar function is performed by a row of linear 
radiating elements or transceivers, totally integrated in the reflector of 
the antenna, along a generatrix intersecting the boresight axis of the 
antenna by passing through the projection of its phase center onto the 
concave reflector surface. In that system the emission-reception source of 
the primary radar function radiates a wave which is polarized 
rectilinearly and orthogonally to the rectilinearly polarized wave emitted 
by the radiating elements of the secondary radar function. 
OBJECT OF THE INVENTION 
The object of my present invention is to extend the principle of 
integration of a radiating network in the reflector of the antenna to 
other types of radiating sources, particularly those emitting circularly 
or elliptically polarized waves. 
SUMMARY OF THE INVENTION 
In accordance with my present invention I provide a common antenna for a 
primary and secondary radar, of the type disclosed in my prior patent, 
comprising on the one hand a single reflector illuminated by an 
emission-reception source for the primary radar function and, on the other 
hand, a row of radiation emitters disposed--as in my prior 
system--entirely within the body of the reflector along a generatrix 
passing through the projection of the phase center of the antenna. 
Whereas, however, the radiation emitters of that prior system comprise 
linear exciters inside prismatic cavities terminating in rectangular slots 
on the reflector surface, the cavities of my present antenna are 
cylindrical and terminate in coaxial circular ports. Advantageously, 
pursuant to a more particular feature of my invention, each cavity 
includes polarizing means rotatable about its axis and interposed between 
the respective exciter and the concave reflector surface, e.g. within the 
circular port itself, to enable the selective generation of rectilinearly, 
circularly or elliptically polarized waves.

SPECIFIC DESCRIPTION 
As was mentioned above, a common antenna for primary and secondary radars 
may require different modes of polarization of the waves emitted by its 
several sources. Thus, if the source of primary radiation emits 
rectilinearly polarized waves, the sources of secondary radiation may emit 
a wave which is rectilinearly polarized and orthogonal to the first one, 
as in the system of my prior patent, or else a circularly or elliptically 
polarized wave, provided that these primary and secondary sources do not 
absorb each other's waves. Similarly, when the source of primary radiation 
emits a circularly or elliptically polarized wave, the sources of 
secondary radiation may emit a counterrotating polarized wave. 
In case the secondary radar function provides a control channel as 
described above, the latter may emit polarized waves different from those 
emitted by the primary source or even by the secondary sources. Various 
wave-control means for establishing the desired mode of polarization for 
the emitted secondary radiation will now be described. 
FIG. 1 shows schematically a sectional view of a common antenna reflector 1 
for primary and secondary radars, comprising an array 2 of radiating 
elements for the selective generation of rectilinearly, circularly or 
elliptically polarized waves. 
Reflector 1 is formed from a metallic or dielectric material 3, e.g. from 
an epoxy-impregnated glass mat covered with a tissue 4 of glass fibers 
carrying crossed, covered metal wires 40 and 41. These wires are generally 
made from copper strands of small thickness. 
The radiating elements of FIG. 1 each comprise a vertical exciter 6 
radially disposed in a cylindrical waveguide generally designated 2i, with 
i ranging from l to n where n represents the total number of elements in 
the group. A polarizing device 13 for the wave emitted by each radiator 2, 
6 is rotatable about the longitudinal axis .DELTA. of each guide, 
extending radially to the reflector surface, and is situated in a circular 
aperture of that surface. This polarizing device 13 is formed by a planar 
grid of parallel metal blades or by several juxtaposed sets of parallel 
metal wires. Such a grouping of wires enables better impedance matching, 
and the spacing as well as the number of these juxtaposed sets can be 
varied to allow adjustment of the width of the operating band of the 
radiation emitters 2, 6. In order to make the polarizing devices 13 
equivalent to a part of the reflector 1 with regard to the polarized wave 
emitted by the source of primary radiation confronting that reflector, the 
dimensions of each waveguide must be such as to establish a short-circuit 
plane at the front face of reflector 1. 
The wavelengths 2i are formed in reflector 1 from the same material as the 
reflector itself and are covered in the same way with a tissue 4 of glass 
fibers carrying peripherally and axially extending metal wires. 
To reduce the volume of waveguides 2i and to form a monolithic unit simple 
to construct, the guides are filled with dielectric 3. The exciting 
elements 6 for these guides 2i, which can be of the plunger or the 
crossbar type, are inserted into the dielectric 3 filling the guides and 
have each a coaxial base 7 allowing impedance matching between the guides 
2i and associated coaxial lines 8 which connect them to a power divider 9, 
placed at the back of the reflector 1 and identical with that described in 
my prior patent. 
When use is made of a control channel whose pattern, given by the radiation 
emitters 2, 6 opening onto the front face of reflector 1, does not ensure 
proper blanking of the rear part of the directional pattern or diagram of 
the interrogation channel, this control channel is provided with one or 
more additional, rearwardly radiating elements such as waveguides 11 
formed in the material of a cap or cover 10. These additional radiation 
sources 11 are similar to and axially aligned with some of the forwardly 
radiating elements. 
FIG. 2 shows another embodiment of my invention having radiating elements 
adjustable to emit rectilinearly, circularly or elliptically polarized 
waves. Here, the polarizing devices of waveguides 2i are inserts 14 in the 
form of generally H-shaped dielectric plates. Each insert 14 is 
symmetrical with respect to the longitudinal axis .DELTA. of the 
respective guide 2i and rotatable about this axis .DELTA. so as to vary 
the angle of inclination of its plane relative to the associated exciter 
6, depending on the channel whose radiation is to be controlled. Thus, a 
directional channel of the secondary radar may emit circularly polarized 
waves through a certain number of the elements 2i of row 2 whose 
polarizing devices 14 have been correspondingly oriented. 
Alternatively, such a polarizing device may be formed by adjustable iris 
diaphragms allowing the aperture of the guide to be modified according to 
the desired mode of polarization. 
If no change in the mode of polarization is required, I may provide the 
secondary-radiation emitters with fixed means for generating circularly or 
elliptically polarized waves, to the exclusion of each other. Such means 
may comprise an excitation element coiled in each cavity about the axis 
thereof. 
In FIG. 3, the radiation-generating elements are formed by helices 15 each 
placed in a cylindrical waveguide or cavity 16, similar to those of the 
preceding Figures, whose dimensions are again such as to establish a 
short-circuit plane at the front face of reflector 1. Each helix is 
connected to the power divider 9 by a coaxial line 150 and tapers toward 
the front aperture or port of its cavity. In FIG. 4, the radiating 
elements are spirals 17 placed in the circular apertures of the front wall 
of the reflector and are each excited by a resonant cavity 18 integrated 
in this same reflector dimensioned, as in the foregoing instance, to 
establish a short-circuit plane at the front face of the reflector. These 
spirals are formed mechanically or are deposited by photogravure on a 
dielectric wafer. The operating energy for each spiral is supplied through 
a coaxial line 170 which connects it to the power divider 9. 
In these two latter embodiments it is possible, as in the preceding cases, 
to place a few radiating elements in the rear cover so as to let the 
pattern of the control channel blank the rear secondary lobes of the 
radiating pattern of the interrogation channel of the secondary radar.