Common antenna for primary and secondary radar system

A bifunctional antenna of a primary/secondary radar system comprises a reflector having a concave front surface formed with a row of slots along a horizontal generatrix, the slots lying in front of respective cavities excitable to radiate interrogation signals in a directive sum pattern and supplemental radiation in a differential control pattern designed to blank minor lobes of the interrogation pattern. Some of the cavities and slots are symmetrically duplicated on a dielectric cap covering the convex rear surface of the reflector. The reflector and its cap form a closed shell of dielectric material, specifically a glass mat impregnated with epoxy resin, overlain at the front by a fiber-glass fabric incorporating orthogonally intersecting insulated copper wires. The fabric also lines the inner walls of each cavity which is filled with dielectric material; its radiating slot is spanned only by horizontal wires paralleling the plane of polarization of target-seeking radiation from a source illuminating the reflector.

FIELD AND BACKGROUND OF THE INVENTION 
My present invention relates to a common antenna for primary and secondary 
radar systems. 
It is frequently necessary in a radar station to combine a number of 
antennas in the same operating location. However, this causes problems 
because the equipment in question has to be located in an area which is 
extremely restricted in the case of, for example, weapon systems. The 
combination of a primary radar antenna and a secondary radar antenna can 
be realized in two different ways. In one instance the antenna of the 
secondary radar is separate from that of the primary radar, the antennas 
installed in this way being essentially of the "beam" type. In the other 
instance the antenna of the secondary radar is integrated into the primary 
radar antenna, thus bringing about a true bifunctional antenna for the 
primary and secondary radars. 
A bifunctional antenna for primary and secondary radars is generally 
constituted by a single reflector illuminated by a confronting source in 
such a way as to radiate energy into space for the purpose of detecting a 
target such as an aircraft, this being called the primary radar function, 
and also to transmit an interrogation signal to an aircraft equipped with 
a transporter which automatically transmits its answer, this being called 
the secondary radar function. 
The radiated beam carrying the interrogation signal is effective in the 
direction where the aircraft has been detected. However, it has been found 
that the transponder of the interrogated aircraft or possibly that of a 
different aircraft could be triggered by secondary lobes of the 
interrogation diagram, whose level is liable to be relatively high 
compared with that of the major lobe. To obviate this disadvantage, the 
single antenna referred to 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 answer from the latter by the 
local receiver; these elements radiate in accordance with a 
quasi-omnidirectional control diagram whose level is such as to blank the 
secondary lobes of the interrogation diagram. 
This arrangement makes it possible, by comparing the amplitude of the 
pulses received from the transponder and those received from the control 
system in the associated circuits, to determine the pulse received in 
reply to the interrogation by the major lobe. 
The means for establishing the control diagram and affecting the 
transmission of an interrogation signal as well as the reception of a 
response signal from an interrogated target must be so designed that the 
gain of the associated control channel is greater than that of the 
interrogation and response channel in the angular zones containing the 
secondary lobes of the directional interrogation diagram, but much smaller 
in the direction of its major lobe. 
In existing constructions the control means comprise radiating members, 
namely wave emitters, whose radiation pattern is of the omnidirectional 
type, positioned on the common reflector close to its boresight axis or on 
its upper part. They may also serve as the transmission source of the 
interrogation signal emitted for a limited time in a directive radiation 
pattern. 
However, despite these precautions the radiation pattern of the control 
means does not completely fulfill its function, either because it is not 
totally omnidirectional or because certain high-level secondary lobes of 
the main directional pattern are not blanked and also because in some 
instances the major lobe may have such a low level as not to be absorbed 
by the omnidirectional diagram. Moreover, the control diagrams are 
disturbed by certain external structures, such as for example radomes 
under which the antennas are placed. 
Finally, all these additional members, such as wave radiators, cause 
masking phenomena of the primary source due to the shadow created by these 
radiators on the surface of the reflector. 
OBJECT OF THE INVENTION 
The object of my present invention is to obviate these disadvantages and to 
provide means for optimizing the diagram of the control channel of the 
secondary radar without disturbing the operation of the primary radar. 
SUMMARY OF THE INVENTION 
A bifunctional antenna according to my present invention comprises an 
arcuate array of radiators integrated into a reflector serving for target 
detection, i.e. for the primary radar function, these radiators performing 
the interrogation function with a sum-type radiation pattern and being 
used at least in part as control means whose radiation pattern is of the 
differential type. 
According to a more particular feature of my invention, the radiators 
serving as secondary radar transceivers are constituted by slots in a 
concave front surface of the reflector which are associated with radiating 
cavities distributed along a generatrix thereof preferably intersecting 
its boresight axis, the control channel being constituted by a certain 
number of slots in this array arranged symmetrically about that axis. 
In order to have an optimum directional pattern in the horizontal or 
azimuthal plane, I prefer to dispose these slots on a horizontal 
generatrix. The cross-section of the reflector in a vertical plane can be 
circular, elliptical or rectilinear.

DETAILED DESCRIPTION 
There is no longer any need to demonstrate the advantage of combining 
primary and secondary radar systems in the monitoring of space, 
particularly at approaches to airports or airfields. The primary radar 
detects the direction and distance of aircraft with respect to the antenna 
system and the secondary radar interrogates them; the transponders 
provided for this purpose on the aircraft transmit to the ground, i.e. to 
the interrogator, data relating to their altitude, identity, speed, etc. 
The interrogation of aircraft by the secondary radar takes place in the 
direction detected by the primary radar, so that it is of advantage either 
the couple the antennas of both radar systems or to use but a single 
antenna able to fulfill the two functions defined hereinbefore. However, 
as has been stated above, a conventional primary/secondary radar system 
has disadvantages which are prejudicial to its satisfactory operation and 
efficiency. Thus, as noted, the radiation pattern of the secondary radar 
has, in addition to a major lobe which transmits the interrogation and 
receives the response from the interrogated aircraft, secondary lobes 
whose level can be sufficient to trigger a transponder, the latter 
belonging either to the aircraft being interrogated or to another 
aircraft. In the latter case this can lead to errors which may have 
dangerous consequences. 
I have found that the inadequacies of prior attempts to obviate these 
disadvantages by suppressing the secondary or lateral lobes of the 
interrogation diagram can be obviated by forming on the one hand an 
interrogation/response radiation pattern of the sum or additive type and 
on the other hand a control-channel radiation pattern of the differential 
or subtractive type. The main advantage of the subtractive type is the 
fact that the centerline of the gap in the differential pattern is 
constant throughout the elevational range, thus giving a better centering 
of the interrogation arc and, in principle, an increased stability of the 
latter along the elevation range. Beyond the central zone of the radiation 
pattern the problem of blanking the lateral lobes of the radiation pattern 
of the primary radar is solved by a suitable choice of the amplitude and 
phase distribution of the radiating elements. For the 
interrogation/response channel the radiators are to be excited with 
additive phasing but with staggered amplitudes, as with a Gaussian 
distribution, to obtain a sum-type radiation pattern; an excitation of a 
certain number of these radiators distributed symmetrically about the 
boresight axis, with subtractive phasing, makes it possible to obtain a 
radiation pattern of the differential type for the control channel. 
The integration of the secondary radiators into the reflector of the 
primary antenna has the advantage of obviating any increase in the volume 
of the primary antenna, and consequently any increase in its weight and 
susceptibility to wind action. The driving mechanism for this device 
remains relatively simple and of small volume, which is particularly 
advantageous in weapon systems. 
FIG. 1 diagrammatically shows a sectional view of a common antenna 
reflector 1 for a primary and a secondary radar system, the reflector 
being concave toward a nonillustrated primary source and having a linear 
row 2 of a multiplicity of slot radiators generally designated 2.sub.i. 
The slots are arranged along a generatrix lying in a horizontal midplane 
of the reflector and preferably extend over the entire aperture thereof. 
The slot spacing h is of the order of 0.6 to 0.8.lambda. in a preferred 
embodiment. Reflector 1 has a body made from a dielectric material 3, 
namely an epoxy-resin-impregnated glass mat, covered by a fiberglass 
fabric 4 carrying two sets of orthogonally intersecting metal wires 40, 
41. These wires are generally made from copper of limited thickness. 
Behind each slot 2.sub.i of the arcuate array 2 is a parallelepipedic 
radiating cavity 5.sub.i whose walls are integral with and made of the 
same dielectric 3 as the body of reflector 1 and are covered by an 
extension of the fiberglass fabric 4 incorporating the wires 40, 41. The 
directions of polarization of the sources of the primary and secondary 
extensions are mutually perpendicular, specifically horizontal and 
vertical, respectively. In order to reflect both types of radiation, metal 
wires 40 and 41 cross one another over the entire surface of the reflector 
1 and also within the cavities 5.sub.i, yet in front of the slots there 
are only wires 40 arranged parallel to the horizontal generatrix and thus 
to the plane of polarization of the target-seeking radiation emitted by 
the primary antenna source illuminating the reflector. 
With a transmission frequency of 10.sup.4 MHz the diameter of metal wires 
40 and 41 may be 0.12 mm and the distance between them may be of the order 
of 1.5 mm. The covering of the metal wires by glass fibers gives the 
fabric a homogeneous elasticity. 
To reduce the volume of cavities 5.sub.i and provide a simply constructed 
monolithic assembly, the cavities are filled with dielectric 3. The 
exciting elements 6 of cavities 5.sub.i, of the piston or crossbar type, 
are inserted in the dielectric 3 filling the cavities and have coaxial 
bases 7 coupling the cavities 5.sub.i to coaxial lines 8 which connect 
them to a power divider 9 on the convex back surface of the reflector 1. 
This power divider 9, which can be constituted by distributors, is 
connected by an ultra-high-frequency feed line to a conventional system 
for generating outgoing interrogation signals and receiving incoming 
response signals. The back of the reflector is protected by a sealed cap 
10 forming therewith a closed shell essentially made of the aforementioned 
dielectric material 3. 
If it is found that the diagram of the control channel established by the 
forwardly radiating slots 2.sub.i does not ensure proper blanking of the 
rear part of the directional diagram of the interrogation channel, that 
control channel is provided with one or more supplementary rearwardly 
radiating elements. These additional radiators may be one or more slots 11 
formed in the dielectric material of cap 10 in line with cavities 12, 
conforming to the forwardly radiating cavities 5.sub.i of reflector 1. 
There are only a limited number of slots 11 and they are placed in cap 10 
in the plane of symmetry of reflector 1 containing the forwardly radiating 
slots. 
As noted above, it is by means of the power divider 9 that the cavities 
5.sub.i and 12 associated with the slots 2.sub.i and 11 are excited in 
order to generate a sum-type directional radiation pattern for the 
interrogation/response channel and a differential type pattern for the 
control channel. The slots of the control channel, no matter whether they 
radiate toward the front or the rear of the reflector 1, are subdivided 
into two equal groups which are excited in phase opposition by means of a 
.pi. phase shifter located in the power divider. 
As can be gathered from FIG. 2, a 0-.pi. hybrid phase shifter 15 has two 
output 13 and 14 which are in phase opposition and are respectively 
connected to terminals 16 and 17 of power distributor 9 for supplying the 
two groups 2', 2" of slots 2.sub.i forming part of the control channel. 
The phase shifter 15 has input terminals 130 and 140. 
FIG. 3 shows the radiation pattern I of the sum or additive type generated 
by the interrogation channel, assigned to the secondary radar function, in 
the azimuthal plane indicated by the abscissa axis .theta. (azimuth 
angle); the ordinate axis represents gain in dB. The width 3 dB of its 
major lobe 18, associated with the desired gain along the 
maximum-radiation direction or boresight axis, is large compared with that 
of adjacent low-level lateral lobes 19 which are flanked by lobes 20 
representing a still lower diffuse-radiation level. 
These characteristics should exist not only in the plane containing the 
boresight axis but also over the entire elevational aperture of the 
operating field of radiation, in order to ensure the blanking of the 
interrogation diagram or pattern by that of the control channel. 
FIG. 4 shows the directional pattern I of the interrogation/response 
channel overlain by a pattern C of the control channel of the differential 
type. The centerline of a gap 21 in the differential pattern C is the same 
as that of the major lobe 18 of the sum pattern I. The lateral lobes 19 of 
the radiation pattern I are submerged in the radiation pattern of the 
control channel C.