The present invention relates to a compact differential coupler for a monopulse radar.
As the accuracy and speed of measurement by the process of angular tracking by scanning are limited, the angular tracking process by monopulse was developed and consists of carrying out angular measurements, whilst processing each pulse back from the target with a multidirectional antenna. For example an antenna formed by a parabolic reflector and two identical horns positioned symmetrically with respect to the focus of the reflector is used.
Each horn is connected to a receiver. If the target is on the focal axis of the antenna, the signals received by the two receivers linked to the two horns are identical. However, if the target is not on the focal axis, the receivers linked with the two horns do not receive the same signals. The comparison of the signals received in each receiver must then make it possible to locate the target with respect to the focal axis, as a result of an appropriate processing of the signals.
In a monopulse tracking radar, the measurement of the angles must take place in site and in bearing. Thus, the antenna is organized to supply a sum channel, a site difference channel and a bearing difference channel.
The primary monopulse source in a monopulse antenna with amplitude comparison supplies four signals on four guides making it possible to carry out radar tracking, following processing of the said signals. Behind the said primary monopulse source is placed a differential coupler constituted by four magic T's grouped in accordance with the diagram of FIG. 1.
The source transmits four waves A, B, C and D respectively on the four channels 1, 2, 3 and 4 of the coupler. Channel .epsilon. receives the sum of the powers of the signals collected by the four channels 1 to 4: A+B+C+D. Channel .DELTA.S effects the high-low difference .DELTA.S: (A+B)-(C+D) and channel .DELTA.G effects the right-left difference .DELTA.G: (A+C)-(B+D). Thus, there is a planar problem for each of the site and bearing angular deviation measurements.
Emission takes place by channel .epsilon., the antenna then behaving like a single lobe antenna and reception takes place on the three channels .epsilon., .DELTA.S and .DELTA.G. When the antenna is perfectly pointed toward the target channel .epsilon. receives a maximum power signal, whereas the difference channels .DELTA.S and .DELTA.G receive nothing. When there is a slight depointing, this signal received by channel .epsilon. is not significantly changed, but site and/or bearing depointing signals appear on channels .DELTA.S and .DELTA.G having a by no means negligible power.
As a function of their sign, it is known whether depointing has taken place in the upward or downward direction for the site difference channel .DELTA.S, and the right or left for the bearing difference channel .DELTA.G.
At present there are two types of construction of a differential coupler constituted by magic T's. According to the first type shown in FIG. 2, the magic T's 5, 6, 7 and 8 are produced separately and then assembled by connecting guides 9, and 10 and joining flanges 11, 12, 13 and 14. A differential coupler produced in this way has large dimensions due to the addition of the guides connecting the T's, is complicated as a result of the large number of parts to be machined and adjusted, and finally does not always have good decoupling characteristics because the symmetry of the parts and connections is not perfect, particularly due to the stray capacitances of the edges of the joining flanges.
According to the second constructional type of a differential coupler, the magic T's forming it are grouped by a mechanical brazing process making it possible to produce relatively small, but expensive assemblies. At the time of brazing, a significant amount of waste is produced and inevitably leads to deformations causing poor symmetry.