Multimode feed for reflector antenna of a monopulse tracking system

A multimode feed for a reflector antenna of a monopulse tracking system, the multimode feed comprising:

• a receiving element having an aperture, configured to receive an electromagnetic wave and to generate, from the electromagnetic wave, a waveguide signal comprising a plurality of higher-order modes as a function of an incidence angle of the electromagnetic wave on the aperture;
• and
• a waveguide structure coupled to the receiving element and having a first output port and a second output port, the waveguide structure being configured to generate, from the plurality of higher-order modes, a first tracking error signal at the first output port and a second tracking error signal at the second output port,
• wherein the waveguide structure comprises:
• a first coupler configured to extract, from the waveguide signal, a first higher-order mode of the plurality of higher-order modes and to provide a first extracted signal;
• a second coupler configured to extract, from the waveguide signal, a second higher-order mode of the plurality of higher-order modes and to provide a second extracted signal;
• a third coupler configured to extract, from the waveguide signal, a third higher-order mode of the plurality of higher-order modes and to provide a third extracted signal;
• a fourth coupler configured to extract, from the waveguide signal, a fourth higher-order mode of the plurality of higher-order modes and to provide a fourth extracted signal;
• a first summing element forming the first output port and configured to sum the first extracted signal and the second extracted signal, thereby generating the first error tracking signal;
• and
• a second summing element forming the second output port and configured to sum the third extracted signal and the fourth extracted signal, thereby generating the second error tracking signal,
• wherein the first, the second, the third and the fourth couplers each comprise a main coupling line, the waveguide structure having a main waveguide configured for propagating the waveguide signal, the main waveguide comprising a first portion, a second portion and a third portion extending along a longitudinal axis, in succession, from the receiving element, wherein the first portion forms the main coupling line of the first coupler, the second portion forms the main coupling line of the second and the third couplers, and the third portion forms the main coupling line of the fourth coupler,
• wherein the first portion of the main waveguide supports the propagation of the first higher-order mode and the second portion of the main waveguide does not support the propagation of the first higher-order mode, and
• wherein the main coupling line of the first coupler has a circular cross-section and extends along the longitudinal axis between a front section having a first diameter and a back section having a second diameter smaller than the first diameter, the second diameter being smaller than the diameter associated with a cut-off frequency of the first higher-order mode,
• wherein the first, the second, the third and the fourth couplers each further comprise a respective secondary coupling line and a respective extraction portion configured to extract a mode propagating in the respective main coupling line into the respective secondary coupling line, the extraction portion being formed by at least one slot extending in a sidewall of the main coupling line,
• wherein the at least one slot of the first coupler, the at least one slot of the second coupler and the at least one slot of the third coupler are longitudinal slots extending along a direction parallel to the longitudinal axis, the at least one slot of the fourth coupler being a transversal slot extending along an axis transversal to the longitudinal axis.