Diplexer for a reflector antenna for transmitting microwave signals, comprising: - a common circular signal waveguide for transmitting a transmitted signal and a received signal, which comprises a first end and a second end, a common gate being formed at the first end; - a waveguide arrangement, which is arranged adjacent to the second end of the signal waveguide, coaxially with respect to the signal waveguide; - a cylindrical coupling section having a number of grooves, which is arranged between the first and the second end of the signal waveguide, joins the second end and connects the waveguide arrangement to the common signal waveguide; characterized in that in order to form a first and a second coaxial waveguide gate (21, 22), the waveguide arrangement comprises: - a first circular waveguide, in which the received signal can propagate during operation of the diplexer; - a second circular waveguide, in which the transmitted signal can propagate with a lower frequency than the received signal during operation of the diplexer, wherein the second waveguide surrounds the first waveguide; - arranged in the interior of the first waveguide there is an internal conductor, which is formed in the manner of a pin with a cylindrical outer contour, by means of which in the received band, as the TM01 mode is fed in at the common signal waveguide, the energy can be transferred into the TEM mode of the first waveguide, and - at the end of the second waveguide that faces away from the common signal waveguide, four transmitting waveguides arranged symmetrically with respect to one another and four receiving waveguides arranged symmetrically with respect to one another are provided, each having a rectangular cross section and being arranged orthogonally with respect to a longitudinal axis of the diplexer. - a common circular signal waveguide for transmitting a transmitted signal and a received signal, which comprises a first end and a second end, a common gate being formed at the first end; - a waveguide arrangement, which is arranged adjacent to the second end of the signal waveguide, coaxially with respect to the signal waveguide; - a cylindrical coupling section having a number of grooves, which is arranged between the first and the second end of the signal waveguide, joins the second end and connects the waveguide arrangement to the common signal waveguide; - a first circular waveguide, in which the received signal can propagate during operation of the diplexer; - a second circular waveguide, in which the transmitted signal can propagate with a lower frequency than the received signal during operation of the diplexer, wherein the second waveguide surrounds the first waveguide; - arranged in the interior of the first waveguide there is an internal conductor, which is formed in the manner of a pin with a cylindrical outer contour, by means of which in the received band, as the TM01 mode is fed in at the common signal waveguide, the energy can be transferred into the TEM mode of the first waveguide, and - at the end of the second waveguide that faces away from the common signal waveguide, four transmitting waveguides arranged symmetrically with respect to one another and four receiving waveguides arranged symmetrically with respect to one another are provided, each having a rectangular cross section and being arranged orthogonally with respect to a longitudinal axis of the diplexer.