A radar level gauge using electromagnetic signals to determine a level of a object, comprising: a transmitter receiver circuit board accommodated in a housing; an antenna unit for transmitting and receiving electromagnetic signals; and a connection arrangement for connecting the transmitter receiver circuit board and the antenna unit with one another; wherein the transmitter receiver circuit board is so disposed in the housing as to extend vertically; wherein the connection arrangement includes a waveguide sequence which is fed at a feed terminal formed on the transmitter receiver circuit board; and wherein the waveguide sequence includes, in sequential order from the transmitter receiver circuit board to the antenna unit: a circular waveguide extending horizontally and perpendicular to the transmitter receiver circuit board; a rectangular waveguide in connection with the circular waveguide and extending vertically; and a second circular waveguide further in connection with the rectangular waveguide and extending vertically, the gauge further comprising: a dielectric body which is disposed in the waveguide sequence at and over the boundary between the rectangular waveguide and the second circular waveguide; characterized in that it further comprises an insulation dielectric body for electrically isolating a ground conductor of the transmitter receiver circuit board and the housing from one another, the insulation dielectric body being disposed at the boundary between the rectangular waveguide and the second circular waveguide and fitted over the dielectric body. a transmitter receiver circuit board accommodated in a housing; an antenna unit for transmitting and receiving electromagnetic signals; and a connection arrangement for connecting the transmitter receiver circuit board and the antenna unit with one another; wherein the transmitter receiver circuit board is so disposed in the housing as to extend vertically; wherein the connection arrangement includes a waveguide sequence which is fed at a feed terminal formed on the transmitter receiver circuit board; and wherein the waveguide sequence includes, in sequential order from the transmitter receiver circuit board to the antenna unit: a circular waveguide extending horizontally and perpendicular to the transmitter receiver circuit board; a rectangular waveguide in connection with the circular waveguide and extending vertically; and a second circular waveguide further in connection with the rectangular waveguide and extending vertically, the gauge further comprising: a dielectric body which is disposed in the waveguide sequence at and over the boundary between the rectangular waveguide and the second circular waveguide; characterized in that it further comprises an insulation dielectric body for electrically isolating a ground conductor of the transmitter receiver circuit board and the housing from one another, the insulation dielectric body being disposed at the boundary between the rectangular waveguide and the second circular waveguide and fitted over the dielectric body. a dielectric body which is disposed in the waveguide sequence at and over the boundary between the rectangular waveguide and the second circular waveguide; characterized in that it further comprises an insulation dielectric body for electrically isolating a ground conductor of the transmitter receiver circuit board and the housing from one another, the insulation dielectric body being disposed at the boundary between the rectangular waveguide and the second circular waveguide and fitted over the dielectric body.