[en] A multi-channel, fixed frequency heterodyne radiometer, operating in the radiation frequency range 73GHz to 127GHz, is used for electron cyclotron emission measurements on the JET tokamak. In a tokamak, the spatial variation of the magnetic field in the plasma provides the means by which spatially resolved measurements of the emission can be made. To obtain well localized measurements, it is necessary to minimize the volume over which the measured emission is averaged. The spatial resolution along the line of sight is determined by the spectral resolution of the radiometer which, in the case of the JET radiometer, is 500 MHz. This corresponds to a spatial resolution of ∼20.mm. The spatial resolution transverse to the line of sight is determined by the antenna pattern of the collection system. At present a waveguide system is used and this has a spot size of ∼20 mm at the plasma centre. A quasi-optical system which will be used to improve this is under construction and is described in this paper. The quasi-optical system has been designed to satisfy certain criteria. It must operate over a wide bandwidth; the bandwidth of the radiometer has been increased to cover 73GHz to 139GHz. It must transmit two orthogonal, linear polarizations with minimal cross-polar scattering because the radiometer measures the intensity of both polarizations independently. The double vacuum window through which the plasma is viewed has a diameter of ∼65mm which means that the beam radius, at the vacuum window, has to be ∼16.mm. In addition, the beam has to couple efficiently to oversized waveguide which is used to transport the radiation along a tortuous path, ∼40m long to the radiometer