Systematic disagreements between ECE and Thomson Scattering diagnostics observed in various fusion devices have substantiated the proposal of the so called Oblique ECE diagnostics on JET. This layout allows the study of the electron distribution function at low energies revealing deviations from the Maxwellian distribution. This paper is devoted to the design of the quasi optical antenna for the diagnostics. The physics requirements imply two lines of sight at about 10 and 20 degrees respectively from the magnetic field gradient. Severe geometrical constraints are imposed by the mounting method of the antenna. The design strategy and physical optics calculations used to validate it are presented.