A novel very broadband termination has been developed, using radial lines in a "shamrock" configuration. Broader bandwidths are achievable than adopting both a conventional straight stub and an "half-moon" configuration. with respect to the latter case, the circumferential mode excitation, that tipically results in a further narrowing of the useful bandwidth, is also avoided. An accurate modelling, developed by the authors, has been used in the design. Good agreement between theory and experiments has been obtained.