A radiator of the thermosiphon type comprising a collector situated in the lowest part of the radiator, and adapted to contain an intermediate vector fluid, an external heat source, placed within the collector, wherein the intermediate vector fluid is adapted to evaporate on contact with a hot surface of the external heat source, at least one vertical tube containing therein one or more channels (4) connected to the collector and communicating with the same, characterized in that said collector and said channels are dimensioned so that each section thereof crossed by the intermediate vector fluid, excluding the thickness of the liquid film of moisture, has the smallest linear direction which is twice bigger than the diameter db of an intermediate fluid vapour bubble which, during operation, detaches itself from the hot surface of the external source during boiling of the intermediate fluid.