Patent ID: 7975646

Claim:
A device for depositing a coating on an internal surface of a container made of a thermoplastic, of the type in which the deposition is carried out by means of a low-pressure plasma, which is created inside the container by excitation of a precursor gas by electromagnetic waves of UHF microwave type, and of the type in which the container is placed in a cavity made of conducting material into which the microwaves are introduced, said device comprising: a UHF electromagnetic wave generator; an electromagnetic waveguide for connecting said generator to a window in the side wall of the cavity, allowing at least one central field to be generated in said cavity; means for injecting the precursor gas, comprising an injection tube extending partly into the container; means for pumping down the cavity and the internal volume of the container; and an internal envelope, coaxial with the cavity and substantially transparent to the electromagnetic waves, for example made of quartz, and defining a chamber in which the container is placed during excitation of the precursor gas; wherein the injection tube dips into the container by a length of between one quarter and one half of the total height of the container considered between the top and the bottom of the container, said length of the injection tube constituting a longitudinal antenna capable of picking up the UHF electromagnetic wave generated by said generator, wherein a UHF short-circuit in the form of a plate is provided on the injection tube in such a way that the face of said plate on the side facing the cavity defines a point of zero amplitude of the electromagnetic wave that propagates along said injection tube, the length between said short-circuit and the free end of said injection tube corresponding to an odd number of quarter-wavelengths so as to obtain a maximum amplitude, that is to say an antinode, at the free end of the injection tube; wherein the short-circuit is a circular radial plate through the center of which the injection tube passes, said circular plate embedded in a dielectric plug through which the injection tube passes axially.