Mechanical vertical stops, in form of metallic or plastic screws or dowel pins are introduced as permanent limits and attached on the tuning probes or the vertical axis of slide screw impedances tuners; the vertical stops are adjusted in order to prevent the probes from getting closer to the center conductor than a certain limit; this limit is calculated to prevent corona discharge at a specified maximum RF power and a required maximum reflection factor (Gamma). The adjustment method uses measured data of Gamma as a function of the vertical position of the probe to estimate and adjust for the requested gap to be maintained between the probes (at their lowest position) and the center conductor at any given RF power. The technique can be used in automatic, stepper controlled, as well as in manual, micrometer screw controlled, tuners.