Topband: Seperation of Radial Ring vs Vertical

[k8do]

It is no different than the spacing on capacitor plates...  A quarter inch
gap between the base and the radials plate, will handle the voltages present
at the base of a series fed, 1/4 wave vertical, nicely... The presence of
rain water and other conductors in the gap will modify this...

Non resonant verticals may have higher voltages present at the feed point
and need a larger gap... Certainly, a half inch will handle amateur power
levels...

Denny