K1TTT wrote:
> Only 1/4" exposed? Not much of a gap for an antenna feedpoint unless you
> operate barefoot or qrp all the time. granted, the dc sparkover voltage of
> a 1/4" gap is on the order of 5kv or so in dry air... and 1500w at 50ohms is
> only 275v or so if I punched that through the calculator right. but you
> are talking the conduction along a rather large surface area that will be
> wet, contaminated with metallic salts that run down the vertical, and may be
> porous if you don't keep it well sealed. Take a good look at how power line
> insulators are designed, the actual gap between the hot and ground metal
> pieces may only be 1/2-1" for 20-40kv of line voltage straight through the
> insulating material... but trace the surface distance between them and you
> get anywhere from 4" to maybe 24" for that same voltage. The extra surface
> area along the glass, porcelain, hard plastic, or silicone rubber surface is
> to make it able to handle that contaminated rain water.. and in most
> insulator designs there is a method used to keep at least part of the
> surface dry. This approach is also seen in commercially available
> insulators for verticals, there is always at least several inches of
> insulating material between the conductors.
The usual design rule of thumb is that the breakdown along a creeping
path (over the surface) is 1/3 that of the free air distance. That's
for clean, dry insulators.
As you've noted, for insulators to operate dirty and wet, you need a
much larger distance (and really, what the power line folks are doing is
reducing leakage current, more than preventing a flashover... if it's
dirty enough to let leakage current flow, then it won't arc.. it tends
to distribute the potential drop evenly.)
And, don't look at power transmission lines in the 100kV+ range for
examples.. the field is so non-uniform that there's a whole different
set of rules for insulation design. Not to mention that they carefully
coordinate the insulation levels through the system so that big
transients cook the expected thing, rather than something else. You'd
rather flashover an insulator string to a supporting tower than through
a transformer, for instance.
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