On Oct 16, 5:13*pm, Jim Lux wrote:
Dave wrote:
A well grounded mast DOES NOT attract lightning any
better than a non grounded mast at the same location.

actually it does. *both the ieee and cigre have been using lightning
statistics data collected mostly from tall masts for many years.
There are well known formulas used to calculate the number of strokes
to a pole or power line, both include the height, and as height
increases so does the number of strokes to the object. *The height
also skews the current distribution with higher structures more likely
to get more high current strokes.

But, is there a difference in strike rate between grounded and
ungrounded towers of the same height. *I would think that the difference
would be very small, and smaller as the height gets bigger.

Since the vast majority of commercial masts,towers,buildings used to
collect the stats are probably grounded (Because the code requires
it...), it might be hard to find decent data for "ungrounded" things.
(for one thing, the equipment used to collect the strike data, until
recently, probably measured the current spike on the grounding wire..
these days, you could use the RF lightning detection systems, and match
up strike locations against structure locations)

Maybe wooden poles? (which are only "sort of grounded")

'sort of grounded' is as good as grounded. The current that builds up
the charge to initiate the upward streamer is relatively small, that
can be seen in the use of the high value resistors commonly used to
'bleed off' charge from antennas. instead of bleeding it off they are
actually just equalizing it with the local ground potential, the same
as happens on a 'properly' grounded tower/vertical. in most cases you
won't find an amateur tower that isn't grounded one way or another
anyway, even if a specific ground rod or other system isn't supplied
at the base there is still a decent ground through the foundation.
and if not then there is through any cable going up the tower that
connects to a rotor or most antennas. it takes real work to really
insulate a tower from ground in such a way that it would not allow
charge attracted to the area under a downward leader from moving up
the tower... the large rf isolators and tower lighting isolators used
on base insulated AM broadcast towers are a good example... and even
on those types of towers you will hear reports of 'lots of snapping
and popping' before a stroke, those sounds are small flashovers of the
guy wire insulators as the charge flows up them toward the tower. So
to do it right would mean using insulating guys also. Then, even if
you got that far a tower of any height would end up going into corona
at the bottom and top anyway due to the high vertical electric fields
under a storm cloud... even if the corona didn't reach nearby ground
conductors it would still collect/dissipate charge on the conducting
vertical section which may still initiate a streamer. So the short
answer is that i have not seen any comparison between grounded and
ungrounded vertical structures.