I am the one who made the original comment regarding the "minimum 50 foot
tower-to-structure" separation. This number was given to be by the folks
at Polyphaser with this explanation (Bob can correct me if I didn't
understand correctly):
There are different considerations depending upon whether you are trying to
protect a "hard structure" (perimeter ground system, lightning suppression
on everything coming in via single-point ground window, etc) or a "soft
structure" (typical residential dwelling).
Hard structures can take a direct hit and keep on ticking. There is no
minimum separation required.
Soft structures are obviously more difficult to protect. Not only do you
need to protect them from direct strikes as much as possible, but you need
to separate them from both the strike energy dissipation field and strike
energy egress via wires/cables.
When a properly grounded tower takes a strike, the energy is directed into
the ground at the base of the tower and from there it begins to saturate
the surrounding earth. As more and more of the energy from the strike
enters the earth, the area of saturation begins to increase and spread
outward away from the tower. It is not a good idea to be sitting,
standing, lying, bathing, eating on this surface as the strike energy tries
to dissipate under you. Hence, the 50 foot minimum distance between the
tower and the unprotected structure and its occupants.
Another consideration is to reduce as much as possible the amount of strike
energy which can travel from the tower through interconnecting wires/cables
to the unprotected structure. At a length of 50 feet, there is typically
sufficient distributed inductance to reduce the amount of strike energy
reaching the residence to a point where consumer-quality lightning
suppression equipment has a fighting chance at handling it.
At my home I maintain not only the 50 foot separation, but I take as many
lightning suppression measures as I can in and around my residence. I have
a partial perimeter ground, single-point ground window and
commercial-quality lightning suppression equipment. If I take chances in
my shack, I risk losing thousands of dollars of equipment. I don't want to
think about what I risk losing in my home.
73, Dick
--
Dick Flanagan W6OLD CFII Minden, Nevada (South of Reno)
Visit http://www.qsl.net/cvrc/
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