[TowerTalk] tower to house spacing

[Jim Jarvis]

TT:

A 25' offset from the house is an arbitrary distance.

I've had house bracketed towers, and not worried about
them...but I've also grounded them well...and been careful
to have a disconnect system for all cables coming into the
house.

The point, however, is that that isn't enough.  In the event
of a lightning strike, you have current flowing in the tower,
which forms the single turn primary of a transformer...the secondary
of which is your house wiring.   Significant currents can be
induced there.

Why 25'?  I can tell you that I've observed lightning strikes
to a water tower 400 yards away, which produced voltage spikes
in excess of 1,000V across coax of "grounded" VHF antenna systems.
 

The short form is...you can't be completely safe.  If you can 
afford the space, get the tower at least one tower height from
the house.  You'll sleep better.  

In VT, with 50' of house bracketed 45G, I had ice falling on my
flat roof with some regularity.  Fortunately it was over the guest
room, and not the master bedroom.  Nearest lightning strike hit a
big tree, not the tower...but the tower circuit was just another
secondary in that case...and there were arc's across the rcs4 
switch at the tower base as a result.  

n2ea
jimjarvis at ieee.org