Just to simply make a few statements on the general questions:
No, lightning doesn't always hit the highest point. In lightning triggered
cameras watching frequently struck high voltage power lines the lightning
often comes in sideways and hits lower down conductors or the tower.
I used to have access to lightning detection network data and tried in
several locations to see if I could find patterns like w3lpl and at least
one other station have claimed about lightning stopping near their station.
I have used ham stations, and large power line corridors, and have not seen
any long term difference in the stroke concentration over those areas. Over
a several year period the stroke density appears to average out most of the
short term differences. We recommend to utilities to use at least 10 years
of lightning stroke recorded data to get a good average because of the year
to year variations that happen because lightning storms are fairly localized
events.
Porcupines don't work. That has been well debunked by nasa and other
studies. I don't have the reports links handy, but they are available on
the web and you can probably find the links in the archives of this list
from last year, and the year before, and the year before that.
The procedure for building mounted antennas is to bond well to the building
structure... since they are obviously well away from 'ground' that won't
help them, their key is to make sure their antennas and equipment stay at
the same potential as the building frame and power feed where they are
housed.
David Robbins K1TTT
e-mail: mailto:k1ttt@arrl.net
web: http://www.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
> -----Original Message-----
> From: Jim Thomson [mailto:jim.thom@telus.net]
> Sent: Saturday, June 19, 2010 12:00
> To: towertalk@contesting.com
> Subject: [TowerTalk] Is lightning hitting the tower..or the yagi's ?
>
> Roger mentioned his tower gets hit 3 x per yr with lightning. Does any
> of the radio gear/coax
> ever get fried ? What is getting hit... the top of the tower, top of
> mast, or yagi ele's ??
>
> If yagi's are stacked on the mast.. is it always the top yagi that gets
> hit ? Does anybody use those porcupine ball things
> that extend above the top of the tower. Tall building's in NYC + chicago
> get hit all the time with lightning.. and they all have
> radio/tv gear on the top floors.... what is their procedure ??
>
>
> I think it was w3lpl a while back, mentioned the lightning would stop
> when it got to his ant farm... pass
> right over it.... then start up again on the other side. That apparently
> will work... IF the towers are well grnded... and
> have lots of well bonded yagi's on the sides of the tower.
>
> Ok, what happens with yagi's like F-12.. where ALL the els are insulated
> from the boom ? On a similar
> note, F-12 uses a hairpin... so both sides of the DE are bonded....via
> the floating hairpin. Both sides of the DE
> end up bonded to braid of coax. in this case, you want the coax braid
> bonded to top of tower [ and bottom of tower]
> On the OWA direct fed yagi's.... no hairpin used... and 1/2 of the DE is
> not bonded to top of tower via braid. Now what do you
> do? Can you install say a 45 uh choke [ made from 12 ga magnet wire,
> wound on single torroid] directly across the ant terminals ?
> [ or one side of ant terminal to boom] At least that would DC grnd the
> center conductor of the coax.. and that 1/2 of the DE. ICE does
> the same thing... but I wouldn't trust their box to be water proof. I use
> the ice box inside the house, on a 1/4" thick AL plate.. just inside
> the basement.
>
> later......... Jim VE7RF
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