On 6/19/2010 7:59 AM, Jim Thomson wrote:
> Roger mentioned his tower gets hit 3 x per yr with lightning.
*Was* getting hit three times a year. AFAIK it has escaped any hits for
nearly 3 years. As is mentioned elsewhere for commercial installations
they take the data over a decade or even several (I forgot the number).
But lightning is not only unpredictable, it seems to run in cycles.
> Does any of the radio gear/coax
> ever get fried ?
Not since the ground system was finished. Before that I lost two
repeater antennas, a run of 7/8" Heliax. It blew out 20 feet or so from
the antenna leaving it too short for anything here. I had one
polyphaser taken out and lost one *new* computer and this was before I
had installed the network.
> What is getting hit... the top of the tower, top of mast, or yagi ele's ??
>
Like the 800# Gorilla, ...It hits any where it wants. Top mast,
antennas, down on the tower...
> If yagi's are stacked on the mast.. is it always the top yagi that gets hit ?
No.
> Does anybody use those porcupine ball things
> that extend above the top of the tower.
Not that I know of. Others have pretty well covered them in this thread.
> 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 ??
>
>
Thorough bonding.
> 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.
>
>
Bonding, bonding, and bonding... But I can think of no real reason why a
storm would just skip over an antenna farm. They, being the tallest
subjects around usually take the brunt of the attack.
> Ok, what happens with yagi's like F-12..
I had a TH-5 "up there" and prior to that on another tower, a Cushcraft
ATB34. Both insulate the driven element halves from the boom and feed
them through a balun. Other than age loosening one of the contact screws
in the ATB34 neither antenna gave a bit of trouble.
> 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]
>
Considering the minimal loss in the connectors at HF and even VHF I
installed a piece of 1 1/2" X 1/8" Aluminum angle at the top and bottom
of the tower. This is drilled out and bulkhead connectors installed to
ground the shield at the top and bottom of the 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
>
Are they constructed like that or is the DE one piece fed with a gama or
T match?
> do? Can you install say a 45 uh choke [ made from 12 ga magnet wire,
> wound on single torroid] directly across the ant terminals ?
>
I think you could install a choke, but I'm not sure it would be warranted.
> [ 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.
>
"I think" the large capacitance in the long run of coax plus the
inductance tends to lower the amplitude of the pulse and round off the
edges. It's normally not enough to blow out the coax in a well bonded
system although how well it'd stand up to one of those super strikes is
anybody's guess. I think the mountain top example in another post is
likely a good example of a super strike which are rare and tend to occur
in spring and fall more often than summer according to what I've read.
Regular lightning does little to airplanes. Super strikes blow holes in
them. Some where around here I have a photo of an airliner with a hole
big enough to climb through right behind the first officer's seat.
Some times you think the storm is trying to get you. Flying at 5000 feet
in clear air, about 12 miles from a very large but isolated
thunderstorm, I saw a bolt come out of the storm horizontally at our
altitude. The bolt then curved down and hit the ground about 2 miles
from us. This happened almost immediately after my passenger (another
pilot - non Instrument rated) asked me how close we'd dare get to a
storm like that. It was like it answered before I could. <:-))
Although we are learning a lot about lightning there is still a lot left
to learn. Super strikes (positive lightning) and sprites were only
recently discovered to exist.
The astronauts photos of sprites are impressive.
73
Roger (K8RI)
> later......... Jim VE7RF
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