> Let's see if we can start another holy war. It's been kinda quiet
around
> here the last couple of days.
Ok, let the holy wars begin!
I'm only jumping into this one to promote the "Holy War" scenario.
Bob Wanderer, AA0CY, should get into this one, yet again,to calrify all of
the the subtleties!
>" A nice low impedance path from the tower to the house" is a good
> start. Yes, it's not a 'ground system' but it's something that needs to
be
> done.
A lightning strike does not respect "A nice low impedence path from the
tower to the house." It just has a @#@#$$%^& of a lot of energy trying to
find a ground connection. It is merely looking for the lowest inductance
path to a low resistance connection to the earth ground!
Impedance may be great for RF grounding, but lightning doesn't care!
Why is inductance so important? Because a typical strike has a 2-10 us rise
time.
We all understand that circuit inductance controls the ability of energy to
make rapid changes. Lots of inductance, in any circuit, reduces its ability
to efficiently transfer energy, when it is changing rapidly. That's how we
make use of RF chokes, to keep our transmissions off of the neighbors phone
lines, etc.
Low inductance lets lots of energy transfer through the circuit, when it is
changing rapidly. Very poor for eliminating RFI!
Because the rise time of the strike is so short, we want to give it the
lowest inductance path to a ground connection that can handle the energy.
The important thing is to realize that inductance to ground and resistance
of the ground connection are two different things.
Flat copper conductors present a lower inductance path than round ones! The
flat conductor path provides lowest inductance to get the strike to go
where you want it to go, and the overall grounding circuit interface with
the earth determines the resistance, which makes things get hot! Too much
resistance at the earth connection will cause high potentials to exist
between the strike energy and your equipment, which is also connected to
the ground system.
>What a 'lightning protection system' should do is have everything bonded
> together to minimize potential differences. It's these potential
differences
> that cause the arcing and assorted damage to equipment. It's the
potential
> difference of the tower and the earth it's sitting in. It's the potential
> difference of the tower and cables. It's the potential difference of your
> telephone line and earth. It's just about everything.
The rapid energy rise time of the typical lightning strike is so short that
the path to ground inductance will control where the energy will try to go!
After that, the resistance of the ground connection will determine how much
potential is developed between the antenna system and ground to destroy
your equipment.
The lightning strike ground system should offer a direct, low inductance
path to ground, accompanied by a low resistance connection to ground.
Potential differences are unavoidable, that's why everyone wants us to use
the surge protection devices to shunt the energy off of the center
conductor of the coax.
The worlds most effective gorunding system can only get lightning energy
off of the tower and the coax shield. The rest is done by the protection
device, to shunt the energy off of the center conductor to an effective
ground, if we've provided one.
> "A nice wide surface area connection to earth will." With 'normal'
> soil, a solid copper wire (large - I can't remember the gauge) ground
system
> provides adequate connection to the earth. Commercial ground systems are
built
> to be 5 ohms or less. The times when 2-4 inch wide copper strap is
helpful is
> when you've got really poor soil, or rocky. It puts more surface area in
the
> system.
Ohms are important for preventing excessive soil heating, which can create
new precious gems, etc, and preventing excessive voltage potentials from
developing between equipment and ground, but inductance is king!
You can actually create a phenomenal low resistance connection to ground
that could take the largest lightning strike in the universe, but try to
connect to it with the the galaxy's highest inductance path (cheap, small,
round wire) and you will end up on the who's who of "I got my butt kicked
by lightning."
> While I appreciate the theory (I'm always looking for good
information),
> I think what most TowerTalkians want is "how do I do it?". You know, the
> PRACTICAL stuff. And that's why I rate Roger Block's Polyphaser grounding
book
> as barely useful. It's real long on the theoretical and the esoteric
> engineering data but it's real short on practical construction tips.
I spent a week at one of Roger's seminars on lightning protection. It's too
bad you weren't there! You'd know how much he cared about this subject, and
how adequately he dealt with the dumb questions guys like us ask! Think
about the professor, who forgot more about the subject, than we'll ever
know!
It's really a shame that Roger has sold the company, and he'll no longer be
teaching the seminar. Yeah, he may have been long on theory, and short on
"tell me just how to do it," but he is one of us, a "ham," and he knew that
there were no quick and dirty solutions to this subject matter.
73, Kurt
YagiStress - The Ultimate Mechanical Design Software for Yagi's
Visit - http://freeyellow.com/members3/yagistress
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