On 5/28/2010 1:39 AM, jimlux wrote:
> Roger (K8RI) wrote:
>
>> On 5/27/2010 9:27 PM, Gary Schafer wrote:
>>
>>> Actually, Polyphaser recommended against putting loops in the coax line. The
>>> thought was that a loop would greatly increase the field and act as an
>>> antenna that would increase the chances of induced current into other
>>> conductors.
>>>
>>>
>> The problem with loops, or I should say the potential problem with
>> loops is lightning does not like to travel in circles.Give it a corner
>> or curve and it's likely to get off the conductor and go some where else.
>>
>
>
> Uhhh... I'd kind of like to see a physics explanation of not like to
> travel in circles or bends and getting off and going somewhere else.
>
>
Well, lightning likes to take the shortest path although that may not be
a straight line
Any conductor, straight, bent, circular, or crooked has inductance.
The rise time of a pulse creates a reverse EMF. The steeper the rise
time the greater the reverse EMF.
A discontinuity in the coax such as a bend or coil increases the
inductance at that point.
In the case of a coil and a rapid rise time the shortest path may be
from turn to turn, to another cable, or to the tower rather than to
continue through the coil.
I've seen lightning strike the top of a tower and get off half or 3/4 of
the way down without following the guy wires. This was explained to me
that the rise time created enough reverse EMF to make it easier for the
lightning to get off the tower and jump to ground rather than proceed
straight ahead even though the tower provided a substantial conductor.
> A loop has inductance. There will be some amount of voltage drop across
> the inductance. But a 1 turn loop doesn't have a heck of a lot of
> inductance (I think 6" diameter is like 0.5 uH)... about the same as a
> half a meter of straight wire.
>
> Lightning has a rise time of about 1 microsecond, and peaks at say, 50kA
> (higher than most strokes, but makes the math easy).. so the voltage
> drop is Ldi/dt is 0.5E-6*50E3/1E-6... about 25kV... that's big, but not
> huge.. not going to leap feet, in any case. Maybe an inch or so.
> Certainly would punch through the jacket on most coax and arc to any
> touching metal. (hence the electrical code requirements to space
> conductors that might carry lightning away from other ones)
>
>
Yet I've seen lightning bolts in both photos and in person, that jumped
many feet, or rather 50 to 60 yards when they had only 50 to 100 foot of
tower between them and ground.
Having worked in industry I've seen electricity do some strange things.
Two memorable ones were an electrician prying the cover off an overhead
buss to add another switch. That buss was composed of 3 1" copper
rods. When he pried off the cover the screwdriver slipped and contacted
one of the phases (3 conductors - 3 phase) At that point the whole
overhead buss with the three conductors and steel enclosure started
burning like a fuse. Man, what an arc. Noise, light, fire, and motion.
The current in those conductors was causing them to jump and arch
between 6" and a foot until some one could make the mad dash to the sub
and throw the "big switch". Needless to say the burnt buss was not the
expensive part. Another was an engineer forgot three rather small
grounding lines in a building sub. Now these things were meant to
prevent static buildup so the wire was less than #16. Probably 20 or
22. When the main was thrown to provide power to the sub it found three
direct shorts to ground. It instantly vaporized the three grounding
lines and actually bent the 1/8" thick steel doors with reinforcing
strips that were on the sub.
The energy in lightning bolts is many times the stuff we were working
with, so I can understand the induced and reverse EMF values being
enough to jump quite a ways. Normally in a transmitter, or a house they
*can't* jump more than a foot or two without running into another
conductor. But lightning is highly variable so the voltages and
currents vary by at least a magnitude and probably more than several
magnitudes when anomalous lighting strikes are taken into account. I
don't worry about the small ones any more. As I've said many times, the
tower here has taken at least 17 verified hits in what is now the last
decade. How many times it has actually been hit is anyone's guess. I do
know that I've had no damage while several neighbors have had extensive
damage, and I saw one pine tree blown apart with enough force to throw a
section of trunk over 50 feet and stick it over 3' deep into the yard
which was not muddy.
> I'm pretty sure the injunction against bends and loops in lightning
> conductors is for mechanical reasons. The force tending to expand the
> loop *is* quite high. A 50kA pulse rising in a few microseconds will
> easily rip a 1" diameter coil of AWG 10 wire apart (see, e.g., quarter
> shrinkers)
>
I've seen less bend one inch buss bar.
> You don't want your lightning conductor ripping off the wall or loose
> from the supports.
>
>
> Running a 6" diameter coil with 2 turns and 1" long through the Wheeler
> formula, I get about 1 uH.. Not a particularly effective choke at 1
> MHz... a Z of about 6 ohms.
>
True, but the one thing about lightning is its inconsistency.
What do you get for a square steel tower, 2' on a side? Yet the
inductance was enough to cause the bolt to get off the tower and jump
sideways.
73
Roger (K8RI)
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