[time-nuts] Aerial coax downlead placement

[jimlux]

On 7/5/19 2:26 PM, Poul-Henning Kamp wrote:
> --------
> In message <28F942E8-B61D-4FA5-929D-923184828FC1 at n1k.org>, Bob kb8tq writes:
> 
>> Energy flow is indeed inside the cable if things are set up and operating correctly.
> 
> Please note in this context that *nothing* about lightning strikes
> works the way you would assume it does.
> 
> Cables run inside steel tubes protect the steel tube from lightning
> current because copper is a better conductor than steel - in
> particular when the leading flank is measured in kV/uS and the
> current in kA.
> 
> Likewise, a 90 degree bend or a loop on the cable is a huge
> inductance to get all that high frequency energy through, so
> lightning tend to jump from bends and loops, to less inductive
> paths if possible

Actually, the inductance of a bend isn't much more than the inductance 
of a straight piece of wire of the same length. You have to have a 
complete loop before the inductance starts to rise, and even then, a 1 
turn loop doesn't have huge inductance, it's multiple loops where the 
inductance starts to rise as Nturns^2.

Straight wire has an inductance of about 1 microhenry/meter (very weakly 
dependent on diameter)

But a more exact calculation says that a 31.4 cm piece of wire, 10mm in 
diameter has a self L of 0.26 microhenry.

A loop that is 100mm in diameter with a 10mm diameter conductor has an 
inductance of about 0.15 microHenry. 100mm diameter is a length of 0.314 
meters, so it actually has *less* inductance that a wire that's the 
length of the perimeter.

A 1 meter diameter loop (perimeter 3.14 meters) has an inductance of 3 
uH.  Which is close to the self inductance of a 3.14 meter straight wire 
(4 uH)

The origin of the "no sharp bends in lightning conductors" is more 
related to the flashover voltage to surroundings - A sharp 90 bend has a 
lower breakdown voltage than a gradual bend because the radius of 
curvature is smaller.

There's also a mechanical stress effect - if you have a corner, the wire 
on one side of the corner is carrying a current at right angles the 
field from the other arm of the corner, and it will tend to move 
(violently, given the large peak currents)




> 
> Be careful with EMI/EMC clam-on ferrites, they can explode in
> lightning strikes.
>