[TowerTalk] Lightning suppression through coax loops

[jimlux]

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.

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)

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)

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.