[TowerTalk] Antenna/Tower Grounding (Lightning Protection)

[K4SAV]

K4FMX wrote:

..."The book "grounds for lightning protection" from polyphaser has those
examples and formulas in it.

They tell you how to estimate how much voltage will appear in different
parts of the system. You can quickly see why low inductance ground leads and
tight low resistance connections are important."......



And if you look at the examples closely you can find some errors. By 
their calculations two runs of RG8 down the tower would have the same 
inductance as a 35 inch-face tower. That leads to the conclusion that to 
half the voltage drop down your tower all you have to do is run two coax 
lines down it. This should be obviously incorrect, even to a casual 
observer. You can end up with some large errors if you neglect mutual 
inductance.

Jerry, K4SAV

Gary Schafer wrote:

>  
>
>>-----Original Message-----
>>From: towertalk-bounces at contesting.com [mailto:towertalk-
>>bounces at contesting.com] On Behalf Of Telegrapher9 at aol.com
>>Sent: Thursday, January 04, 2007 4:08 AM
>>To: ersmar at comcast.net; w1ksz at earthlink.net; towertalk at contesting.com
>>Subject: Re: [TowerTalk] Antenna/Tower Grounding (Lightning Protection)
>>
>>Gene,
>>
>>does anyone run simulations or calculations on lightning strikes. A common
>>direct stike is 100 kA with a risetime of 8 us, correct? If so, using an
>>inductance of 200 nH/ft, the voltage drop along 1 foot of wire is 2.5 kV.
>>And we can
>>calculate the low frequency impedance of a ground system. Let's say it is
>>1
>>ohm. Now we have 100 kV between the tower base and earth ground.
>>
>>Using some ball park numbers like this we should be able to determine the
>>currents and potentials at various points in the system. Then appropriate
>>series
>>Z and shunt Z can be applied to design for survival of equipment.
>>
>>I'm thinking of doing it via design rather than blindly by code. I suppose
>>that there is nearly 100 years of experience contained in the codes that
>>will
>>lead one to success. But perhaps a good understanding of the nuances can
>>help
>>one to gain several dB of margin by some simple additions.
>>
>>Can someone steer me to some numbers on lightning? Thanks.
>>
>>    Dave WX7G
>>    
>>
>
>The book "grounds for lightning protection" from polyphaser has those
>examples and formulas in it.
>
>They tell you how to estimate how much voltage will appear in different
>parts of the system. You can quickly see why low inductance ground leads and
>tight low resistance connections are important.
>
>73
>Gary  K4FMX
>
>
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