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@contesting.com [mailto:towertalk-
>>bounces@contesting.com] On Behalf Of Telegrapher9@aol.com
>>Sent: Thursday, January 04, 2007 4:08 AM
>>To: ersmar@comcast.net; w1ksz@earthlink.net; towertalk@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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