[BC] Re: Lightning and grounded masts

[Al Wolfe]

    I believe one important point has been missed in this whole discussion. 
Towers are steel and skirts tend to be copper. Lightning generally behaves 
like low frequency RF. Ferris materials generally impede the flow of RF. 
This being the case, one can deduce that, given a choice, lightning will 
take the copper path most often. Therefore, usually the skirt takes the 
brunt of the lightning bolt.

Al, retired
K9SI


> Date: Thu, 12 May 2005 12:29:00 -0400
> From: Cowboy <curt at spam-o-matic.net>
> Subject: Re: [BC] Lightning and grounded masts
> To: Broadcast Radio Mailing List <broadcast at radiolists.net>
> Message-ID: <200505121229.00087.curt at spam-o-matic.net>
> Content-Type: text/plain;  charset="iso-8859-1"
>
> On Wednesday 11 May 2005 20:59, Clive Warner wrote:
>> > Actually, Grounded towers don't do much better. It seems that when the
>>
>> charge
>>
>> > travels down the tower to ground, it induces voltage in the outrigger
>>
>> wires.
>>
>> > -D
>>
>> Well, let's see if I can put some figures to that assertion.
>
> And you did quite well, with one criticle error.....
>
> And it is....
>
>> The mast end is earthed,
>
> It may appear grounded at visual observation of the obvious, but it is
> some number of degrees, and some turns ratio above ground, and
> therefore your voltage calculation is actually in series with, and 
> additive
> to, the lightning voltage at that point on the tower, stepped up by the
> transformer effects of transmission line theory !
>
> ONE end of the tower is grounded, but that's not even ( electricly ) close 
> to where
> the shunt tap is located.
>
> In all my years, dating back to a casual conversation with Mr. Franklin
> one stormy afternoon in Philly, the Congressman agreed that this was not 
> intuitive,
> and tried to use that as a basis to sell me one of his "Franklin Rods" !
> ;-)
>
> Shunt fed towers are no gaurantee, nor even a suggestion, of lightning
> protection.
> The slant-wire seems to be a better performer than a skirt, however, and
> sometimes better than a traditional series fed radiator.
>
> On Wednesday 11 May 2005 12:41, DANA PUOPOLO wrote:
>> Actually, Grounded towers don't do much better. It seems that when the
>> charge travels down the tower to ground, it induces voltage in the
>> outrigger wires.
>
> Among other things.
> Skirt fed towers generally do worse, and there is much empiracle evidence
> to suggest that skirts are many things, but lightning protection is NOT 
> one of them !
>
> In my eons of experience, bottle brush disipators do appear to work, and I 
> generally
> install them immediately below the Franklin Rods !
>
>