[BC] lightning

[Phil Alexander]

On 11 May 2005 at 9:41, 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. 
> 
> 
> ------ Original Message ------
> From: <clive at citiria.com>
>
> Seems like another recommendation to use grounded antennas 
> like the shunt fed. NO lightning problems at all since the 
> structure is solidly attached to the earthing radials.

Not only do TRUE shunt fed towers (which I very much like for 
other reasons) not fair much better than series fed, in many 
cases they have MORE problems with lightning that conventional 
series fed radiators.

I realize this is non-intuitive unless given some thought, 
however, as anyone who has cleaned up the aftermath of a 
strike on a shunt fed tower can attest, it is none-the-less 
true.

First, consider the shunt fed tower as an autotransformer.

Second, remember that lightning is an ALTERNATING current.

Third, consider that the autotransformer "primary" is quite
sympathetic to ground strikes in the near vicinity.

The saving grace is most shunt feeds are quite inductive at
the drive point, hence they typically have a large series
capacitor bringing the feedpoint reactance to null.

However, it is vitally important to have a good spark gap,
preferably of the Jacob's Ladder variety at the feed point
end of the feed wire to prevent development of a pulse
sufficient to break through the series capacitor dielectric.

In the case of a direct heavy strike on the tower, you may 
find the arc gap is overwhelmed and the capacitor arced.
Thus begins the "interesting" project. Fortunately, the odds
of a strike of this magnitude are low. :)

Skirt fed monopole/unipole radiators generally do not exhibit
quite the same behavior as the older slant wire feeds, and are
best understood using a somewhat different concept. Owing to
installation variations, these can be somewhat unpredictable 
WRT lightning, however the hazards for the ATU are similar to 
those of a series fed owing to the impedance of the supporting 
structure below the attach point of the drape wires.

The bottom line is that if you can transmit RF, you are also
a lightning "receiving" station. <g>


Phil Alexander, CSRE 
Broadcast Engineering Services and Technology 
(a Div. of Advanced Parts Corporation) 
Ph. (317) 335-2065   FAX (317) 335-9037




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