[BC] More sparks

[Cowboy]

On Thursday 12 May 2005 20:15, clive at citiria.com wrote:
> >First, consider the shunt fed tower as an autotransformer.
>
> *** Eh? It is not an autotransformer. Picture a narrow vertical triangle.
> The right side is vertical. That side is earthed at the base. The left
> (slant) side, at the base, is the driven part. The lines go all the way to
> the top of the antenna. It is a folded dipole with top loading. I don't
> know what kind of antenna you might be referring to.
>
>                   /|   (Top loading radials not shown)
>                  / |
>                 /  |
>                /   |
>               /    |
>      50 ohm Feed   |
>  ==================!======= Ground ===

 The art-work suggests you may have missed your calling !  ;-)

 Picture this..... ( I'm no artist, so I won't try )

 A coil of wire, center tapped.
 A voltage impressed across the center tap and one end, will result
 in double that voltage appearing end to end, yes ?

 Now, take that coil, streatch it out straight, and bend it in half.
 Tie one end to ground, and the other end to the ATU.
 Is this not what you've drawn so well above ?

 ;-)

> Now regarding the next one:
> >> 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 !
>
> *** The base of the tower sits directly on the terminating plate for all
> the ground radials. If that isn't earth, tell me what is, please.

 OK. I read it wrong. I read the meaning to be the mast end of the shunt wire,
 which is far above ground potential.

> But I
> think I know what you mean. You mean the antenna is not a perfect ground
> because of its self-inductance.

 That's also true, and especially true the way I read it.

> My calculations relate strictly to the assertion made by Dana that the
> magnetic field induced by the lightning current racing down to ground,
> induces a high voltage in the slant wires. I calculated 800 V and that
> voltage is between the feed point and the top of the tower.
>
> > ONE end of the tower is grounded, but that's not even ( electricly )
> > close to where the shunt tap is located.
>
> *** Correct. There is some missing information, as I said above, relating
> to the voltage at the top of the tower being faced with a self-inductance
> (the tower) in its path to ground. A good question is whether this voltage
> is additive to or subtractive from the induced voltage, because I did leave
> out one item from my Faraday equation: the sign of the induced voltage is
> negative. (- dW/dT) volts.

 Well, we're pretty much on the same page now.
 In order for a shunt feed to work at all, the current in the shunt must
 be flowing in parallel to, and in phase with, the current in the stick.
 This parallel flow is what causes the step-up in feed point Z from the
 zero +j zero at the base of the grounded stick.

 If they were out of phase, then the radiation would cancel, as in a de-tuning
 situation. Even there, while the current is minimized, there is still substantial
 voltage across the end of the shunt, and the "grounded" tower base.

> But I am mainly a transmitter person; I need to talk to some dedicated
> antenna engineers.

 So.....
 You don't design 'em, but only clean up the mess when it doesn't work ?
 :-)

 The short version is still that a shunt feed is no protection against lightning.
 ( although, if the lightning current is parallel and in phase between the skirt
 and the stick, then the Z transform occurs, and the voltage =may= be less than
 a direct strike on a series fed. )

 It's that missing information that tends to cause parts that are normally located
 in one spot to occupy several spots all around the room !