Tod,
Picture a dipole with no feedline; say the feedpoint impedance is 75
Ohms (SWR=1.5:1).
Suppose we now connect a length of coax a multiple of a halfwave long,
which is well earthed at the shack end; we will be connecting one side
of the feedpoint to ground through a low impedance path and the
resulting feedpoint impedance will be very different from 75 Ohms. Now
change the coax length by a quarterwavelength and the braid path is
quite high impedance and the feedpoint impedance will be restored to
close to 75 Ohms.
So even if the SWR meter is not affected directly by CM current flowing
through it, it can still show a significant change in SWR reading when
we alter the coax length simply because the feedpoint impedance has changed.
It helps to picture the coax braid as a third radiating antenna wire. We
should expect that if we add a third dipole leg and start altering its
length we would measure a changing SWR.
73,
Steve G3TXQ
On 20/04/2012 18:14, Tod Olson wrote:
> Whoops.
>
> Net impedance at the 'feed point'S( ?
>
> Of the antenna, of the feed line at the station?
>
> I am not sure I understand the use of the term "net impedance' Steve.
> Mentally I am picturing two impedances the are combined to give a single
> impedance. The individual impedances are associated with the Common Mode
> [CM] path and the inside of the coax RF path.
>
> The inside the coax path is the one that the SWR measurement is taken on.
> The CM path is isolated from the SWR measurement path [if the SWR meter is
> a good one]. No matter what happens with the CM impedance, how does that
> change the 'independent' SWR impedance? Maybe they are not independent?
>
> Somehow I think that there are a lot of variables in play here and it gets
> challenging to figure out what is doing what and when it is doing it.
>
> Where are the guys who comfortably handle the equations? They should be
> jumping in here to set us straight.
>
> Tod
>
>
>
>
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