I think what we want to say is, the SWR meter meaures SWR of the line,
minus small errors caused by small line losses which reduce slightly the
indicated swr from the true swr which exists, and the SWR is only caused
by the ratio of the antenna impedance to the characteristic impedance of
the transmission line.
I think we can say that a properly operating SWR
meter does not measure the ratio of the feedpoint impedance at it's
location to 50 ohms, even if the feedpoint impedance varies (as it will)
along the line when there is reflected power caused by a mismatch at the
antenna feedpoint.
73, DX, de
Pat, AA6EG/N6IJ;
aa6eg@hh.tmx.com;n6ij@hotmail.com
599 DX Drive, Marina CA 93933
See us on the web: www.polkinghorn.org/n6ij
"The Contest Station from the Government"
On Fri, 1 Oct 1999, Gilmer, Mike wrote:
>
> On Friday, October 01, 1999 2:28 PM, Jon Ogden [SMTP:jono@enteract.com]
> wrote:
> > Gilmer, Mike wrote:
> >
> > >This doesn't say I thought the SWR varied ALONG the cable. I'll
> > >rephrase: In a system where (at the far end) the load (antenna) and
> > >cable have different impedances, the impedance seen looking into the
> > >near end of the cable will change depending on cable length.
> Correct?
> > >
> > >Therefore, does that not imply that the SWR at, say, the amp/cable
> > >interface (50 ohm source, changing load), also change depending on
> the
> > >length of said cable?
> >
> > No, that's not correct. That is the same as saying that SWR varies
> along
> > the length of the cable.
>
> Yup, I guess it does. :(
>
> OK, so somewhat rhetorically...
>
> If the cable/antenna were treated as a black box, with some impedance, Z
> (caused by the cable length's transforming effect on the antenna
> impedance), and some VSWR (caused by the mismatch of the cable and
> antenna), what does the "amp" see when it looks into the black box?
> It sees, of course, the same VSWR and the same impedance.
> So this means I can't expect a SWR meter between the amp and the black
> box to read 1:1 even if the black box is 50 ohms. Is this true?
>
> What changes/happens now as I try to "tune" the amp into the black box?
> I'm not changing the black box at all (so the SWR meter doesn't change).
> I'm only changing what the tube sees looking into its tank, correct?
>
>
> SWR is the ration of the forward and reflected
> > voltage. This ratio is constant along the cable regardless of length.
>
> > So it is possible the to tap into a 50 Ohm real resistance spot on the
>
> > cable and still have a high SWR.
> >
> > 73,
> >
> > Jon
> > KE9NA
>
> 73
> Mike N2MG
>
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