On Monday, October 04, 1999 2:56 PM, Jon Ogden [SMTP:jono@enteract.com]
wrote:
>
> Vic Rosenthal wrote:
>
> >>
> >> >What is the SWR as measured at the generator with a 50-ohm
> >> >characteristic-Z SWR meter?
> >>
> >> ? At the end of an any characteristic.-Z halfwave transmission
line, the
> >> termination Z repeats itself -- with a reactance reversal. .
Since
> >> there is no reactance in this termination, the Z at the end of the
93-ohm
> >> halfwave line is 50 +/- j0 ohms, the SWR is 1:1.
> >
> >No. It is correct that the impedance seen by the generator is 50
ohms but
> >the
> >SWR is unchanged, regardless of the line length. The SWR is
entirely
> >determined by the impedances of the line and the load.
>
> This isn't correct, Vic.
>
> In a 93 Ohm system, the SWR is unchanged. However, we are attempting
to measure a 50 Ohm load in a system where 50 Ohms is our base (the SWR
meter has been specified as being 50 Ohms and the exciter is also
assumed to be at 50 Ohms).
>
> At a half wavelength of coax, you rotate completely around the VSWR
circle and thus see the impedance of the load at the input of the coax.
In this case, the load is 50 Ohms. At the half wavelength of coax, the
load is 50 Ohms as well. The SWR in a 50 Ohm system then is 1:1. In a
93 Ohm system you would have the SWR mismatch of 1.86:1, but in a 50 Ohm
system, our VSWR is 1:1.
>
Jon - Your statements seem to contradict the most important concept I've
learned in the last few days.
The SWR in the cable is set by the load/cable mismatch.
PERIOD.
If this isn't true, then I haven't learned a thing :(
If it is, I'll continue:
Remove the SWR meter and the generator. Cable and load only. What is
this black box?
It's 1.86:1 VSWR regardless of line length. The cable could be 2 meters
or two miles (yes, ignoring attenuation).
Why would putting a SWR meter at one end change this?
In general you normalize to the cable, in this case 93 ohms.
> So the bottom line is that if you have a 50 Ohm load and a 1/2
wavelength of coax that it matters not what the impedance of that coax
is! It's inherently narrow band, but that's the case.
>
Yes, to transform the impedance back to "itself" it matters not what the
cable Z is.
The SWR is a different matter.
> 73,
> Jon
> KE9NA
73
Mike
N2MG
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