I haven't run any experiments, but I'll concede that some methods of
measuring VSWR are less than perfect, in which case that is a definite
possibility.
Bob, N7XY
On Sun, Oct 10, 2004 at 01:50:27PM -0500, Chris Boone wrote:
> Ok...what I should have said was "apparent" SWR changes according to the
> metering.....
> If the cable length changes and the antenna is NOT a resistive only
> component, the coax will cause an apparent change in the reactance, thus
> making the SWR meter change...
>
> Any argument with that??
>
> Yes, TRUE SWR itself does not change....but the metering is not perfect
> :)
>
> > -----Original Message-----
> > From: antennas-bounces@mailman.qth.net
> > [mailto:antennas-bounces@mailman.qth.net] On Behalf Of Bob Nielsen
> > Sent: Sunday, October 10, 2004 12:12 PM
> > To: antennas@mailman.qth.net; 'TowerTalk List'
> > Subject: Re: [Antennas] coax 'sweet lenght'
> >
> >
> > WRONG! Ignoring cable losses, the VSWR is independent the
> > length of the cable. What changes with length are the
> > complex components of the impedance, while the absolute value
> > (VSWR) remains constant.
> >
> > If you plot the impedance on a Smith chart, you get a circle
> > whose origin is at the center of the chart where the
> > impedance vector rotates as the cable length changes, but the
> > length of the vector is constant while the resistive and
> > reactive components of the impedance vary.
> >
> > However, because a matching circuit may not be able to
> > compensate for all possible resistance and reactance values
> > for a given VSWR (particularly in the case of a very high
> > VSWR, such as an end-fed half-wavelength antenna), a
> > different cable length may transform the complex components
> > of the impedance, as seen at the transmitter, to a value
> > which can be more easily matched.
>
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--
Bob Nielsen, N7XY n7xy (at) n7xy.net
Bainbridge Island, WA http://www.n7xy.net
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