Richard Clark wrote in
:

On Thu, 01 Mar 2007 19:21:43 GMT, Owen Duffy wrote:

If you are asserting that VSWR on a real or even theoretical line
varies sinudoidally with displacement, it is time to go back to
basics. You need some time with a reputable text book.

Hi Owen,

Under the right circumstances (and they have been presumed in some
discussion here), then the power terms (as expressed by a power meter
inserted into the line) will vary sinusoidally with displacement, even
if the SWR does not.

Richard,

I am not sure of what you mean by the "right circumstances".

Firstly, except in the very special case of an almost purely inductive
load on a lossy cable at low frequencies, the VSWR calculated from Vf and
Vr at a point on a line, decreases smoothly from the load end to the
source end. (Vf and Vr each decay exponentially in magnitude from source
to load, and the forward and reflected power values calculated from those
samples will vary as the square of the exponentially decaying Vf, Vr from
load to source.)

If you use an instrument that is calibrated for an impedance other than
the line under test, your measurement does not indicate VSWR on the line
under test, and the instrument readings will be different than I outlined
in the previous paragraph. Fig 3 in my article at
http://www.vk1od.net/VSWR/VSWRMeter.htm shows a line labelled "VSWR(50)"
that indicates the values that would be indicated / calculated using a 50
ohm instrument in a 75 ohm cable with a 1.5:1 VSWR.

Owen