On Thu, 01 Mar 2007 21:04:36 GMT, Owen Duffy wrote:

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

Hi Owen,

It should be under "all circumstances." However, to reveal it
requires the "right circumstances." This again returns us to the
discussion of source resistance/impedance. A matching source driving
a mismatched load through a line of indeterminate length can exhibit
this variation, but it will require considerable skill to see it. If
you force the problem by mismatching the source as well (this then
means that the line is mismatched at both ends, much like your
halfwave model); then you can observe a variation in power readings
along its length that vary sinusoidally.

If you use an instrument that is calibrated for an impedance other than
the line under test,

That is not the case, although there are occasions where power has to
be determined in a heavily mismatched situation - this is done with
considerable error if the line lengths are unknown. If they are, then
corrections can be made.

your measurement does not indicate VSWR on the line

I have restricted myself to the cyclic display of powers.

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.

Well, at a quick glance and noting no tabular form of data, what is
presented wouldn't reveal cyclic variation anyway for two reasons:
1. It lacks resolution (not enough places);
2. It lacks sufficient mismatch.
This is not a complaint, merely an observation because the evidence
for your example is hidden deep in the decimal places.

The solution is simple conceptually and mathematically. Reference any
discussion of Two Beam Interference as treated in Optics. For the
special case the math devolves to:
I = 2 · I1 · (1 + cos(theta2 - theta1 - delta))

Conceptually, it is only the combination of phase and amplitude from
two sources (each reflecting interface on the ends of the line).

73's
Richard Clark, KB7QHC