Richard Harrison wrote:
Cecil, W5DXP wrote:
"How is it possible to use a signal (standing wave current) that is
known not to change phase, to measure the phase delay through a wire or
coil?"

Ignore it.

Lissajous figures result from applying signals to the vertical and
horizontal deflection circuits of an oscilloscope simultaneously. Phase
difference between signals of the same frequency make a distinctive
pattern.

One can use coax lines with identical delays to couple the inputs with
phase sampling loops. Take samples of the currents at the two points
where the phase difference would be known. Amplitudes can be adjusted
for a suitable pattern. It will be destinctive.

Then take samples from the same source. Add a known delay to one channel
until you have reproduced the distinctive pattern you had observed when
testing the felay between the points that have the unknown phase
difference.

With a few elaborations, that`s how a phase monitor works.

Many analog scopes aren't capable of producing a meaningful Lissajous
figure at HF because of the limited bandwidth of the horizontal channel.
Significant phase delays occur at frequencies well below the nominal
cutoff frequency, which is often much lower than the vertical channel.
Before believing in the validity of any figure, you should look at the
figure you get when you apply the signal to both axes at the same time.
If it deviates significantly from a single diagonal line, you won't be
able to trust other patterns.

It would be a simple matter for a digital scope to present a good
Lissajous figure, since the bandwidth is determined solely by the input
samplers rather than a series of amplifiers and the CRT deflection
structure as in an analog scope. I haven't looked closely at digital
scopes lately, but I'd be surprised if most don't have the capability of
making a good Lissajous figure at HF. It would be simply a matter of
internal firmware programming.

Of course, a dedicated phase monitor would be designed for good phase
and amplitude match between channels at the frequencies it's specified
to be used at.

Roy Lewallen, W7EL