Cecil Moore wrote:
John Popelish wrote:
Cecil Moore wrote:
John Popelish wrote:

If the sensor sits at a single point and sees an AC current,
you have no way, from this one measurement, if this current
is the result of a standing wave (two oppositely traveling
equal waves adding), or a single traveling wave,
or any combination of traveling waves of different amplitudes.
You know only the net current at that point.

But if one it smart enough to slide the sensor up and down
the wire and note the phase is fixed and unchanging,
one knows he is dealing with a standing wave.

Another point, entirely.
My point is that current has a point definition,
and standing wave current is certainly indistinguishable
from traveling wave current, at a point. Current is current.

Patterns of current over length is another subject. But you keep
saying that there is something different about current in a standing
wave. There isn't.

Do you really think that func(kx)*func(wt) is the same thing as
func(kx +/- wt)? If so, time to dust off the old math books.

( I restored some context)

func(kx)*func(wt) describes the instantaneous current if you pick a
point along dimension, x, and a moment in time, t. It is a map of the
pattern of current over these two dimensions.

func(kx +/- wt) describes a different pattern of the instantaneous
current if you pick a point along dimension, x, and a moment in time, t.

If you put a tiny current transformer around some point of the
conductors in question, (pick an x) and watch the pattern of current
through time (without comparing the phase to any reference) you will
see a sinusoidal current variation for both the standing and traveling
wave cases. The amplitude will vary in a different way, over x, for
the traveling and standing wave cases. If you include comparing the
phase of sinusoidal current cycle you see, to a reference phase, that
will also vary in a different way over x, for the traveling and
standing wave cases.

But regardless, at a point (any particular x) the pattern of current
variation as time passes, will be a sinusoid, in either case. There
is no difference in kind of current you would measure.

The pattern of how this sinusoidal current varies in both phase and
magnitude is very different in the two cases (standing and traveling
waves), but you need both a phase reference and multiple locations to
see the differences.

The the definition of the word "current", in simplest form, is, the
rate of charge movement past a point at some moment in time.

An extension of this instantaneous and point definition might include
a sinusoidal cyclic variation through time, by adding a frequency,
phase and amplitude, to specify a common pattern of current over time,
but still at a point.

Adding in an additional function of position allows the extension of
this definition of current over time to also include spacial variation
of the time dependent pattern.

But if you say the words "the current is different", and don't include
a lot of additional verbiage to indicate that you are talking about
the two dimensional pattern of the variation of current over time and
location, some people are going to misunderstand you and argue based
on picturing another definition of what might be legitimately meant by
the word, "current". I made it clear what definition I was using for
the word "current" (the instantaneous point definition) and you are
arguing with me, while using some different definition.

I realize that I am being pedantic, here, and stating the painfully
obvious. But if your goal is to have other minds synchronize with the
abstract thoughts rippling through your mind, you have to be pedantic.

If you are just using this topic to argue, because you enjoy argument,

then never mind.

John Popelish wrote:
The pattern of how this sinusoidal current varies in both phase and
magnitude is very different in the two cases (standing and traveling
waves), but you need both a phase reference and multiple locations to
see the differences.

Exactly! And the multiple locations are available for us to
measure.

Since you like handicaps so much, how about just plucking
out your eyeballs and chopping off your hands? :-)
--
73, Cecil http://www.qsl.net/w5dxp

John P. wrote, among other things,

"The pattern of how this sinusoidal current varies in both phase and
magnitude is very different in the two cases (standing and traveling
waves), but you need both a phase reference and multiple locations to
see the differences. "

Do you really need the phase reference? Traditionally (since the
beginning of measuring them, and sometimes still today), standing waves
on a uniform transmission line have been measured by finding a point of
minimum amplitude (as measured by voltage, or alternatively by current)
and a point of maximum amplitude, with no reference to phase. In fact,
SWR was reasonably defined as the ratio of max/min amplitudes. If you
know that the wave you're observing is a sinusoid and you have min and
max amplitudes along the line, then you can resolve the wave into two
travelling-wave amplitudes; you won't know which is which but you will
know the two amplitudes. If there is but one source in the system,
it's reasonable to think that the higher amplitude travelling wave was
the one coming from the direction of that source.

In fact, you don't even need to find the minimum and the maximum
points. Again, given sinusoidal excitation and a uniform line, some
small set of points with accurate amplitude measurement at each will
suffice, since they will uniquely determine the amplitudes of the two
waves and the line attenuation. You would have to know the spacing of
the points and that they were dense enough that there is not a spacial
aliasing problem (points distributed over more than 1/4 wavelength...).

It's common to think of a standing wave as the result of two travelling
waves, one in each direction, but another way to think of a standing
wave pattern is as a pure standing wave plus a pure travelling wave.
The minimum-amplitude represents the amplitude of the travelling-wave
portion. The difference between max and min represents the amplitude
of the standing wave portion. For some folk, it's enlightening to see
an animation of the waves for several different values of SWR.

Cheers,
Tom

K7ITM wrote:
John P. wrote, among other things,

"The pattern of how this sinusoidal current varies in both phase and
magnitude is very different in the two cases (standing and traveling
waves), but you need both a phase reference and multiple locations to
see the differences. "

Do you really need the phase reference? Traditionally (since the
beginning of measuring them, and sometimes still today), standing waves
on a uniform transmission line have been measured by finding a point of
minimum amplitude (as measured by voltage, or alternatively by current)
and a point of maximum amplitude, with no reference to phase. In fact,
SWR was reasonably defined as the ratio of max/min amplitudes.
(snip)

What I was trying to say is that to completely see (measure) all the
differences between the current pattern in a standing wave versus a
traveling wave (or any combination of traveling waves of different
magnitudes in opposite directions, with or without losses, especially
when there are discontinuities in the conductor, like loading coils)
those observations would include phase versus position.

In many practical cases, you can infer what you need to know about the
two traveling waves by just taking amplitude measurements, as you suggest.

K7ITM wrote:
In fact, you don't even need to find the minimum and the maximum
points. Again, given sinusoidal excitation and a uniform line, some
small set of points with accurate amplitude measurement at each will
suffice, since they will uniquely determine the amplitudes of the two
waves and the line attenuation. You would have to know the spacing of
the points and that they were dense enough that there is not a spacial
aliasing problem (points distributed over more than 1/4 wavelength...).

Which points out, once again, that the phase information in a
standing wave is contained in the amplitude, not in the phase.
W7EL measured the *phase* of the standing-wave current which
is known not to contain any information as it is close to
unchanging all along a 1/2WL dipole or 1/4WL monopole. Yet he
reported it as meaningful. So far, nobody has made meaningful
phase shift measurements through a loading coil.

It's common to think of a standing wave as the result of two travelling
waves, one in each direction, but another way to think of a standing
wave pattern is as a pure standing wave plus a pure travelling wave.

One cannot get away from the fact that the pure standing wave
is the superposition of equal amplitude traveling waves flowing
in opposite directions. Some part of the forward traveling wave
must be allocated to the standing wave function. That part of
the traveling wave transfers no energy.

|Ifor| - |Iref| = |Ifor'| the part of the forward traveling wave
that is transferring energy.

|Ifor| - |Ifor'| = |Ifor''| = |Iref| the part of the forward
traveling wave that is contributing to the pure standing wave
and transferring no energy.
--
73, Cecil http://www.qsl.net/w5dxp