Roy Lewallen wrote in news:132fvs4qvp5je04
@corp.supernews.com:

I believe there's at least one basic fallacy in your development.

The problem is that a directional antenna can't be made to take up zero
space. Let's consider a situation where we can have complete

Roy, the type of probe I was considering does take up space, and I
understand your point that therein lies a possible / likely explanation
for its behaviour.

I was thinking along the lines of the superposition occuring within the
directional antenna where segment currents would each be dependent on the
field from each of the sources (and to some extent field from other
segments of itself), and the antenna was where the superposition mainly
occurred. But you are correct that the antenna is of non zero size, and
the segments that I refer to are not all located at a point where the
field strength from each source is equal and opposite.

....

I maintain that there is actually zero field at a point of
superposition
of multiple waves which sum to zero, and that no device or detector can
be devised which, looking only at that point, can tell that the zero
field is a result of multiple waves. This is a very important and
fundamental point, and I'm glad you brought it up. If you or anyone can

I understand the second point.

Extended to transmission lines, I think it means that although we can
make an observation at a single point of V and I, and knowing Zo we can
state whether there are standing waves or not, we cannot tell if that is
the result of more than two travelling waves (unless you take the view
that there is only one wave travelling in each direction, the resultant
of interactions at the ends of the line).

I will think some more about the "actual zero field", but that cannot
suggest that one wave modified the other, they must both pass beyond that
point, each unchanged, mustn't they? If that is so, the waves must be
independent, but the resultant at a point is something separate to each
of the components and doesn't of itself alter the propagation of either
wave.

Owen

On Fri, 20 Apr 2007 00:52:32 GMT, Owen Duffy wrote:

Roy Lewallen wrote in news:132fvs4qvp5je04
:

I believe there's at least one basic fallacy in your development.

The problem is that a directional antenna can't be made to take up zero
space. Let's consider a situation where we can have complete

Roy, the type of probe I was considering does take up space, and I
understand your point that therein lies a possible / likely explanation
for its behaviour.

I was thinking along the lines of the superposition occuring within the
directional antenna where segment currents would each be dependent on the
field from each of the sources (and to some extent field from other
segments of itself), and the antenna was where the superposition mainly
occurred. But you are correct that the antenna is of non zero size, and
the segments that I refer to are not all located at a point where the
field strength from each source is equal and opposite.

Hi Owen,

Why would you think that superposition fails for this?

73's
Richard Clark, KB7QHC

Richard Clark wrote in
:
....
Why would you think that superposition fails for this?

Richard, I don't... but the failure was to think that such an experiment
indicated that the two interfering waves could be isolated at a point.

Owen

On Fri, 20 Apr 2007 05:40:13 GMT, Owen Duffy wrote:

Richard Clark wrote in
:
...
Why would you think that superposition fails for this?

Richard, I don't... but the failure was to think that such an experiment
indicated that the two interfering waves could be isolated at a point.

Hi Owen,

I presume all of this flows from your statement:
A practical example of this is that an omni directional receiving antenna
may be located at a point where a direct wave and a reflected wave result
in very low received power at the antenna, whereas a directional antenna
that favours one or other of the waves will result in higher received
power. This indicates that both waves are independent and available to
the receiving antenna, the waves do not cancel in space, but rather the
superposition occurs in the antenna.

As Roy did not quote any of your material, I must presume this. Am I
correct?

73's
Richard Clark, KB7QHC

Richard Clark wrote in
news
On Fri, 20 Apr 2007 05:40:13 GMT, Owen Duffy wrote:

Richard Clark wrote in
m:
...
Why would you think that superposition fails for this?

Richard, I don't... but the failure was to think that such an
experiment indicated that the two interfering waves could be isolated
at a point.

Hi Owen,

I presume all of this flows from your statement:
A practical example of this is that an omni directional receiving
antenna may be located at a point where a direct wave and a
reflected wave result in very low received power at the antenna,
whereas a directional antenna that favours one or other of the waves
will result in higher received power. This indicates that both waves
are independent and available to the receiving antenna, the waves do
not cancel in space, but rather the superposition occurs in the
antenna.

As Roy did not quote any of your material, I must presume this. Am I
correct?

Yes

On Fri, 20 Apr 2007 06:49:46 GMT, Owen Duffy wrote:

Richard Clark wrote in
A practical example of this is that an omni directional receiving
antenna may be located at a point where a direct wave and a
reflected wave result in very low received power at the antenna,
whereas a directional antenna that favours one or other of the waves
will result in higher received power. This indicates that both waves
are independent and available to the receiving antenna, the waves do
not cancel in space, but rather the superposition occurs in the
antenna.

As Roy did not quote any of your material, I must presume this. Am I
correct?

Yes

Hi Owen,

And you have already allowed that superposition does not fail. Thus
there must be some other failure to be found in the choice of antenna.
From other correspondence, it is asserted that a gain antenna, by
virtue of its size, cannot be placed in null space (that point wherein
all contributions of energy sum to zero) which is planar and
equidistant between sources (there being two of them for the purpose
of discussion).

Have I described this accurately?

73's
Richard Clark, KB7QHC

Richard Clark wrote:

Hi Owen,

And you have already allowed that superposition does not fail. Thus
there must be some other failure to be found in the choice of antenna.
From other correspondence, it is asserted that a gain antenna, by
virtue of its size, cannot be placed in null space (that point wherein
all contributions of energy sum to zero) which is planar and
equidistant between sources (there being two of them for the purpose
of discussion).

Have I described this accurately?

I think it might be more fundamental and perhaps subtle than just a
limitation of size. If the null space is a whole plane, as with the two
radiating elements of my example, you have an infinite area on which to
construct your antenna, although it would have to have zero thickness.
But even allowing infinitely thin elements, I don't see any way you can
construct it entirely on the plane so it will be more sensitive to
signals coming from one side of the plane than the other. That is, use
any number of elements you want, oriented and phased any way you want,
and as long as all elements lie entirely on the plane, I don't think you
can make it favor the signal from one of the radiators over the other. I
believe you'll find this same problem with any region of total wave
cancellation. I don't have any rigorous proof of this, just intuition
from observing the symmetry, and would be glad to see an example which
would prove me wrong. (It might reveal a whole new class of directional
antennas! Maybe one of Art's Gaussian marvels would do it?) But if I'm
right, then there's no way to do as Owen originally proposed, namely to
determine entirely from a null space that the null is the sum of
multiple fields, let alone the nature of those fields -- at least with a
directional antenna. It has to extend out where it can a sniff of the
uncanceled fields to do that.

Roy Lewallen, W7EL

Owen Duffy wrote:
Richard Clark wrote:
Why would you think that superposition fails for this?

Richard, I don't... but the failure was to think that such an experiment
indicated that the two interfering waves could be isolated at a point.

Doesn't

b1 = s11(a1) + s12(a2) = 0

indicate that the two interfering waves are isolated
to a point?
--
73, Cecil http://www.w5dxp.com

Owen,

It's a pleasure to have a rational discussion. We will both learn from
this, and perhaps some of the readers will also.

Owen Duffy wrote:
Roy Lewallen wrote in news:132fvs4qvp5je04
@corp.supernews.com:

I believe there's at least one basic fallacy in your development.

The problem is that a directional antenna can't be made to take up zero
space. Let's consider a situation where we can have complete

Roy, the type of probe I was considering does take up space, and I
understand your point that therein lies a possible / likely explanation
for its behaviour.

I was thinking along the lines of the superposition occuring within the
directional antenna where segment currents would each be dependent on the
field from each of the sources (and to some extent field from other
segments of itself), and the antenna was where the superposition mainly
occurred. But you are correct that the antenna is of non zero size, and
the segments that I refer to are not all located at a point where the
field strength from each source is equal and opposite.

Yes, each element is seeing a different field from the other. Those
induce different currents in the elements. The sum of those is what
ultimately gives you the output from the antenna. If the two elements
both were at a point of complete wave cancellation, both would produce zero.

. . .

Extended to transmission lines, I think it means that although we can
make an observation at a single point of V and I, and knowing Zo we can
state whether there are standing waves or not, we cannot tell if that is
the result of more than two travelling waves (unless you take the view
that there is only one wave travelling in each direction, the resultant
of interactions at the ends of the line).

Hm, let's think about this a little. In my free space example, we had
two radiators whose fields went through the same point, and those two
radiators were equal in magnitude and out of phase. The sum of the two E
fields was zero and the sum of the H fields was zero, so there was no
field at all where they crossed.

But now let's look at a transmission line with waves created by
reflections from a single source. I believe that there is no point along
the line where both the E and H fields are zero, or where both the
current and the voltage are zero. (Please correct me if I'm wrong about
this.) That's a different situation from the free space, two-radiator
situation I proposed. So in a transmission line, we can find a point of
zero voltage (a "virtual short"), say, but discover that there's current
there. There will be an H field but no E field. And conversely for a
"virtual open". So there is a difference between those points and a
point of no field at all. And there is energy in the E or H field. (This
also occurs in free space where a wave interferes with its reflection or
when waves traveling in opposite directions cross.) Now, if you could
feed two equal canceling waves into a transmission line, going in the
same direction, then you would have truly zero E and H fields, and zero
voltage and current, like the plane bisecting the two free space
antennas. You couldn't tell the difference between that and no waves at
all. But as Keith recently pointed out, superposition of two parallel
equal voltage batteries would show large currents in both directions.
But they would sum to zero, which is what we observe. And as long as the
batteries remain connected, we can never detect those supposed currents.
The two-wave scenario I described is in the same category, I believe.

We can readily concede that there is no field, voltage, current, or
energy beyond the point at which the two canceling waves meet, without
having to invoke any interaction or seeing any violation of energy
conservation. Show me the whole circuit which produces this overlaying
of canceling waves, and I'll show you where every erg of energy from
your source(s) has gone. None of it will be beyond that canceling point.

I will think some more about the "actual zero field", but that cannot
suggest that one wave modified the other, they must both pass beyond that
point, each unchanged, mustn't they?

Absolutely!

If that is so, the waves must be
independent

Absolutely!

, but the resultant at a point is something separate to each
of the components and doesn't of itself alter the propagation of either
wave.

Sorry, I don't fully understand what you've said. But it is true that
the propagation of neither wave is affected in any way by the presence
of the other.

Roy Lewallen, W7EL

Roy Lewallen wrote in
:

Owen,

It's a pleasure to have a rational discussion. We will both learn from
this, and perhaps some of the readers will also.

Thanks Roy.


Owen Duffy wrote:
....

Extended to transmission lines, I think it means that although we can
make an observation at a single point of V and I, and knowing Zo we
can state whether there are standing waves or not, we cannot tell if
that is the result of more than two travelling waves (unless you take
the view that there is only one wave travelling in each direction,
the resultant of interactions at the ends of the line).

Hm, let's think about this a little. In my free space example, we had
two radiators whose fields went through the same point, and those two
radiators were equal in magnitude and out of phase. The sum of the two
E fields was zero and the sum of the H fields was zero, so there was
no field at all where they crossed.

But now let's look at a transmission line with waves created by
reflections from a single source. I believe that there is no point
along the line where both the E and H fields are zero, or where both
the current and the voltage are zero. (Please correct me if I'm wrong
about this.) That's a different situation from the free space,

Yes, I agree with you, and I think the key factor is that waves are only
free to travel in two directions, and if multiple coherent waves can
travel in the same direction, they are colinear.

two-radiator situation I proposed. So in a transmission line, we can
find a point of zero voltage (a "virtual short"), say, but discover
that there's current there. There will be an H field but no E field.
And conversely for a "virtual open". So there is a difference between
those points and a point of no field at all. And there is energy in
the E or H field. (This also occurs in free space where a wave
interferes with its reflection or when waves traveling in opposite
directions cross.) Now, if you could feed two equal canceling waves
into a transmission line, going in the same direction, then you would
have truly zero E and H fields, and zero voltage and current, like the
plane bisecting the two free space antennas. You couldn't tell the

But is it possible to inject two coherent waves travelling independently
in the same direction? Could I not legitimately resolve the attempt at a
circuit node (line end node) of two coherent sources to drive the line to
be the superposition of the voltages and curents of each to effectively
resolve to a single phasor voltage and associated phasor current at that
node, and then the conditions on the line would be such as to comply with
the boundary conditions at that line end node. Though I have mentioned
phasors which implies the steady state, this should be true in general
using v(t) and i(t), just the maths is more complex.

I can see that we can deal mathematicly with two or more coherent
components thought of as travelling in the same direction on a line (by
adding their voltages or currents algebraicly), but it seems to me that
there is no way to isolate the components, and that questions whether
they actually exist separately.

So, whilst it may be held by some that there is re-reflected energy at
the source end of a transmission line in certain scenarios, a second
independent forward wave component to track, has not the forward wave
just changed to a new value to comply with boundary conditions in
response to a change in the source V/I characteristic when the reflection
arrived at the source end of the line? I know that analysis of either
scenario will yield the same result, but one may be more complex, and it
is questionable whether the two (or more) forward wave components really
exist independently.

....
I will think some more about the "actual zero field", but that cannot
suggest that one wave modified the other, they must both pass beyond
that point, each unchanged, mustn't they?

Absolutely!

If that is so, the waves must be
independent

Absolutely!

, but the resultant at a point is something separate to each
of the components and doesn't of itself alter the propagation of
either wave.

Sorry, I don't fully understand what you've said. But it is true that
the propagation of neither wave is affected in any way by the presence
of the other.

I am saying that resolution of the fields of two independent waves at a
point in free space to a resultant is not a wave itself, it cannot be
represented as a wave, and it does not of itself alter the propagation of
either wave. It may be useful in predicting the influence of the two
waves on something at that point, but nowhere else.

Having thought through to the last sentence, I think I am agreeing with
your statement about free space interference "I maintain that there is
actually zero field at a point of superposition of multiple waves which
sum to zero, and that no device or detector can be devised which, looking
only at that point, can tell that the zero field is a result of multiple
waves."

And we haven't mentioned power, not once!

Owen