Revisiting the Power Explanation
 

On Sun, 01 Apr 2007 18:18:53 GMT, Walter Maxwell
wrote:

Richard, also consider an open-wire transmission line, equal and opposite currents flowing on each wire, and
no common-mode currents. There is zero radiation, because the opposing fields developed by the current flow
cancel. Are you denying that the zero radiation results from interferece?

Hi Walt,

Zero radiation results from a particularly small in relation to
wavelength separation of wires. In a very close proximity, all loads
see the same phase balance (0), hence the combination of these issues
of distance with regard to wavelength render a null in any load remote
from the line you have described.

Also, consider standing waves on on a line, resulting from the superposition of the forward and reflected
waves, where the maximum amplitude results from constructive interference and the minimum amplitude results
from destructive interference. Are you denying the existence of interference in this case?

In a lossless line, you can't tell the difference. In a lossy line,
you have found a load. As any lossy line generally presents the same
loss on a per-length basis, it is obvious that phase combines to
create loss in only certain positions with regard to distance and
wavelength. Mapping will follow the load.

Barring either of these cases, you have to insert a load into the line
to force the interference. Sure, you can anticipate it
mathematically. And certainly it will be borne out in practice with
lossy line melting or arcing at cardinal points. Without that loss -
who cares? Nothing happened or will happen and the waves will never
interact to give a new product as (I imagine we have agreed) this is
linear space.

The operation of TR/ATR tubes in transmission line systems works with
this to force interference to steer radiation. Remove the tubes and
the action ceases. The fields in an identical transmission line
environment cannot do it themselves without that load present. This
is classic RADAR systems topology.

Imagine what this would be like with the "Magic T?"

73's
Richard Clark, KB7QHC

Revisiting the Power Explanation
 

On Sun, 01 Apr 2007 18:30:06 GMT, Cecil Moore
wrote:
Cecil Moore wrote:

Richard, did you unethically edit my posting to make
it appear that I said something different from what
I said?

So, are you still sleeping with Hecht?

Revisiting the Power Explanation
 

On 1 Apr 2007 18:05:39 -0700, "walt" wrote:

On Apr 2, 1:51 pm, Richard Clark wrote:
On Sun, 01 Apr 2007 18:30:06 GMT, Cecil Moore
wrote:

Cecil Moore wrote:

Richard, did you unethically edit my posting to make
it appear that I said something different from what
I said?

So, are you still sleeping with Hecht?

Richard, it's very uncommon, but on this issue I'm having a difficult
time following you. The only reason that I can conclude for my lack of
understanding is that our definition of 'interference' must be
divergent. So I'll just drop the discussion--OK?

Hi Walt,

It's fine, except you were the only lucid respondent in this recent
line of inquiry.

73's
Richard Clark, KB7QHC

Revisiting the Power Explanation
 

On 1 Apr 2007 18:25:19 -0700, "Jim Kelley" wrote:

Also note that 'Watt' isn't a unit of irradiance or current.

Also note that irradiance also demands an area unit. Same old errors.

Maybe absolute braces could gussy up the treatment.

73's
Richard Clark, KB7QHC

Revisiting the Power Explanation
 

Walter Maxwell wrote:
On Wed, 28 Mar 2007 16:26:10 -0700, Jim Kelley wrote:


Hi Cecil -

We've been over this a hundred times already. The only way to get
past it is for you to try to understand that my comment and Eugene
Hecht's are both true. You need to find a way to understand that
there is no contradiction. You could start by noting that Hecht does
not contradict anything that I said. Nowhere does he claim that
interference redirects energy. That's your claim! And I haven't said
that energy isn't redirected. If redirection of energy takes place,
it takes place by reflection - not interference. It's just basic optics.

73, ac6xg


Sorry Jim, but I take exception to your statement, "If redirection of energy takes place,
it takes place by reflection - not interference."

It is the interference between the forward and reflected voltages and beween the forward and reflected
currents that yields the resultant voltage and current values of rho at the matching point which produces
either a virtual short or a virtual open circuit that causes the re-reflection. I have shown this to be true
in my QEX article of Mar/Apr 1998, entitled, "Examining the Mechanics of Wave Interference in Impedance
Matching. It is also Chapter 23 in Reflections 2.

Using the complex values of rho I have shown the magnitude and phase relationships of the aforementioned
voltages and currents at the stub point that result in a virtual open circuit at the stub point to waves
reflected from a 3:1 mismatched load. The result is no reflections on the line between the stub and the
source, but a 3:1 SWR on the line between the mismatched load and the stub. If you don't have a copy of this
article please let me know and I'll send you one via email.

Walt, W2DU

Hello Walt,

Please know that all of my comments are offered with all due respect,
and there is a lot of respect due, and sincerely felt. However,
though the numbers work out as one would expect given the effects
that are observed, cause can only be attributed to
phenomenon which is observed in nature. Electromagnetic
waves can reflect only from real media. Though I admire the procedure
you have devised for describing the complex effects of these
reflections, the attempts to extend those ideas to describe real (not
virtual) physical phenomena are unsupported from a scientific
standpoint. Inference is insufficient proof.

Specifically: The nature of reflective surfaces does not depend on
whether or not steady state has been reached. Their reflectivity does
not change as a function voltage. Their nature does not depend on
things which lie at the other end of the transmission line - even
though the overall performance of the system certainly does. The
nature of reflective surfaces does not depend on how many times the
wave has bounced back and forth since the signal was initiated. These
things are implied by your claim, unfortunately.

In the case of optical media, reflectivity is determined by the
relative indices of refraction of the optical media comprising the
reflective surface. Neither the
indices, nor their ratio changes in response to illumination.
Likewise, the reflective nature of impedance discontinuities on a
transmission line depend entirely on physical constants analogous to
optical indices of refraction. These attributes do not tend to change
when illuminated by RF.

I thought we had covered this ground back a couple of years ago when
the topic of reflections from virtual shorts and opens came up on this
newsgroup. At that time it became clear that certain of the
predictions made under the model did not match well with reality.
For example, the virtual short circuit which can appear at the
entrance to a stub on a transmission line would, according to the
model, prevent the very currents necessary for creating the virtual
condition from entering the stub in the first place. Reflections can
occur only at physical discontinuities, not at a voltage to current ratio.

I was encouraged by the work we did on your transmission line
impedance matching transformer diagram. In that, it seemed we agreed
that the reflective coefficients were constant, were determined by the
characteristic impedance's of the transmission lines, and that steady
state was in fact comprised of the summation of a long series of
multiple partial reflections. And, it correlates exactly with the
descriptions and drawings of the analogous optical phenomena in the
physics texts.

I really admire your work, Walt, but I feel this one point is clearly
inconsistent with nature. Let me hasten to add that I see nothing at
all wrong with making calculations based on this model. Its utility
and beauty lies in its accuracy at the macroscopic level. I also
think that your treatise on interference as it applies to the
reflections found in RF systems is well done. The error I find is in
your notion of interference as a cause of reflection, which I assume
is extrapolated from your notion that virtual shorts and opens cause
reflections, and, that real reflective coefficients may be calculated
from virtual impedance's.

Best Regards,

Jim Kelley

Revisiting the Power Explanation
 

Jim Kelley wrote:
On Mar 29, 7:33 pm, Cecil Moore wrote:
Unless it is located at a
physical impedance discontinuity, absolutely nothing happens
because of the V/I ratio.

The last half of the sentence is absolute correct. The V/I ratio is a
result, not a cause.

Yep, I didn't say that very well. Let me try again.
EM wave energy in a transmission line can only change
directions at a physical impedance discontinuity.
--
73, Cecil http://www.w5dxp.com

Revisiting the Power Explanation
 

walt wrote:
Richard, it's very uncommon, but on this issue I'm having a difficult
time following you. The only reason that I can conclude for my lack of
understanding is that our definition of 'interference' must be
divergent. So I'll just drop the discussion--OK?

I am ignorant of any technical words (if they exist)
for what I am about to describe so bear with me.

Interference can have totally different outcomes so I
have to ask, are there different kinds of interference?

For instance, the interference between the forward wave
and reflected wave that causes standing waves has no
effect on either the forward wave or the reflected wave.
In an unchanging Z0 environment, the forward wave and
reflected wave pass like ships in the night. For want
of a better term, I will call this type of interference
"temporary interference".

The other type of interference occurs when reflections
are eliminated at a Z0-match or a non-reflective thin-
film. This is *wave cancellation* between two coherent
waves of equal magnitude and opposite phase traveling
in the same path in the direction. For want of a better
term, I will call that type of interference "permanent
interference" since the two waves are canceled and
disappear. Their energy components are redistributed.

Are there any technical words to differentiate between
the two types of interference?
--
73, Cecil http://www.w5dxp.com

Revisiting the Power Explanation
 

Jim Kelley wrote:
Cecil Moore wrote:
Note that I is "irradiance", not current.

Also note that 'Watt' isn't a unit of irradiance or current.

Close enough for a laser beam. The unit of irradiance is
watts/unit-area. The square of the radius of the laser
beam times pi can be considered to be the unit area. Thus
the unit area is implied and fixed but not specified.
The same is true for a coaxial transmission line. Given the
line, the unit area term can be dropped without error.
--
73, Cecil http://www.w5dxp.com

Revisiting the Power Explanation
 

Keith Dysart wrote:
This means that reflecting back into the generator from the
left end of the 75 Ohm line will be the same Pref2 = 229.6 W
that exists on the right side of the 75 Ohm line section.

Since ZERO reflected energy exists anywhere on the
75 ohm line, your assertion doesn't make any sense.

Now consider that the 75 ohm line can be one foot
long and everything is the same as the 1WL of 75
ohm line (except the delays).

If adding one foot of 75 ohm line inside the source
completely eliminates reflections, then the source
impedance doesn't matter at all because all it is
delivering is a voltage.
--
73, Cecil http://www.w5dxp.com

Revisiting the Power Explanation
 

Richard Clark wrote:
So, are you still sleeping with Hecht?

As a matter of fact, I often fall asleep with
"Optics" clutched between my hands. You really
should try reading it sometime.
--
73, Cecil http://www.w5dxp.com