Revisiting the Power Explanation
 

On Apr 2, 9:40 pm, Jim Kelley wrote:
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- Hide quoted text -

- Show quoted text -

Hi Jim,

Thanks for taking the time to write such an insightful response--I
was
expecting it.

At the moment I'm in a hotel in Jacksonville, going through pre-op for
spinal
surgery. I'll be home Wednesday, and that will be the earliest I'll
have the
opportunity to review your comments. So I'll get back with you after
having the
time to review it.

Thanks again, Jim,

Revisiting the Power Explanation
 

Cecil Moore wrote:
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.

Accordingly, the fraction of energy that changes direction at a
physical discontinuity is dictated by the physical constants of the
physical discontinuity; index of refraction in the case of optics;
impedance in the case of transmission lines.

73, Jim AC6XG

Revisiting the Power Explanation
 

Cecil Moore wrote:

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.

Given the
line, the unit area term can be dropped without error.

In the engineering profession, it would probably mean without job; in
science, without publication.

ac6xg

Revisiting the Power Explanation
 

Jim Kelley wrote:
"The error I find is in your notation that virtual shorts and opens
cause reflections, and that real reflection coefficients may be
calculated from virtual impedances."

Impedance is a voltage to current ratio. A low impedance allows much
current from a low impressed voltage. A high impedance only allows a low
current from a high impressed voltage.

A transmission line with a mismatched load, has a reverse or reflected
wave traveling back from the load in addition to the incident wave
traveling toward and impressed on the load. Both waves, incident and
reflected, have the same voltage to current ratio. It equals the Zo of
the line and is enforced by the construction of the assumed uniform
line.

Superposition of the forward or incident wave and the reverse or
reflected wave produces periodic variations in the combined volts and
amps along the line. The rms values of the constituent volts and amps
are likely very steady. It`s their combination which varies.

High voltage points are high impedance points. Low voltage points are
low impedance points.

Severity of voltage and impedance variations along the line depends on
how different the load impedance is from Zo.

A short or an open on the line can produce segments analogous with
series and parallel resonant LC circuits (which behave as shorts and
opens with respect to impedance for example).

Input impedance of an open circuited line of length less than a quarter
wavelength is capacitive. Input impedance of an open line of length
greater than a quarter wavelength but less than a half wavelength is
inductive.

An open-circuited quarter wavelength of line is practically a short
circuit at its input.

A quarter wavelength back from a line short, its impedance is is an open
circuit, and as above, a quarter wavelength back from an open circuit,
the line impedance is a short circuit.

At a line short circuit, incident and reflected current phasors are
in-phase while the incident and reflected voltage phasors are
out-of-phase.

A quarter wave back from the short, the incident and reflected current
phasors are out-of-phase while the the incident and reflected voltage
phasors are in-phase.

A quarter wave back from a hard short on a good line, the reflected
voltage is equal to and of the same phase as the incident voltage.
Therefore there is no potential difference between the incident and
reflected voltages at this point and the current is zero. This is
analogous to connecting identical battery cells in parallel. No current
flows between them. The same can be said of connecting correctly phased
identical transformer windings in parallel.

High voltage and almost zero net current means the impedance is nearly
infinite. This is similar to a good parallel resonant circuit. It is
almost an open circuit. Quarter wave shorted stubs have been used as
"metal insulators" for line support and other purposes.

I have no problem with "virtual shorts and opens". We so called them and
used them in school when I was there over a half century ago.

Examine RADAR TR and anti-TR circuits for examples of virtual shorts and
opens which automatically route the energy to the right places and keep
it out of the wrong places.

Best regards. Richard Harrison, KB5WZI

Revisiting the Power Explanation
 

On Apr 2, 5:19 pm, Cecil Moore wrote:
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.

Please try again after adding 1 more wavelength of 450
Ohm line between the generator and the 75 Ohm line you
added. Kindly explain where the 'reflected power' on
this new section of 450 Ohm line goes.

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).

This would be quite incorrect. The impedance presented to
the generator is quite different. Do recall that by design
the generator has a 450 Ohm output impedance so there is
a physical discontinuity at the connection to the 75 Ohm
line. In this case the 75 Ohm line is acting as an impedance
transformer and its length is very relevent. Make it a
multiple half wavelength and the transformation is unity.
I had assumed that this was why in your first analysis
you chose to add one wavelength of line and state that
this would not alter the steady state response, to which,
of course, I agree, though it quite alters the transient
response.

....Keith

Revisiting the Power Explanation
 

Cecil Moore wrote:
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?

When you think of interference as being the instantaneous sum of waves
at a given position and time, then there is really only one kind of
interference to be had - though there are a variety of results which
can be obtained from it. It is useful to bear in mind that fields,
voltages, and currents are what give physical form to the waves in
transmission lines. Whether in a transmission line, or free space it
is the fields which interfere - not power, or energy.

73, Jim AC6XG

Revisiting the Power Explanation
 

On Apr 2, 7:35 pm, (Richard Harrison) wrote:
Jim Kelley wrote:

"The error I find is in your notion that virtual shorts and opens
cause reflections, and that real reflection coefficients may be
calculated from virtual impedances."


I have no problem with "virtual shorts and opens". We so called them and
used them in school when I was there over a half century ago.

Examine RADAR TR and anti-TR circuits for examples of virtual shorts and
opens which automatically route the energy to the right places and keep
it out of the wrong places.

Best regards. Richard Harrison, KB5WZI

Hi Richard,

If I gave the impression that I have a problem with virtual, or
effective impedances then I apollogize. I tried to make it clear that
the only problem is in improperly attributing cause and effect. While
I am not familiar with RADAR TR or anti-TR, I am quite sure that it is
subject to the same physical laws as any other technology.
Electromagnetic waves reflect only from real physical boundaries. JC
Maxwell took great care to describe exactly how that works. If you
feel that RADAR is somehow exempt from these physical laws, then this
might be an opportunity for us both to review our understanding of the
technology.

Best Regards,
Jim AC6XG

Revisiting the Power Explanation
 

On 3 Apr 2007 07:28:45 -0700, "Jim Kelley" wrote:

Electromagnetic waves reflect only from real physical boundaries.

TR/ATR tubes (by their very description and certainly operation)
fulfill that condition.

73's
Richard Clark, KB7QHC

Revisiting the Power Explanation
 

On 3 Apr 2007 07:28:45 -0700, "Jim Kelley" wrote:

I am not familiar with RADAR TR or anti-TR

Hi Jim, and others similarly unfamiliar,

Radar transmission systems have a wide and diverse design topology
that reveals all of the characteristics in an easy and accessibly
small space. Among these diverse applications are the already
mentioned TRansmit and AntiTRansmit tubes. Also are mode shifters,
polarization shifters, choke joints, directional couplers (classic,
not Bruene), isolators, circulators, and separators - a class that
includes what I alluded to, the "Magic T." This last is something
like the classic Hybrid Coupler.

Within these lines you can add either resistive, conductive or
dielectric windows, steps, vanes, and other configurations to create
tuned sections or transitions between sections. Each and all of such
elements readily reduce to wavelength and transmission line mechanics.
Their scale makes them "hands-on."

Returning to the TR/ATR tubes, they simply reside within the path of
the transmission line at a critical harmonic dimension. They consist
of a glass envelope, much like an acorn tube, and it contains a gas
and a simple spark gap and possibly a third exciter electrode. When a
sufficient electric field causes the gap to discharge, this creates
the short that is reflected to a nearby junction. The exciter
electrode is used with a bias to create a very low threshold for
firing. Needless to say, received signals are of insufficient
amplitude to fire the tube, hence the duplexing action.

This, then, creates a different topology between transmit and receive,
and it keeps the MW peak amplitudes out of the receiver front end as
the receive and transmit signal paths are identical otherwise.

Choke joints are passive in nature, but they also exhibit the use of a
tuned cavity that creates a conductive bridge across an otherwise open
gap between transmission line elements (this is a classic mechanism in
the rotary joint of the turning radar antenna).

The RADAR Transmission line systems offers the student vastly more
about transmission line concepts than the rather boring Lecher lines.

73's
Richard Clark, KB7QHC

Revisiting the Power Explanation
 

Jim Kelley wrote:
Accordingly, the fraction of energy that changes direction at a physical
discontinuity is dictated by the physical constants of the physical
discontinuity; index of refraction in the case of optics; impedance in
the case of transmission lines.

Yes, all explained in my energy analysis article.
Personally, I wish whoever introduced virtual
reflection coefficients had not done so.
--
73, Cecil http://www.w5dxp.com