-
"The Third Term"
-
Roy Lewallen wrote:
-
| No, the average Poynting vector points toward the load.
| Power leaves the line and enters the load, as it should.
| ...
| I imagine your problem
| with the solution is your notion that
| the total average power
| is the difference between the
| "forward power" and "reverse power".
|
| But it's not.
|
| I gave the equation showing what the total power
| is, and as you can see,
| there's a third term involved.
| When this is
| taken into consideration, you see that there's a net power flow out of
| the line into the load, as there should be.
| ...
-
As usually,
Mr. Roy Lewallen,
points the right direction.
And this time,
it is of:
-
"The Third Term".
-
In the whole of the book by R.A.Chipman,
a phrase, less than a printed line,
is proved enough to cause a major upset:
-
"The third term
on the right
represents interaction
between the two waves."
-
But when there is such a steadfast loyalty
to the existence of some kind of
"interference"
between two,
rather clearly distinct waves,
the incident and the reflected one,
it is difficult for anybody
to compromise himself and accept
that the same two waves,
so clearly distinct until now,
when are coming along a line with complex Z0,
have to bear in addition
some kind of
"interaction".
-
Very difficult, indeed.
-
Sincerely,
-
pez
SV7BAX
-

Johnson uses the same term of "interaction" to describe the origin of
the extra power term. However, you can hopefully see from the analysis I
posted that only ordinary superposition of the forward and reverse
voltage and current waves is necessary for the term to appear. So I
don't feel that "interaction" is entirely appropriate. The extra term
actually is a result of the calculation of average power. I've said many
times that it's risky to abandon the time information in the power
waveform and deal only with averages. If the voltage and current in each
wave aren't in phase with each other, there are components of the total
VI product that add together but don't show up in the averages of the
individual forward and reverse average powers. There's no mystery or
true "interaction" involved. The "problem" lies simply in calculating
average "forward power" and "reverse power" separately, throwing away
all time related information, then expecting them to add or subtract to
get the total.

Roy Lewallen, W7EL

pez wrote:
-
"The Third Term"
-
Roy Lewallen wrote:
-
| No, the average Poynting vector points toward the load.
| Power leaves the line and enters the load, as it should.
| ...
| I imagine your problem
| with the solution is your notion that
| the total average power
| is the difference between the
| "forward power" and "reverse power".
|
| But it's not.
|
| I gave the equation showing what the total power
| is, and as you can see,
| there's a third term involved.
| When this is
| taken into consideration, you see that there's a net power flow out of
| the line into the load, as there should be.
| ...
-
As usually,
Mr. Roy Lewallen,
points the right direction.
And this time,
it is of:
-
"The Third Term".
-
In the whole of the book by R.A.Chipman,
a phrase, less than a printed line,
is proved enough to cause a major upset:
-
"The third term
on the right
represents interaction
between the two waves."
-
But when there is such a steadfast loyalty
to the existence of some kind of
"interference"
between two,
rather clearly distinct waves,
the incident and the reflected one,
it is difficult for anybody
to compromise himself and accept
that the same two waves,
so clearly distinct until now,
when are coming along a line with complex Z0,
have to bear in addition
some kind of
"interaction".
-
Very difficult, indeed.
-
Sincerely,
-
pez
SV7BAX
-

Roy Lewallen wrote:
The "problem" lies simply in calculating
average "forward power" and "reverse power" separately, throwing away
all time related information, then expecting them to add or subtract to
get the total.

The s-parameter analysis doesn't have a problem doing that, Roy.
Your analysis won't either when you include all the appropriate
terms.
--
73, Cecil http://www.qsl.net/w5dxp



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No, no, Cecil. *You* need to include all the separate voltages and
currents, to show us. I'm glad that you find the s-parameter analysis to
be more trouble free. So do it, and when you're done, substitute back in
for transmission line characteristic impedance, length, and loss; and
load impedance, then show us the resulting voltages, currents, and
powers. Should be easy, no?

Roy Lewallen, W7EL

Cecil Moore wrote:
Roy Lewallen wrote:

The "problem" lies simply in calculating average "forward power" and
"reverse power" separately, throwing away all time related
information, then expecting them to add or subtract to get the total.


The s-parameter analysis doesn't have a problem doing that, Roy.
Your analysis won't either when you include all the appropriate
terms.

Roy Lewallen wrote:

No, no, Cecil. *You* need to include all the separate voltages and
currents, to show us.

Do you think I am capable of inventing something so complex just to
pull your leg, Roy? What I am saying is grounded in physics. You
simply made a simple mistake in assuming one term was forward voltage
when actually, there are two terms when added together, yield the
forward voltage. You didn't realize that you were doing a 2-port
analysis.
--
73, Cecil http://www.qsl.net/w5dxp



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Cecil:

[snip]
forward voltage. You didn't realize that you were doing a 2-port
analysis.
--
73, Cecil http://www.qsl.net/w5dxp
[snip]

I was just castigated by Roy and Dave Robbins on another thread for making
postings about
"ports", apparently we are not allowed to discuss "ports" when we are
discussing transmission
lines since ports have only to do with networks and transmission lines have
no ports!!!

;-)

--
Peter K1PO [A guy who believes that transmission lines have two ports.]
Indialantic By-the-Sea, Fl

Peter O. Brackett wrote:
I was just castigated by Roy and Dave Robbins on another thread for making
postings about
"ports", apparently we are not allowed to discuss "ports" when we are
discussing transmission
lines since ports have only to do with networks and transmission lines have
no ports!!!

Darn Peter, I was hoping you could help me explain to Roy what is wrong with
his analysis - that he is using a 2-port analysis and getting four power terms
as a result, two of which have to be added to get forward power and the other
two of which have to be added to get reflected power.
--
73, Cecil http://www.qsl.net/w5dxp



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Dave Shrader wrote:
Or, how about: one with an in'port' and the second with an out'port'?

Speaking of which, I just thought of a way to alleviate the confusion
about the earlier example which was:

------lossy feedline---x----10+j60 ohm load

I hope my memory is correct about the load value. If we add one wavelength
of lossless feedline to the experiment, we don't change anything but
things become a lot clearer.

---lossy feedline--+--1WL lossless 50 ohm feedline--10+j60 ohm load
Pfwd1-- Pfwd2--
--Pref1 --Pref2

It is readily apparent that Pref2 cannot be greater than Pfwd2.
If my math is correct, at the load, |rho|=0.85 and SWR=12.3:1
|rho|^2=0.7225 so Pref2 = 0.7225(Pfwd2)

The main thing to gather from the above example is that:

(Pfwd2 - Pref2) = (Pfwd1 - Pref1)

Therefore if Pfwd2 Pref2 then Pfwd1 Pref1

Conclusion: This is NOT an example of Pfwd Pref.
--
73, Cecil, W5DXP

"Cecil Moore" wrote in message
...
Peter O. Brackett wrote:
I was just castigated by Roy and Dave Robbins on another thread for
making
postings about
"ports", apparently we are not allowed to discuss "ports" when we are
discussing transmission
lines since ports have only to do with networks and transmission lines
have
no ports!!!

Darn Peter, I was hoping you could help me explain to Roy what is wrong
with
his analysis - that he is using a 2-port analysis and getting four power
terms
as a result, two of which have to be added to get forward power and the
other
two of which have to be added to get reflected power.

i like ports, i use them all the time in network analysis, they are an
important part of the tcp/ip protocol!. for transmission lines all you
really need is voltage OR current waves, everything else falls out from
those. computing power and trying to reflect power can only lead to
confusion, because unless you use the complete formula for complex powers
you are losing important information.... and don't even start on computing
rms or other average powers, then you have totally lost the physical
significance of the waves.

Dave:

[snip]
Is that one on the East Coast and one on the West Coast??

Or, how about: one with an in'port' and the second with an out'port'?

:-)

Deacon Dave, W1MCE
[snip]

I always have a port before dinner.

And often I have one sitting on the operating desk while I am transmitting.
I like the fortified ports best.

I always love the hotels in Portugal, cuz they always seem to leave a nice
decanter of port on the night stand every day. Helps to put you to sleep.

--
Peter K1PO
Indialantic By-the-Sea, FL