Cecil, W5DXP wrote:
"Net current doesn`t flow."

I`m inclined to agree, but at first current flows with a volts to amps
ratio rqual to the Zo of the line until a reflection returns to the
connection point of the stub. Then the total phase rotation within the
stub has reached 360-degrees. The complete reflection supplies a
reflected voltage wqual to the incident voltage. There is no difference
of potential or phase to evoke current. It`s the equivalent of a very
high impedance. Almost no more current is motivated to flow, once the
steady-state condition is reached.

Alexander Wing wrote on page 29 of "Transmission Lines, Antennas, and
Wave Guides":
"Suppression of Even Harmonics.- An application of a short-circuited
quarter-wavelength line is to suppress any unwanted even harmonics in
the output of a radio transmitter. A short-circuited one-quarter
wavelength long at the desired output frequency may be connected across
the output terminals or across the antenna feeder at any point without
placing much load on the transmitter at the fundamental or desired
output frequency, since at this frequency such a section has an
impedance ideally infinite, actually about 400,000 ohms."

How much current flows into 400.000 ohms?

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:

Cecil, W5DXP wrote:
"Net current doesn`t flow."

I`m inclined to agree, but at first current flows with a volts to amps
ratio rqual to the Zo of the line until a reflection returns to the
connection point of the stub. Then the total phase rotation within the
stub has reached 360-degrees. The complete reflection supplies a
reflected voltage wqual to the incident voltage. There is no difference
of potential or phase to evoke current. It`s the equivalent of a very
high impedance. Almost no more current is motivated to flow, once the
steady-state condition is reached.

Richard, some people use that exact same argument to try to prove
that no current is flowing in the middle of a transmission line
where forward power equals reflected power. The strange thing is
that the current 1/4WL away from that zero net current point is
sky high.

If the current 1/4WL away from your above source output terminals
is indeed sky high, it is because the forward and reflected currents
are in phase at that point which means they are 180 degrees out of
phase at the source output which means they are both still there.
If the net current is zero, all it means is that |Ifor|-|Iref| = 0.
It tells us nothing about the magnitudes of Ifor and Iref.

Trying to treat a distributed network as a lumped circuit can lead
to mistakes.

Alexander Wing wrote on page 29 of "Transmission Lines, Antennas, and
Wave Guides":
"Suppression of Even Harmonics.- An application of a short-circuited
quarter-wavelength line is to suppress any unwanted even harmonics in
the output of a radio transmitter. A short-circuited one-quarter
wavelength long at the desired output frequency may be connected across
the output terminals or across the antenna feeder at any point without
placing much load on the transmitter at the fundamental or desired
output frequency, since at this frequency such a section has an
impedance ideally infinite, actually about 400,000 ohms."

How much current flows into 400.000 ohms?

How much current flows in the shorted end of the stub? Let's say we
measure it at 2 amps. Where does that current come from?

The 400,000 ohms is (Vfor+Vref)/(Ifor+Iref) at the mouth of the stub
where V's and I's are phasors. Knowing that (Ifor+Iref) is a small
value doesn't tell us anything about the magnitudes of Ifor and Iref
except that they are nearly equal as is always the case for a
low-loss stub. Knowing (Inet=|Ifor|-|Iref|) is small doesn't tell us
a thing about the magnitude of (|Ifor|+|Iref|) that exists at the
shorted end of the stub. Does that make sense?
--
73, Cecil http://www.qsl.net/w5dxp

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On Sat, 13 Aug 2005 16:57:52 -0500, Cecil Moore
wrote:
some people use that exact same argument to try to prove
that no current is flowing in the middle of a transmission line
Or uncanceled reflections brighter than the sun are black.

Richard Clark wrote:
Cecil Moore wrote:
some people use that exact same argument to try to prove
that no current is flowing in the middle of a transmission line

Or uncanceled reflections brighter than the sun are black.

In a lossless system sourced by monochromatic
coherent light, if the 1st medium is air and if the
index of refraction of the 3rd medium is the square of
the index of refraction of the 1/4WL 2nd medium, then
all reflections are canceled in the 1st medium. That's
one of the rules of optics for non-reflective coatings.
--
73, Cecil http://www.qsl.net/w5dxp

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On Sat, 13 Aug 2005 22:34:38 -0500, Cecil Moore
wrote:
then all reflections are canceled in the 1st medium
"all," of course, if you discard precision to force the theory to fit
the expectations.
About as respectable as E = M · C² ±3dB

Richard Clark wrote:
Cecil Moore wrote:
then all reflections are canceled in the 1st medium

"all," of course, if you discard precision to force the theory to fit
the expectations.
About as respectable as E = M · C² ±3dB

Given an ideal system with carefully selected boundary conditions,
all reflections are canceled. That is a concept, not something
that can be achieved in reality. Your height and weight on your
driver's license is not 100% accurate either but it is good
enough for government work.
--
73, Cecil http://www.qsl.net/w5dxp


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On Sun, 14 Aug 2005 07:23:56 -0500, Cecil Moore
wrote:
Your height and weight
±3dB? All the expectation of a Dr. Atkins diet.

Richard Harrison wrote:

Cecil, W5DXP wrote:
"Net current doesn`t flow."

How much current flows into 400.000 ohms?

Let's take a look at a simplified system. The SGCL source is
a signal generator equipped with a circulator and load resistor.
Each segment in the following 50 ohm lossless coax is 1/4WL long.

|--------1.25WL 50 ohm coax-------|
200W SGCL------A------B------C------D------short

The signal generator is sourcing 200W and the circulator load
resistor is dissipating 200W.

What is the net current at the short and at A and C?

What is the net voltage at D, B, and the source?

What is the forward power and reflected power?

What is the net current at the signal generator output?

What magnitude of current is the signal generator sourcing?

What magnitude of current is flowing through the circulator
load resistor?

What is the phase difference between the forward current and
reflected current?
--
73, Cecil http://www.qsl.net/w5dxp

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