Roy Lewallen wrote:
This isn't quite correct. A standing wave isn't the sum of traveling
waves. It's a description of the envelope of the current distribution
that sum produces. . .

Look at this standing wave:

http://www.chemmybear.com/standing.html

The equation for that standing wave is:

Ex = E*e^j(wt-Bz) + E'*e^j(wt+Bz) [see quote below]

At one time in the cycle, the standing wave equals zero
all up and down the line.

A STANDING WAVE *IS* THE SUM OF TRAVELING WAVES.

On page 285 of "Fields and Waves in Modern Radio",
2nd edition, by Ramo and Whinnery, it gives the
equations for the forward wave, the reflected wave,
and the standing wave. Begin quote:
--------------------------------------------------
Incident Wave --- E*e^j(wt-Bz)

Reflected Wave --- -E*e^j(wt+Bz)

If Ex = 0 at z = 0 for all values of time, E' = -E.

[Standing Wave equation]
Ex = E*e^j(wt-Bz) + E'*e^j(wt+Bz) = -2jE*sin(Bz e^jwt)

[Standing wave envelope equation on page 42]
V = -2jV1*sin(Bz)

End quote:
--------------------------------------------------
The standing wave equation is simply the sum of the
traveling wave equations.
--
73, Cecil http://www.w5dxp.com

"Keith Dysart" wrote in message
...

text cut.....
The directional ammeter measures instantaneous
Vt and It, does the above arithmetic and presents
If. A directional ammeter that presents a single
number rather than the time varying If has probably
converted the instantaneous values to RMS.
text cut......

...Keith

I don't think that the directional ammeter reads instantaneous Vt and It.
The circuits I am thinking of sample a length of line (NOT A POINT) so the
sample records average voltage (or current) from a period of time.

If I understand the methodology of the directional ammeter correctly, it
extracts energy from the wave from both magnetic (current) and voltage
components. If the components are in phase, they add, and that only occurs
with the wave going in the design direction. Yes, this is a reading of
power, not voltage or current individually. Current and voltage are
related by the Zo of the transmission line, so if we know current, we also
know voltage, and visa versa.

Agreed?

73, Roger, W7WKB

On Dec 10, 10:08 am, "Roger Sparks" wrote:
"Keith Dysart" wrote in message

...

text cut.....

The directional ammeter measures instantaneous
Vt and It, does the above arithmetic and presents
If. A directional ammeter that presents a single
number rather than the time varying If has probably
converted the instantaneous values to RMS.
text cut......

...Keith

I don't think that the directional ammeter reads instantaneous Vt and It.
The circuits I am thinking of sample a length of line (NOT A POINT) so the
sample records average voltage (or current) from a period of time.

If I understand the methodology of the directional ammeter correctly, it
extracts energy from the wave from both magnetic (current) and voltage
components. If the components are in phase, they add, and that only occurs
with the wave going in the design direction. Yes, this is a reading of
power, not voltage or current individually. Current and voltage are
related by the Zo of the transmission line, so if we know current, we also
know voltage, and visa versa.

Agreed?

Only partly. If you look at the element on an instrument
like a Bird 43, you will find that it is both capacitively
and inductively coupled to the line. The capactive
coupling is sensitive to the total voltage on the line
at the point of the element, while the inductive coupling
is sensitive to the total current in the line.

The subtraction (or addition) is done in the element
where the voltage sample and the current sample
(scaled by Z0) are subtracted before being applied
to the diode. The output of the diode is the
rectified instantaneous difference of the voltage and
scaled current from the equations originally provided.
This is fed to an average responding meter which has
a scale marked to show (Vf**2)/Z0 (i.e. power).

You are correct that the element does not sample
at a point, but rather over the width of the coupling
element. This is done because of design
limitations and is one of the sources for error in the
instrument, though small if the wavelength is long
compared to the element.

There are many ways to obtain the instaneous
voltage and current for the subtraction (or addition).
Some designs measure the voltage by using
an electrical connection to the line, so these are
essentially measuring at a point. Other designs
measure the current by detecting the voltage
drop across a resistor in series with the line.

Diagrams of the internals of the Bird 43 element
are available in the Operations Manual he
http://www.bird-electronic.com/produ...uct.aspx?id=81

....Keith

Keith Dysart wrote:
There are many ways to obtain the instaneous
voltage and current for the subtraction (or addition).

Here are the associated equations:

Vz = V*e^-jBz + V'*e^+jBz

Iz*Z0 = V*e^-jBz - V'*e^+jBz

Current is sampled in such a way as to perform
the multiplication by Z0. That's where the calibration
to Z0 comes in.

If one adds the two equations (samples) the reflected
terms drop out and the result is a voltage proportional
to the forward wave.

If one subtracts the two equations (samples) the
forward terms drop out and the result is a voltage
proportional to the reflected wave.
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote:
If one adds the two equations (samples) the reflected
terms drop out and the result is a voltage proportional
to the forward wave.

If one subtracts the two equations (samples) the
forward terms drop out and the result is a voltage
proportional to the reflected wave.

Continuing: The phase of the standing-wave current
in a 1/4WL stub is constant from feedpoint to tip.
However, two directional couplers, one placed at the
1/3 point and the other placed at the 2/3 point
would allow one to see the 30 degree phase shift in
the traveling-waves at the points before diode
rectification takes place.
--
73, Cecil http://www.w5dxp.com

Keith Dysart wrote:

Only partly. If you look at the element on an instrument
like a Bird 43, you will find that it is both capacitively
and inductively coupled to the line. The capactive
coupling is sensitive to the total voltage on the line
at the point of the element, while the inductive coupling
is sensitive to the total current in the line.

The subtraction (or addition) is done in the element
where the voltage sample and the current sample
(scaled by Z0) are subtracted before being applied
to the diode. The output of the diode is the
rectified instantaneous difference of the voltage and
scaled current from the equations originally provided.
This is fed to an average responding meter which has
a scale marked to show (Vf**2)/Z0 (i.e. power).
. . .

Another common directional wattmeter circuit is the Bruene type circuit.
This uses an ordinary current transformer to get the current sample and
a direct connection for the voltage sample. The voltage sample is
reduced to the correct proportional value via a transformer or
capacitive voltage divider. It shouldn't be hard to find a diagram of
one on the web.

Roy Lewallen, W7EL

Roy Lewallen wrote in news:13lr7u2mueltrb3
@corp.supernews.com:

....
Another common directional wattmeter circuit is the Bruene type circuit.
This uses an ordinary current transformer to get the current sample and
a direct connection for the voltage sample. The voltage sample is
reduced to the correct proportional value via a transformer or
capacitive voltage divider. It shouldn't be hard to find a diagram of
one on the web.

The article at http://www.vk1od.net/VSWR/VSWRMeter.htm includes a simple
circuit analysis of the Breune design and some comment on the application
of the instrument.

Owen

"Roger Sparks" wrote in
:


"Keith Dysart" wrote in message
.
..

text cut.....
The directional ammeter measures instantaneous
Vt and It, does the above arithmetic and presents
If. A directional ammeter that presents a single
number rather than the time varying If has probably
converted the instantaneous values to RMS.
text cut......

...Keith

I don't think that the directional ammeter reads instantaneous Vt and
It. The circuits I am thinking of sample a length of line (NOT A
POINT) so the sample records average voltage (or current) from a
period of time.


Many simple reflectometer designs do indeed sample the line over a short
length of line, and that short length may be 100mm or more.

Ideally, they would take the sample at a point. (Since a point has zero
length, I can't quickly think of a sampling technique that truly takes a
point sample.)

Although sampling over a non-zero length limits their accuracy somewhat,
if that length is kept sufficiently short, they are still able to provide
sufficiently accurate measurements.

Owen

Owen Duffy wrote:
"Roger Sparks" wrote in
:


"Keith Dysart" wrote in message
.
..

text cut.....

The directional ammeter measures instantaneous
Vt and It, does the above arithmetic and presents
If. A directional ammeter that presents a single
number rather than the time varying If has probably
converted the instantaneous values to RMS.

text cut......

...Keith

I don't think that the directional ammeter reads instantaneous Vt and
It. The circuits I am thinking of sample a length of line (NOT A
POINT) so the sample records average voltage (or current) from a
period of time.



Many simple reflectometer designs do indeed sample the line over a short
length of line, and that short length may be 100mm or more.

Ideally, they would take the sample at a point. (Since a point has zero
length, I can't quickly think of a sampling technique that truly takes a
point sample.)

The voltage sample is easy... measure the voltage using an infinitely
thin probe.

The current sample is measured in a similar way by measuring the
magnetic field over a infinitely small segment of the conductor. There
are sensitivity issues or bandwidth issues, but there are lots of very,
very small magnetic field probe schemes around.


If one says, "point sample" == "less than 1/1000 wavelength), I think
it's actually pretty straight forward, certainly for 100 MHz or less.
(3mm is 1/1000 lambda).






Although sampling over a non-zero length limits their accuracy somewhat,
if that length is kept sufficiently short, they are still able to provide
sufficiently accurate measurements.

Owen

Cecil Moore wrote:
Gene Fuller wrote:

So how many nanoseconds does that 36.6 degree phase shift represent?


As far as impedance discontinuity *points* go, a nonsense
question.

The question was excellent. 'Impedance discontinuity points' is nonsense.

How many nanoseconds does it take for a signal to travel
through a dimensionless point???? Well, let's see. What
is the speed of light multiplied by zero? Hmmmm, that's
a really tough one.

The hard part would be inverting zero sec^-1 in order to get units of
time. :-)

73, ac6xg