On Dec 9, 9:36 pm, Roger wrote:
The constantly-in-phase traveling wave concept requires the
difficult-to-believe observation that a directional ammeter placed very
near the end of an open transmission line will read the same current as
if it were placed at the source end. Perhaps someone can perform that
experiment some day, but I can not imagine how it can be done without
placing a load on the line, thus invalidating the initial assumptions.

The experiment will show the expected result but will
not help understand why. For that, examination of
the measurements and arithmetic performed by
a directional ammeter is useful.

Below, all voltages and currents are instantaneous.

Total voltage, Vt = Vf + Vr
Total current, It = If - Ir

Vf = If * Z0
Vr = Ir * Z0

Substituting....

Vt = (If + Ir) * Z0
Ir = Vt/Z0 - If

If = It + Ir
If = It + (Vt/Z0 - If)
If = (It + Vt/Z0)/2

Similarly, Ir = (It - Vt/Z0)/2

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.

Examing It and Vt at various points on the line
and doing the above arithmetic will reveal why
the same value for If is obtained everywhere.

Directional wattmeters are more common
than directional ammeters. A directional
wattmeter does the above arithmetic then
squares If, multiplies by Z0 and presents
the results in watts.

All this from just measuring Vt and It.

....Keith

"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

Jim Lux wrote:

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.

Agreed; we're discussing principles here, and the issue of single-point
sampling is mostly a practical one.

In principle, we can always choose a method of sampling that doesn't
require a finite length of line. Within the limits of our skill and
imagination, we can also make the current and voltage pickups physically
smaller, so that they occupy less length along the line. Or if skill and
imagination fail, we can shift the whole discussion to longer and longer
wavelengths, to make the error as small as we like. It may not be
practical, but no general principles are being broken.

The issue of single-point sampling is interesting in its own right, but
in this much wider discussion it is only a minor detail. In order to
move on with the wider discussion, let's agree to assume that
single-point sampling always *can* be achieved, within the accuracy that
we require.



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).

Even for practical instruments, this particular source of error is
usually quite small. At any one frequency, it is always possible to null
the bridge in the reverse direction, so that the voltage and current
samples (as described by Cecil) will cancel. How well the cancellation
holds over a wider frequency band will depend on the choice of bridge
circuit and the way it is constructed.


--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek