Current through coils
 

wrote:
I've solved the measurement problem. I measured current and voltage
levels and phase of each.

There you go again, "I, I, I". This is not about you. This is
about valid measurements. I concede that when one measures
the current phase shift in a standing wave environment, that
the result will be zero or close to zero. But we are not
interested in measuring a constant phase whether the coil
is in the circuit or not. We are interested in measuring the
phase shift through the coil and it is NOT zero.

Reference: Kraus' "Antennas for All Applications", 3rd edition,
Figure 14-2. Kraus clearly shows if you measure the phase shift
between any two points on a 1/2WL dipole, that measured shift
will be close to zero degrees whether a coil is present or not.
There is no need to keep performing those same measurements.
I agree with Kraus.

We know a 1/2WL dipole is 180 degrees long. The fact that the
standing wave current doesn't change phase from end to end
doesn't mean the 1/2WL dipole is zero degrees long. The fact
that the standing wave current doesn't change phase on each
side of a coil doesn't mean the coil is zero degrees long.

I've measured time delay of current appearing at the coil output
compared to input.

There you go again, "I, I, I". It doesn't matter who does the
measurement or whether a coil is in the circuit or not. The
standing wave current's phase doesn't change in the antenna's
180 degrees of length. That has nothing to do with anybody's
measurements. That's just a fact of physics. One cannot
use standing wave current to measure the delay through
a coil OR A WIRE in a standing wave environment.

If one takes a known 30 degrees of a 1/2WL dipole and uses
standing wave current to measure the phase shift through
that 30 degrees of wire, the measurement yields zero
degrees. Does that mean the phase shift in 30 degrees
of wire in a 1/2WL dipole is zero? Of course not. One
simply cannot ascertain the phase shift in a piece of
wire (or coil) by measuring the phase of the standing
wave current.

Current is current.

On the contrary, one can look at the formula for standing wave
current and see that standing wave current is NOT like traveling
wave current. Traveling wave current is of the form f(z+wt) or
f(z-wt) depending upon the direction of travel. Standing wave
current is of the form f(z) + f(wt) so they are quite different
and therefore have *different* characteristics.

As you can see from the functions, magnitude and phase are
interlocked for a traveling wave. Magnitude and phase are
unlocked for a standing wave. With a phasor fixed at zero
degrees, how does a standing wave phasor manage to flow?

As is my custom, I am going to trim the part of your posting
with which I agree.

In every single device we would be able to build, we would never be
able to sort reflected current from forward because current is current.

On the contrary, we do it all the time for transmission lines
with a known Z0. We separate forward power from reflected power.
It is trivial to take forward power in a certain Z0 feedline
and convert that value of power into forward current. It is
trivial to take the reflected power in that same line and convert
that power into a reflected current. Here are the formulas:

|Ifor| = SQRT(Pfor/Z0) |Iref| = SQRT(Pref/Z0)

In our directional couplers, we throw away the phase when we
rectify it, but we don't have to throw away the phase. We
can look ahead of the diodes with an o'scope probe and actually
compare the phases of the two waves.

You have taken this argument to an absolute dead end, because you
insist current can flow two directions at the same time at one single
point in a system.

There you go again, "you, you, you". Everyone has requested that
we cease and desist from the personal attacks. "We" means "you
and me".

But it is well accepted in the distributed network model that
two currents can flow in opposite directions at the same time.
There's probably no other way to get standing waves and a
75m mobile bugcatcher antenna system *IS* a standing-wave
antenna.

You are demanding a measurement method that uses a device that cannot
be built to measure something that does not exist. That is either
humorous, sad, or frustrating. It sure isn't science.

There you go again, "you, you, you". This is not about you or
me or our feelings. We can certainly measure the two currents
flowing in opposite directions in a transmission line. Not having
a voltage reference common is why it's hard to do in an antenna
but I suspect it could be done with E-field and H-field probes
and a little superpositioning.

We are sometimes a rational species. We can perform our
experiments on a transmission line with known Z0 and if we
are careful, project out results on a standing wave antenna
with an unknown Z0. Actually, the Z0 for a 1/2WL dipole made
from #14 wire at 30 ft. from the ground is pretty well known
to be about 1200 ohms.
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

Tom, W8JI wrote:
"A traditional directional coupler works by comparing voltage across the
line at any one point to current in the line at that same point."

Almost. It compares a voltage sample to a current sample, both of which
have been converted into d-c voltages. These have been carefully crafted
to be exactly equal d-c voltages regardless of the power level in the
line.

I`m giving up on correcting line by line.

Important fact is that a reflection reverses the phase between the
voltage and current produced by a wave.

So when the samples from the forward wave are siummed, their total is
exactly 2x the value of either the voltage-derived sample or the
current-derived sample.

When the samples from the reflected wave are summed, being equal but
opposite in polarity, they add to ZERO. Calibration is so the total
produces the correct value on the power scale for the wave in the
forward direction.

To get the power in the reverse direction, the input and output are
effectively exchanged so that the forward power indication cancels and
the reverse power indication is produced by the sum of its voltage and
current d-c sample outputs.

Best regards, Richard Harrison, KB5WZI

Current through coils
 

Cecil Moore wrote:
Tom Donaly wrote:

What's the formula, Cecil?


http://www.ttr.com/TELSIKS2001-MASTER-1.pdf equation (32)

The velocity factor can also be measured from the self-
resonant frequency at 1/4WL. VF = 0.25(1/f)

I suppose you also
have something that will tell us how to find your coil's characteristic
impedance; o.k., out with it.


http://www.ttr.com/TELSIKS2001-MASTER-1.pdf equation (43)

The characteristic impedance can also be measured at
1/2 the self-resonant frequency at 1/8WL. For a lossless
case, the impedance is j1.0, normalized to the
characteristic impedance so |Z0| = |XL|.
For a Q = 300 coil, that should have some ballpark accuracy.

We don't need extreme accuracy here. We just need enough to
indicate a trend that the velocity factor of a well-designed
coil doesn't increase by a factor of 5 when going from 16
MHz to 4 MHz.

In "Antennas for All Applications", Kraus gives us the phase
of the standing wave current on standing wave antennas like
a 1/2WL dipole and mobile antennas. 3rd edition, Figure 14-2.
It clearly shows that the phase of the standing wave is virtually
constant tip-to-tip for a 1/2WL dipole. It is constant whether
a coil is present or not. There is no reason to keep measuring
that phase shift over and over, ad infinitum. There is virtually
no phase shift unless the dipole is longer than 1/2WL and then
it abruptly shifts phase by 180 degrees.

I agree with Kraus and concede that the current phase shift in
the midst of standing waves is at or near zero. There is no
need to keep providing measurement results and references.

You load your antennas with a Tesla coil? Did you read the part
about a Tesla coil going to a lumped inductor when it was shortened?
73,
Tom Donaly, KA6RUH

Current through coils
 

Richard Harrison wrote:
Tom, W8JI wrote:
"A traditional directional coupler works by comparing voltage across the
line at any one point to current in the line at that same point."

Almost. It compares a voltage sample to a current sample, both of which
have been converted into d-c voltages. These have been carefully crafted
to be exactly equal d-c voltages regardless of the power level in the
line.

That's absolutely incorrect Richard.

If you get out the schematic of ANY directional coupler, you will see
the current sampling device is in series with a voltage sampling
device.

The radio frequency voltage ratios of sampling system are combined
BEFORE detection.

The dc voltage level does vary with both voltage and current (power),
and that is why the meter on the front of your watt meter goes up and
down with power levels.

Only a phase detector levels voltages.

73 Tom

Current through coils
 

On Tue, 14 Mar 2006 17:03:49 -0600, (Richard
Harrison) wrote:

Tom, W8JI wrote:
"A traditional directional coupler works by comparing voltage across the
line at any one point to current in the line at that same point."

Almost. It compares a voltage sample to a current sample, both of which
have been converted into d-c voltages. These have been carefully crafted
to be exactly equal d-c voltages regardless of the power level in the
line.

When you convert a sample of the RF voltage at a point to DC voltage
you have a DC voltage proportional to the magnitude of the RF voltage
at the point.

When you convert a sample of the RF current at the same point to DC
voltage you have a DC voltage proportional to the magnitude of the RF
current at the point.

You cannot use these two samples to determine generally the magnitude
of the reflection coefficient (and hence VSWR), much less independent
samples of the travelling waves.

No, neither a directional coupler nor a common reflectoter work as you
describe Richard.

Owen
--

Current through coils
 

Current through coils
 

Richard Harrison wrote:
Tom, W8JI wrote:
"A traditional directional coupler works by comparing voltage across the
line at any one point to current in the line at that same point."

Almost. It compares a voltage sample to a current sample, both of which
have been converted into d-c voltages. These have been carefully crafted
to be exactly equal d-c voltages regardless of the power level in the
line.

I`m giving up on correcting line by line.

Important fact is that a reflection reverses the phase between the
voltage and current produced by a wave.

So when the samples from the forward wave are siummed, their total is
exactly 2x the value of either the voltage-derived sample or the
current-derived sample.

When the samples from the reflected wave are summed, being equal but
opposite in polarity, they add to ZERO. Calibration is so the total
produces the correct value on the power scale for the wave in the
forward direction.

To get the power in the reverse direction, the input and output are
effectively exchanged so that the forward power indication cancels and
the reverse power indication is produced by the sum of its voltage and
current d-c sample outputs.

Thank you for this concise summary.

Current through coils
 

Hello Tom,

I understand that on page 6, the reference qualifies the statement in
the abstract by saying that for heights " . . . less than 15 degrees . .
.. one passes to the lumped element regime . . ."

I thought Cecil was drawing examples for heights greater than 15
degrees. Have I misunderstood?

73,

Chuck, NT3G


Tom Donaly wrote:
Cecil Moore wrote:

Tom Donaly wrote:

What's the formula, Cecil?



http://www.ttr.com/TELSIKS2001-MASTER-1.pdf equation (32)

The velocity factor can also be measured from the self-
resonant frequency at 1/4WL. VF = 0.25(1/f)

I suppose you also
have something that will tell us how to find your coil's characteristic
impedance; o.k., out with it.



http://www.ttr.com/TELSIKS2001-MASTER-1.pdf equation (43)

The characteristic impedance can also be measured at
1/2 the self-resonant frequency at 1/8WL. For a lossless
case, the impedance is j1.0, normalized to the
characteristic impedance so |Z0| = |XL|.
For a Q = 300 coil, that should have some ballpark accuracy.

We don't need extreme accuracy here. We just need enough to
indicate a trend that the velocity factor of a well-designed
coil doesn't increase by a factor of 5 when going from 16
MHz to 4 MHz.

In "Antennas for All Applications", Kraus gives us the phase
of the standing wave current on standing wave antennas like
a 1/2WL dipole and mobile antennas. 3rd edition, Figure 14-2.
It clearly shows that the phase of the standing wave is virtually
constant tip-to-tip for a 1/2WL dipole. It is constant whether
a coil is present or not. There is no reason to keep measuring
that phase shift over and over, ad infinitum. There is virtually
no phase shift unless the dipole is longer than 1/2WL and then
it abruptly shifts phase by 180 degrees.

I agree with Kraus and concede that the current phase shift in
the midst of standing waves is at or near zero. There is no
need to keep providing measurement results and references.


You load your antennas with a Tesla coil? Did you read the part
about a Tesla coil going to a lumped inductor when it was shortened?
73,
Tom Donaly, KA6RUH

Current through coils
 

John Popelish wrote:
To get the power in the reverse direction, the input and output are
effectively exchanged so that the forward power indication cancels and
the reverse power indication is produced by the sum of its voltage and
current d-c sample outputs.

Thank you for this concise summary.

Except it is actually an incorrect concise summary.

The directional coupler adds RF voltage from a sampling across the line
directly to a sampling of RF current past that point.

It is only after the voltages, one proportional to current and one
proportional to voltage, are added that the resulting voltage is
rectified and used to drive a meter.

The directional effect can be analyzed using wave theory or simple
circuit theory. The results are the same.

73 Tom

Current through coils
 

Tom wrote, "The directional effect can be analyzed using wave theory or
simple
circuit theory. The results are the same."

Of course, "the directional effect" depends completely on having the
sampler calibrated to the impedance of the line into which it's
inserted. Otherwise, it's just resolving "forward" and "reverse"
_as_if_ the signal is in a line that has a characterisitc impedance
equal to the sampler's calibration impedance.

To the extent the samples are accurate for instantaneous currents and
voltages, the sampler does NOT depend on sinusoidal excitation. The
result is accurate for the current and voltage that exist at each
instant in time. Some directional couplers are very broadband; others
are not. We made the ones in the 8753 that Tom uses to be accurate
over a wide frequency range. And of course, if you don't just rectify
the output, you can extract phase information from it as well as
amplitude.

Cheers,
Tom