RadioBanter - View Single Post

March 14th 06, 10:29 PM posted to rec.radio.amateur.antenna

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