Gene Fuller wrote:
I was going to drop this discussion, but I will respond to your request
to share physics knowledge.
Thanks, Gene, Please be patient with me.
1) I will repeat. E-fields, H-fields, voltages, and currents are all
related through some very profound equations. However, shout THEY ARE
NOT INTERCHANGEABLE. /shout
Is the H-field around a wire proportional to the RF current in the wire?
Is the E-field around a wire proportional to the RF voltage between the
wires? Is the ratio of E-field to H-field fixed by Z0? Do I^2*R losses
affect the E-field and H-field by equal amounts?
This is not just a matter of semantics. These entities have different
physical meanings, different units, and different dimensionalities.
Of course that's true. However, they are not unrelated.
2) I offered a physics-based explanation for your proposed "current
drop" in the 440 MHz RG-58 example a few days ago. Did you not read that
message before responding to it?
Yes, and I have been thinking about an example that would better illustrate
what I was asking. A 1000 wavelength dipole located in outer space would
have less current at the ends than at the source. Since there is no other
path for current, what is the explanation for the decrease in the current
at the ends?
3) What is not correct is the assertion that the coil
exhibits a phase shift consistent with, for example, 20 feet of wire
used to make the coil.
Because nobody has made that assertion since the original eHam article, it
appears to be a straw man. The coil occupies whatever number of degrees
that it occupies and it does NOT occupy zero degrees.
For instance, using a particular EZNEC segment model of a coil, the
current at the bottom is 1.0 amp and the current at the top is 0.5
amp. Assuming the cosine distribution of standing-wave current is
accurate, the coil occupies about 60 degrees. The whip would occupy
about 30 degrees, the rest of the 1/4WL.
Nobody has attempted to explain how one can obtain 90 degrees of a
1/4WL antenna on 4 MHz using a ten foot (15 degree) whip. That is
one hell of a velocity factor. If the bottom-loading coil really
occupies zero degrees, then the ten foot whip would be forced to
occupy 90 degrees. That is so impossible as to be laughable.
The notion that a coil replaces some sizable
portion of the total phase shift in an antenna has been shown to be
incorrect. Experiments reported by Roy and Tom R. convincingly
demonstrate the phase shift behavior of coils.
The total current undergoes virtually no phase shift since it is a
standing wave. That's in the textbooks and nobody is arguing that
point so it's just another straw man.
It's the forward current and reflected current that is undergoing a
phase shift through the coil just like they do on a wire standing-
wave antenna. Nobody has measured those two current components so
the jury is still out on that subject. There is no argument about
the phase of the total current that Roy and Tom measured. Please,
there are enough arguments already without having to introduce
straw men.
If you will look at my phasor diagrams of forward and reflected
currents at:
http://www.qsl.net/w5dxp/current.htm
you will see that the phase of the total current is exactly the same
in both cases. That's the phase that Roy measured. Since it is a
standing wave current, the phase of the standing-wave current is almost
constant. It is the magnitude of the standing-wave current that
changes and it changes as a cosine function of electrical length
in degrees. The coil has an electrical length in degrees. That's
what causes the current to be different at the bottom and at the
top in a 1/4WL antenna. Assuming the phase shift from the feedpoint
current to the tip of the antenna is 90 degrees, if an accurate
measurement of the current at the top and bottom of a bottom-loaded
antenna coil is made, the number of degrees occupied by the coil can
be calculated from
arccos(Itop/Ibottom)
just as it can be calculated between two points on a wire. This
assumes that Ibottom is an Imax point on the standing wave.
--
73, Cecil
http://www.qsl.net/w5dxp
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