Asimov wrote:
"Roy Lewallen" bravely wrote to "All" (07 Apr 05 15:08:40)
--- on the heady topic of " VF, low-loss line, high-impedence line -
relationship"

RL From: Roy Lewallen
RL Xref: aeinews rec.radio.amateur.antenna:28088

RL Asimov wrote:

Since a portion of the EM field in open wire line is free to travel
outside the conductor into the environment then we may safely assume
there is an exchange between the environment and the conductor.

RL If the conductors are perfectly conducting, no part of the field at
RL all exists within the conductor. With good conductors like copper and
RL at HF and above, there's very little penetration of the conductor by
RL the fields, either electric or magnetic.

Is there any electron current in the conductor or not?

In a perfect conductor, no. In a real but good conductor like copper,
it's confined to a very thin layer at the surface. Look up "skin effect"
in any electromagnetics or basic text on RF, or google it.



RL First of all, a mismatch doesn't cause loss.

An impedance mismatch in any medium causes a scattering of the energy.
Of course, it isn't a net loss as far as the universe is concerned but
some of the energy doesn't arrive where it was intended.

If I connect a 50 ohm source to a one wavelength, 300 ohm transmission
line and connect the other end of that line to a 50 ohm resistor,
there's a 6:1 mismatch at both ends of the line. The power supplied by
the source and the power delivered to the load are exactly the same as
if I had used a 50 ohm line instead. This is, of course, overlooking
resitive loss in the line. If you consider the resistive loss, it can be
greater in one line than the other (the 300 ohm line might be less
lossy), depending on the physical construction of the line.

No loss is caused by the mismatch. No "scattering of energy occurs". All
of the energy from the source arrives at the load, where it was intended.



RL Secondly, as I explained in my last posting, the characteristic
RL impedance of a transmission line isn't the same thing as the
RL characteristic impedance of free space.

May I suggest you make up your mind whether the electric energy is
travelling in a conductive medium or not?

I'm sorry, I don't understand what you're asking. No RF energy exists in
or travels in a perfect conductor.


RL It has to do with the reflective
coefficient where the energy is returned.

RL Well, no. There isn't a bundle of energy trying to escape the line and
RL bouncing off the air, or bouncing off the air as it travels along the
RL line, or bouncing off the conductors into the air. So reflection
RL coefficient isn't applicable here.

What makes you so sure?

A basic understanding of electromagnetics derived from an electrical
engineering education, extensive additional reading and study, and about
30 years of engineering design experience including design of microwave
and very high speed time domain circuitry.


RL I'm afraid that the conclusions you've reached about loss and
RL characteristic impedance are based on a poor understanding of
RL fundamental transmission line operation. The result is some
RL conclusions that are, and are well known to be, untrue.

I think you are only concerned with modeling of transmission lines as
lumped constants but models can only go so far in explaining how
something works. Models are like analogies and we all know no analogy
is perfect even this one.

I have no idea what makes you think that my modeling or understanding of
transmission lines is limited to lumped constant models -- it's
certainly not true. Indeed, no analogy is perfect, but some are
certainly better than others, and some are demonstrably false. I'm
afraid that some of the ones you've put forth are in the latter category.

Roy Lewallen, W7EL