On Sun, 17 Aug 2003 13:56:11 GMT, tad danley
wrote:

Roy Lewallen wrote:


I've tried to point out on this thread that although the feedpoint
impedance is an impedance with the units of ohms, and the impedance of a
plane wave in free space also has the units of ohms, they're not the
same thing.


This may not be a good analogy, but Specific Impulse of rocket motors
helps me to remember that the 'units' of something have to be considered
in the context of what is being measured. Specific impulse is a measure
of the performance of a rocket motor. It measures the thrust obtained
from a single kilogram of propellant burned in one second. The 'units'
of Specific Impulse are seconds, but we're not measuring 'time'.

73,

Hi Tad,

Your point is well taken. ALL physical phenomenon can be expressed
through a chain of conversions in the MKS system of units. When
someone tells you that their terminology is inconsistent between
disciplines (as such offered in this and other threads); it must then
be amenable to reduction to MKS terms or one of the two conflicting
expressions is invalid.

That is to say to the specific matter about the usage of "ohms:"
Here, the unit of ohm must be reduced to Meters, Kilograms, and
Seconds for both usages (electrical and radiative). At that point,
both will have a common basis for comparison and if in fact their
reduced terms are identical, then their common usage is also
identical.

One simple example is with the measurement of body weight on the
bathroom scale (a torsion or compression device) as opposed to the
weight measured on the doctor's scale (a beam balance). Let's say
before you go to the doctor's, you weigh yourself in around 165
pounds. When you arrive at the doctor's, his scale says you weigh
around 75 kilograms.

Let's remove this same scenario to the moon (you live in one of those
futuristic 1990's colonies forecast by the space race back in the
60's). Before you went to the doctor's you weighed in around 33
pounds. When you arrive at the doctor's, his scale says you weigh
around 75 kilograms.

Here we find the expression "pounds" suffers what appears to be the
same plight of "ohms" in that the determination of a value is
inconsistent. You may also note constants of proportionality on earth
and the moon. These constants when expressed as a ratio also describe
the significant differences between the earth and the moon.

The problem is that the term "weight" has a hidden association to the
constant of Gravity. The expression Gram is one of Mass, not weight.
The expression pound is not an expression of Mass unless you expand it
to include the term for the particular constant of Gravity. Mass is
constant in the Newtonian Universe, and weight is not.

If you were to have reduced the pounds to the MKS system both times,
you would have found it consistent both times (here on earth, and on
the moon).

If you reduce the "ohms" to the MKS system both times.... Well I will
leave that for further deliberation as some are sure to be surprised.
:-)

73's
Richard Clark, KB7QHC