Hi Dan,

Taking up your postings in reverse order, I can see one problem and
that is emblematic in the question above. Basically it defines a
solution in search of a question. The problem is to figure out what
question you answered by taking the measurements.

This is not an auspicious beginning.

The antenna is a loaded vertical over a set of untuned radials. The feed
is 50
Ohm coax with a 1:1 isolation transformer at the antenna.

I intend to run a separate coax for measurement purposes to the feed
point,

This single statement introduces too much complexity for your stated
goal of taking measurements. There are rather more conventional ways
to accomplish this.

stimulate the antenna
with a 10 mw signal at the operating frequency, and measure the voltage
across
the antenna feed and the ground connection.

"And ground connection?" If your isolation transformer is working,
there is no need to - unless, of course, you are simply interested in
knowing this fact, or are going to put it to some particular purpose.
However, you are approaching this with no stated purpose, so this
ambiguity is simply accelerating your Drift velocity.

Will the voltages indicate the relative RF impedance of antenna (including
loading coil) versus the ground?

If you are treating the untuned radials in relation to ground, you
really need to describe ground. It is not so easily achived as
driving a nail into the mud and declaring that's your reference.

If the ground were near perfect the voltage at
the feedpoint should be close to zero.

"If" comes with magnitudes of error that can only be imagined. From
this point on, you can make your measurements and be guaranteed they
can fit any purpose - "If" you ignore the error.

These voltages should indicate the power
disapation difference between the antenna and the ground.

They will simply represent the potentials distributed throughout an
imaginary land mine field. Tap dance with care or you may have to
sign off as Stumpy.

73's
Richard Clark, KB7QHC

Dan, The only way you will get a valid measurement is with your
1:1 transformer, coax, and your vector voltmeter. I guess if you
are having problems with the vector voltmeter you can do the
same, with less accuracy, using a dual channel scope, and
directional coupler.

73,

Frank