"dansawyeror" wrote in message
. ..
Thank you all for your replys, they were all interesting.

The first set of test measurements were taken on a 2-meter loaded
vertical. Two
voltage readings were taken via a high impedance scope probe, one at the
feedpoint of the coax center connection, and the other from the connection
of
the coax shield to the antenna ground. In this case the ground is a simple
raised metal 1/4 inch screen mesh about 10 square feet.

The antenna was tuned to resonance. The relative voltage readings were the
center conductor was 2x the ground reading. I do not have a way to measure
current at the base at this time.

However looking ahead, with power = iv would this imply about 2 times the
energy
is being radiated from the vertical element as through the base?

Thanks Dan - kb0qil

dansawyeror wrote:
Good morning,

This question is: Taking measurements at the antenna and what they mean?

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

Will the voltages indicate the relative RF impedance of antenna
(including loading coil) versus the ground? If the ground were near
perfect the voltage at the feedpoint should be close to zero. These
voltages should indicate the power disapation difference between the
antenna and the ground.

Thanks,
Dan


Dan,
I'm echoing the others on this. Using scope probes for this kind of
measurenet is a very touchy undertaking at the very best, even with the best
quality equipment and much experience. Other methods of measuting RF are
required and used by those who know. That's why they were developed. If
you do this at 10M or below, then you have more chance of getting correct
results.
That being said, it is not clear what you are trying to measure since you
say you are, in essence, measuring "ground". It is not clear just what you
are doing. Unfortunately, from your description, the way you describe this,
I feel you do not have enough technical background to understand the
technical concepts well enough. You appear to have some knowledge, but not
enough, in what appear to be basics, to do this.
The way you describe your technique, you are not connecting the probe ground
to anything. This makes no sense. RF measurements may seem like "black
magic" but it takes nothing more than *really* understanding all the basics
and all the stray effects really, really well.

At least you seem to understand that "ground" is not the absolute zero
reference that others seem to believe it is / can be / needs to be.

A few comments in general:
You *MUST* connect *BOTH* parts of the probe tip if you hope to get anything
close to true results.
Both these connections must have no effect on the measurement.
The probe must be capable of measuring what you are attempting.
There are high frequency 10x probes, but all the other sonsiderations are
even more important because they will be even more susceptable to the side
effects just because they *are* capable of making the measurements.
The Shield / ground part of the scope probe and cable leading back to the
scope is something which can have a significant effect on what you see on
the scope face. The antenna can be inducing current into this and giving
false readings. This is a major problem in measuting any high frequency
signals. There must be no possibility of the scope cable picking up any
signal. I have seen very misleading results when this technique is tried on
digital circuits which have fast rise times and therefore high frequency
components.

Good luck & 73, Steve, K9DCI