On Sun, 11 Jun 2006 17:53:00 -0700, dansawyeror
wrote:

The frequency is 147 MHz. The
system is near resonance. The voltage measurements are taken with a pair of 10x
probes, connected to a dual channel plug in (B plug in).

Hi Dan,

The use of 10x scope probes is an illusion at best, and certainly not
the way the "pros" do it by a long stretch.

There is no significant change to the signal or the vector voltmeter readings
when the probe leads are attached.

Compared to what? This is again an example of ambiguity that offers
nothing tangible except a "feel-good" kind of experience. Do you know
why there is "no significant change?" If you replace a 1x feed with a
10x probe, wouldn't you expect change? Your faith is based on what
would normally set off red lights and have warning flags being waved.

The phase changes by less the 10 degrees and
none of the amplitudes change noticeably. I do not believe the measurement
system is materially influencing the measurements.

This is a faith-based feeling instead of rational expectation. It is
overwhelmingly obvious by your statement above that the system is
wholly out of whack with expectations.

A simple review of the actual probe can be instructive in this case to
explain why you see little change, and why you wouldn't use a scope
probe in the first place. Most 10x scope probes are not rated into
the VHF. The 10x probe is generally a 10 MOhm resistor with a
parallel 10pf variable capacitor. Simple math would reveal that at
147 MHz that cap presents about 100 Ohms reactance - hardly worth the
effort and certainly no where near a 10x function. The equipment 10x
probe (not a 10x scope probe) has only 2pF of capacitance (5 times the
reactance of a scope probe) and a tenth of the 10x scope probe
resistance. What you are reading as scaled voltage is not what is
present to be measured, but a complex, phase shifted value.

You have absolutely no rational basis to use a 10x probe of any kind.
Skip this unnecessary elaboration.

73's
Richard Clark, KB7QHC

In addition to the weaknesses of scope probes mentioned by Richard is
the ground lead. Even a short one has considerable reactance at VHF, and
its use will promote common mode current on the outside of the probe
cable. Any decent scope measurement at very high frequencies is done
with an adapter which connects directly to the ground ring near the end
of the probe, through a very short conductor. An example is the one sold
by Tektronix, P/N 013-0085-00 the last time I checked. Even it must be
used with some care.

I don't see any hope in your making meaningful measurement with the
setup you describe -- there are too many potential sources of serious error.

Roy Lewallen, W7EL

"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