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