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