"Cecil Moore" wrote in message
m...
Dave wrote:
don't bother, its a figment of cecil's imagination and creative
vocabulary.

Actually, the question logically follows from the reflection model.
Under what boundary conditions does a piece of transmission line
force Vfor/Ifor = Vref/Iref = Z0 to a specified accuracy?

then look up article 3.19 in 'fields and waves in communications
electronics' which is where they derive the fringe effects for a step change
in spacing between two plates. this is where they refer you for calculating
the effects of evanescent modes from a more complex problem stating that the
results are identical with the static case in 3.19... in the derivation of
the complex case of a step change in a cable they show that a single lumped
capacitance added at the step is an adequate representation of the
discontinuity when calculating the evanescent modes below cutoff. in 3.19
if you disregard the fields along the length of the step it ends up in an
equation: Z=h/pi(exp(pi*W/V0)-1-pi*W/V0_j*pi) the important part is
obvioulsy the decay factor in the exponential which goes as exp(-x/h) where
h is the separation in the planes... or you get one 1/e reduction for each
distance equal to the spacing which in rg-58 or rg-8x that i used is
something like 1mm or less.. so how far down do you want to be? in 4.5mm
you are down to 1% which is well under the accuracy of these cheap meters,
and much less than the length of even the connectors on the meters.