Cliff Curry wrote:
In all transmission lines, including coax, there are various shapes of
transverse electric and magnetic fields that can exist for the particular
transmission line geometry. For each shape, the "propagation constant" can
be calculated. Many transmission lines (at lower frequencies) have only one
shape with propagates with low attenuation. The other shapes can exist, but
their "propagation constant" is such that they decrease exponentially with
distance. The propagation constant for each shape can be calculated, and is
often a function of frequency.
When there is a discontinuity in a line, other shapes than the usual
one must exist at the point of the discontinuity. (for example, in order to
ensure that the transverse electric field is zero the surface of a
conducting shape that is part of the line discontinuity). Thus, these other
shapes exist (at a certain amplitude) at the point of discontinuity. The
amplitude of the other shapes decreases exponentially at distances away from
the discontinuity. The rate of the fall-off will depend on the particular
shape, according to its propagation constant.
Thus, the distance needed to be back to regular old TEM propagation in a
coax will depend on the particular discontinuity, and the propagation
constants of the "higher order modes" or different field shapes, of a coax
line.
I have seen examples worked out for waveguide propagation and a step
change in waveguide width. There are probably worked examples of coax
discontinuities in the literature, also.
These non-propagating shapes are usually called " evanescent modes", and
this would be a good search term to use to investigate this further.


All agreed. Along with the math that Cecil has retrieved and quoted
again, everything points towards the distance in question being a
function of coax diameter only; and not wavelength.


--
73 from Ian G/GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek