Owen Duffy wrote:
On Tue, 27 Sep 2005 02:04:46 GMT, Cecil Moore wrote:



The transmission line length must only be long enough such that
the V/I ratio is forced to the Z0 value. According to some pretty
smart guys I asked, that's about 2% of a wavelength.


Cecil, do you have some quantitative explanation / support for this?

The treatments that I have seen of transmission line tuners where
different Zo lines are directly connected do not suggest corrections /
tolerances of the type you imply.

(IIRC, Terman discusses a fringing capacitance as a means of allowing
for a physical discontinuity.)

I am not asking whether or not field conditions (and V/I on the
conductors) immediate to the discontinuity are not Zo of either of the
lines, just where has the 2% of a wavelength come from?

Owen

As I recall it came from someone on sci.physics.electromag.

But think about it. The surge impedance (Zo) is basically just the
ratio of the capacitance per unit length to the inductance per unit
length. Those quantities might vary a little bit from one place to
another, but probably not by much. And there are undoubtedly end
effects which locally pull the capacitance and inductance values away
from the ideal. So the length really need only be long enough for the
variations to average out and for the total values to become large
enough to swamp the end effects.

ac6xg