RadioBanter - View Single Post

September 10th 03, 10:20 PM

(Garvin) wrote in message . com...
....
If we are talking about only DISSIPATED power, do we have
to say P=Vrms**2/Re(Zo)?

Well, you'll probably find a way to argue about it, but for a long
time a lot of people have been using P=Irms^2*Re(Z). Note that that
equation does NOT tell you where the power actually ends up.

If you want dissipated power in a TEM line, then P=Irms^2*R+Erms^2*G
(at a particular frequency where R and G have fixed values). Since
Irms and Erms are functions of position along the line, P is a
per-unit-length quantity like R and G, and total power is found from
integrating the incremental P over the length of line you're
interested in. Note that Zo isn't directly involved. In fact, if you
do it that way, you don't even need to use uniform line: R and G can
vary with distance along the line, as can L and C. (Knowing the exact
value of Zo DOES NOT give enough information to find the dissipation
in a line, even knowing Irms and Erms at every point along the line.)

If you do THAT instead of worrying your head over rho applied to
power, you'll get the right answers. Use rho, if you wish, to help
you find Irms and Erms along the line. Those right answers are all
self-consistent, and consistent with what's predicted by all the usual
transmission line equations which are all derived from the normal
L,C,R,G TEM model, and it's all verifiable by experiment, if you wish.
Have fun.

Cheers,
Tom