|
|
|
|
How to measure speaker cable inductance and capacitance?
(Bob-Stanton) wrote in message om...
At 20KHz they act mostly (sort of)
like "high frequency" transmission lines, at 200 Hz they act like
discrete components.
At 20 kHz, the wavelength at speed of light is 3*E8/20*E3=1.5*E4m,
i.e. 15 km or about 9 miles. Your 100 ft of cable is 2/1000th of that,
hardly a transmission line, even if we take into consideration a
somewhat slower speed in the cable.
Or do you say that it still will act as a transmission line? It is
not, according to the books I own.
Are you familiar with the Smith Chart? It is probably the most usesful
graphical tool available to the RF designer. It was originally
conceived by Phil Smith, in Bell Labs, in the 1930's and it is still
widely in use.
The Smith chart is readable down to about 1/2 degree, which equals
0.0014 wavelength. The chart will accurately show the change of
impedance along a transmission line that is only 0.002 wavelengths
long.
I used an RF computer program to calculate the impedance(and loss) of
100 ft (of 100 Ohm) speaker cable terminated with an 8 Ohm load. Then,
I checked the calculations by hand, using a Smith Chart. The Smith
Chart results agreed with the computer program. They both showed that
fractional wavelengh transmission lines "work".
NO, they absolutely do NOT. They simply show that you have two
conceptual models that give similar answers. You yourself have admitted
in a previous post in this thread that you do not have any idea if
the answers you are getting are right or wrong.
If you feel that fractional wavelengh transmission lines don't "work",
you should throw away your Smith Chart, or perhaps just whiteout the
fractional wavelength part of the Smith Chart, that you don't agree
with. :-) As for me, I'll keep my old Smith Chart, because it shows
that fractional wavelength transmission lines do "work" and it can
calculate how they work.
No, it shows nothing of the kind. It shows that you have a model
of a physical phenomenon that gives an answer that satisfies you.
Until such time as you check those answers against the real physical
behavior in a way that is clear, unambiguous, consistent and predictive,
your models are worthless.
|