On Tue, 14 Jul 2009 20:35:48 -0700, Richard (Rick) Karlquist wrote:
>Good points, but here is some more evidence: I have 350 feet of cable
>feeding my MonstIR. Initially, it wouldn't work because the
resistance
>was too high. I installed two more runs and tied everything in
>parallel. This tripled the capacitance and cut the resistance to 1/3
>of the original value. The MonstIR works perfectly now. I have as
>much capacitance as a normal run of 1000 feet.
I agree with your conclusion.
>But actually, the cable is a transmission line, and like all
>transmission lines, has both capacitance and inductance.
Only if the line is a significant fraction of a quarter wave at the
frequency of interest. Shorter than that, it behaves as lumped R, L,
and C. No control cable that either of us is likely to run is going to
be a transmission line for those SteppIR pulses.
>If you terminate a transmission line with less than 30 ohms,
>and the transmission line has a characteristic impedance of more
>than 30 ohms, the input impedance will be inductive, not capacitive,
The equation for Zo of transmission line is complex -- sqrt [(R+jwL) /
(G + JwC)]. Only at high frequencies does it simplify to sqrt (L/C). At
audio frequencies, the impedance is high and capacitive (approximately
sqrt (R/jwC), gradually transitioning to the familar value by the time
you hit 100 kHz. For most practical cables, this transition occurs in
the mid to high audio spectrum. BTW -- Vp also goes through the same
sort of transition and in the same frequency range. Vp is typically
0.05 at low audio frequencies, more like 0.5 at high audio frequencies.
When telephone lines are equalized, the time response can be more
important than the amplitude response!
At radio frequencies, Zo of most practical audio cables is between 70
and 110 ohms.
There is a tutorial note about this on my website.
http://audiosystemsgroup.com/TransLines-LowFreq.pdf
73,
Jim K9YC
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|