[time-nuts] 50 vs 75 ohm cables
Hal Murray
hmurray at megapathdsl.net
Fri May 11 15:26:55 EDT 2007
phk at phk.freebsd.dk said:
> I can confirm that the choice of 75 Ohm for telecom use indeed is
> because of the low attenuation. The first use of coax was for
> "Carrier Frequency" systems, where a number of telephone conversations
> were AM modulated on individual carriers, usually 4 kHz apart.
What's the attenuation mechanism?
I thought the old 10 megabit vampire-tap Ethernet picked 50 ohms because of
lower attenuation. The story I remember is that for a given outside
diameter, the inside diameter was bigger at a lower impedance. The main
losses were resistive on the center conductor due to skin effect. A bigger
center conductor had more area at a given skin depth and hence lower losses.
--
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