[TowerTalk] Open-wire transmission line

[Jim Brown]

On 12/31/2023 7:55 AM, Jim Lux wrote:
> I suspect the popularity of 600 ohms is because that is the standard Z
> of an open wire voice/telegraphy pair from the 19th century. And*that*
>   is probably determined by the physical size of insulators, with the
> wire size chosen for the mechanical properties to accommodate the pole
> to pole span.

YES! I did some research on this a couple of decades ago. Twisted pair 
is well known to have a powerful rejection of noise/crosstalk. Since the 
earliest days of telephony, long distance (cross country) lines were run 
on power poles, below the lines carrying power. 60 Hz was the only noise 
source, and twisting to reject that very long wavelength was achieved by 
crossing over the pair every few poles.
> 
> All those 600 ohm transformers in telephones and such:  even though
> they’re actually fed by twisted pair, which is almost certainly in the
> 100-150 ohm range.

YES again. The Zo of common audio twisted pair is in the range of 70 
ohms. CAT5 and digital audio cables are in the range of 100-110 ohms, 
which is about the limit of practical cable pairs. CAT5, by the way, is 
excellent transmission line, with the four pairs twisted at different 
rates to minimize crosstalk between them.

600 ohm matching has not been used in pro or consumer audio since solid 
state took over from hollow state devices in the '70s. Audio circuits 
use a very low-Z source impedance and a high-Z load impedance, so that 
very little current flows.

73, Jim K9YC