[TowerTalk] Open-wire transmission line

[Michael Tope]

This guy has a whole bunch of information on how and why the twisting 
was done:
https://the-electric-orphanage.com/wp-open-wire-transpositioning-systems/

Credit to Google University :-)

73, Mike W4EF..............

On 12/31/2023 12:34 PM, Jim Brown wrote:
> 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
>
>
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