[time-nuts] 50 vs 75 ohm cables

Chuck Harris cfharris at erols.com
Thu May 10 10:31:05 EDT 2007


If you factor in all of the ohmic losses, ~70 ohm is the
lowest loss ratio of inner to outer conductor diameters
in an air dielectric coaxial transmission line.

The original 70 ohm line, invented by Western Electric,
was air dielectric with a thin polyethylene disk every foot.

50 ohm comes about when you replace the air, in a 70 ohm cable,
with a polyethylene dielectric.  Why add a polyethylene dielectric?
To gain the better flexibility needed in airplanes and other
mobile equipment.

75 ohms came about when you adjust the inner and outer conductor
diameters to come up with a low loss polyethylene dielectic

-Chuck Harris

Peter Vince wrote:
> I came across some telecom equipment the other day which had 
> reference outputs marked as 75 ohms.  I work in television, not 
> telecoms, and we use 75 ohm connections for video, but with most RF 
> stuff being (I believe) 50 ohms, and certainly all the HP and other 
> counters seem to have 50 ohm inputs, I rather assumed telecoms used 
> 50 ohms - obviously not!  Can anyone tell me how and why the 50/75 
> ohm distinction came about?  Was it perhaps a VHS/Betamax type issue 
> with different manufacturers going their own ways, and then with so 
> much equipment out in the field,  neither side was willing to change?
> 	Thanks,
> 		Peter (London)
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