> >The impedance should be about 150 ohms (balanced).
>
> It turns out to be somewhat less than twice the impedance of the lines
> taken separately. Double 75 coax used as a balanced feedline was called
> 130 or 135 ohm at AT&T. Someone know the reasons why?
Not so.
If the lines are operated in parallel, the impedance is 37.5 ohms.
If they are operated in push-pull, they are 150 ohms.
The loss is exactly the same as a single cable of the same length
with the same VSWR. While loss does not improve, power
handling doubles (as a general rule) because you have two lines
dissipating the same total power.
73, Tom W8JI
w8ji@contesting.com
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