Guy K2AV said:
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?
(Was used a lot in baseband analog video where pickup of hum noise was
a real problem. They've gone digital now, of course...)
=====snip======
The 135 ohm cable Guy references is (was) twisted pair in a single
braid shield rather than 2 coaxs. It carried RF 60KHz-108KHz, what
we called group level frequencies, which was the modulated channel
bank spectrum of 12 combined voice grade channels allotted a 4 kHz
band width each. The shield was as good a braid shield as could be
manufactured then, to prevent picking up or radiation of RF sigs from
or to other similar circuits. In a Long Lines toll office, we had
thousands of these circuits. Each had a separate 135 ohm cable for
transmit and one for receive.
73
Pete K4LDR Citrus County, FL on the Gulf
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