I know that stuff too. In an analog LD switching office there were
tons of it at the channelbank/group level. 135 ohm shielded twisted
pair.
But what I was talking about was two pieces of coax held side-by-side
with a woven cover. It terminated in a double coaxial jack or plug.
Under the woven cover it was the same stuff as the single 75 ohm coax
that piped supergroups in the supergroup bank bays and mastergroups to
the line bays of analog message systems. This double coax was used for
the analog/baseband side of FMT's and FMR's (converted broadband
baseband analog to a broadband FM signal at 70 Mhz for piping to
microwave channels, & reverse). I know we *didn't* call it 150 ohm.
On Tue, 27 Apr 1999 17:39:00 -0400, you wrote:
>
>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
73, Guy
--. .-..
Guy Olinger, K2AV
k2av@qsl.net
Apex, NC, USA
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