[TowerTalk] 600 Ohm Line

[EUGENE SMAR]

TT:

----- Original Message ----- 
From: "Rick Karlquist" <richard at karlquist.com>
To: "Jim Lux" <jimlux at earthlink.net>
Cc: <towertalk at contesting.com>; "Jim Brown" <jim at audiosystemsgroup.com>
Sent: Thursday, July 29, 2004 6:26 PM
Subject: Re: [TowerTalk] 600 Ohm Line


> Turns out you can't consider the phone line as a lumped component
> even to get audio to the CO; moreover, the characteristic impedance
> is complex.  This is why the phone company has to use loading
> coils.
>
> Rick N6RK
>
>

Rick is correct.  The copper twisted-pair (TP) is considered a distributed
reactance element.  The loading coils are actually series inductors
(remember the 88 mH toroidal coils we all used in the 70's?)  (Remember the
70's?)  These series inductors cancelled a bunch of the capacitive reactance
of those parallel conductors on long (> 1 mile) lines so that T-carrier
devices (1.544 Mbps) data signals could pass from customer premises (usually
businesses) to the CO.  Otherwise, the phase shift and attenuation of the TP
would obliterate the T-1 signals.

When I worked for a local phone company (AFTER the power company), we were
using ADSL over TP (duh) to attempt to deliver video.  The biggest hurdles
we encountered were not so much with the ADSL modems, but with the condition
of the outside plant (OSP) itself.  We had things such as undocumented
splices, unterminated branch taps (wire pairs installed just-in-case...),
corroded mechanical splices.  We had to await improvement of the
error-correction algorithms in the modems (and faster modem microprocessors)
to compensate for the poor condition of much of the old OSP.  It was and
still is too expensive to reconstruct the copper OSP.

73 de
Gene Smar  AD3F