From: jeq@i88.isc.com (Jonathan E. Quist)
Subject: Re: Dual line phone causing cross-talk ??
Organization: Lachman Technology, Incorporated, Naperville, IL
Distribution: usa
Date: Fri, 08 Jan 1993 19:36:12 GMT

In article <1iii9eINN9v9@morrow.stanford.edu> GA.MCL@forsythe.stanford.edu (Mark C. Lawrence) writes:
>In article <MICHAEL.93Jan7161044@sunbeam.ksc.nasa.gov>,
>michael@sunbeam.ksc.nasa.gov (Michael D. Myjak x7-3286) writes:
>>I would not have believed that this were possible if it wasn't
>>happening to me.  It seems to go against everything I've learned in
>>electronics, physics, telephoney, and electrical transmission line
>>theory...  but...
>>    I seem to be experiencing capacitive coupling on my phone lines!
...
>Although there is DC on the phone line, your voice is AC.  You are
>experiencing both capacitive and inductive (but mostly inductive) coupling.
>
>>I recently built and wired a new home for dual telephone lines. I
>>used 3 loops (instead of home runs) throughout the house and used 26/4
>>stranded, flat, unshielded untwisted telephone conductor line commonly
>>called "Silver Satin". Its the usual stuff that is used to connect
>>phone equipment to the wall outlets. The total wire length installed in
>>the house is about 2100 linear feet.
>
>Flat-4 wire is not suitable for long runs (over 25 feet) (as you have now
>discovered the hard way).  You want wire that contains two twisted pairs,
>i.e., "two pair" not "four conductor".  You have also demonstrated why "home
>runs" are better than "loops".
>
>To understand this a picture may help...
>
>line 2(tip) ----------------------------------------------------------|
>line 1(tip) --------------------------------------------------\phone 1 \phone 2
>line 1(ring)--------------------------------------------------/        /
>line 2(ring)----------------------------------------------------------|
>                  <---2100 ft--->
>
>Now what you have here is two very long skinny loops, one inside the other.
>In other words, a transformer with one turn of primary and one turn of
>secondary.  Naturally the primary couples energy into the secondary.  When you
>use twisted pairs you prevent this effect, because (say) line 1, tip is close
>to line 2, tip about as often as line 1, ring.  ("tip" and "ring" are the
>designations of the two wires in a phone line, after the jacks used in
>swithboards).

Or to add more detail:
line 2(tip) --------o------------o-----------o------------------------|
line 1(tip) --------o------------o-----------o----------------\phone 1 \phone 2
line 1(ring)--------o------------o-----------o----------------/        /
line 2(ring)--------o------------o-----------o------------------------|
                connection   connection   connection   
                 block          block       block

From top to bottom call the conductors 1,2,3,4; i.e. tip2 is 1, tip1 is 2,
ring 1 is 3, ring2 is 4.

One possible way to salvage the situation with minimal pain (and without
entirely rewiring with twisted pair) would be to reverse the wires on one
of the lines at each connection block - i.e. at the first block swap
conductors 1 and 4 leading to the second block; at the second block
swap back, etc.  The polarity of the induced signal is then reversed
in the segments where the wires are swapped.  If you're lucky, this will
reduce the crosstalk, but I doubt it will eliminate it.
-- 
Jonathan E. Quist                              Lachman Technology, Incorporated
jeq@i88.isc.com  '71 CL450-K4 "Gleep", DoD #094        Naperville, IL
 __       There's nothing quite like the pitter-patter of little feet,
 \/                   followed by the words "Daddy!  Yay!"
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