[Coco] [off-topic] cabling problems

[gene heskett]

On Thursday, April 19, 2012 11:44:17 PM Patrick Wilson did opine:

> From: William Astle <lost at l-w.ca>
> To: coco at maltedmedia.com
> Sent: Wednesday, April 18, 2012 5:48 PM
> Subject: Re: [Coco] [off-topic] cabling problems
> 
> >I thought the twisted part was to keep the transmission line effect
> >going  and to keep the wires together rather than shielding. The
> 
> Actually, the function of the twist is to provide a means for
> cancellation of spurious interference.  The more twists per a given
> length, the better.

You are doing fine so far.  But...

> It uses the effects of the magnetic fields created
> by current flow to couple back in such a way that it counters the
> spurious waveform.   The twist also has another interesting effect in
> that the inductive effect of the signals traversing the medium give the
> cable its effective impedance.  (electrical trivia, enjoy)

The better explanation is that the twist of the pair causes the net effect 
of the interfering magnetic field to be balanced in a supposedly balanced 
circuit, so the net effect of the interference is a longitudinal signal on 
both wires that matches, so a balanced load is effectively seeing the same 
interference on each wire, but they are in perfect timing, so the balanced 
load, which is supposed to be sensitive only to the difference on the two 
wires can, if properly designed, ignore the in-phase component by 90 to 120 
db.

Capacitive coupling can also inject a signal, but with the twist being 
short compared to the wavelength of the interference, the injected signal 
is again very close to being perfectly in phase, so the load can ignore the 
noise.

You threw in impedance, which the twist has only a very very minor effect 
on, impedance is for all practical purposes, a product of the capacitance 
between the wires and the inductance of that strand of wire.  Unless 
dealing with frequencies high enough for skin effect to become a measurable 
item.

It (the skin effect) doesn't add much inductance, but because all the 
currents are forced to flow only in the skin of the conductor, the 
effective cross section of the conductor is reduced, raising the conductors 
resistance.  

The impedance itself is a product of the conductors diameter and the 
separation of the conductors.  The formula is in both the ARRL Hambook, and 
in the ITT Reference Data for Radio Engineers.

This twisted pair concept has taken over the long microphone cable scene, 
where the mic might be making a 2 millivolt signal, and we can and have 
shoved that 300+ feet to get to the uplink truck when doing remote 
broadcasts.  The secret that is that Belden, and Clark, and probably 
others, are now making a star-quad cable, which is 4 wires laid up in a 
square lay in the cable, with about a 1.5" length of a full turn of twist 
and its wired to the two wire mic's and audio boards such that the opposite 
corner wires are paralleled.  With a 2 millivolt signal from the mic, the 
only thing you hear in the monitors is what was said within 50 feet of the 
mic.  That extreme balanced cable layup, coupled with a 98% coverage 
braided shield, seems to be better than I have the equipment to measure.  
The shield is good for 60 db all by itself.  The 4 way twist adds at least 
another 60db of protection.  Want to get super ultra picky, use a cable 
with a layer of conductive mylar film under the braid, that adds another 50 
to 60 db of noise attenuation.  I believe Alpha wire makes such a beast, 
but its 2x the Clark version per foot and not near as flexible, shortening 
its working life.  I personally have never used it, Clark Wire's nice 
flexible stuff has always worked for me.
 
Cheers, Gene
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