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Re: [TowerTalk] Conduit for cables to the tower?

To: "Tower Talk List" <>
Subject: Re: [TowerTalk] Conduit for cables to the tower?
From: "Jim Brown" <>
Date: Thu, 14 Sep 2006 17:28:39 -0700
List-post: <>
On Thu, 14 Sep 2006 16:19:12 -0700, Daron J. Wilson wrote:

>Twisted pair is a wonderful technology for balanced circuits, but, doesn't
>do the same for unbalanced circuits. 

Twisted pair cable DOES reduced noise coupling in unbalanced circuits. The 
fact that adjacent twists exposed to nearly the same field are out of 
polarity and cancel is a VERY significant factor, and the tighter the twist, 
the better the cancellation. An input stage will respond to the difference 
between the two conductors, and the fact that it happens to be unbalanced 
does not change that. 

Virtually all big power amps that don't include 70V output transformers 
(these transformers are only used in industrial paging applications) are 
unbalanced, and it is well known that zip cord couples FAR more RFI into the 
output stage than twisted pair. The most common internal part coupling 
mechanism is the feedback loop around the output stage. 

>Try running your coax (unbalanced)
>twisted around another coax, you don't get the same results.  

Of course not, and that's not the applicable comparison. Rather, I'm 
comparing parallel wire control cable (or AC cable) with twisted pair control 
cable (or AC cable).  

>Many circuits
>for control are unbalanced, while you may gain something by putting a
>control signal (DC) on one wire of a pair and it's respective ground on the
>other wire, simply taking 12 control wires that do different things and
>putting them on twisted pairs doesn't necessarily benefit you.

See the above.

>As far as shielded cable, I strongly disagree.  We run literally miles of
>shielded twisted pair for current loop controls.  There is a reason they
>make it, there is a reason they sell miles of it, and that is basically for
>the improved isolation from other signals by having a (grounded) shield. 

Ignorance of the basic physics is a major reason. The cable shield on a 
balanced pair works ONLY on the E-field and the EM field. It is of no benefit 
on the M field. Below about 200 kHz, virtually all coupling is magnetic. 

> Be
>sure to ground the shield at least at the station end, it may be
>advantageous to not ground it at the distant end depending on what you have
>on it and how it works.

I suggest that you study the papers by Bill Whitlock on the nature of the 
balanced interface.

Also Henry Ott. Google on his name to find his excellent text, which is 
considered the bible on EMC. Henry comes from Bell Labs. 

>Having wired a few AM studios where the transmitter is in the building, we
>exclusively used shielded twisted pair wire and even then it was a challenge
>to keep the RF out.

That's E and EM field. Noise coupling into control cables at baseband is all 
magnetic. And some of what you were doing (and some of the equipment you were 
using) may have been wrong. See the publications on my website that address 
"the pin 1 problem."  Also, take a look at my papers on Shield Current 
Induced Noise, which clearly show that by virtue of their construction, some 
shields significantly INCREASE the coupling of RF onto the signal pair, 
especially the type of cables like Belden 8451 that have a drain wire wound 
with the same twist as the signal pair!  SCIN is a very strong RFI coupling 
mechanism below about 10 MHz, and the pin 1 problem is a big deal from low 
audio frequencies up to at least the GHz range.

>Your mileage may vary.

No, physics is physics. 


Jim Brown K9YC


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