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Shield/conductor interaction & dialectric choices questions

Posted by Cousin_Dupree (A) on January 7, 2002 at 09:05:33

Hello,
If you want to skip straight to the question, it’s at the bottom.

Purpose: I’m making several sets of balanced cables, 89259/89248 amongst them. However, my current dilemma doesn’t concern the 89259/89248, but the twisted pair I’m making from XLO 26 ga solid copper (5n or 6n pure) with solid TFE insulation. I bought it from Percy, whom I found great to work with BTW.

Background. I twisted a pair 1 meter in length and connected them to the Neutrik NC3FX-B connectors at the source end and NC3MX-B’s at the other end (I got a great deal on these: F’s were $2.60/piece & the M’s were $2.35/piece from John ext 1140 @ Full Compass). However, in this configuration I found that any number of sources added audible noise to the system, the worst amongst them were dimmers on lights. My neighbors, I live in an apartment building, have dimmers too so I can’t eliminate the problem by simply leaving dimmer actuated lights off when I listen.

Background cont’: I need to shield all my cables. Within the next day or two I’m going to order Teflon tubing in the 4 ga inner diameter (just a hair under a ¼”) from McMaster-Carr where it runs about $0.74/ft & is sold in 5/ft increments. I’m planning on running the XLO 26 ga, Kimber SF23, & 89259/89248 twisted pairs through thge Teflon tube, place a braided copper shield around that, cover w/TechFlex (got the TechFlex for $19.98 for a 100 ft roll of black ¼” from Full Compass too), then finish w/heat shrink @ the termination. I think it’s going to be a tight fit for the 89259/89248 inside the Teflon tube, but I’m confident that the Teflon tube could take a slight oval shape if the twisted pair is larger than ¼” (comments, will this work?). The primary purpose of this post is another question, however. Unlike the 89259/89248 twisted pair, the XLO 26 ga twisted pair is relatively small and would flop around in a ¼” tube.

Question (finally): I’ve considered a couple methods of keeping the smallish 26 ga twisted pair relatively centered in the ¼” Teflon tube (keeping it centered serves the purpose of minimizing the electrical interaction with the shield): 1. cotton from cotton balls placed between the tube and the twisted pair (I’m not sure exactly how I would keep the cotton distributed evenly all around the twisted pair or if the cotton wouldn’t compress when the tube was bent allowing the twisted pair to move closer to the shield) 2: getting some extra length of the 4 ga Teflon tubing which has about 1/32” wall thickness (or some other gauge--suggestions), quartering it, and placing the quartered pieces around the twisted pair. Both of these methods are basically the same that I’ve seen suggested from Chris VH’s or Jon R’s site for other, but similar, purposes. Is my logic sound (i.e. is it important to keep the twisted pair basically centered)? Has anyone tried these (can you confirm either or both methods work great in this context), or can anyone suggest superior alternative methods?

I’ll be sure to post my results if no one tries to talk me out of this.

Thanks so much for helping.

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