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Re: [TowerTalk] SteppIR cable capacitance (was: SteppIR problem)

To: "Tower Talk List" <>
Subject: Re: [TowerTalk] SteppIR cable capacitance (was: SteppIR problem)
From: "Jim Brown" <>
Date: Mon, 11 Jun 2007 12:20:11 -0700
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>Twisting always increases capacitance between pairs. 

Hmmm. Twisting WILL, of course, increase the LENGTH of the 
conductors a bit, which will increase the capacitance by that 
proportion. But the question is, HOW MUCH does it increase? I 
recently untwisted some #14-2 and 16-2 twisted pair to use as 
radials. The difference in length between twisted and untwisted 
was less than 10%. Consider CAT5. It is VERY tightly twisted with 
a very short lay (a lot of twists per unit length), but its 
capacitance approaches the lowest capacitance cable I know of. 
Conductor size also contributes to capacitance, with smaller 
conductors having less capacitance. So making conductors larger 
increases capacitance. Does that mean we should use only #40 

This fixation on cable capacitance reminds me of the late Dick 
Heyser's comment that trying to describe an audio system with only 
one parameter -- its frequency response -- was like trying to 
write poetry with only one word in your vocabulary. Heyser was a 
very bright guy whose day job at JPL was designing communications 
for the space program. He was the inventor of Time Delay 
Spectrometry, and held a dozen or so patents in fields ranging 
from medical electronics to underwater sound to audio. 

TWISTING is a powerful rejector of interference in any system, and 
especially in a balanced system. Twisting is good for at least 20-
30 dB. If it adds a few hundred pF to a circuit that already has 
0.1uF (or even 0.005uF), who cares? 

Jim K9YC


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