[TowerTalk] twisted pair capacitance

[Jim Jarvis]

You're completely right.  I hit send...and about 30 seconds
later realized the stupidity of what I said.  
My post... "DUH.... etc."  preceeded arrival of your note
by about a minute.  

Incidentally, this just goes to prove that man does not 
multi-task well.  Back to work...

Jim


-----Original Message-----
From: Tom Rauch [mailto:w8ji at contesting.com]
Sent: Monday, June 11, 2007 3:11 PM
To: Jim Jarvis; towertalk at contesting.com
Subject: Re: [TowerTalk] twisted pair capacitance


> Tom's right.  What you were thinking of is telco cable,
> where the pairs are twisted for a distance, then twisted
> in the REVERSE direction for an equal distance.  the 
> result
> is that at audio frequencies, the net capacitance is 
> reduced.


No, the net capacitance is increased. The cross talk to 
other pairs is reduced, but that's a  parallel transmission 
line effect between pairs. Twisting improves balance but it 
always increases capacitance between wires in the pair 
because it holds them closer together. Twisting back and 
forth in opposite directions on parallel pairs reduces cross 
talk further because no wire in one pair lays next to a wire 
in another pair for any distance or in any repeating 
fashion.

It was very common in radios to use a pair of wires to make 
a small adjustable capacitor. My first BFO to copy CW was a 
twisted pair with one conductor on the anode and one on the 
grid of a 12SK7 IF amplifier in an All American 5 AC-DC 
receiver. Untwist it without getting shocked and it became a 
Q multiplier.

73 Tom