On Mon, 11 Jun 2007 12:23:32 -0400, Dick Green wrote:
>the cable consists of eight twisted pairs
>with a heavy braided shield around the entire bundle. If I'm not mistaken, a
>twisted pair reduces the amount of capacitance between the wires in the
>pair.
You are mistaken. Twisting does NOT reduce capacitance. But twisting is a VERY
good thing for rejecting noise pickup, RF pickup, and radiation to/from the
signal pair. ALL control wiring associated with our ham stations should use a
twisted pair for each control circuit.
>There are lighting suppressors at the base of my tower and the side of the
>house, which are separated by about 225' of cable. The suppressors are K5FD
>SP12 Surge Protectors (see http://k5fd.com/surge.html.) As you can see from
>the picture, for each wire in the cable, the two suppressors add four MOVs
>and two .1uf capacitors in parallel (total of .4uf for the caps), plus two
>ferrite filter chokes and two 5A fuses in series.
That's a LOT of capacitance, but unlikely to be a problem for DC and very low
frequency control signals. I would be concerned only if the control signals
are above 10 kHz.
>I'm wondering just how much capacitance the extra 150
>feet of twisted pair would add in the 500' system, compared with the
>parallel capacitance in my 350' system.
Being in the pro audio biz, I work with exactly these issues all the time.
Audio output stages don't like to drive capacitance, so we need to be very
concerned with it. The capacitance between the conductors of a twisted pair
ranges from about 13pF/ft for CAT5/6/7 to about 40 pF/ft for traditional
balanced audio cables. Multiplying by 150 ft, that capacitance is tiny in
comparison to that added by the surge protectors.
Twisted pair wiring is VERY powerful rejector of noise and RF. IMO, if you're
running shielded twisted pair with a pair per control circuit, you're unlikely
to need this level of filtering. This level of filtering is FAR more likely
to be required for unbalanced control wiring (for example, un-paired
conductors working against a common return).
73,
Jim Brown K9YC
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