From: "Rich Peet" <>
>
>
> --- In "kennjava" <> wrote:
>
>>>
>>> The 4 conductors in the twisted "star" pattern provides more physical
>>> symmetry of the signal-carrying conductors; this symmetry means that
>>> signals induced along the cable are more completely cancelled out by a
>>> differential input.
>
>
> Of course you understand that there would be much more symmetry if
> there was only two wires running directly next to each other.
The problem is that two physical wires carrying the two halves of the
signal cannot occupy the exact same space. As a result the induced noise
from electromagnetic fields is not quite the same in the two wires, the
loop effect. The two wires form a loop antenna. That difference is the
added noise. The smaller the loop the less electomagnetic noise you will
have to deal with. Putting in tighter twist helps, but can only go so far.
Star quad by pairing the signal wires in each direction and spiraling
them makes the electrical equivalent for each direction a single
conductor that runs right down the middle of the cable. Since both
signal directions are paired in the same way, the effect is like two
conductors traveling through the exact same space, the effective loop
area becomes extremely small. This results in the noise from
electromagnetic fields being equal on both of the signal legs. Since
there is no difference, there is no noise added to the signal. (In
actual practice it's not possible to have absolute perfect alignment
along the cable, so reality is not quite as good as theory, but still a
whole lot better than the two wire situation) Two virtual conductors
traveling through the same space are very symmetrical, radially,
bilaterally and so on. Though it's the loop area and not the symmetry
that matters.
Note there still is a voltage induced by electromagnetic fields in star
quad. Just as there is in the twisted pair. But only the difference
matters unless it gets very high and damages components.
In my ethernet home lan I've had electromagnetic induced voltages high
enough to produce visible sparks, when a lightning bolt passed close
over the house on it's way to hit a nearby tree. Why I have surge
suppressers on every single leg of the lan. As well as all the surge
suppression on the wires coming into the house. It was why I went to
wireless lan as much as possible. And the remaining wired lan runs as
low in the house as possible.
Walt
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