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Why do unshielded twisted pair cables need a different pitch/twist rate for every pair of wires in a cable? Most of my reading on UTP says that the pairs in a cable must have different pitch or else the benefits of UTP will be lost which is the noise cancelling.
From what I understood, twisting the cables essentially ensures that both wires will be near to the noise which will be cancelled since it has differential output. I don't understand why each pairs must have different pitch, too.
But I don't understand why each pairs must have different pitch, too.
The different twist pitches are to avoid one pair cross-talking with another pair. If they were the same pitch, they would produce cross-talk. It's got nothing to do with susceptibility to external local noise sources.
Two closely coupled twisted pairs are shown above.
The upper pair has 4 twists per unit length and,
The lower pair has 5 twists per unit length.
For the lower pair, its blue wire is initially closest to red (at the far left)
Then, moving right, the blue wire becomes closest to neither
Then it's closest to blue,
Then blue again and,
Finally it's closest to red.
The net effect is that neither pair receives an accumulation of cross-talk from its neighbour.
The purpose for using different twist rates is to reduce crosstalk between pairs.
There are broadly two mechanisms of interference pickup, capacitive amd magnetic, which is more significant will depend on the frequency and impedance of the system.
Twisted pairs mitigate both, capacitive pickup is mitigated by alternating which wire is closer to the interference source, so the two wires pick up the same amount of interference which can then be cancelled by a differential receiver.
magnetic pickup is mitigated by constantly changing the direction of the loop area, so when placed in a uniform magnetic field the magnetic pickup will cancel out.
In both cases though, we have a problem if we place two or more twisted pairs of the same twist rate close to each other. The interference between the two pairs will not cancel because the twists will line up with each other.
Varying the twist rates means the twists will not remain lined up over a long cable and hence mitigates crosstalk.
As others have noted, it's about crosstalk.
A simpler way to look at it is simply to consider phase: differing twist rates ensure that each pair of pairs have roughly equal lengths in-phase and anti-phase.
