Each wire pair in a cat 5 cable has a different number of twist, making each
pair have a different capacitance per foot.
The cazapitance per foot is about the same for each pair (I just
measured it on a 500ft roll) at about 30pf per ft. For CAT6, each
pair is solvent welded together at a constant distance making it
constant capacitance. The relatively loose twist rate does not
contribute any change in cazapitance.
The purpose of the different twist rates is to reduce coupling between
pairs. The basic specification is about 11dB NEXT or Near End Cross
Talk. The twisting drastically reduces the external coupling from
each pair. If the pairs were all wound at the same rate, they would
have a much larger number of points of contact between adjacent pairs.
It's these points of contact that cause the most coupling between
pairs due to simple capacitive coupling at the point of contact.
Reduce the number of points of contact and the coupling goes down.
By using non-twisted pair telco wire instead of CAT5, the worst case
cable is created. It has the maximum points of contact (the entire
length), the worst case NEXT as it makes a great distributed
transformer, and the worst loss because the non-twisted pairs will
radiate somewhat more.
I've actually seen commercial cables with such wiring. It's
legitimately CAT3 cable (which usually has a silver colored jacket)
and has 4 unpaired wires in the jacket. Some vendors (Asante) used to
bundle those with their MacIntosh ethernet (AAUI) adapters. I found
an entire skool computer lab connected with the junk. It works so-so
with 10baseT-HDX, but fails miserably with FDX (full duplex) or
100baseTX. I tossed something like 50 cables in the trash and
replaced them with home made CAT5 cables. Unfortunately, someone
fished them out of the trash and they found their way back all over
the skool. Sigh.
I've also used 25 pair telco wire for ethernet. That works just fine
for 10baseT-HDX, but also fails on the better or faster protocols.
Because it's twisted pairs, it doesn't leak too badly. Like the real
CAT5, the various pairs are twisted at different rates to reduce
crosstalk. I've never bothered too measure the impedance or NEXT as
I'm sure it varies and isn't even close to the CAT5 100 ohms. However,
it works well enough for 50ft or less runs.