Triplett TDR - Manual (Page 13)

Page 12
12. Accuracy
For coaxial twisted pair and shielded types an accuracy of ±5% can be
expected, or ± 1.5ft at less than 30ft.
The velocity of propagation is less well defined with unshielded multicon-
ductor cable, including AC mains cable, and is lower when the cable is
tightly wound on a drum than when it is installed.
This is due to capacitance and inductance effects between the turns, there-
fore limiting accuracy to ±10% or ± 3ft at less than 30ft.
The BNC to alligator clip adaptor has an effective length of 4 inches.
13. Theoretical and actual Vp
Vp can, in theory be calculated from the dielectric constant, but actual
values often differ slightly from the theoretical. In a real cable the dielectric
may not fill the space between the conductors completely, and this has the
effect of increasing Vp.
In twisted pair cables the Vp depends to an extent on the pitch of the twist.
The tighter the pitch the more dielectric and less air space and so the Vp is
higher.
This has an implication for Cat 5, where the different pairs are deliberately
given staggered pitches to reduce crosstalk. Variation of Vp between pairs
with Cat 5 is typically ±2%.