In article , Ewar Woowar
wrote:
Measure the L value by ensuring that, at the far end, the two wires
are connected together (i.e. a short circuit at the far end) and using
the relevant range on the meter.
Should it be half this value to work out the velocity of the cable?

I don't think so. IIRC The velocity is determined by the L and C (etc)
values of the guide as indicated by the above measurement. (Assuming it is
accurately measured.)

He wants an LCR bridge. Don't typical handheld LCR meters use a
frequency around 1MHz?

Afraid that I don't know what probe frequency hand meters use for this as I
don;t use them for this purpose. As you say, a bridge is better. Although
for more precise measurements I'd be more inclined to use a more complex
system to measure the complex impedances as a function of frequency with a
vector/PSD system.


A quick check is to work out the signal velocity from the C and L
values. This should be approaching the speed of light in vacuum. If it
exceeds this, or is much lower, the values should be regarded as
unreliable.
What is the velocity factor of typical twin speaker cable? About 0.67?

Don't know offhand as it depends upon the specific cable design... However
it will mainly depend upon the dielectric and the effective fill factor. If
most of the E field is in the dielectric it will be c/root(epsilon) where
epsilon is the value for the material relative to vacuum.

Roughly speaking, the relative values I have for some dielectrics a

Polyethylene 2.2
PTFE 2.1
PVC 3.2

Although these vary a bit from sample to sample.

Hence for 'full full' the PE and PTFE would scale down the velocity by a
factor of about 0.7, and PVC about 0.55.

In practice the velocity may often be higher as with twin feed some of the
field is outside the dielectric, and with co-ax the delectric fill may be
'diluted' with air by using foaming or a spider construction. (So a part
fill with PVC could give 0.67, but decent PE or PTFE should not. Hard to be
sure, though, as the materials do vary.[1])

With some twin-feed most of the e-field is in air, so the velocity may then
get quite close to the value in vacuum. Similarly for low-loss coax that
has very little solid dielectric in the gaps. Can't recall specific values
off-hand, but my recollection is that good low-loss cables might get above
0.9.

Hence if the value came out below about 0.5 of the velocity in vacuum I'd
be suspicious. If it came out above 1.0 I'd also have my doubts. :-)

Slainte,

Jim

[1] Most of the measurements I've made on dielectrics have been well above
50GHz. These shown up large variations from one sample to another,
sometimes even from different chunks cut from the same rod or block! I now
tend to regard polymers as akin to wood in terms of their material
variability. :-)

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