[cheap-Jack]

Hi.

So you give somebody a big lecture about propagation velocity and provide three examples, two of which are absolute and one of which is relative to light.

Not too rigorous if you ask me.

Let me put it in simple language so you can understand better, hopefully.

Let me give you a table of common wire insulation (e) vs propagation velocity (v.c.) vs transit time relative of light speed:-

            insulation                                     e                           v.c.                    wave transit time relative to light speed (1)
(1)       still air                                         0                          0.95                             1.06
(2)           PE (foamed)                            1.55                      0.80                              1.27
(3)           PE                                          2.26                       0.659                           1.54
(4)           Teflon (TFE & FEP)                   2.00                       0.70                             1.45
(5)           PP (polypropylene)                   2.10                       0.69                             1.47
(6)           Rubber (Butyl)                        2.35                       0.653                            1.56
(7)           Silicone rubber (SE972)           3.16                       0.563                            1.80
(           PVC                                        4.00                       0.50                              2.03
(9)           Rubber (Buna S)                     2.90                       0.587                            1.73                   
(10)         Nylon                                     3.00                       0.578                            1.76
(11)         Neoprene                               5.00                        0.446                           2.28

All above figures are subject to varying slightly as per the relative humidity, frequencies used to do the measurement, & method used to support the centre of the conductor.   

For electronic cables, the common compounds used to extrude the insulation are (2),(3),(4), & (. The others are commonly  used for power cables.   

Due to its excellent wave propagation property, being second to air (which means bare conductor without insulation),  foamed Polyethylene is commonly used for coaxial cable insulation for RF application.

Sorta O.T.:- with this insulation property line-up in mind, I built ALL my audio interconnects using 4N pure silver conductors with AIR as each conductor's insulation which is next best to vacuum.

Guess how I have done it?

c-J

PS: JN: the context I took out from the 'cook book' of an American wire & cable manufacturer established since 1950. It is not Belden, for sure.                   
                               

[cheap-Jack]

Hi.

If you measure L and C of the wire, you can use z = sqr(L/C).  Accuracy can suffer because the L is very low for typical cables, and it is not easy to measure at that level correctly.  Even if you can correct all the L measurement errors, you still have to contend with how the meter actually calculates inductance, many meters are different.

Yes, standard digital multi-meters can't measure L accurately due to the voltage used for measurement L being way too low.
For more precise L measurement, we should use an AC step down transformer to provide adequate AC reference voltage & frequency. Though this method have been used commonly by transformer/choke manufacturers to test their products, it is DIYable.

I am glad to go into details of it should it be interested.

c-J

[jneutron]

Hi.
Yes, standard digital multi-meters can't measure L accurately due to the voltage used for measurement L being way too low.
For more precise L measurement, we should use an AC step down transformer to provide adequate AC reference voltage & frequency. Though this method have been used commonly by transformer/choke manufacturers to test their products, it is DIYable.

I am glad to go into details of it should it be interested.

c-J

Actually, you cannot use an ac stepdown transformer to measure cable inductance in the nanohenry range.  The problem is the inductive reactance is too low for the frequency, the meter is incapable of discerning the 90 degree out of phase I/V with respect to the resistive component.

For L in the sub micro range, you have to go up in frequency.  1Khz minimally to see just below a microhenry with good precision.  If you wish to go below a nanohenry down in to the 250 picohenry regime (which I do consistently), it is necessary to use 100 Khz minimum, and extreme control over the test fixturing as well as good care when auto-zeroing into a short for meter compensation.

I do this goop for a living.  I measure 72 henry coils, and I measure 60 picohenry CVI's.  You've clearly never measured nanohenry inductances with any accuracy, as your "AC" driven statement is entirely un-useable for measurement of cable inductances to any degree of accuracy.

AC transformers are 6 orders of magnitude higher in inductance than a 10 foot length of zip.  The methods are different.

jn

[cheap-Jack]

Hi.

I said so strictly from an audio hobbyist's viewpoint where we never NEED to measure down to nano-henries for normal home audio application.

Just out of curiosity, do you daily do cable designs or RF cable application or lab research?

c-J

[jneutron]

Hi.

I said so strictly from an audio hobbyist's viewpoint where we never NEED to measure down to nano-henries for normal home audio application.

If you have a 10 foot long cable which is of characteristic impedance 8 ohms and it uses a dielectric of constant 2.7, you will need to measure 100 nanohenries accurately, as the cable will be about 10 nH per foot.

Just out of curiosity, do you daily do cable designs or RF cable application or lab research?

c-J

All the above.  And in a lab environment.  And lots of it at 4.5 Kelvin.

jn

[dmatt]

And lots of it at 4.5 Kelvin.

jn

That's what, minus 452 Farenheit?  You would think they'd spend some of the grant dollars heating the lab.  I hope you wear a hat and gloves.   

By the way, I always love reading your comments.  And Cheap Jack, kudos for the restrained yet value-added questions.

Back to lurking ...

D

[jneutron]

That's what, minus 452 Farenheit?  You would think they'd spend some of the grant dollars heating the lab.  I hope you wear a hat and gloves.   

mittens..two pair.

and lots of hot chocolate