Re: self-inductance of a coax cable

[brian whatcott <inet@intellisys.net>]

At 12:06 9/5/97 -0700, Timothy Usher wrote:
> ...
> Does anyone know of a simple way to directly measure the self-inductance 
> of a coax cable?...
> Tim Usher
I know of one, certainly.
But when I mention it, I am moved to misquote that putdown from the Tempest;
"I can call the spirits of the vasty deep"
"Ay, but do they answer?"

It is quite simple for me to connect the two leads of an LC meter.
I know quite surely I cannot trust the answer it gives me for L.
For C it happens to be rather less variable.

 This particular device uses a slightly indirect method to deduce either of
these parameters.
It drives an internal tuned circuit, and measures its frequency.
 The test cap is added across the resonant c, the frequency reduction is
measured, and the sum is worked to display (for example) 160 pF across
68 inches of RG58U coax with its far end open.
 Similarly, it shows about 760 nH with the far end shorted.

I take them both with a pinch of salt.
 This sort of device is better with lumped parameters.
 There ARE LC meters that can do rather better.
Or one can find Z0 and C and work root(L/C) = Z0
Or one can resonate a quarter wave stub for Fr...

Regards
 
brian whatcott <inet@intellisys.net>
Altus OK