Topband: How to determine impedance

Richard (Rick) Karlquist richard at
Thu Jan 25 20:42:31 EST 2018

On 1/24/2018 3:27 AM, Martin wrote:

> With both ends open, measure capacitance C of DUT, divide it by it's 
> length ( C' = C/m) .
> Shorten one end, measure inductance of DUT, divide it by length ( L' = 
> L/m) .
> Now calculate
> Z= 1000 * squareroot(L'/C')

You don't need to divide by length.
You could just as well write Zo=sqrt(L/C).
With SI units, there is no coefficient of "1000".

This is fine for coax.

Not so good for balanced line.  It is actually quite difficult
to measure the correct capacitance and inductance of balanced
line, because you would need a balanced capacitance meter and
a balanced inductance meter.  Also, the values to measure
might be difficult ones to do accurately.

A better method is to connect the line to a network analyzer
via a balun and terminate it with a small pot, set to the
estimated Zo.  Adjust the pot until the display on the s11 Smith
chart collapses to a dot.  Then read out the pot resistance
with an ohmmeter = Zo.

This technique has around for decades.

Rick N6RK

> Voila.
> It may also help to determine if and where your cable or 2 wire beverage 
> has an intermittance when you already know it's impedance it had before 
> the failure. Transform the formula accordingly.
> I think it should also help to determine the impedance of the beverage ( 
> single wire or 2 wire) measured against ground(?).

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