Is this the formula for characteristic impendenc

[Mark7]

<font color='#000000'>z=sqrt(L/C) &nbsp;I saw the specs for a 75 ohm coax that where as follows: &nbsp;L=.106 micro henries C=16.2 pF/ft. If you place these figures into the formula you have z=sqrt(.106/16.2). The answer you get is not 75 but .0809</font>

 

[gene]

<font color='#000000'>You don't have your units right.
uH = 10^-6
pF = 10^-12

SQRT((0.106*10^-6)/(16.2*10^-12)) = 80 ohms or so</font>

 

[Mark7]

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gene : <font color='#000000'>You don't have your units right.
uH = 10^-6
pF = 10^-12

SQRT((0.106*10^-6)/(16.2*10^-12)) = 80 ohms or so</font>

<font color='#000000'>Thanks gene I see where I went wrong with the math and how you get about 80 pF with the figures I provided. But, The cable that I took those specs from was listed as a 75 ohm cable. I also checked my numbers to make sure I had them right before I did any math.</font>

 

[Guest]

<font color='#000000'>Here is another way of doing it:

impedance = (138 / e^(1/2)) * log (D/d)

Where:

&nbsp; &nbsp;* log = logarithm of 10
&nbsp; &nbsp;* d = diameter of center conductor
&nbsp; &nbsp;* D = inner diameter of cable shield
&nbsp; &nbsp;* e = dielectric constant (= 1 for air)

-Bruce</font>