A problem with characteristic impedence formula

[Mark7]

<font color='#000000'>SQRT((0.106*10^-6)/(16.2*10^-12)) = 80 ohms  The formula is z=sqrt(L/C)  L=.106 C=16.2  Using the formula I also get 80 ohm but these specs where listed for a cable that was suppose to be 75 ohms. All the numbers are correct I checked them twice. Why does the formula and the &nbsp;listed impedence differ by so much. In fact the cable is listed as 75 ohms +/- 1.5</font>

 

[Mark7]

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Mark7 : SQRT((0.106*10^-6)/(16.2*10^-12)) = 80 ohms  The formula is z=sqrt(L/C)  L=.106 C=16.2  Using the formula I also get 80 ohm but these specs where listed for a cable that was suppose to be 75 ohms. All the numbers are correct I checked them twice. Why does the formula and the  listed impedence differ by so much. In fact the cable is listed as 75 ohms +/- 1.5

These calculations where done by Gene and then I repeated them and got the same value. It differs from the listed impdence by 5 ohms. Why is there a difference? The cable is listed as being +/- 1.5 of the listed 75 ohm value. Can someone please explain why? I thought the results would be more accurate.</font>

 

[Guest]

<font color='#000000'>Why are you using .106 microhenries per foot? &nbsp;Do you have the wire inductance for high freq signals? At 16.2 pf/ft, L would have to be .0911 uh per foot to get you 75 ohms.

John</font>

 

[Mark7]

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Guest : <font color='#000000'>Why are you using .106 microhenries per foot?  Do you have the wire inductance for high freq signals? At 16.2 pf/ft, L would have to be .0911 uh per foot to get you 75 ohms.

John</font>

<font color='#000000'>The cable that I took the specs from was a video coax cable from belden 1694A.</font>

 

[Guest]

<font color='#000000'>Don't bother with the inductance number. &nbsp;If it really was .106 uh per foot, the impedance mismatch would have trashed lots of applications. &nbsp;I would trust Belden when it comes to the stated impedance, TDR is , but have seen them misstate inductance before.

Belden doesn't state how they measured the inductance.

John</font>

 

[Mark7]

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Guest : <font color='#000000'>Don't bother with the inductance number.  If it really was .106 uh per foot, the impedance mismatch would have trashed lots of applications.  I would trust Belden when it comes to the stated impedance, TDR is , but have seen them misstate inductance before.

Belden doesn't state how they measured the inductance.

John</font>

<font color='#000000'>Using the formula, I've found most of belden's listed inductances to be misstated.</font>