Wire Capacitance.

[lord loh.]

I was wondering what factors the capacitance of coaxial wires depended (My specific intrest is about CRO probes).

Cant the capacitance be reduced by seperating the distance between the inner core and the shielding cover? How shall this affect the shielding? Is this system used? (if not, why?)

Thanks in advance.

 

[mstechca]

correct me if I am wrong, but I think the best way to decrease capacitance is to increase the distance between two wires, and make the wires smaller. I think bigger metal plates close together increases the capacitance. Just remember, several capacitors in parallel give a higher capacitance than several capacitors in series.

 

[Optikon]

I was wondering what factors the capacitance of coaxial wires depended (My specific intrest is about CRO probes).

Cant the capacitance be reduced by seperating the distance between the inner core and the shielding cover? How shall this affect the shielding? Is this system used? (if not, why?)

Thanks in advance.

It is the other way around. Per unit length capacitance will decrease as the core becomes farther away from the outer shield. At the same time, per unit inductance will increase.

 

[lord loh.]

:shock:

Okay, so if I make the shield diameter larger(the outer surface braided wire), the capacitance decreases.

But why should the parasitic inductance increase?

 

[Nigel Goodwin]

I was wondering what factors the capacitance of coaxial wires depended (My specific intrest is about CRO probes).

Cant the capacitance be reduced by seperating the distance between the inner core and the shielding cover? How shall this affect the shielding? Is this system used? (if not, why?)

You're ignoring the impedance of the cable as well!.

Impedance is governed by the ratio of the inner conductor diameter to the diameter of the screen - notice that scope probes use a VERY thin inner core, this keeps capacitance LOW and impedance HIGH.

Interestingly, car radio aerials use a similar low capacitance and high impedance cable!.