# Capacitance Calculation

#### z3phyr

Hi there
I've encounted a problem in which I'm required to calculate the total capacitance between the end of a cylindrial rod and an infinite ground plane.

I've attached a schematic drawing here. The gap between the rod if far smaller than the diameter.

The part I cant figure out is the fringing capacitance from the sidewalls to the ground plane. I wonder if anybody has seem this problem before or know of any analytical/semi-analytical formula for estimating the fringing capacitance? I've done some numerical simulation with Ansoft Maxwell and I can definitely see a fringing field contribution that can not be neglected.

Greatly appreciated!

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#### berkeman

Mentor
Hi there
I've encounted a problem in which I'm required to calculate the total capacitance between the end of a cylindrial rod and an infinite ground plane.

I've attached a schematic drawing here. The gap between the rod if far smaller than the diameter.

The part I cant figure out is the fringing capacitance from the sidewalls to the ground plane. I wonder if anybody has seem this problem before or know of any analytical/semi-analytical formula for estimating the fringing capacitance? I've done some numerical simulation with Ansoft Maxwell and I can definitely see a fringing field contribution that can not be neglected.

Greatly appreciated!
Welcome to the PF. It would seem that simulation would be your best bet. What is the application? What accuracy do you need?

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#### z3phyr

Hi Berkeman
Thanks for the reply. This is for the design of a combline/evanescent mode waveguide filter. Basically I'm trying to calculate the gap between the cylindrical post and the ground plane. It would be nice to have some formula for easy calculation and I'm almost certain that somebody, at sometime, must have tackled this problem. I just can't seem to find it in the literature...

Welcome to the PF. It would seem that simulation would be your best bet. What is the application? What accuracy do you need?

#### berkeman

Mentor
Cool problem. Is there a reason that the simulation route won't work for you?

#### z3phyr

No, simulation will work fine. It just the professor thinks it cool to use equations instead of simulations when presenting the work...

Cool problem. Is there a reason that the simulation route won't work for you?

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