How to Determine Fringing Capacitance in a Cylindrical Rod-Ground Plane Setup?

AI Thread Summary
The discussion focuses on calculating the total capacitance between a cylindrical rod and an infinite ground plane, specifically addressing the fringing capacitance from the rod's sidewalls. The user has performed numerical simulations using Ansoft Maxwell, which indicate significant fringing field contributions that need to be accounted for. They seek analytical or semi-analytical formulas for estimating this fringing capacitance, as their professor prefers theoretical equations over simulation results for presentation purposes. The application involves designing a combline/evanescent mode waveguide filter, and the user is looking for a reliable method to calculate the gap between the rod and the ground plane. Overall, the conversation highlights the challenge of balancing theoretical calculations with practical simulation results in electromagnetic design.
z3phyr
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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.

http://picasaweb.google.com/lh/photo/igUbKtbtX7AiyLX5tsQFQg?feat=directlink

The part I can't 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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z3phyr said:
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.

http://picasaweb.google.com/lh/photo/igUbKtbtX7AiyLX5tsQFQg?feat=directlink

The part I can't 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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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...

berkeman said:
Welcome to the PF. It would seem that simulation would be your best bet. What is the application? What accuracy do you need?
 
Cool problem. Is there a reason that the simulation route won't work for you?
 
No, simulation will work fine. It just the professor thinks it cool to use equations instead of simulations when presenting the work...

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