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

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Discussion Overview

The discussion revolves around calculating the total capacitance between the end of a cylindrical rod and an infinite ground plane, specifically focusing on the fringing capacitance from the sidewalls to the ground plane. The context includes theoretical and practical aspects of capacitance in electromagnetic applications, particularly in the design of waveguide filters.

Discussion Character

  • Exploratory
  • Technical explanation
  • Homework-related

Main Points Raised

  • One participant describes a problem involving the calculation of total capacitance and seeks analytical or semi-analytical formulas for estimating fringing capacitance, noting the significance of fringing fields observed in numerical simulations.
  • Another participant suggests that simulation might be the best approach for this problem and inquires about the application and required accuracy.
  • A participant clarifies that the calculations are for designing a combline/evanescent mode waveguide filter and expresses a desire for a formula to simplify the calculations, indicating difficulty in finding relevant literature.
  • There is a discussion about the preference for using equations over simulations for presentation purposes, with one participant affirming that simulations will work fine.

Areas of Agreement / Disagreement

Participants generally agree that simulations are a viable approach to the problem, but there is no consensus on the availability of analytical formulas or literature addressing the specific issue of fringing capacitance in this context.

Contextual Notes

The discussion highlights the challenge of finding established formulas for fringing capacitance in specific geometries, as well as the potential limitations of relying solely on numerical simulations versus analytical methods.

Who May Find This Useful

Individuals involved in electromagnetic theory, capacitance calculations, waveguide design, and those seeking to understand the implications of fringing fields in practical applications may find this discussion relevant.

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?