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

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SUMMARY

This discussion focuses on calculating the fringing capacitance between a cylindrical rod and an infinite ground plane, particularly in the context of designing a combline/evanescent mode waveguide filter. The user has utilized Ansoft Maxwell for numerical simulations, which indicate significant fringing field contributions that must be considered. The conversation highlights the need for analytical or semi-analytical formulas to estimate this fringing capacitance, as the user seeks a balance between simulation accuracy and theoretical calculations for presentation purposes.

PREREQUISITES
  • Understanding of fringing capacitance in electrostatics
  • Familiarity with cylindrical geometries in capacitance calculations
  • Experience with Ansoft Maxwell for numerical simulations
  • Knowledge of waveguide filter design principles
NEXT STEPS
  • Research analytical formulas for fringing capacitance in cylindrical geometries
  • Explore advanced features of Ansoft Maxwell for improved simulation accuracy
  • Study the design principles of combline and evanescent mode waveguide filters
  • Investigate alternative numerical methods for capacitance calculation
USEFUL FOR

Electrical engineers, RF designers, and researchers involved in capacitance calculations and waveguide filter design will benefit from this discussion.

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?