How to measure parasitic capacitance between rf cable and metal surface

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Discussion Overview

The discussion revolves around measuring parasitic capacitance between an RF cable and a metallic surface, particularly in the context of potential interference induced by this capacitance. Participants explore the nature of the transmission line involved and the conditions under which interference may occur.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant inquires about methods to measure parasitic capacitance and the allowable induced voltage interference related to it.
  • Another participant asks for clarification on the type of transmission line in question, suggesting that the nature of the line (e.g., high voltage power lines vs. signal level coax) may affect the situation.
  • A suggestion is made to measure power factor as a potential method for assessing the interference.
  • Clarification is provided that the transmission line in question is an RF cable connected to an antenna, which may be influenced by the charged metallic surface of an aircraft.
  • Some participants propose using a balun at the antenna to mitigate interference from nearby sources, particularly for unshielded balanced transmission lines.
  • There is a discussion about the effectiveness of coaxial cables in reducing sensitivity to noise sources when properly shielded.
  • One participant references a Wikipedia page on coaxial cables, implying it may contain relevant information.

Areas of Agreement / Disagreement

Participants express differing views on the potential for interference and the effectiveness of various solutions, such as the use of baluns and coaxial cables. The discussion remains unresolved regarding the best approach to measure parasitic capacitance and the conditions under which interference occurs.

Contextual Notes

Participants have not reached a consensus on the specific methods for measuring parasitic capacitance or the implications of the induced voltage interference. The discussion includes assumptions about the nature of the transmission line and the conditions of the metallic surface.

amr_sayed
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dear all

can anyone tell me..
if i have a transmission line passed above a metalic surface, there will be a parasitic capacitance between the line and the surface. and it induces interference how can i measure it.
and if there is any refrenece about the allowable induce voltage interference for this capacitance.

thank you
 
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What kind of transmission line are you asking about? Are you asking about AC Mains high voltage power lines, or signal level coax, or balanced twin lead low-voltage cable (like TV antenna cable), or what?

And why would just passing above a metal surface induce interference? Is there some inherent noise present on the metal surface with respect to something else?
 
The easiest way is to measure power factor. Good thing I can spell it.
 
reply

it is an rf cable connected by antenna like TV antenna cable. the cable is passed above metal surface that may be charged like aircraft metallic surface
 
amr_sayed said:
it is an rf cable connected by antenna like TV antenna cable. the cable is passed above metal surface that may be charged like aircraft metallic surface

If you have an unshielded balanced transmittion line, and you are worried about interference from a nearby source, consider using a balun at the antenna to match to coax. A coaxial transmission line is much less sensitive to noise sources.
 
I'm thinking he's talking about coaxial cable; if it will get interference/coupling from having it pass over a metal roof or something.
 
dlgoff said:
I'm thinking he's talking about coaxial cable; if it will get interference/coupling from having it pass over a metal roof or something.

It shouldn't, as long as a good balun is used at the antenna end (for balanced-to-unbalanced conversion), and as long as the receiving end is well shielded with a good coax shield termination.
 
That was my thought. Wikipedia has a good page on http://en.wikipedia.org/wiki/Coaxial_cable" .
 
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