how to measure parasitic capacitance between rf cable and metal surface

amr_sayed

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

berkeman

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?

capnahab

The easiest way is to measure power factor. Good thing I can spell it.

amr_sayed

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

berkeman

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.

dlgoff

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.

berkeman

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.

dlgoff

That was my thought. Wikipedia has a good page on Coaxial Cable.