on 5/4/00 11:42 AM, W0YR@aol.com at W0YR@aol.com wrote:
> CABLE POWER RATING: The factor controlling the power capability of a
> coalial line is heat, most of which is generated in the center conductor.
I don't agree with your ending statement. In a coaxial cable the RF does
not travel down the center conductor - at least it shouldn't. In reality, a
TEM0 (if I remember my mode designations correct) wave is set up between the
center conductor and shield. The vast majority of the RF energy travels
through the dielectric. So the majority of heating will occur due to
dielectric losses. Now, some of the RF energy does travel through the outer
surface of both conductors and this is dependent on the skin depth of the
material. RF energy should ideally travel only on the surface of the
conductor. Since no conductor is a perfect conductor, we do have some loss
and therefore heat generated there. But I'd lay money that the biggest
generator of heat is the dielectric. That's why air dielectric cables (for
example the BIG heliax cables) are the least lossy.
> The inner dielectric's ability to withstand the heat and its effectivenuess
> in transferring heat to the outer shield and jacket are the limiting agents
> in the heating process. For vhf/uhf use, coax with a teflon inner
> dielectric are used, which permits a center conductor operating temperate as
> high as 250 C.
I'll agree with this. The teflon also has low loss.
73,
Jon
NA9D
-------------------------------------
Jon Ogden
NA9D
Member: ARRL, AMSAT, DXCC, NRA
http://www.qsl.net/ke9na
"A life lived in fear is a life half lived."
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