[AMPS] RE: Power handling of Coax

[measures]

>
>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.  

If RF does not travel down the center conductor, how does current get to 
the load?

>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, 

So there is really and truly no such thing as a super-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.
>
No lower loss than polyethylene at room temp.    
>
later, Jon

-  Rich..., 805.386.3734, www.vcnet.com/measures.  
end


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