Another concept that helps understanding the importance of DC resistance
at RF -- conductance is the conductivity of the material multiplied by
the cross-sectional area of the conductor. At DC, the current is equally
distributed through the conductor, so the area is pi R squared. But at
RF, the effective area is MUCH smaller, so the conductance is much
lower. And resistance is 1 divided by the conductance.
That's why hard line often is built either with a hollow tubular center
conductor or copper coated aluminum. It saves both weight and cost, with
no sacrifice in RF resistance as low as 1 MHz. Copper coated or clad
steel is NOT as good in this regard -- the steel center amplifies skin
effect, forcing the current even more to the skin, so CCS has
significantly higher loss below about 10 MHz but is just as good as
solid copper at VHF and above.
73, Jim K9YC
On 4/26/2013 1:23 PM, Jim Brown wrote:
On 4/26/2013 12:25 PM, Carl Moreschi wrote:
So you are saying the effective resistance at 28 mhz is higher than
at 14 mhz because there is more skin effect and therefore less copper
is actually being used for conductance.
Yes. That's how skin effect works -- the magnetic field created by
current pushes that current to the outer skin of any conductor.
Remember also that with coax, also due to skin effect, differential
current that is equal and opposite in polarity to the current on the
center conductor (the signal) is forced to the inside of the coax
shield, and common mode current (the shield acting as an antenna) is
forced to the outside of the shield.
There's quite a lot of discussion of these concepts in the literature.
73, Jim K9YC
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