[TenTec] [SMC] OT: Half inch Andrews Heliax LDF4-50A vs LMR-400

[Jim Brown]

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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