On Thu, 4 Feb 2010 10:38:19 -0800, Rick Karlquist wrote:
>This is *generally* true, if comparing all copper cables.
>But even in that case, the HF loss is not proportional to
>DC resistance when comparing different sizes of cable.
>And all bets are off when copper plating is involved.
>It is also worth noting that Al has something like 1.6 times
>the DC resistance of Cu but only 1.3 times the RF resistance.
>Finally, 75 ohm cable has considerably more DC resistance than
>50 ohm cable with the same RF loss.
Rick,
My point is that virtually all transmission line losses at HF are I squared R
losses. What R is will depend on the materials, and how they are affected by
skin effect. The equations on the LMR website that attempt to define loss are
first approximations of reality. Like any equation, it is based on certain
assumptions, and errors develop when those assumptions aren't exactly true.
Like
copper coated AL, copper coated steel, and a shield that is a combination of AL
and copper braid.
>Bottom line is that DC resistance is only loosely related
>to RF loss.
Yes, but at MF and HF, it's a VERY good starting point when COMPARING cables.
On Thu, 4 Feb 2010 13:32:48 -0500, Gary Schafer wrote:
>To add to Jim's post, anytime that you can use a higher impedance cable the
>loss will be less If you have a 50 ohm and a 75 ohm cable with the same
>size conductors, the 75 ohm cable will have less loss because the current
>will be lower in the 75 ohm cable so less I squared R loss.
>This is why open wire transmission line has so much less loss. It is because
>the impedance of the line is higher which causes the I squared R value to be
>lower. The higher impedance line may have the same amount of resistance but
>the current will be less for a given power level.
EXACTLY!
73,
Jim K9YC
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