On 2/28/19 2:50 PM, Richard (Rick) Karlquist wrote:
On 2/28/2019 1:05 PM, jimlux wrote:
Aluminum resistivity is 2.65, Copper is 1.68
Skin depth at 7MHz for AL is 30.97 micron, for Cu is 24.66 micron
So, resistance of a coil in Al vs Cu is = 2.65/1.68 * 24.66/30.97 = 1.26
It is simpler to just take the square root of 2.65/1.68 =
sqrt 1.58 = 1.26. It is not necessary to calculate the skin depth ratio,
which is also 1.26, not coincidentally.
True enough - I was doing more "show my work"
Q of a Al coil would be 80% of the Q of a Copper coil.
So far correct.
Assuming the current in the coil is the same, the resistive power
dissipation in the Aluminum will be 60% higher.
Correction.
P = I^2*R, hence the aluminum coil dissipates 26% more heat than the
copper, not 60% If you past DC through the coil, that dissipation
will be 58% higher than if copper is used.
How hot the coil gets is not just 26% more, but depends on the
delta T, and the emmissivity of the respective coils. The
aluminum should ideally be black anodized, not shiny.
I suspect that in amateur use, conduction cooling (to air and the
chassis, through the mounting) is the dominant thermal path.
But yes.. the temperature rise will be less than sqrt(resistivity) - the
radiation loss goes as T^4 - the conducted loss goes as T - this is an
interesting physics problem
And definitely not shiny - there's a reason seat belt tabs and shiny
handrails get hot in the sun.
In any case, I suspect that in an antenna application aluminum would
work as well as copper, and if you got nervous, you could increase the
diameter of your tubing. And save money on the materials to boot, which
you will then spend on a TIG welder so you can make good connections to
the aluminum. ANd then a lot of aluminum scrap to practice welding on, etc.
I don't notice many hams building antennas out of copper (or wrought
silver) tubing. (except compact loops, which are high Q)
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