Jim,
Sounds like copper tubing (without cool-amp) is the way to go.
I am impressed with the JK coils. Solid aluminun rod and
welded connectors, very nice ! I assume the solid rod coils are pretty
rigid and
hold their shape and turn spacing nicely.
thanks,
Bob
K6UJ
On 1/4/16 7:17 AM, jimlux wrote:
On 1/4/16 6:24 AM, Peter Voelpel wrote:
With wider turn spacings capacity between turns is lower and coil Q
higher.
Centre loading is more sufficient then base loading on shortened
antennas.
Silver plating of coils with cool-amp is for the birds.
I'll agree with Peter here.. Skin depth at HF is large enough (about
20 microns, 0.0008" in copper at 10 MHz) that a thin layer of silver
isn't going to make very much difference. Silver is only 5% lower
resistivity, and skin depth goes as square root of resistivity, so
skin depth is pretty much the same for silver as copper.
Typical silver plating thickness runs from 1-2 micron (typical for
terminals and components which you want to solder to) to 40 microns
(wearing surface, like switch contacts).
Worse than that, usually, Silver is plated over Nickel over Copper.
Nickel is magnetic (mu > 50, as I recall) and fairly resistive (8.67
for Ni vs 1.67 for Cu and 1.58 for Silver). Many a microwave engineer
has been surprised by the loss of the plated waveguide being higher
than the unplated aluminum. Same for PCBs: the usual gold plating is
actually gold over nickel on the copper. Maybe 0.05 dB/inch increased
loss at 10 GHz. (I generally figure FR-4 with 1 oz copper traces is
about 1 dB/inch at 10 GHz, almost entirely due to the substrate)
http://www.taconic-add.com/pdf/technicalarticles--effectsofsurfacefinish.pdf
shows 0.8 dB/inch
There are silver plating processes directly onto copper, but they have
reliability and durability problems: it's hard to get the silver to
stick. There's also a diffusion effect over the long term, but I
suspect that for ham applications, this wouldn't be significant.
I think silver plating is chosen because it's easier to solder to,
looks nice, etc. I'll bet an inexpensive dinner that you couldn't
tell the difference in performance in an antenna application without
resorting to truly extreme measurement methods.
Jim
W6RMK
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