[TowerTalk] Force 12 loading coils

[jimlux]

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