Topband: Stainless Steel for coil taps.

[Richard (Rick) Karlquist]

I need to correct some misconceptions:

On 11/16/2020 6:24 AM, Tim Shoppa wrote:
> Conductivity of Copper: 5.85 x 10^7 mho/m
> 
> Conductivity of 301 Stainless: 0.14 x 10^ mho/m
> 
> Stainless is 42 times worse conductor than copper.
> 

1.  Because of skin effect, the surface resistance is
inversely proportional to the SQUARE ROOT of the conductance
at RF.  There, in the calculation above, the square root
of 42 should have been used.

2.  But that would still be incorrect because any magnetic
material will have additional loss at RF above and beyond
mere RF skin effect.  This loss is difficult to quantify
but should be assumed to be substantial.

[BTW, does anyone know if so-called "non magnetic" stainless
steel is free of this effect at RF by virtue of not being
attracted to a magnet?]

There was also some mention to the effect that nickel was
a "non-ferrous" metal and therefore would be good at RF.
Nothing could be further from the truth.  Although technically
"non-ferrous", nickel is nevertheless "ferromagnetic".
I once measured some nickel strap material for RF loss
and it was horrific.  Much worse than even what its DC
resistivity would suggest.

An additional factor with various stainless steels is that
some alloys naturally form an oxide layer, along the lines
of aluminum.  I can easily imagine wrapping a stainless
clamp around copper tubing and have high contact resistance
on account  of this.  This is no mechanism for puncturing
the oxide and forming a gas tight seal like you would have
in a crimped connection.  IMHO, this is the more likely
reason for the pyrotechnics.  My conclusion is that the
technique is in the "too clever by half" category.

73

Rick N6RK