[TowerTalk] hose clamps and end caps

[Jim Lux]

donovanf at starpower.net wrote:
> Oops...
> 
> While stainless steel screws may perform useful mechanical functions
> if applied correctly, they can NOT perform electrical functions at RF
> because of skin effect.
> 
> 73
> Frank
> W3LPL


Oh.. I would think it more depends on what sort of electrical function 
you're worried about.  SS isn't a great conductor, it's true, but, OTOH, 
the RF resistance of a clamp is still going to be a pretty small number.

(SS316 is 70-80 microohm-cm, Al is 2.65 microohm-cm

Skin depth at 14 MHz for Al is about 22 microns (0.86 mil)
for SS 316, about 120 microns, 4.7 mils (assuming it's non-magnetic, 
which it's not, but I don't have the data handy)

So, considering a large (compared to skin depth) tube, the conductive 
layer is about 5-6 times thicker for SS than for Al, which will help 
make up for the 30:1 resistivity ratio..  The net effect is that a SS 
tube would be about 6 times the resistance of an aluminum one.

However, the resistance of a 1" diameter aluminum tube (at 14 MHz) is 
about 0.2 milliohms per centimeter of length. SS would be roughly 5 
times that: 1 mOhm/cm.   A clamp (which might be a cm "thick") isn't 
going to do that much....


I'd worry more about things like corrosion on the surfaces involved.

(FWIW, the resistivity difference between SS and Copper does get 
important when you talk about large spans.. say you're building a 30 
foot long dipole from AWG 10 wire (10 gauge just because the math is 
easy.. 0.1" diameter.. 2 mOhm/cm for Cu, 10 mOhm/cm for SS.. 10 meters 
is 1000 cm, so 2 ohms vs 10 ohms...That's a fairly significant 
difference on an antenna with a feedpoint impedance of 70 ohms)

(Apologies in advance if I screwed up the math...
A handy skin depth calculator can be found on the microwaves101 website..
http://www.microwaves101.com/)


Jim, W6RMK