[Antennas] Stainless Steel Conduits

Fri Mar 13 19:11:04 EDT 2015

On 3/12/2015 6:33 PM, charles berry via Antennas wrote:
> Is Steel Conduit suitable for J-Poles and Active Booms? The price of copper has made it extremely problematic in building antennas. How is steel's velocity factor different than copper?
> Thanks,Chuck BerryN7CHS
>
Much confusion posted here. Let's clear this up.
The web page cited is full of errors and misleading
commentary. Pure marketing, not science.
Aluminum is the tried and true material for building
antennas. Its RF resistance is only 25% greater than
copper, and in nearly all practical cases has
negligible effect. The RF resistance of materials
scales as the square root of the DC resistance, which
will come as news to the author of the web page.
Copper is expensive, heavy, corrodes to lossy copper
oxide, is structurally weak, and is difficult to
machine unless alloyed with tellurium, which lowers
the conductivity. Any alloying metal that makes the
copper into a brass or bronze lowers the conductivity
to below that of aluminum. I don't know that copper
water pipe is pure copper rather than an alloy.
In conclusion, there is virtually
no reason to consider copper for the vast majority
of antennas. About the only thing copper really has going
for it is that it can be soldered.
I don't know what the title of this posting:
"Stainless Steel Conduits" is referring to; they don't exist.
OTOH, (conventional) steel conduits are galvanized,
so that for RF purposes they are essentially zinc.
The DC conductivity of zinc is 27% of copper.
Therefore, galvanized conduit will have
about twice the RF resistance compared to copper.
Thus if you really want to use galvanized conduit
or pipe for antennas, the additional RF resistance
is probably not a show stopper in most cases.
Now if you did try to use a stainless steel tube for
RF purposes, that is a problem. Not only is the DC
resistance much higher than aluminum, but some SS is
magnetic, which makes it even lossier at RF.
I had a client once who wanted to use some nickel
tubes to make a transmission line. (I know, dumb
idea, but the client (who has money) is always right.)
I measured the RF resistance of the nickel and found
that it was much higher than would be predicted by
DC resistance. This indicated that the magnetic
permeability was a big factor. Stainless steel has
both nickel and chromium in it.
You do see stainless steel CB whips (102 inch) which
have to be made of SS for mechanical reasons. You
just have to live with the extra RF loss. Mobile
installations have many other sources of loss, so
in the overall scheme of things, the SS whip is not
so bad. But it is a special case. I love my 4 inch
Hi-Q screwdriver with a 102 inch whip on top.
Finally, velocity factor is a property of dielectrics
not conductors. A stainless steel CB whip resonates
at 27 MHz, just like it would if it were made of copper.
Rick N6RK