On 3/1/19 9:49 AM, Jim Thomson wrote:
From: jimlux <jimlux@earthlink.net>
To: towertalk@contesting.com
Subject: Re: [TowerTalk] OT: Inductor Calculator
< And how to get something silver plated once out of high school
<chemistry class I don't know?
< Silver plating doesn't affect the RF resistance very much unless the
<silver is VERY thick - particularly at 2 MHz.
<Copper skin depth at 2 MHz is 1.82 mils (46.1 microns)
<Silver skin depth at 2 Mhz is 1.77 mils (44.9 micron)
<Resistivity is 1.59 for silver, 1.68 for copper
<QQ-S-365, ASTM B700 silver plating thickness is .05 mil, 13 microns
<Complicating this is there's usually a nickel flash/strike plate under
<the silver, to prevent silver/copper migration, and nickel is magnetic
<and lossy.
<But for low frequencies, you're probably better off just making your
<conductor 6% bigger in diameter and that will provide lower overall AC
<resistance.
## personally, I hate the look of bare copper, drives me nuts...so I silver
plate all my cu strap, coils, etc, etc.... with the ..cool amp goop from the
cool amp company in Ore. I also use it on brass machine screws like
10-32, and .25-20, and also phosphor bronze etc. Also used it on contactors
and relays, fuse holders etc, and dc resistance drops like a rock.
Silver plating is much easier to solder than copper
## 44.9 microns = 0.001767717 inches. Im sure I can pile on at least
.001 inch..or more of silver plating..esp with multiple applications.
The paper that was linked by another list member comments that plating
conductivity is often (much) lower than the plain metal, because of
stuff added to make the plating actually work.
And you really need 5 skin depths - 1 skin depth only gets the first 37%
of the current - 5 skin depths is >99%.
There's a whole raft of icky things too about silver migration, weird
alloys, and so forth.
## what about freqs higher than 2 mhz ??
skin depth goes as the square root - 20 MHz is about 1/3 the thickness.
When you get to microwave frequencies, or even VHF, UHF plating starts
to make sense for things like resonant cavities (which are high Q).
There's a whole bunch of folks trying to make resonant cavities &
filters with high temperature superconductors plated on the inside.
Liquid nitrogen temps are fairly straightforward.
A .25 inch OD tubing coil will handle
41 % more current, due to skin effect, on 7 mhz... vs the same .25 inch OD
tubing coil
on 14 mhz. The same .25 inch OD tubing coil on 1.8 mhz will handle a
whopping 2.82
X more current... vs the same .25 inch OD tubing coil on 14.350 mhz.....again
due to
skin effect. It goes to the sq rt of the ratio of the freqs.
I don't know about "handle more current" - the RF resistance goes up as
the square root of frequency, so the dissipation goes up as sqrt(f) too.
## re Aluminum vs copper. You require 58-60 % more cross sectional
surface area
if Aluminum is used vs copper. Again, taking skin effect into account,
the tubing used
could be paper thin...like aprx .00176 inches thick..on 2 mhz. The only
way to increase the
puny cross sectional surface area of the tubing is to increase its
circumference by 58-60 %.
And since circumference is directly proportional to diameter, the diameter has
to be
increased by 58-60 %. IE instead of say using .25 cu tubing, you would
have to use .4
Al tubing. Closet thing to that would be .375 al tubing. If instead of
using say .375 cu tubing,
you would have to use .6 inch Al tubing. Closest thing to that would be
.625 Al tubing.
You can also do similar calcs for solid copper wire vs solid AL wire.
Same deal, the AL wire
has to have its diameter increased by 58-60%.
Sure - it's a classic engineering tradeoff - if you look at a dollars
per dissipated watt sort of metric, it takes 60% more aluminum than
copper to get the same resistance, but copper costs a lot more than 60%
more than aluminum. So if you've got room, it's cheaper to make it
bigger out of aluminum. And, in self supporting antennas, aluminum is a
better structural material than copper<grin>
I might note that you shouldn't build antennas out of stainless steel or
titanium. Titanium has a great strength to weight ratio, and is fairly
inexpensive, but is a terrible RF material.
## I have run loads of tests to verify the above over the last 10 years, and
also again recently.
But you also dont want to lose it all by using lousy connections to the given
coil...or using puny gauge
wire to make taps on a tubing coil. I use cu strap for connections to
tubing coils..or other RF
connections and terminations. Strap is unique in that it conducts RF current
on both sides of the strap.
Tubing only conducts RF on the outside. .375 wide cu strap is aprx the eq
of .25 tubing.
And, because the skin depth is so shallow, you can use *really thin*
strap - it becomes more of a mechanical issue than a electrical one.
Lots of high performance inductors are wound with tape, not round
conductors for all sorts of reasons. Tubing *is* nice for HV - edges
are bad.
I think my take home is that silver plating is nice for appearance and
solderability, but probably not worth it for antenna components at HF,
and maybe not worth it for high Q resonant circuits (you'd have to
analyze it).
For a lot of things, whether to use copper or aluminum is going to be
determined by mechanical considerations (or convenience... If you've
already got 10,000 ft of copper or aluminum wire, you'll tend to use it)
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