On 2/27/19 11:35 AM, terry burge wrote:
Hey folks,
I look at these coil calculators and come to the only conclusion is the wrong
question is being answered. Example:
If I have 130 feet of 12 gauge multi-strand insulated house wire (from Lowes) how
large of 3-4" coil wound on a PVC form do I need to resonate it at 1830Khz?
1850Khz? 1880Khz? 1920Khz?
Or using a commercial coil from what I believe was an AM broadcast station like
pictured on my qrz page? (qrz/KI7M)
Or if you are real ambitious using some small Aluminum tubing from
DXEngineering or similar source?
These coil formulas always seem to want some factors that may not be known about using
the available material like 1/4" or 1/8" copper tubing. Or number 8 gage copper
I see in the local Lowes or Home Depot.
The effect of conductor diameter on inductance is small - I would
suspect that in a matching transformer/loading inductor sort of
application it would be overwhelmed by things like the inductance of the
wires connecting the inductor in the circuit.
Simple formulas like Wheeler assume an infinitely thin wire at the
center of the conductor.
The dominant effect in a close-wound coil is to "squeeze" the current to
the surface of the coil, but that doesn't change the inductance very
much, although it does change the AC resistance. When looking at the
turn to turn mutual inductance, a closewound coil starts to look more
like a pair of turns (in parallel) at the inner and outer surface, and
if the conductor is sufficiently large in diameter, relative to the
spacing you wind up with 4 terms to calculate:
Inner (turn 1) - Inner (turn 2)
Inner (turn 1) - outer (turn 2)
outer (turn 1) - inner (turn 2)
outer (turn 1) - outer (turn 2)
The distance of each of these is either s or sqrt(s^2+dw^2), where s is
the turn spacing and dw is the diameter of the wire.
Just because you can calculate more precisely doesn't mean you *should*.
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.
Maybe you might want to put a silver plating on the inside of Aluminum
waveguide at high frequencies.
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.
Just a thought or two.
Terry
KI7M
On February 27, 2019 at 7:26 AM "Richard (Rick) Karlquist"
<richard@karlquist.com> wrote:
This may be helpful:
http://hamwaves.com/antennas/inductance.html
Rick N6RK
On 2/26/2019 9:06 PM, Gedas wrote:
Once upon a time I found a site that had an awesome online calculator
for calculating the inductance of single layer coils where you could
Gedas, W8BYA
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