# making a 0.01 mH

Discussion in 'The Projects Forum' started by tune, Nov 6, 2009.

1. ### tune Thread Starter New Member

Oct 30, 2009
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i intend to use a o.o1 mH for my oscillator circuit which transmits at 160.7 MHz what are the parameters which l need to consider

Apr 5, 2008
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3. ### Ron H AAC Fanatic!

Apr 14, 2005
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I hope this is used as an RF choke. If it is in the tank (resonant) circuit, the value is much too high.

4. ### KL7AJ AAC Fanatic!

Nov 4, 2008
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He he~! Yeah, I hope he means uH too.!

Eric

5. ### Ron H AAC Fanatic!

Apr 14, 2005
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Yeah, even as an RF choke, it's too high. It would be operating well above self-resonance.

6. ### SgtWookie Expert

Jul 17, 2007
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Get a cheap Bic pen. Take the ink tube out using a pair of pliers or dikes.
These pen barrels measure 0.330" in diameter.

Wind 28 gauge magnet wire on the pen barrel in a single layer, tightly spaced, 18 turns. Apply a little Superglue or the like to keep it from spreading apart/opening up. Then on top of those 18 turns, wind another 17 turns. The finished coil should be 0.230" long, and measure about 9.98uH. You can leave the pen barrel inside the inductor.

7. ### steveb Senior Member

Jul 3, 2008
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Can you double check your math? I get about 1 uH for that design. One of us lost a decimal place.

8. ### SgtWookie Expert

Jul 17, 2007
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Yes, you did.

I've just checked using a few online multilayer air coil calculators. Seems that there is a difference in opinion between myself and several calculators about the physical diameter of AWG 28 magnet wire; all somewhat different, likely due to the thickness of insulation. This of course affects the calculation results. One site thought 37 turns were needed, another calculated 39 turns. I don't happen to have any AWG 28 magnet wire on hand.

Of course, different gauge magnet wire could be substituted, or a different size core, but the turns/layers would have to be re-calculated.

9. ### steveb Senior Member

Jul 3, 2008
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Yep, I did. Sorry about that. I used a metric version of the Wheeler formula and must have screwed up the inch to meter conversions somehow. That coil size struck me as being too small for 10 uH, but my intuition is just wrong.

10. ### Xantor123 New Member

Nov 1, 2009
4
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If it's for the tank circuit of a 160MHz osc, 0.01uH would be more likely, let's say 4-5 spaced windings of silver coated copper wire, airform.

11. ### SgtWookie Expert

Jul 17, 2007
22,201
1,806
Actually, for a 0.01uH (10nH) inductor, 1 turn of AWG 22 around an 0.200" form would do it.

12. ### SgtWookie Expert

Jul 17, 2007
22,201
1,806
Wheeler's formula won't work for multilayer coils, which is what I was describing (a 2-layer coil). Wheeler's is pretty handy for single-layer coils though.

13. ### steveb Senior Member

Jul 3, 2008
2,432
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It provides a reasonably estimate for a 2 layer coil. In this case we calculate about 8.6 uH. I was just trying to get an order of magnitude estimate to check my intuition that the coil seemed too small.

Of course, both my intuition and my calculation were wrong. I usually make mistakes in pairs, which really gets me into some trouble.

14. ### SgtWookie Expert

Jul 17, 2007
22,201
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I hear you about the mistakes in pairs.

Fiddling around a bit more with my spreadsheet, two turns of AWG 24 wire around a 0.070" form (a #50 drill bit would be perfect) works out to 9.98nH.