# Air cored coil - Low Inductance measurement

#### brianmk

Joined Dec 23, 2016
21
I want to make a simple single layer air cored coil:-
Inductance: 3.9uH. Former diameter: 16mm. Wire: 0.6mm enamelled copper.
Coil calculators on the web all give roughly the same information:-
18 turns should give about 3.7uH.

So I wound 18 turns on a 16mm hollow paxolin former.
I then tried to measure the inductance using a vintage Marconi TF2700 bridge. The lowest inductance range on the bridge is 10uH. I measured 1.35uH.
The bridge is quite old, so I to validate the measurement I tried measuring a commercial 4.7uH inductor. It measured 4.15uH which seems reasonable.

I'm baffled as to why the measured inductance of my coil is so far out.

#### ci139

Joined Jul 11, 2016
1,696
try them in series $$L_N=\sum_{i=1}^NL_i$$ . . . ..

#### brianmk

Joined Dec 23, 2016
21
The Marconi TF2700 normally operates using an internal 1kHz oscillator.
It includes the facility to plug in an external oscillator up to 20kHz.
When I tried that today, I found that if I use 20kHz, the measured inductance of my coil is now 3.9uH as expected.

I still don't understand this result.
I thought the frequency would only affect the coil losses (Q factor).
It shouldn't affect the inductance.

Why does the frequency affect the measurement of an air cored coil so much more than when I measure a commercially made choke coil with a similar inductance (4.7uH)? I'm guessing the choke has a ferrite core.
I can only assume it's down to deficiences within the bridge when measuring small inductances rather than the coil itself.

#### Papabravo

Joined Feb 24, 2006
14,664
The Marconi TF2700 normally operates using an internal 1kHz oscillator.
It includes the facility to plug in an external oscillator up to 20kHz.
When I tried that today, I found that if I use 20kHz, the measured inductance of my coil is now 3.9uH as expected.

I still don't understand this result.
I thought the frequency would only affect the coil losses (Q factor).
It shouldn't affect the inductance.

Why does the frequency affect the measurement of an air cored coil so much more than when I measure a commercially made choke coil with a similar inductance (4.7uH)? I'm guessing the choke has a ferrite core.
I can only assume it's down to deficiences within the bridge when measuring small inductances rather than the coil itself.
As the frequency increases, the impedance of the inductor increases while the impedance of the parasitic capacitor decreases, so at some high frequency the impedance of the capacitor is much lower than the impedance of the inductor, which means that your inductor behaves like a capacitor. You may or may not be experiencing this behavior. It normally occurs at much higher frequencies than 20 kHz. The only way to be sure is to put it on a VNA. You could also be measuring other parasitic effects of a non-ideal inductor.

The other test you could try is to measure the voltage drop and time rate of change of current. This ratio will pretty accurately tell you the practical inductance value.

#### ci139

Joined Jul 11, 2016
1,696
Last edited:

#### MrAl

Joined Jun 17, 2014
7,849
I want to make a simple single layer air cored coil:-
Inductance: 3.9uH. Former diameter: 16mm. Wire: 0.6mm enamelled copper.
Coil calculators on the web all give roughly the same information:-
18 turns should give about 3.7uH.

So I wound 18 turns on a 16mm hollow paxolin former.
I then tried to measure the inductance using a vintage Marconi TF2700 bridge. The lowest inductance range on the bridge is 10uH. I measured 1.35uH.
The bridge is quite old, so I to validate the measurement I tried measuring a commercial 4.7uH inductor. It measured 4.15uH which seems reasonable.

I'm baffled as to why the measured inductance of my coil is so far out.
Hi,

One of the things about old bridge type testers is that the phase angle of the measurement sometimes affects the measurement. The old one i used to have would show a very clear indication on the "eye" tube it used (electron ray indicator tube) to indicate a balance if the inductance was relatively pure, but would show a smear like indication if there was a phase shift. When there was a phase shift it was harder to read and be sure of the value.

With your inductor, 18 turns of 0.6mm wire on a 16mm diameter core i assume leads to an inductance of 3.9uH or thereabouts, but it also leads to a DC resistance that is TWICE that of the inductive reactance at 1kHz. At 20kHz, the DC resistance appears as roughly 1/9th (one ninth) of the inductive reactance. This would mean the measurement could be off at 1kHz because the bridge detector mechanism is having a hard time distinguishing the resistance from the reactance and using some amplitude that is a result of the combination of the two in order to determine the inductance.
The reason you may not see this behavior with the commercial inductor could be because the Q of the commercial inductor is much higher than the hand wound coil which at 1kHz has a fairly low Q.

To find out for sure, try to measure the DC resistance of the commercial coil if you can although it might be very very low though and so very hard to measure, but if you can you may find that the DC resistance of the commercial coil is lower than the hand wound coil and thus the commercial coil measures more accurately.

You could also try winding a second hand wound coil with heavier gauge wire, or maybe wind it bifilar. That will lower the DC resistance and thus may show a more accurate reading.

You could also test your existing coil with a range of frequencies, say 10kHz, and see if the measured inductance goes up smoothly with frequency. It would be interesting to see more readings at 2kHz, 5kHz, and 10kHz as well as the 1kHz and 20kHz.