Air coil versus straight wire

A straight wire of 1mm, 16cm makes approximately the same nH inductance as an air coil of 25mm diameter and 2 windings with the same length of wire. The inductance rises with square of the winding number and of course with winding.

This is funny.

I found the same results on other applications but I dont believe.

The calculator does not include the length of the coil.

I dont critisize, I am curious.

Please - I am curious abt. correct values of the inner resistance of an air transformer, that has best possible coupled primary to secondary wires but seems to have a very high inner resistance at VHF and UHF. In Power electric we say it has zero short circuit current.

Hi Franz,

Why cant i get the same result basing from this online calculator?

1. Air Core Inductor Inductance Calculator
2. Straight Wire Inductor Calculator


Thanks,

Thanks for reply, Emmanuel.

The coil with 2 windings 1 inch diameter (approximately 25mm) and 0.4 inch long coil gives 118 nH result.

The straight line with 6.3 inch wire length (the same as in the coil) and 0.02 inch wire radius (1mm diameter) results in 182 nH.

I am amused about the result that the coil with 2 windings makes less inductance than the straight wire.

I dont believe it. *I dont believe the formula.*

73 de Franz OE5FGL

Firstly, how good is your inductance measurement, it takes a lot of care, and a very good piece of instrumentation to measure low inductance's. Secondly, most of the formula for calculating low values of inductance, where mutual inductance starts having an effect, as in a loop, are empirical, and may be somewhat inaccurate,
Cheers,
Richard

Oh i see now., thanks for the enlightenment.

Hello Richard,

I dont measure.

I talk about the 2 calculator applications (here in EEWEB and in similar application progams with similar results) with the same wire straight and compared in a coil with 2 windings.

I cannot believe that the coil has less inductance than the straight wire.


73, Franz

A straight wire by itself has no inductance, it only exists if there is a loop, a return path for the current flow, this could be infinitely large, or quite small, depending on how it is defined and measured. Therefore a calculator for a straight wire has to make an assumption with regards to where the return path is. If it is on a PCB, for example, as a trace, then the loop size may be defined. For a coil of a few turns, the loop size is more obviously defined. My point about measuring inductance, is that with a piece of wire, the return path for the current is ill defined, and often determined by the measurement technique. In the case of coils, depending on the geometry, the inductance can vary widely, since the mutual inductance is determined by this. Take for example the difference between a long solenoid and large diameter loop (wire diameter

Inductance has nothing to do with "a return path for the current flow, "

Inductance has to with the fact that the magnetic flux produced by each turn in a coil cuts the other turns and produces a voltage in the opposite direction.

That's why inductance does not really start until there is two turns.

Hello All, Air core coil describe an inductor that does not depend upon a ferromagnetic material to achieve its specified inductance. This covers the cases where there really is just air inside as well as windings upon a different insulator such as bakelite, glass or PTFE etc. 

Richard, thank you for the link, I have been battling to find the formula for mutual inductance on parallel wires for some time now! The original question is about partial inductance and I am also trying to fathom why the "calculators" are giving higher values for straight wires of equivalent length to coiled wires. Something does not make sense.