More about air core inductors
What is an air core inductor?
An "air core inductor" is an inductor that
does not depend upon a ferromagnetic material to achieve
its specified inductance. Some inductors are wound without
a bobbin and just air as the core. Some others are wound
on a bobbin made of bakelite, plastic, ceramic etc.
Advantages of an air core coil:
Its inductance is unaffected by the current it carries.
This contrasts with the situation with coils using ferromagnetic
cores whose inductance tends to reach a peak at moderate
field strengths before dropping towards zero as saturation
approaches. Sometimes non-linearity in the magnetization
curve can be tolerated; for example in switching power
supplies and in some switching topologies this is an
In circuits such as audio cross over filters in hi-fi
speaker systems you must avoid distortion; then an air
coil is a good choice. Most radio transmitters rely
on air coils to prevent the production of harmonics.
Air coils are also free of the "iron losses"
which a problem with ferromagnetic cores. As frequency
is increased this advantage becomes progressively more
important. You obtain better Q-factor, greater efficiency,
greater power handling, and less distortion.
Lastly, air coils can be designed to perform at frequencies
as high as 1 Ghz. Most ferromagnetic cores tend to be
rather lossy above 100 MHz.
And the "downside":
Without a high permeability core you must have more
and/or larger turns to achieve a given inductance value.
More turns means larger coils, lower self-resonance
due to higher interwinding capacitance and higher copper
loss. At higher frequencies you generally don't need
high inductance, so this is then less of a problem.
Greater stray field radiation and pickup:
With the closed magnetic paths used in cored inductors
radiation is much less serious. As the diameter increases
towards a wavelength (lambda = c / f), loss due to electromagnetic
radiation will become significant. You may be able to
reduce this problem by enclosing the coil in a screen,
or by mounting it at right angles to other coils it
may be coupling with.
You may be using an air cored coil not because you require
a circuit element with a specific inductance per se
but because your coil is used as a proximity sensor,
loop antenna, induction heater, Tesla coil, electromagnet,
magnetometer head or deflection yoke etc. Then an external
radiated field may be what you want.