Hi Wavewatcher,
I managed to access to an old inductance meter that can measure the Q factor of the coil too.
(I refer to my earlier post here I quoted below.)
I used a small multilayer, multiturn air core coil, measured 3.6mH at the 37.6kHz measuring frequency.
The Q measured about 47. (The L meter forms a calibrated oscillator from the coil to be measured,
oscillating frequency always depends on the actual value of the coil with this meter.)
I placed a cylinder Neo magnet to one of the coil ends and noticed the inductance slightly decreased,
maybe a few tens of uH, judged from the mechanical scale. The Q went down to about 38.
I managed to reach about the same reducement in inductance with a bronze screw when I inserted it
about halfway into the air coil (bronze does not attract to magnets) but the Q reduced to about 43.
(The size of the bronze screw was OD=3mm, length=20mm.)
Next I placed a rectangular ceramic magnet to the same end of the air coil. The inductance increased
maybe a few uH, yes, increased. The Q stayed at about the same value, 47, it may have changed very little
upwards but on the mechanical scale it could not be read.
I think the Neo magnet reduced the inductance mainly due to its metal body presence just like the
bronze screw did: due to the eddy current induced in the metal by the coil's field.
It seems the eddy current effect is stronger than the very slightly higher than one permeability effect of
the Neo material's small Fe content.
Ceramic magnets are basically (saturated) ferrite materials, no or only very little eddy currents, so the
Fe content can have a slight increasing effect on inductance.
Notice: I always used the same pole in case of both types of magnets and changing the poles did not
make any difference in the above results.
rgds, Gyula
Hi WaveWatcher,
Thank you for clarifying my questions. In the next couple of weeks I will have access to a Q meter and can check how the Q of an air core coil may change when a strong Neo magnet is inserted or brought very near to it. Will return to this then.
You are right, the inductance decreases when a copper slug is inserted into it (I erred) and the explanation is that copper is diamagnetic and has a relative permeability of just below 1: ur= .999994
For aluminum which is paramagnetic, the ur=1.000022
I mainly found Alu cores in the oscillators of Philips FM tuners of table, portable and car radio sets (manufactured mainly for the European market in the 70's and 80's).
Thanks, Gyula