I recall reading about someone who increased the inductance of an iron core solenoid by placing it in a box and covering with iron filings. I don't have access to iron filings to try this, but does this method have any basis in fact? IOW could the frequency response of a capacitor-tuned detector coil be lowered significantly by adding more inductance in this way?
David King
Sure, it will work. The key here, is that the 'solenoid' is a nonshielded inductor, and has field in the space outside the solenoid. A toroid or pot core inductor does NOT have any significant field leakage outside the core, so would not respond.
Hello David,
When it is a magnetically open construction with straight fat core (so not too large length/D ratio), the increase in inductance can be significant.
The theoretical basis lies in the overall magnetic resistance seen by the H-field. When adding the material, the field lines that normally would go through air, will now go through material with good magnetic permeability. This reduces the total magnetic resistance. Therefore a certain current through the winding will cause more flux through the core, hence more inductance.
There are two things to consider: By adding the iron shield, detection efficiency may reduce, as external applied field will not reach the solenoid,
when it is a HF detector, the added iron will loose all of its magnetic permeability because of eddy current. This may even reduce the inductance. At HF, magnetic field will not go through conductors (eddy current, skin depth issue).
In case of an MF, HF detector, you should use appropriate ferrite or powdered iron instead of normal iron or steel. Such material keeps its magnetic properties well into MHz frequencies.
Best regards,
Wim PA3DJS
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