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I am measuring magnetic fields from 50kHz to 500kHz with this search coil (air coil.) It is a wound of bell wire from the hardware store connected to an oscilloscope probe tip and ground:

wounded bell wire from hardware store

I have built a new one with a ferrite core Fair-Rite 2631814002 with 1500 relative magnetic permeability for enhanced sensivity (with approximatley the same length of wire):

approximatley the same length of wire

The ferrite core search coil isn't sensitive. In a situation where the air coil picks up 35dbU above noise 50kHz to 500kHz, the ferrite core search coil doesn't pick up anything.

Why?

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  • \$\begingroup\$ For the coil to pick up anything the magnetic flux in the core at any instant would have to be greater in one direction than in the other. Why do you think it would be? \$\endgroup\$
    – Transistor
    Commented Sep 5, 2020 at 15:28
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    \$\begingroup\$ The reason ferrite toroids are used is typically because they do not leak RF Fields. So using that same logic they will not absorb any either. The only field the wire really sees is whatever is flowing through the ferrite, and the only way to have something flowing through the ferrite would usually be to induce it via wire coiled around it. \$\endgroup\$
    – MadHatter
    Commented Sep 5, 2020 at 15:47
  • \$\begingroup\$ Instead of making me better coil, you're probably better off improving whatever analog amplifier you're using to get a signal out of that coil... \$\endgroup\$
    – MadHatter
    Commented Sep 5, 2020 at 15:51
  • \$\begingroup\$ Try a ferrite rod instead. They are commonly used to pick up electroagnetic fields : like AM radio. \$\endgroup\$
    – user16324
    Commented Sep 5, 2020 at 20:52

1 Answer 1

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No it won't pick anything up as you saw. In any direction and, in any prevailing magnetic field, the lines of flux will concentrate in two halves of the ferrite in the same direction and, you will get cancelling voltages induced: -

enter image description here

Thus, flux entering from the left will congregate in the shorter paths and use the ferrite core to do so. This will produce equal but opposite voltages in the turns around the top compared to the turns around the bottom.

Flux will exit and start to spread out again but, there will be no net signal in your receiver coil. Ideally, you need something like this to enhance the flux and voltage sensitivity: -

enter image description here

The big red centre is a piece of solid ferrite but, you could use your toroid to prove this.

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  • \$\begingroup\$ Andy, don't some toroids act as a 1-turn sensor? where changing flux through the area give a current signal in the wrapped wire? That would be perpendicular to the flux path in your diagram. Very insensitive, especially for ferrites that are mostly non-conductive. \$\endgroup\$
    – glen_geek
    Commented Sep 5, 2020 at 16:15
  • \$\begingroup\$ @glen_geek I can't visualize what you mean. \$\endgroup\$
    – Andy aka
    Commented Sep 5, 2020 at 16:28
  • \$\begingroup\$ well its a moot point - the resistivity of OP's ferrite is 3kohm-cm (fairly high). So not much current flowing around that one-turn ferrite ring. It would be a very insensitive sensor. \$\endgroup\$
    – glen_geek
    Commented Sep 5, 2020 at 17:02
  • \$\begingroup\$ I have proven your answer, Andy. I have only used one half of my wiring on one half of the toroid and it works. Isn't this sensitive though, I will definitely go for a solid ferrite. I have put all my 3 suitable toroids in the middle of my air coil and it raised the signal by 4dB. \$\endgroup\$ Commented Sep 6, 2020 at 7:46

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