The near-field measurement was performed in the 200kHz-20MHz range and various ferrite plates were tested by placing between two loop probes.
Thanks Sofoklis. But this is an approximation law and only refers to DC-low frequencies. How about RF? Is it only a matter of the geometry of the shield?
The magnetic materials at DC and low frequency has very poor response at high frequency. As frequency increases, its behavior doesn't remain like magnetic material. For example, magnetic material having AL value say 1000 at 10 KHz, will not be useful at 1MHz. This at high frequency spectra it will have constant behavior
Also at high frequency, radiation is Electromgnetic field which predominates over magnetic field
In your test, the dominant shielding mechanism above 200 kHz is likely due to the eddy currents. In this case, the shielding is a stronger function of the conductivity of the material than of the permeability. An aluminum plate with no relative permeability would perform pretty well in the test you described.
Dear Mr. Amipara, the response was not poor. Actually, 15-20dB power attenuation was achieved. But this attenuation is similar for all materials under test.
Dear Mr. Hubing, I think the electrical resistivity is too high to blame eddy currents for this attenuation. Handbooks and theory describe the effect of magnetic losses on shielding as well, that's why I put permeability in discussion.
If the resistivity is high, this is not a good choice of material to shield magnetic fields above 200 kHz. Nevertheless, that doesn't answer your original question. If I understood your original question correctly, you have data that describes the permeability of these materials between 200 kHz and 2 MHz, but that data does not correlate to the results of your measurements with the two loop probes. Is that correct? If so, how was the permeability data obtained?
Exactly. That is my query. The permeability data were taken from similarly processed ring samples.
Since you are seeing 15 - 20 dB of attenuation, you are already capturing 80 - 90% of the magnetic flux and redirecting it before it passes through the receiving loop. This suggests you already have a pretty high relative permeability. Increasing the permeability will result in diminishing returns (e.g. you should not expect a doubling of permeability to double the measured attenuation). Note that even with infinite permeability, there would be measurable coupling between the two loops.
Yes. Permeability ranges from 200 to 1700 in these materials, but attenuation remains high even when permeability drops and losses rise (resonance).
Dear Sir,
no, not all ferrite with different permeability in the dependence frequency range have the same shielding effects.
It is a difference if you use a anisotroic ferrite and a isotropic ferrite. These ferrites have different results in the shielding effect over the frequency range.
Yours Sincerely
F. Gräbner
Dear Mr Grabner,
Thank you for your response. In my tests all ferrites are isotropic. I also want to clarify that this is a measurement in the near field of loop probes.
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