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In a video about ferrite rod antennas it was stated that the antenna is most sensitive to the direction perpendicular to the axis of the rod, and it is insensitive along the axis. These are shown with the green markers in the screenshot below. I found it surprising, because the induced voltage is equal to the change in the magnetic flux, which goes through the loop (the scalar product of the magnetic flux with the normal vector of the "surface"). Where is the mistake? Which argument is the correct one?

All the literature I've found was either very high level, or formulate the explanation in a very scientific, complex, mathematical way, with lots of backreferences, which I was not able to fully trace back and understand.

screenshot

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  • \$\begingroup\$ quora.com/… \$\endgroup\$
    – Antonio51
    Commented Jan 24, 2022 at 8:36
  • \$\begingroup\$ @Antonio51 I am not sure how the link is relevant, since the question was about small loop antennas, not about a straight conductor. Could you elaborate it? \$\endgroup\$ Commented Jan 24, 2022 at 8:42
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    \$\begingroup\$ For the "navigation" with the radio-compass, we also use, as a reference, a vertical antenna to "remove doubt". \$\endgroup\$
    – Antonio51
    Commented Jan 24, 2022 at 9:11
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    \$\begingroup\$ A propagating radio wave consists of electric and magnetic field vectors, both perpendicular to the direction of travel. The small ferrite/coil antenna is most sensitive to magnetic fields along the ferrite rod's axis, therefore it's most sensitive to waves arriving from directions perpendicular to the rod's axis. \$\endgroup\$ Commented Jan 24, 2022 at 16:53
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    \$\begingroup\$ Yes. Just so. Remember that it is in "far-field approximation", in "near-field", it is a bit complicated. \$\endgroup\$
    – Antonio51
    Commented Jan 24, 2022 at 21:39

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For help understanding, one can find here some good simulations about antenna (transmitting)... and others things.

http://amanogawa.com/archive/Antenna1/Antenna1-2.html

enter image description here

One can see that the H vector is "circular" around the antenna.

So, for receiving something, "ferrite rod" must be "perpendicular" to the "direction line" of the emitter.

NB: This is the reason why two inductors that must not be "coupled" magnetically are arranged perpendicular to each other.

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