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If we measure AC resistance of a coil using an LCR meter (or impedance analyser), does it include both ohmic resistance and radiation resistance? I understand that the radiation resistance is negligibly small for coil at low frequencies, however, for example, if we submerge a coil underwater, radiation resistance can be comparable to ohmic resistance.

I am measuring resistance of a coil underwater and I noticed a significant increase in resistance compared to the air surrounding. I wanted to confirm that the increase is due to the increase of radiation resistance. Maximum frequency is 1 MHz, and coil self resonance is way above the measurement range - we can safely neglect the effect of parasitic capacitance.

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I am measuring resistance of a coil underwater and I noticed a significant increase in resistance compared to the air surrounding. I wanted to confirm that the increase is due to the increase of radiation resistance.

Water conducts quite well and of course sea water conducts a lot better. What you are seeing is the effect of induced eddy currents (electrical) in the water. Literally, your coil creates a changing magnetic field and this induces an electric field in the water and the water acts as a resistor and electrical currents are produced.

It has nothing to do with radiation resistance - radiation resistance is a phenomena associated with the transmission of EM power through a medium. What you are seeing is a resistance due to near-field effects and eddy currents in the water.

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  • \$\begingroup\$ Thanks, Isn't those eddy current losses correspond to radiation resistance? Isn't it still a "transmission of EM power through a medium" regardless of near field or far field? @tomnexus \$\endgroup\$ – Pojj Jul 8 '18 at 20:48
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    \$\begingroup\$ No it’s different to radiation resistance. Proper rad resistance involves permeability and permittivity of the medium aka magnetic and electric field constants. Eddy current losses are due only the magnetic field interactions. \$\endgroup\$ – Andy aka Jul 8 '18 at 21:23
  • \$\begingroup\$ Is there any way to decouple these three resistance effects? Can we assume copper losses to be same as it in air? Then can we decouple radiation resistance and eddy current losses in the media? I have seeen in literature that radiation resistance increases in water significantly - any thoughts about how can we measure them? \$\endgroup\$ – Pojj Jul 9 '18 at 6:35
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    \$\begingroup\$ Rad resistance in water decreases because the permittivity of water is a lot higher than that of air/vacuum (Link). The impedance of a media is \$\sqrt{\dfrac{\mu}{\epsilon}}\$. So if permittivity (\$\epsilon\$) increases as it does in water, impedance of the media drops and that impedance, when projected to the terminals of an antenna, also drops. \$\endgroup\$ – Andy aka Jul 9 '18 at 9:15
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Yes, radiation resistance is indistinguishable from other impedance, it appears at the terminals of the coil/antenna.

But watch out - at high frequencies, the impedance and radiation resistance of the loop created by the wires from meter to coil could be significant compared to that of the coil itself. Once you are in the regime of measuring antenna properties, the entire circuit is an antenna. Careful comparison of a circuit with and without the coil might help.

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  • \$\begingroup\$ Thanks, I am not particularly interested in high frequencies - max up to 1 MHz. Electrical length of the coil will still very small. Instead I am measuring resistance of a coil underwater and I noticed a significant increase in resistance compared to the air. I wanted to confirm that the increase is due to the increase of radiation resistance. \$\endgroup\$ – Pojj Jul 6 '18 at 6:45

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