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Earlier, I asked a question about an induction loop being used to transmit Wi-Fi, or internet to anybody who was in the loop. I then realized that the loop could not pick up the data for uploads, because it is the big loop transmitting, and the small loop in the wi-fi device would not be able to send data back to the big loop.

I then commented

I also forgot about upload, I only thought of download!

But then someone commented back

skyler that's actually not so much of a problem, if you could solve all the others. If an antenna can transmit, it can usually receive as just as well.

How could the induction loop pick up the data from the small coil if it takes a huge loop around the house to send the data?

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  • \$\begingroup\$ I think WiFi requires bi-directional communication in any case - the sender sends a data packet, the receiver says "got it", sender sends another ... \$\endgroup\$ – Peter Bennett Jan 6 '14 at 17:34
  • \$\begingroup\$ It's not that it "takes" a large TX antenna to send and a small one to receive, but that the small antenna can be located anywhere within the large one. If both were small their '"fields"' may not overlap. \$\endgroup\$ – Russell McMahon Jan 6 '14 at 19:04
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It's called the principle of antenna reciprocity, which basically states that the signal transfer gain/loss from one antenna to another is the same as a transfer in the other direction.

In other words, it doesn't matter which antenna is transmitting — the amount of energy transferred to the other antenna will be the same.

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Imagine one big loop and one small loop with the small loop inside the big loop. The magnetic field from the big loop might be "X" with only a small fraction picked up by the small loop (say) X/1000. It's nearly the same in the other direction with one constraint; as the small transmitting loop moves towards the centre of the much bigger loop, nearly all the magnetic field produced from the small loop is cancelled out - the small loop produces magnetic flux within its own loop-area and this will always equal the flux lines returning (remember iron filings, paper and bar magnets) on the outside of the outside of the small coil - those two fluxes nearly cancel. They would totally cancel if the big loop was infinitely wide but, providing the loops are not totally-massively different in size, there will still be a net flux received up by the big loop from the smaller loop.

Things get better as the small loop moves off centre and moves towards the circumference of the bigger loop but, as the small loop passes over the big loop, there will be a total null i.e. nothing can either be sent or received - bear this in mind. This phenomenon is used in balanced-coil metal detectors - a big field is produced by one coil but the other coil only receives a signal when the magnetic-null is upset by localized ferromagnetic or conductive material.

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