Electronics > Projects, Designs, and Technical Stuff

Current Induced in a Coil of N Loops

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Chet T16:
I'm doing a presentation on RFID for university and i've been poking around my RFID reader and oscilloscope.

I decided to see if i could detect the alternating field by attaching the ground clip to the end of the probe and sure enough it worked. Going back to faraday i tried making an antenna and compared the voltages against the number of turns

voltage on the Y, turns on the X

So the theory tells us it should be proportional to N which obviously isn't the case here. I didn't have any thin wire so i used single core hookup wire. Is the non-linear result because of the thinkness of the wire or because the readers field isn't strong enough or both? Also do the loops need to be tight? i.e. together, not the radius

IanB:
I'm no EE, but with RF things aren't so simple. You are getting into antenna design theory, where resonance and capacitance effects come into play.

For instance if you place the turns of the coil close together you introduce some parasitic capacitance between adjacent turns that can have a negative impact on performance. That's why some RF coils are deliberately made with a spacing between adjacent turns rather than winding them next to each other.

Flip the graph horizontally, and you have a classic power transfer graph. You will remember that for maximum power transfer, the impedance of a load will equal the impedance of the source. You have found that the characteristic impedance is achieved at around 3 or 4 turns.

http://en.wikipedia.org/wiki/Maximum_power_transfer_theorem

slateraptor:

--- Quote from: Zad on November 12, 2011, 10:06:36 pm ---Flip the graph horizontally, and you have a classic power transfer graph. You will remember that for maximum power transfer, the impedance of a load will equal the impedance of the source. You have found that the characteristic impedance is achieved at around 3 or 4 turns.

http://en.wikipedia.org/wiki/Maximum_power_transfer_theorem

--- End quote ---

Nice observation. ;)

Chet T16:

--- Quote from: slateraptor on November 13, 2011, 10:54:59 am ---
Nice observation. ;)

--- End quote ---

Indeed!

I've flipped the graph now, it gives me something nice to point out.