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NFC field generator

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T3sl4co1l:
Taking NFC literally, no.  You can make an asymmetrical field, but you can't make a field stop arbitrarily (no matter how many times the textbooks draw diagrams as if you could).  To be exact, the field satisfies Laplace's equation, \$\nabla^2 \phi = 0\$.

A very large antenna (in human terms; a dozen meters or so) can have directional gain (i.e., better than dipole) in a propagating field, but that's not near-field.

The usual approach is to place a coil over a ferrite plate, which acts to short out the magnetic field lines on that side.  This reduces the volume of field (reducing the magnetic path length more than the effective area is reduced, hence the inductance goes up), giving less range on the open side, and higher mutual inductance up close.

The shield is as much practical as anything: it improves coupling between coil and target, and shields the circuitry underneath, allowing a compact build.

Tim

t1d:
That's a good explanation, Tim; thanks!

Berni:

--- Quote from: Ammar on December 16, 2019, 10:42:42 pm ---Only 10W??

This would really get things going: https://au.mouser.com/pdfdocs/IXYS_PRF-1150.pdf

Really depends what the op is after. By distance do they mean a cm or like 10 m?

--- End quote ---

Wow that is quite the powerful little RF amp.

I feel like the datasheet for that should include at least a few big red warning boxes around this monstrosity.

judge:

--- Quote from: T3sl4co1l on December 17, 2019, 12:09:30 am ---Switching times are limited by device capacitances alone -- RF amplifiers' bandwidth is subject to the same limit, but the center frequency is subject to more detailed characteristics (in this case, limited by the dropping gain / rising losses of the MOSFET with increasing frequency, and stray inductance affecting stability).

An oscillator with something like IRF530, at 13MHz, is pretty typical. :)  Get out some wire, some smaller capacitors, and you're good to go.  (Would recommend a Colpitts type.)

Tim

--- End quote ---

Fortunately bandwidth isn't something I need to be worried about. I assume. I wish I knew more about how these LED tags work. There's a coil and an LED of course, but there are some discretes on there too, and not just capacitors! It would be nice to know what sort of voltage a typical NFC reader shoves through its coil so I know what I am aiming for. And I am assuming voltage is important in this application because although LEDs are current-limited devices, they also need a minimum voltage to get them going. And, of course, I am assuming here that it is the 13.56MHz carrier that is lighting these things up.

Also, I read a lot about 'antenna tuning' for NFC readers, but my understanding is that these coils aren't antennas at all, just inductors, and I never read about 'inductor tuning', so I am curious as to what the deal is here. There is clearly more about getting the efficiency of these coils up, like apparently wanting the self-resonant frequency to be above the driving frequency, balancing range v. efficiency, high 'Q' v. bandwidth. It is hard to know which of these are relevant to my application, which has no requirement for bandwidth. Plenty to learn, and experiment with.

judge:

--- Quote from: Berni on December 17, 2019, 06:09:13 am ---
--- Quote from: Ammar on December 16, 2019, 10:42:42 pm ---Only 10W??

This would really get things going: https://au.mouser.com/pdfdocs/IXYS_PRF-1150.pdf

Really depends what the op is after. By distance do they mean a cm or like 10 m?

--- End quote ---

Wow that is quite the powerful little RF amp.

I feel like the datasheet for that should include at least a few big red warning boxes around this monstrosity.

--- End quote ---

I read elsewhere that 13.56MHz is used for more than just NFC. Something about welding and induction coils?

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