NFC antenna connection to ground midway through loop

Question

I'm designing a PCB etched NFC antenna. I'm confused about a design element I've seen in several production boards. I haven't seen this element explicitly discussed in any app notes or design guides.

On many of these nfc readers, there's a connection to ground on one of the loops in the antenna. On the pictured board, the next-to-outermost loop has a ground connection. What is this? In antenna design calculators basic loops are always implied, and I'm hoping to use these calculators to assist in my design. How will this ground connection affect my calculations for the matching circuit, etc?

Answer 1

From AN1445 - Antenna desugn guide for MFRC52x, PN51x and PN53x

Antenna Symmetry

The symmetry in antenna design is absolutely necessary with respect to tuning and EMC behavior. Otherwise common mode currents are generated to parasitic capacitances from the antenna to ground. Theses currents can cause emissions that hurt EMV regulations

It can be seen that the center tap of the antenna is connected to ground. Basically. we do not recommend grounding the center tap, but leave it floating. This has the advantage of a virtual ground point which is floating to achieve symmetry of the antenna.

Odds are the antenna is radiating noise to the surrounding environment (as shown in the picture), so they ground the virtual ground point to reduce electromagnetic emissions. They are releasing a demo board with no concern for EMC emissions. It is a compromise to placate EMC regulations.

Easy to verify, cut the ground trace and use a radio with antenna to verify noise output.

Answer 2

It looks to me like that is the ground return path for the loop. If you'll notice that on the other side of the board the last loop crosses over and the 3rd coil is actually the last coil before ground.

I think the main reason this was done was purely asthetical, as the current through the loop would be the same. It could change the mutual inductance between coils. I think the idea behind this design was to put the loop crossover point on the opposite side of the board from the filter, which makes the loop a little more compact.