Does a PCB NFC antenna need an impedance-controlled board?

[HwAoRrDk]

As per subject, if one has an NFC antenna on a PCB, does the board need to be of impedance-controlled construction? We're talking typical 2-layer boards, nothing fancy.

Initially, I would have guessed "no", because as far as I understand, your typical NFC antenna design always has no ground plane underneath (or around) it, so how can the substrate characteristics matter?

However, I've been looking at a reference design for an NFC controller, and the design files for the PCB layout do specify a particular PCB material and stack-up (Isola FR-370HR, each layer 1.5oz plated, core 0.058" 0.5oz), as well as an impedance for the antenna traces ("50 ohms +/- 10% ... referenced to layer 1 ground"). I'm wondering whether this is to do with the fact that the antenna design they've implemented on this reference design is not single-sided, but rather has traces across both sides. It's approx. 3.5 turns total, with the first 2 turns on the top side, and the remainder on the bottom. For an antenna of this configuration, is a controlled-impedance stack-up of significance because of turns overlapping each other, or is it just the chip vendor being overly-specific with their manufacturing instructions?

[twospoons]

I wouldn't bother with controlled impedance. An NFC antenna has a pretty low Q (>30), and will be somewhat detuned by the presence of the second antenna anyway (Near field ...).
The slight variations in tuning that will occur with dielectric variation are going to be lost amongst that.