I have an S4 GT-i9505.
With the standard case on my phone I can get the ring to work very unreliably in one spot, not enough to be useable. If I put the S4’s protective case on (an Incipio) it does not work at all anywhere.
The loose NFC tag that came with the ring works fine even with the protective cover on. I even bent the loose NFC tag into a ring like curve and that worked fine too, again even with the protective cover on.
I then tried holding the loose NFC tag against the ring (on a bare part) and that then didn’t work. Hmmm, the last thing I tried was wrapping the loose NFC tag around a similarly sized plain silver ring and that didn’t work either.
I have several 25mm NFC stickers that work up to 20mm away from the back of my S4 with the protective case on.
I have read through the forums and it sounds like I need to swap my Classic ring for an Alpha??