Following this post, I figured I’d leave this here in a separate thread if someone wants to make one for themselves, since it is generally useful for all kinds of purposes.
A NFC range extender is a bit of a misnomer. It extends the range as in “you can read a tag some distance away from the reader / cellphone” or as in “the tag you read at the other end can be read better or farther away than directly with the cellphone”. This little homebrew gadget does the former without even trying very hard, and the latter if it’s properly tuned:
You can buy them too, but where’s the fun in that? And for what they are, commercial range extenders are fiendishly expensive. Also, the few I’ve tried kind of kill the signal more than they help it.
What you need:
- A roll of 24 gauge / 0.5 mm-diameter magnet wire
- Something cylindrical with a 1 3/4" / 45-mm diameter (small spray can or something)
- 6’ / 180 cm of electrical cord, like the one on grandma’s bedside table lamp from the 60’s.
Wrap 5 turns of magnet wire around the spray can to make one coil. Make two coils. Connect them with the electrical cord. Shorten or lengthen the cord for best results. It’s that easy 
And look, it works pretty good:
It can make my unimpressive cellphone read a difficult glassie, my IAR M1k glassie at a respectable distance, and of course thin and large flexies without any trouble.