How does antenna shape affect performance?
Enclosed area sets the potential and overlap with the reader decides how much of it you get. A compact shape matching the phone coil outperforms a long thin one of the same area, because the phone only excites the part of the loop sitting in front of it.
Area sets the ceiling
A tag draws power from the field passing through its loop, so the area the loop encloses sets how much it can collect. This is why a card outperforms a coin sized sticker.
Area is the ceiling rather than the result. A loop can enclose a large area and still collect little, if that area sits where the field is weak.
Overlap decides what you reach
The field from a phone is concentrated around its own small coil and falls away quickly beyond it. Only the part of a tag loop sitting within that region contributes.
A long thin antenna makes this visible. Its enclosed area can match a card, and most of that area extends beyond where the phone produces any useful field, so it behaves like a much smaller tag.
A compact shape roughly matching the phone coil uses almost all of its area. This is why cards, squares and round tags of a sensible diameter behave predictably, and why unusual outlines disappoint against their footprint.
Shapes forced by the object, such as the band of a ring or a narrow label on a cable, are working against this rather than around it.
Turns and tuning
The loop has several turns, and the number is chosen with the chip to tune the tag near the operating frequency. More turns raise the voltage available and change the tuning, so it is not a free improvement.
This is a design decision made by the manufacturer, not something a buyer adjusts. What a buyer should take from it is that a tag is tuned as a whole, chip and antenna together.
Which is also why nearby materials matter. Metal, water and even a dense stack of other cards shift the tuning away from where it was set, and a detuned tag reads short or not at all.
What this means when buying
Compare outlines rather than areas. A tag whose shape is compact and close to a card format will behave better than a long thin one of the same surface.
Be suspicious of tags shaped to fit an object rather than to work. They can be the right answer, and they should be tested rather than assumed.
And test on the real mounting, since the tuning that matters is the tuning in place. A tag measured on a desk and deployed on a machine housing is two different tags.
Frequently asked questions
Is a round tag better than a square one?
Not meaningfully, at similar sizes. Both are compact shapes that overlap a phone coil well, and the difference between them is far smaller than the difference either has from a long thin tag.
Why do some large labels read badly?
Usually because they are long and narrow. Their area is spread beyond where the phone produces a useful field, so most of it contributes nothing.
Can I improve an antenna myself?
No. The tag is tuned as a chip and coil together, so adding wire or foil detunes it rather than helping. Choosing a different tag is the only real adjustment.
Does the antenna material matter?
Etched aluminium, etched copper and printed conductive ink all work, with differences too small to guide a purchase. The geometry matters far more than the metal.