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They are really small footprint Sptit Ring antennas, mostly ideal for PCB

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8659506/figure/sensors-21-07779-f007/
Impedance matching

Concept and analysis of a coupled split-ring resonator for wide-/dual bands, self-filtering, high out-of-band suppression and highly efficient antennas