patch antenna
A patch antenna is a flat metal rectangle printed on a circuit board, sitting above a ground plane like a postage stamp glued to a sheet of foil. It is the unglamorous hero inside GPS receivers, Wi-Fi cards, and car keyfobs, because it is cheap, low-profile, and made with the same etching process as any printed circuit.
The patch works as a tiny resonant cavity: the half-wavelength-long metal traps a standing wave, and the fringing fields that leak out at its two open edges do the actual radiating, beaming energy upward away from the board. Its flatness is also its weakness — a single patch is narrow-band and radiates mostly in one hemisphere, so designers tile many patches into an array to widen bandwidth or steer the beam.
Because the substrate's dielectric shrinks the effective wavelength, a patch is physically smaller than a half-wave dipole at the same frequency — a handy fact when packing antennas into a tiny gadget.