A microstrip antenna is not, as the name seems to imply, a very thin linear antenna. It is actually a common form of planar antenna. A microstrip antenna is made up of a dielectric substrate sandwiched between a square radiating patch (it can be other shapes as well) and a ground-plane. For those of you who are not electrical engineers, a dielectric substrate is essentially an insulator, while a ground-plane is a flat conducting surface that reflects the radio waves from the other antenna elements. So a microstrip antenna is a very thin planar antenna. The transmission layer can be created by etching away metal or by printing metallic inks. I have not seen many of these antennas on the market.
—Mark Roberti, Founder and Editor, RFID Journal