Short note: Microstrip (printed) antenna

A microstrip (patch or printed) antenna is a thin metallic patch etched on one side of a dielectric substrate whose other side is a ground plane. It is made by printed-circuit techniques.

        patch (L x W)
     _______________
    |_______________|___ feed line
 ///////////////////////////  substrate εr, h
 ===========================  ground plane
  fringing fields at the two radiating edges

Construction and working

• Patch shapes: rectangular, square, circular, triangular. Substrate εr ≈ 2.2–12, height h ≈ 0.003λ–0.05λ.
• The patch behaves as a cavity; for the dominant mode its length is about half a guided wavelength:

L ≈ λ₀ / (2√εr)   (slightly less, due to fringing)

• Radiation comes from the fringing fields at the two open edges, which act like two slots in phase; the beam is broadside (normal to the patch).

Feeding methods: microstrip line (inset), coaxial probe, aperture coupling, proximity coupling.

Advantages: low profile, light, cheap, conformal to curved surfaces, easy to integrate with circuits and to form arrays, linear or circular polarization possible.

Disadvantages: narrow bandwidth (≈ 1–5 %), low efficiency and gain (≈ 6–8 dBi), low power handling, surface-wave losses.

Applications: mobile phones, GPS, Wi-Fi, RFID, aircraft and missile antennas, satellite terminals.