Field Guide · term

Also known as: microstrip antenna, printed patch antenna, planar patch

A patch antenna (or microstrip antenna) is a flat sheet of conductor mounted a small fraction of a wavelength above a ground plane, separated by a thin dielectric board.1 It is a resonant antenna: the patch is sized so that its length is about half a wavelength in the dielectric, and the fringing fields at its two radiating edges launch a beam broadside — straight up, away from the board. Because the whole structure can be etched onto a circuit board or moulded as a ceramic block, the patch is cheap, flat, and mechanically robust, which is why it is the antenna hidden inside GPS units, phones, Wi-Fi cards, and satellite terminals.

ground plane substrate patch (~λ/2) feed broadside lobe
A patch resonates at roughly half a wavelength in the substrate; its edge fields radiate a hemispherical beam broadside to the board.

How it works

Think of the patch and the ground plane as the two plates of a very leaky, resonant cavity. When the patch length is close to a half wavelength (shortened by the substrate’s dielectric constant, so a high-permittivity ceramic makes the patch physically small), the voltage swings to opposite polarity at the two ends. The fields fringe out past each radiating edge, and because the two edges are half a wavelength apart their contributions add in the broadside direction. The result is a single main lobe pointing away from the ground plane, covering roughly the upper hemisphere.

Key consequences of that cavity behaviour:

  • Narrow bandwidth. A thin, high-Q cavity is only well matched over a small band — often a few percent. Thicker or foam substrates widen it at the cost of profile.
  • Low, forgiving gain. A single patch gives roughly 5–8 dBi with a broad beam, ideal when you need coverage of a whole hemisphere (a GPS satellite can be anywhere overhead) rather than a pencil beam.
  • Feed sets the match. Moving the feed point in from the edge finds the spot where the patch’s impedance matches the 50 Ω line; feeds can be a probe through the ground plane, an inset microstrip line, or aperture coupling.

Variants

Feeding two adjacent edges with a 90° phase offset (or trimming opposite corners) makes the patch radiate circular polarization — essential for GPS, whose satellites transmit right-hand circular so the receiver keeps a steady signal regardless of orientation. Stacking two patches, or arraying many of them on one board, widens bandwidth or raises gain and narrows the beam, forming the building block of printed phased arrays.

Relevance to SDR

The patch is one of the most widely deployed antennas on earth precisely because it disappears into a device. In SDR terms it is the natural front end for anything at L-band and up where you want a flat, cheap, hemispheric antenna: GPS/GNSS reception (an SDR fed by a ceramic active patch is the standard way to sample the L1 band), Inmarsat and other L-band satellite downlinks, ADS-B ground stations, and Wi-Fi. Its circular-polarization option is what makes it the default for satellite work.

GopherTrunk targets VHF/UHF land-mobile trunking (P25, DMR, NXDN, TETRA), where wavelengths are long and a resonant patch would be large and narrowband, so scanners use verticals rather than patches. The patch is documented here as the canonical low-profile printed antenna and the reason a fingernail-sized ceramic block can pull a GPS satellite out of the noise.

Sources

  1. Patch antenna — Wikipedia, for the microstrip cavity model, resonant sizing, and circular-polarization feeds. ↩

See also