What is a Microstrip Antenna?

Microstrip antenna is a relatively recent invention, mainly used in microwave band. It is possible to integrate a microstrip antenna along with driving circuit on a common printed circuit board or in a semiconductor chip. Microstrip antenna consists of a dielectric substrate with thickness much less than the wavelength having a metal on the whole of one side which acts as a groundplane and a metal patch on the other side. Hence it is also called a patch antenna. Shape of the patch can vary, though a rectangular patch is commonly used.

Microstrip antenna can be connected with microstrip line and the signal source can be fed between the microstrip line and the ground plane on the reverse side. There could also be a feeding circuitry in between. Maximum radiation of the microstrip antenna is along its broadside, with not much of radiation along the direction of the dielectric substrate, which would be perpendicular that of the broad side. Length of the antenna would be about a half wavelength considering the velocity factor of the dielectric medium. Input impedance depends on the width of the antenna.

Multiple patch antennas on the same substrate can be used for making high gain arrays and phased arrays. Phased arrays can steer the beam electronically as in radars. Microstrip antennas have been used in Global Positioning System (GPS) receivers. As the microstrip antenna can be mounted on the surface, it has been used on missiles, aircrafts and spacecrafts.

A variant of patch antenna used in mobile phones is called shorted patch antenna with one corner or edge grounded with a ground pin. The type of patch antenna commonly used in mobile phones is usually the planar inverted F antenna (PIFA). Directive gain of a single patch antenna is 6-9 dBi. Downsides of microstrip antenna are low efficiency, low power handling capacity and narrow bandwidth.
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