What Are Microstrip Antennas?

Microstrip antennas are low-profile radiating elements fabricated by printing a conducting patch on one side of a thin dielectric substrate while the opposite side serves as the ground plane. The patch, typically rectangular or circular, resonates at a frequency determined by its dimensions and the substrate's electrical properties, radiating primarily through the fringing fields at its edges. Microstrip antennas are chosen for applications where antenna thickness, weight, and conformability to curved surfaces matter, including aircraft, satellites, consumer wireless devices, and wearable electronics.

The technology emerged from microstrip transmission line work in the 1950s and became a practical antenna form in the 1970s, as substrate materials with well-controlled dielectric constants became commercially available. Their compatibility with printed circuit manufacturing allows patch antennas to be integrated with feed networks and active components on a single board, reducing assembly complexity in wireless systems.

Patch Antennas

The rectangular microstrip patch is the most common form. Its resonant frequency is set primarily by the patch length, which is approximately half the guided wavelength in the substrate. The patch width controls the input resistance at resonance and influences the radiation efficiency. Substrates with low relative permittivity (around 2 to 4, such as PTFE composites or Rogers laminates) are preferred because they increase the fringing fields at the edges, improving bandwidth and radiation efficiency, though at the cost of a physically larger patch.

A single rectangular patch supports linear polarization along the feed axis; circular polarization is achieved by truncating opposite corners of the patch or by feeding with two orthogonal ports shifted by 90 degrees in phase. Gain for a single element is modest, typically 5 to 8 dBi, which makes individual patches most useful in applications that rely on arrays to achieve higher directivity, as described in the IEEE Xplore paper on wideband aperture-coupled line feeding designs for microstrip patches.

Aperture Coupled Antennas

Aperture coupling, introduced in 1985 by David Pozar, separates the radiating patch from its feed line using a slot cut in the ground plane. The feed microstrip line lies on the opposite side of the substrate from the patch; electromagnetic energy couples through the aperture to excite the patch. This configuration eliminates direct contact between the feed and the patch, reducing spurious radiation from the feed network and making it possible to use different substrate materials for the feed and the radiator, each optimized for its function.

As surveyed in Pozar's review of aperture coupled microstrip antennas, impedance bandwidths exceeding 50 percent are achievable with stacked patch designs fed through apertures, compared to 1 to 5 percent for a simple edge-fed patch on the same substrate. Dual-polarization aperture coupled patches achieve port isolation of 18 dB or better with linear polarization and up to 27 dB with a crossed-slot aperture, making them suitable for polarization diversity and MIMO systems. The dual-polarization aperture coupled stacked patch paper in IEEE Xplore demonstrates how stacking a parasitic patch over the driven element simultaneously improves bandwidth and maintains good impedance matching.

Applications

Microstrip antennas have applications in a range of fields, including:

• Mobile handsets and tablets, as the primary antenna element for cellular, Wi-Fi, and Bluetooth bands
• Satellite navigation receivers, using circularly polarized patches for GPS, GLONASS, and Galileo signals
• Aircraft and missile systems, conforming patches to fuselage surfaces to minimize drag
• Medical devices, where body-worn patches monitor vital signs or deliver therapeutic microwave energy
• Ground-penetrating radar, providing compact planar antennas suitable for handheld and vehicle-mounted systems