What Are Patch Antennas?

Patch antennas, also called microstrip antennas, are planar radiating elements consisting of a thin metallic conductor bonded to one face of a dielectric substrate, with a ground plane on the opposite face. The conductor is shaped, most often as a rectangle, circle, or ring, and is fed by a microstrip line or a coaxial probe. The element resonates when its length is approximately one half-wavelength of the target frequency in the effective medium formed by the substrate and air, radiating a broadside beam perpendicular to the patch surface. Patch antennas emerged as a practical antenna class in the early 1970s, driven by demand for low-profile, lightweight radiators for aircraft and satellite platforms.

The terms patch antenna and microstrip antenna are used interchangeably in the technical literature. Both refer to the same physical structure: a metallic patch printed or etched on a substrate material, with the ground plane completing the transmission-line-like structure below. The manufacturing process is compatible with standard printed circuit board fabrication, which is a primary reason these antennas are found in high-volume consumer electronics as well as aerospace systems.

Resonant Structure and Operating Principle

A rectangular patch antenna functions as an open-ended microstrip resonator. The patch length L is chosen so that the structure supports a standing wave at the desired frequency; at resonance, two radiating slots form at the open ends of the patch, each approximately L/2 apart, and radiate in phase to produce a broadside pattern. The fringing fields at these edges extend the effective electrical length of the patch slightly beyond its physical length, requiring a small shortening correction in the initial design. Substrate permittivity controls the effective wavelength inside the resonator, so higher-permittivity materials allow smaller patch dimensions but also tighten bandwidth and reduce radiation efficiency. Multiple publications on IEEE Xplore document parametric studies relating substrate choice, patch geometry, and radiation performance across the microwave spectrum.

Design Parameters

The principal design variables are the substrate dielectric constant, substrate thickness, patch geometry, and feed method. Substrate materials range from inexpensive glass-epoxy laminates (FR4, with a relative permittivity near 4.4) to low-loss PTFE-based composites such as Rogers RT/Duroid 5880 (permittivity approximately 2.2), which are preferred when radiation efficiency is critical. Substrate thickness affects both bandwidth and surface-wave losses: thicker substrates improve impedance bandwidth but launch more power into surface-wave modes that degrade gain. Feed options include edge-feed microstrip lines, inset-feed lines recessed into the patch, coaxial probe feeds that connect through the ground plane, and aperture-coupled or proximity-coupled configurations that electrically separate the feed from the radiating element to control unwanted radiation. A design published through IEEE Xplore for 5G applications at 28 GHz illustrates how these design variables are balanced for millimeter-wave use.

Performance Characteristics

Patch antennas offer several attractive properties: a gain of 5 to 8 dBi for a single element, predictable broadside radiation with low back-lobe levels, and the ability to produce linear or circular polarization depending on feed symmetry. Their primary limitation is narrow impedance bandwidth, typically 2 to 5 percent for a single layer at reasonable substrate thickness. Techniques to extend bandwidth include stacking multiple resonant patches separated by air or foam layers, adding parasitic elements, and slotting the patch to introduce secondary resonances. Arrays of patch elements, fed through corporate or series distribution networks, are widely used to achieve the gain levels required for radar, satellite tracking, and base station applications. The design and analysis of microstrip arrays for these high-gain configurations is a well-established area of antenna research.

Applications

Patch antennas have applications in a wide range of disciplines, including:

• Mobile handset and tablet wireless communications (Wi-Fi, LTE, 5G)
• GPS receivers in navigation and surveying equipment
• Satellite communication terminals and direct broadcast receivers
• Radar systems requiring conformal or low-profile radiating apertures
• RFID reader antennas for inventory and logistics management