Stacked PatchEdit

Stacked Patch

A stacked patch is a configuration in RF engineering where two or more microstrip patch radiators are placed in close vertical proximity, usually separated by a dielectric spacer. This arrangement is designed to widen the operational bandwidth, improve gain and directivity, and provide more robust impedance matching than a single patch. In practice, a stacked patch typically consists of a lower, actively excited patch and one or more upper patches that are either parasitic or separately fed. The result is a compact, planar antenna that can be integrated into a variety of systems, from satellites to ground-based communications.

The concept grew out of the need to overcome the narrow bandwidth of conventional patch antennas. By coupling the resonances of multiple patches, designers can create a broader, flatter impedance response and tailor the radiation pattern more effectively. This approach has made stacked patches a staple in aerospace, defense, cellular infrastructure, and handheld devices where space and performance are at a premium. For related concepts and variations, see Patch antenna and Microstrip antenna.

Principles of operation

In a typical stacked patch, the bottom patch is the primary radiator and is excited through a feeding network. The upper patch (or patches) is/are positioned parallel to the lower one, separated by a spacer of dielectric material. The two patches couple electromagnetically, modifying the current distribution and effectively creating a broader resonance than what a single patch could achieve. Depending on the design, the upper patch can be:

Parasitic stacked patch: the upper element is not directly fed but is excited through mutual coupling.
Electrically coupled stacked patch: the upper patch has its own feed path that interacts with the lower radiator.

This coupling alters the input impedance and the radiation pattern, typically increasing the bandwidth and peak gain. The exact effects depend on patch shapes (rectangular, circular, or other geometries) and the dielectric stack, including the substrate materials and their loss characteristics. See also Rectangular patch antenna and Circular patch antenna for common geometries.

Configurations

Two-patch configurations: the most common form uses a primary bottom patch and a single top patch. The spacing between patches (often a fraction of a wavelength at the center frequency) is a critical design parameter.
Multi-stack designs: adding a second or third patch can further extend bandwidth and provide more control over the radiation pattern, at the cost of additional complexity and thickness.
Feeding schemes: stacked patches can be fed in several ways, including a single feed to the lower patch with the upper patch coupled, or separate feeds for each patch using a corporate or series-fed network. See Feeding network and Microstrip line for related concepts.

Common practical choices include rectangular stacks for ease of fabrication and predictable performance, while circular or other shapes may be used to tailor polarization or pattern characteristics. See Patch antenna for foundational geometry and performance considerations.

Design considerations

Key factors that influence performance include:

Patch dimensions: the length and width set the resonant frequency(s) of each patch and, in combination with the spacer, determine the overall bandwidth.
Dielectric properties: the relative permittivity (εr) and loss tangent of the substrata affect efficiency and losses. Higher εr can reduce the physical size but may narrow bandwidth and increase fabrication sensitivity.
Spacer thickness and material: the distance between patches controls coupling strength. Too little spacing can over-couple; too much can degrade coherence and reduce gain.
Spacing between patches: typically a fraction of a wavelength at the center frequency. Proper spacing helps achieve the desired impedance match and avoids unwanted resonances.
Feeding strategy: a robust feeding network minimizes reflections and ensures stable impedance across the band. Techniques range from simple microstrip feeds to more complex phase-controlled networks.
Fabrication tolerance: stacked structures are sensitive to manufacturing tolerances, especially at higher frequencies. Tolerances in spacer thickness and conductor thickness can shift resonances and change gain.
Polarization and pattern control: by adjusting geometry and the coupling, stacked patches can support linear or circular polarization and can shape beams for specific mission requirements.

Designers often use electromagnetic simulation tools (notably methods like the Method of Moments or finite-difference time-domain techniques) to predict performance before fabrication. See Antenna simulation and Antenna gain for related topics.

Performance characteristics

Bandwidth enhancement: stacking typically increases fractional bandwidth compared with a single patch, enabling broader coverage in a single antenna footprint.
Gain and directivity: stacked configurations can provide higher gain than a lone patch by combining radiated fields more effectively, though the exact improvement depends on patch geometry, spacer properties, and feeding.
Radiation pattern: the coupling between patches can shape the main lobe, with opportunities to suppress side lobes or tailor beamwidth. Careful design is required to avoid grating lobes at higher frequencies.
Efficiency and Q factor: while stacking can improve bandwidth, it can also introduce extra loss pathways through the spacer and feed network. High-quality substrates and clean interfaces help preserve efficiency.
Polarization control: with appropriate geometry, stacked patches can support stable linear polarization or engineered circular polarization.

For broader context on how patch-based radiators behave, see Patch antenna and Microstrip antenna.

Applications

Stacked patches are deployed across a range of applications that value compact, planar form factors with reliable broadband performance:

Satellite communications and ground terminals, where space and weight constraints demand efficient, broadband antennas. See Satellite communication.
Airborne and spaceborne radar systems that require robust performance across a band without increasing antenna height significantly. See Radar.
Cellular and wireless infrastructure, including base stations and backhaul links, where compact, wideband antennas help support evolving standards. See Cellular network.
Global navigation satellite systems and precision timing where stable gain and impedance behavior matter across band. See Global Positioning System and Timing.