Generating more (or the maximum number of) resonant frequencies in multiband microstrip antennas is a key objective in modern antenna design, especially for applications like wireless communication, IoT, and 5G. To achieve this, several techniques are employed to introduce multiple resonances and enhance the antenna's performance. Below are the main techniques used:

1. Multi-Resonant Structures

• Multiple Patches: Use multiple radiating patches (e.g., stacked patches or parasitic patches) to create additional resonances.
• Slot Loading: Introduce slots (e.g., U-slot, E-slot, or H-slot) in the radiating patch or ground plane to generate additional resonant frequencies.
• Fractal Geometries: Use fractal shapes (e.g., Koch, Sierpinski, or Minkowski fractals) to create self-similar structures that support multiple resonances.

2. Defected Ground Structure (DGS)

• Ground Plane Modifications: Introduce defects (e.g., slots, etched patterns, or periodic structures) in the ground plane to create additional resonances and improve bandwidth.
• Resonant Cavities: Use DGS to form resonant cavities that interact with the radiating patch to produce multiple resonant frequencies.

3. Parasitic Elements

• Parasitic Patches: Add parasitic patches near the main radiating patch to create coupling effects, which introduce additional resonances.
• Stub Loading: Attach stubs (e.g., open or shorted stubs) to the radiating patch or feedline to generate extra resonant frequencies.

4. Multi-Layer Designs

• Stacked Patches: Use multiple layers of radiating patches stacked vertically to create additional resonances.
• Substrate Layers: Employ multiple substrate layers with different dielectric constants to enhance bandwidth and introduce multiple resonances.

5. Frequency Selective Surfaces (FSS)

• Periodic Structures: Integrate FSS (e.g., periodic arrays of metallic or dielectric elements) to create bandpass or bandstop characteristics, which can introduce additional resonances.
• Metamaterials: Use metamaterial-inspired structures (e.g., split-ring resonators or complementary split-ring resonators) to achieve multiple resonances.

6. Feedline Modifications

• Multi-Feed Techniques: Use multiple feedlines (e.g., dual-feed or triple-feed) to excite different modes and create multiple resonances.
• Impedance Matching Networks: Incorporate impedance matching networks (e.g., stubs, transformers, or LC circuits) to improve bandwidth and introduce additional resonances.

7. Reconfigurable Antennas

• Tunable Components: Use tunable components (e.g., varactor diodes, PIN diodes, or RF MEMS) to dynamically adjust the resonant frequencies.
• Switching Mechanisms: Incorporate switches to activate or deactivate different parts of the antenna structure, enabling multiple resonances.

8. Hybrid Techniques

• Combination of Techniques: Combine multiple techniques (e.g., fractal geometry with DGS or parasitic elements with multi-layer designs) to maximize the number of resonant frequencies.
• Optimization Algorithms: Use optimization algorithms (e.g., genetic algorithms, particle swarm optimization) to fine-tune the antenna design for maximum resonances.

9. Substrate and Material Selection

• High-Permittivity Substrates: Use substrates with high dielectric constants to reduce the antenna size and introduce additional resonances.
• Metamaterial Substrates: Employ metamaterial-based substrates to achieve unusual electromagnetic properties and enhance resonant behavior.

10. Multi-Mode Excitation

• Higher-Order Modes: Design the antenna to excite higher-order modes (e.g., TM10, TM20, TM30) in addition to the fundamental mode, creating multiple resonances.
• Mode Coupling: Use techniques to couple different modes (e.g., even and odd modes) to generate additional resonances.

Summary of Techniques

TechniqueDescriptionMulti-Resonant StructuresUse slots, fractal geometries, or multiple patches to create resonances.Defected Ground Structure (DGS)Modify the ground plane to introduce additional resonances.Parasitic ElementsAdd parasitic patches or stubs to create coupling effects.Multi-Layer DesignsStack patches or use multiple substrate layers for additional resonances.Frequency Selective SurfacesUse periodic structures or metamaterials to enhance resonant behavior.Feedline ModificationsUse multi-feed techniques or impedance matching networks.Reconfigurable AntennasIncorporate tunable components or switching mechanisms.Hybrid TechniquesCombine multiple techniques for maximum resonances.Substrate and Material SelectionUse high-permittivity or metamaterial substrates.Multi-Mode ExcitationExcite higher-order modes or couple different modes.

Example: Multiband Antenna Design

Here’s an example of how to design a multiband microstrip antenna using slot loading and parasitic elements:

Many thanks for all details, Pr. Maher.

best regards,

In 5G patch antennas, achieving multiple resonance frequencies involves techniques like multilayer structures, optimizing patch geometry (such as adding slots), different feeding methods, parasitic elements, frequency-selective surfaces, and reconfigurable antenna designs.

To maximize resonant frequencies in multiband microstrip antennas, key techniques include slot loading, parasitic elements, defected ground structures (DGS), stub loading, metamaterials, stacked patches, high-permittivity substrates, fractal geometries, reactive loading, and multi-feed techniques. These methods modify current distribution, impedance, and electromagnetic coupling to enable multiband operation.