Introduction to Microstrip Antennas
A microstrip patch antenna (MSA) is a type of radio antenna with a low profile, which can be mounted on a flat surface. It consists of a rectangular (or other shaped) metallic patch placed on one side of a dielectric substrate, with a ground plane on the other side [^2^].
Key Characteristics
- •Low profile and conformal structure
- •Lightweight and compact size
- •Compatible with integrated circuits
- •Moderate bandwidth (typically 1-5%)
- •Easy to fabricate using PCB technology
Microstrip antennas operate as open resonant cavities. The field confinement between the patch and ground plane determines the eigenmodes, with the antenna typically operating in the fundamental mode [^3^].
Applications
Frequency Range
Microstrip antennas are most useful at microwave frequencies (f > 1 GHz) where their electrical size becomes practical [^2^].
Structure and Geometry
The basic microstrip antenna consists of three layers: the ground plane, dielectric substrate, and radiating patch. The patch is typically made of copper or gold and can take various shapes, though rectangular and circular are most common [^2^].
Geometric Parameters
Cavity Model Concept
The patch acts approximately as a resonant cavity with Perfect Electric Conductor (PEC) walls on top and bottom, and Perfect Magnetic Conductor (PMC) walls on the edges [^2^]. Radiation is accounted for by using an effective loss tangent for the substrate.
Key Insight: As the substrate gets thinner, the patch current radiates less due to image cancellation. However, the Q of the resonant cavity increases, making patch currents stronger at resonance. These two effects cancel, allowing the patch to radiate well even for thin substrates [^2^].
Figure 1: Rectangular Microstrip Patch Antenna Structure
Design Equations and Formulas
The design of a microstrip patch antenna follows a systematic procedure using transmission line model and cavity model approximations [^3^][^4^].
Design Procedure
Select Operating Frequency (f₀)
Determine target resonance frequency based on application requirements.
Calculate Patch Width (W)
Calculate Effective Dielectric Constant (εeff)
Calculate Length Extension (ΔL)
Calculate Patch Length (L)
Key Parameters
Design Note
The width W is usually chosen to be larger than L to obtain higher bandwidth. However, typically W < 2L to avoid excitation of higher order modes [^2^].
Field Distribution (TM₁₀ Mode)
Figure 2: Electric Field Distribution in Microstrip Patch Antenna (Side View)
The electric field is maximum at the radiating edges (x = 0 and x = L) due to the open-circuit boundary condition. The fringing fields at these edges are responsible for radiation [^7^].
Feeding Techniques
The performance of microstrip antennas depends significantly on the feeding technique. Methods are classified as contacting (direct connection) and non-contacting (electromagnetic coupling) [^1^][^5^].
Microstrip Line Feed
ContactingFigure 3: Microstrip Line Feed Configuration
- ✓ Easy to fabricate (planar structure)
- ✓ Simple etching process
- ✗ Spurious feed radiation
- ✗ Bandwidth limited to 2-5%
Coaxial (Probe) Feed
ContactingFigure 4: Coaxial Feed Configuration
- ✓ Easy impedance matching
- ✓ Low spurious radiation
- ✗ Requires drilling hole
- ✗ Probe inductance limits bandwidth
Aperture Coupled Feed
Non-contactingFigure 5: Aperture Coupled Feed Configuration
- ✓ High bandwidth possible
- ✓ Isolates feed from patch radiation
- ✗ Requires multilayer fabrication
- ✗ Alignment critical
Proximity Coupled Feed
Non-contactingFigure 6: Proximity Coupled Feed Configuration
- ✓ Very low spurious radiation
- ✓ Bandwidth up to 13%
- ✗ Multilayer fabrication required
- ✗ Poor polarization purity
Inset Feed for Impedance Matching
An inset cut is integrated into the patch to achieve impedance matching without additional matching elements. The input impedance varies with inset depth [^3^]:
where y is the inset distance from the edge, and Rin(0) is the input resistance at the edge.
Figure 7: Inset Feed Configuration
Radiation Characteristics
The radiation from a microstrip antenna can be modeled using either the electric current model (physical currents on the patch) or the magnetic current model (equivalent currents at the edges) [^2^].
E-Plane and H-Plane Patterns
Figure 8: Radiation Pattern (Linear Scale)
E-Plane (φ = 0°): Contains the electric field vector and direction of maximum radiation. Pattern is broader.
H-Plane (φ = 90°): Contains the magnetic field vector. Pattern is typically narrower.
Radiation Parameters
Directivity
For thin substrates, directivity is essentially independent of substrate thickness [^2^].
Beamwidth
The half-power beamwidth (HPBW) depends on the patch dimensions and frequency. Typical values range from 50° to 100°.
Bandwidth
The fractional bandwidth is inversely proportional to the quality factor Q [^2^]:
Efficiency
Radiation efficiency depends on conductor losses, dielectric losses, and surface wave excitation. Typical values: 80-95%.
Surface Wave Effects
As substrate thickness increases, surface wave excitation becomes significant, degrading pattern performance and causing mutual coupling in arrays. Techniques to reduce surface waves include using photonic bandgap structures or Reduced Surface Wave (RSW) antenna designs [^2^].
Interactive Design Calculator
Input Parameters
Common Substrates
Calculated Dimensions
Figure 9: Patch Geometry Visualization (Scale Approximate)
Quick Reference Table
| Parameter | Symbol | Formula/Value | Units |
|---|---|---|---|
| Speed of Light | c | 2.998 × 10⁸ | m/s |
| Free-space Wavelength | λ₀ | c/f₀ | m |
| Patch Width | W | c/(2f₀√((εr+1)/2)) | m |
| Effective Dielectric Constant | εeff | (εr+1)/2 + (εr-1)/2(1+12h/W)^(-1/2) | -- |
| Length Extension | ΔL | 0.412h(εeff+0.3)(W/h+0.264)/(εeff-0.258)(W/h+0.8) | m |
| Patch Length | L | c/(2f₀√εeff) - 2ΔL | m |
| Input Resistance (edge) | Rin | 90(λ₀/W)² (typical) | Ω |