Comprehensive study guide covering patch antenna theory, design equations, radiation patterns, and practical implementations for modern wireless systems.
History, advantages, and basic concepts of microstrip antennas
Rectangular, circular, and triangular patch configurations
Cavity model, transmission line model, and radiation mechanisms
Step-by-step design procedure and key formulas
Real-time patch dimension calculator with visualization
Linear arrays, planar arrays, and beam steering
A microstrip antenna (also known as a patch antenna) consists of a metallic patch placed above a ground plane with a dielectric substrate in between. First proposed by Deschamps in 1953 and practically developed by Howell and Munson in the early 1970s, these antennas have become ubiquitous in modern wireless communication.
The patch is typically made of conducting material such as copper or gold and can take various shapes, though rectangular and circular configurations are most common due to their ease of analysis and fabrication.
Rectangular Microstrip Patch Configuration
Most common shape. Length L ≈ λ/2, Width W controls input impedance and bandwidth.
Symmetric radiation pattern. Radius a determined by TM11 mode resonance.
Compact size compared to rectangular. Useful for dual-frequency operation.
Conducting strip connects directly to patch edge. Easy to fabricate but narrow bandwidth due to surface wave excitation.
Inner conductor extends through substrate to patch. Low spurious radiation but narrow bandwidth and difficult to model.
Non-contact feeding through slot in ground plane. Higher bandwidth (~10-20%) but multi-layer fabrication.
Two dielectric layers with feed line on bottom and patch on top. Very high bandwidth up to 13%.
The rectangular patch is viewed as two radiating slots separated by a low-impedance transmission line of length L. Each slot radiates with equivalent admittance:
G = W/(120λ₀) × [1 - (1/24)(k₀h)²] for h << λ₀
B = W/(120λ₀) × [1 - 0.636ln(k₀h)]
where k₀ = 2π/λ₀ is the free-space wavenumber
The region between patch and ground plane is treated as a cavity bounded by magnetic walls (open circuits) at the edges and electric walls (conductors) top and bottom.
Resonant Frequency (TM₁₀₀ mode):
fᵣ = c/(2L√εᵣₑff)
εᵣₑff = (εᵣ + 1)/2 + (εᵣ - 1)/2 × [1 + 12h/W]^(-1/2)
Electric fields fringe at the patch edges, effectively increasing the electrical length. This is accounted for by the effective length:
where ΔL/h = 0.412 × (εᵣₑff + 0.3)(W/h + 0.264) / [(εᵣₑff - 0.258)(W/h + 0.8)]
The two radiating slots act as radiating apertures with fields perpendicular to the ground plane. The far-field pattern is the vector sum of fields from both slots separated by distance L.
| Directivity | ~6-8 dBi |
| Beamwidth (E-plane) | ~100°-120° |
| Beamwidth (H-plane) | ~70°-80° |
| Input Resistance | 150-300 Ω (edge) |
| Bandwidth (VSWR < 2) | 1-5% |
For efficient radiation, W should be slightly less than λ/2 in the dielectric. Wider patches provide better bandwidth but excite higher order modes.
Accounts for fringing fields extending into air above the patch.
The actual resonant length is shorter than λ/2 due to fringing.
For 50Ω impedance match, the inset feed position y₀ is:
Patch Geometry:
Patch Width (W)
--
mm
Patch Length (L)
--
mm
εᵣₑff
--
ΔL Extension
--
mm
Inset Feed Position (y₀)
--
mm from edge
Single microstrip elements provide modest gain (6-8 dBi). To achieve higher gains for radar and communication systems, multiple patches are arranged in arrays. The total field is the product of the element pattern and the array factor.
where β = phase shift between elements. For broadside: β = 0, For scanning: β = -k₀d sinθ₀
Patches arranged along a line. Simple feeding network but limited to one-dimensional scanning.
Two-dimensional matrix of patches. Allows scanning in both azimuth and elevation.
Parallel feed network using quarter-wave transformers. Wide bandwidth but physically large.
Smartphones, WiFi routers (2.4/5 GHz), Bluetooth devices, and wearable technology.
GPS receivers, satellite radio (SDARS), and direct broadcast satellite (DBS) systems.
77 GHz collision avoidance systems, adaptive cruise control, and parking sensors.
Microwave imaging, hyperthermia treatment, and wireless capsule endoscopy.
Electromagnetically coupled parasitic patches above driven element. Bandwidth: 10-20%.
U-slots, L-slots, or E-shaped patches create multiple resonant modes. Bandwidth: 20-40%.
Low dielectric constant foam substrates reduce Q factor. Bandwidth: 10-15%.
Mushroom-like EBG structures or metamaterial loading. Bandwidth: Ultra-wideband possible.
Required for satellite communications to mitigate Faraday rotation and polarization mismatch.
Microstrip antennas are essential components in modern microwave engineering, offering low-profile, conformal solutions for wireless systems. Key design considerations include substrate selection (balancing bandwidth vs. size), feeding technique (matching and efficiency), and array configuration (gain and beam steering requirements).
Key Formula
L = λ/2√εᵣₑff - 2ΔL
Typical Gain
6-8 dBi (single)
Bandwidth
1-5% (standard)