Microstrip Patch Antenna
Design rectangular patch antennas for PCBs.
What is a Microstrip Patch Antenna?
A Microstrip Patch Antenna is a type of radio antenna that can be fabricated directly onto a printed circuit board (PCB). It consists of a flat rectangular sheet of metal (the "patch") mounted over a larger sheet of metal called a ground plane, separated by a dielectric substrate.
Because they are low-profile, lightweight, and inexpensive to manufacture using standard etching techniques, patch antennas are ubiquitous in modern wireless devices. They are the hidden heroes inside your smartphone, WiFi router, GPS receiver, and even radar systems used in automotive cruise control.
Design & Calculation Formulas
Designing a patch antenna is a careful balance of physics. The dimensions of the patch control the resonant frequency. The "Length" determines the frequency, while the "Width" controls the input impedance and bandwidth. We use the Transmission Line Model for these calculations.
1. Calculating Width (W)
For efficient radiation, the width is calculated first:
W = (c / (2f₀)) × sqrt(2 / (εᵣ + 1))
Where c is the speed of light, f₀ is the target frequency, and εᵣ is the dielectric constant of the substrate.
2. Effective Dielectric Constant (ε_eff)
Since some of the electric field travels in the air above the patch and some in the substrate below, we calculate an "effective" dielectric constant:
ε_eff = (εᵣ + 1)/2 + (εᵣ - 1)/2 × [1 + 12(h/W)]⁻⁰·⁵
3. Effective Length & Fringing
The patch looks electrically longer than its physical length due to "fringing fields" at the edges. We calculate this extension (ΔL):
ΔL = 0.412h × [(ε_eff + 0.3)(W/h + 0.264)] / [(ε_eff - 0.258)(W/h + 0.8)]
Finally, the physical length (L) is:
L = (c / (2f₀sqrt(ε_eff))) - 2ΔL
Practical Applications
- GPS Systems: Ceramic patch antennas are standard in almost all GPS receivers due to their circular polarization capabilities.
- Mobile Phones: While often replaced by PIFA antennas, patches are still used in 5G mmWave arrays.
- Radar: Phased arrays composed of hundreds of small patches can steer a radar beam electronically without moving parts.
FAQ
Why use Rogers substrate instead of FR4?
FR4 is cheap but inconsistent. Its dielectric constant can vary from 4.2 to 4.8 across a single board, and it is very "lossy" at high frequencies (it absorbs signal energy). Rogers materials (like RO4350B) are engineered ceramics with precise εᵣ and very low loss, essential for antennas above 2 GHz.
What is the "Feed Point"?
You cannot just connect the wire to the edge of the patch; the impedance there is too high (hundreds of Ohms). To match a standard 50Ω cable, you must feed the signal at a specific point inset from the edge, or use a quarter-wave transformer.
Does the ground plane size matter?
Yes. An infinite ground plane is assumed in the math. In reality, if the ground plane is too small (less than 6h larger than the patch + 6h), the radiation pattern will distort, and the antenna will radiate backwards (backlobes).