PCB Antenna Length Calculator
Use this tool to estimate a starting trace length for common PCB antennas and a rough 50 Ω microstrip feed width.
Note: These values are first-pass estimates. Final antenna dimensions should always be tuned with VNA measurements and your final enclosure/battery in place.
What this PCB antenna calculator does
Designing a PCB antenna is usually a mix of physics, layout constraints, and practical tuning. This calculator gives you a reliable starting dimension by combining free-space wavelength, effective dielectric loading, velocity factor, and an empirical trim. The output is intended for early design, not final certification.
It is especially useful when you are building boards for 2.4 GHz, sub-GHz ISM bands (915 MHz/868 MHz/433 MHz), or other RF products where board space is tight and you need a quick first estimate before simulation and lab tuning.
How the math works
1) Free-space wavelength
The calculator first computes free-space wavelength: λ0 = c / f, where c is the speed of light and f is frequency in Hz.
2) Effective dielectric loading
A PCB antenna does not radiate in free space only; fields partly travel in dielectric material. That slows wave propagation. The calculator uses an effective permittivity estimate and converts λ0 into a guided wavelength λg.
3) Antenna-type factor and trimming
Different antenna styles use different fractions of wavelength. For example, quarter-wave traces are based around λg/4, while compact meanders are usually shorter due to loading. A trim percentage then reduces physical length for real-world fringing and nearby metal/plastic effects.
Input guide
- Operating Frequency (MHz): Your target center frequency.
- Antenna Style: Sets the electrical length factor used in the estimate.
- Dielectric Constant (Er): FR-4 is often 4.0 to 4.6 depending on vendor and frequency.
- Substrate Thickness: Board thickness from signal layer to reference plane for feed estimate.
- Velocity Factor: Helps account for practical slowing from geometry and nearby objects.
- Empirical Trim: Extra reduction to get closer to measured resonance on first spin.
Quick reference values
| Band | Frequency | Free-Space λ0 | Quarter-Wave in Air |
|---|---|---|---|
| 2.4 GHz ISM | 2400 MHz | ~124.9 mm | ~31.2 mm |
| 915 MHz ISM | 915 MHz | ~327.6 mm | ~81.9 mm |
| 868 MHz ISM | 868 MHz | ~345.4 mm | ~86.3 mm |
| 433 MHz ISM | 433 MHz | ~692.4 mm | ~173.1 mm |
Practical PCB layout advice
Keep-out region matters
Maintain a copper keep-out near the antenna trace (all layers if possible). Ground under the radiator can shift resonance and lower efficiency.
Use a short matching section
Reserve a pi-network footprint (series + two shunt pads) between RF pin and antenna feed. You may only populate one or two components after VNA tuning.
Watch enclosure and battery effects
Plastic type, battery position, user hand proximity, and nearby cables can detune the antenna significantly. Tune with the final mechanical stack-up, not with a bare board only.
Common mistakes to avoid
- Copying a reference antenna shape without copying the same ground geometry and clearances.
- Skipping matching pads, then having no practical way to retune late in the cycle.
- Routing noisy digital clocks or switching currents near the feed point.
- Assuming all FR-4 has the same Er and loss tangent.
- Evaluating radiation only on bench without final enclosure installed.
Final note
This calculator is built for fast engineering estimates. For production-grade RF performance, combine this result with EM simulation (or vendor reference layouts), then perform VNA/S11 tuning and OTA testing in the final product form factor.