This guide is for anyone using an SMD chip antenna for the first time and wondering why board layout affects RF performance just as much as the antenna choice itself.
Introduction
You picked a good SMD chip antenna, checked its datasheet and placed it on your board exactly where the reference design shows. However, depending on your layout, the real-world range or received signal strength (RSRP) can be disappointing. One common reason for this behavior is an unfavorable ground plane. For SMD chip antennas, the PCB ground plane serves as a current return path and is therefore a crucial part of your system performance. Antenna manufacturers typically show reference board designs or evaluation boards for each specific antenna. Below you can see an example from Antenova.
Source: Inpai - SR4L099 - Embedded 4G LTE / Cellular / LPWA / NB-IoT Passive FR4 Chip Surface Mount SMD Antenna - Antenova
Typical problems
A few patterns show up regularly in early designs:
- Copying a reference layout’s antenna placement without caring about its board dimensions.
- Assuming any “big enough” ground plane will do, without checking against the actual wavelength used.
- Wrong antenna position on the PCB.
- Treating antenna efficiency as a constant datasheet number, ignoring how sensitive it actually is to several layout parameters.
Background: why the ground plane matters
Most SMD chip antennas are electrically short structures that rely on the PCB ground plane to complete their radiating system. In effect, the ground plane acts as the counterpoise. It’s functionally similar to the radials of a classic quarter-wave vertical antenna. The antenna itself only forms one part of the radiating structure. This means two boards with the identical antenna part number, but different ground plane dimensions, can show meaningfully different efficiency.
Source: Own illustration
The λ/4 rule of thumb
A simple starting point for sizing your ground plane comes from the quarter-wavelength criterion: an effective electrical counterpoise should be on the order of a quarter wavelength of the target frequency to support efficient radiation.
The wavelength follows the standard relationship:
λ = c / f
where c is the speed of light (≈3×10⁸ m/s) and f is your frequency.
Example
For the popular 868 MHz ISM band used across Europe for countless LPWAN and short-range devices:
λ = 3×10⁸ / 868×10⁶ ≈ 345 mm
λ/4 ≈ 86 mm
So, as a rule of thumb, your board should provide an effective ground plane length in the range of at least ~86 mm at 868 MHz to avoid obvious geometric efficiency losses. Below that, your design may still work, but you’re fighting against physics with a shrinking counterpoise.
Summary and practical guidelines
- SMD chip antennas rely on the PCB ground plane as their counterpoise. The ground plane length is comparable to the radials length in a quarter-wave antenna.
- Rule of thumb: effective ground plane length ≥ λ/4, calculated at your lowest frequency used.
- Respect the antenna vendor’s recommended keep-out zone and account for this extra clearance in your overall board dimensions. Antenna vendors typically specify a clearance area beneath and beside the antenna with no ground fill.
- Placing the antenna at a board corner tends to make better use than centering the antenna on an edge.
- If your board is significantly smaller than the antenna vendor’s reference layout, treat published efficiency figures with caution. They were likely measured on a board that met or exceeded the λ/4 guideline.
- If space constraints force a smaller board, expect an efficiency decrease and budget for it (e.g. shorter range, higher transmit power to compensate).
Further information
The antenna manufacturer Ignion offers a free online course on antenna design that I can personally recommend. You can access the course here: https://virtualantennamooc.salle.url.edu/