As a continuation of our previous discussion about selection criteria of BLE SoC for building BLE based IoT devices, this guide focuses on BLE PCB antenna impedance matching design — the most critical factor for reliable wireless range. Proper BLE PCB antenna impedance matching design ensures that maximum RF power from the SoC reaches the antenna, directly determining the communication range of any wireless device. The communication range depends mainly on the RF layout, antenna design, and enclosure. Increased operating distance can be achieved with the right antenna type together with carefully designed BLE PCB antenna impedance matching design, using few matching components to minimise signal reflections and maximise radiated power. Our Digital Transformation Services and mission-critical embedded systems teams apply these principles across a wide range of wireless product programmes.
RF Circuit Design: Transmission Line Types for BLE PCB Antenna Impedance Matching Design
Sound rf circuit design begins with routing transmission lines from the BLE SoC to the antenna through a small network of matching components. Transmission lines carry RF power from the SoC to the antenna, and they must be routed with tight constraints to achieve the target 50-ohm characteristic impedance. Selecting the correct transmission line topology is the first step in BLE PCB antenna impedance matching design. Two types dominate practical RF circuit design for BLE applications:
- Microstrip Line
- Coplanar Wave Guide (CPWG)
Both are PCB traces that differ in how they maintain 50-ohm characteristic impedance. Online impedance calculators allow designers to verify trace parameters before fabrication, accelerating the RF circuit design iteration cycle.
Microstrip Line — This transmission line type places a signal trace on the top of a substrate with a ground plane beneath the substrate. A microstrip line is simple to construct, simulate, and fabricate. The characteristic impedance of a Microstrip line depends on the following factors:
- Substrate height (H)
- Dielectric constant of the substrate (εr)
- Width of the trace (W)
- Thickness of the RF trace (T)
MicroStrip Line
CPWG — This is similar to the microstrip, but it has a copper pour on either side of the RF trace with a gap between them. It provides better isolation for RF traces, better EMI performance, and makes it easier to support the grounding of matching components on an RF trace. CPWG is favoured in professional RF circuit design because it reduces radiation from the feed line and simplifies component placement around the matching network. The characteristic impedance of a CPWG depends on the following factors:
- Substrate height (H)
- Dielectric constant of the substrate (εr)
- Width of the trace (W)
- The gap between the trace and the adjacent ground fill (G)
- Thickness of the RF trace (T)
Coplanar Wave Guide
SoC manufacturers provide reference designs that give routing guidelines and the values (typically capacitance and inductance values) of matching components. The designer must also follow the recommended PCB stackups for desired performance, since the impedance of RF lines changes with PCB layer stackups. In many cases the PCB fabricator may not match the exact stackup as recommended; at these conditions there will be a need for slight changes in RF trace width, gaps, or thickness to preserve correct impedance and achieve the target BLE PCB antenna impedance matching design.
ANSYS HFSS and CST Studio Simulation for RF Performance
Before committing to a PCB build, engineers can validate their BLE PCB antenna impedance matching design using electromagnetic field solvers such as ANSYS HFSS and CST Studio Simulation, which accurately model transmission line impedance, S-parameter behaviour, and antenna radiation patterns — significantly reducing costly board re-spins.
Custom Antenna Design Services: BLE Antenna Selection
An antenna is a critical part of any wireless device that transmits and receives electromagnetic radiation in free space. An antenna is a conductor exposed in space: when the length of a conductor is a certain multiple or ratio of the signal wavelength (λ) it radiates electrical energy into free space at that frequency. Delivering custom antenna design services for BLE applications requires balancing range, cost, and form factor. Our custom antenna design services cover all common BLE antenna topologies for the 2.4 GHz band:
- Wire antenna
- PCB Trace Antenna
- Chip antenna
- Wire antenna: A piece of wire rises from the PCB plane and protrudes into free space over a ground plane. Wire antennas produce the best performance and RF range due to their physical dimension and better exposure. They can take different forms such as straight wire, helix, and loop. A through-hole pad is sufficient to solder the wire antenna, saving board area and reducing PCB cost. Custom antenna design services often recommend wire antennas when maximum range is the priority.
Wire Antenna – Straight and Helix Type
- PCB antenna: This is a copper trace drawn on the PCB. These antennas are inexpensive and easy to design because they are part of the PCB itself, and they provide good performance. Meandered trace and Inverted-F trace are the most popular PCB antenna types used in many designs. Meandered trace antenna is recommended for applications that require minimum PCB area, and the Inverted-F antenna offers better radiation than the meandered type, though it requires more board space. The main drawback of PCB antennas is that two or more board revisions may be needed to achieve target range; this risk is reduced by following chip vendor application notes and recommended stackups as part of a disciplined RF PCB design process.
PCB Antenna Types
- Chip antenna: For applications where the PCB size must be extremely small, a chip antenna is a good choice. They are commercially off-the-shelf antennas that occupy very small PCB area and offer reasonable performance. The disadvantages are increased BoM and assembly cost since they are external components that must be purchased and assembled. Chip antennas are also very sensitive to RF ground clearance, and the manufacturer's ground clearance recommendations must be followed strictly to avoid degrading the BLE PCB antenna impedance matching design.
Chip Antenna
Some applications require antennas placed on or outside the enclosure for better reach. In such cases, antenna connectors can be mounted on the board to route the signal to an external antenna through shielded wire, completing the BLE PCB antenna impedance matching design with a low-loss coaxial feed path.
Antenna Connectors
RF PCB Design: Antenna Connector Options
Selecting the correct RF PCB design for the antenna connector is important for signal integrity. The two most common options used in professional RF circuit design are the U.FL and MMCX connectors.
- U.FL connector: These are miniaturised RF connectors for high-frequency signals. The male connectors are generally surface mounted and soldered directly to the PCB. The female connectors are crimped at one end of the shielded wire; the other end may terminate to a PCB antenna or a mating connector such as SMA or MMCX.
U.FL Connectors
- MMCX connector: These are Micro-Miniature Coaxial connectors with a lock-snap mechanism that allows 360-degree rotation. MMCX connectors are comparatively better than U.FL in terms of insertion and removal lifetime — offering over 10 times the cycle life of U.FL connectors. They are also available in surface-mount packages, making them a solid choice in RF PCB design where frequent cable changes are anticipated.
MMCX Connector
About Embien
Embien Technologies is a leading provider of embedded design services for the Semiconductor, Industrial, Consumer and Healthcare segments. Our team delivers end-to-end BLE PCB antenna impedance matching design, RF circuit design, and custom antenna design services for IoT products across a wide range of industries. Embien has successfully executed many projects based on IoT such as healthcare wearables, gateways, and data analytics. Embien also offers wearable design collections complete with electronics, firmware, and cloud integration to shorten product development costs and time significantly.