PCB Antenna Layout & Keep-out Design: Best Practices for IoT Hardware

Introduction: Wireless Hardware

On-board antennas are the silent heroes of modern IoT devices, bridging the gap between digital data and the physical world. For a device to achieve its full potential, from a simple BLE sensor in a smart home to a long-range LoRa tracker, antenna design must be a core consideration, not an afterthought. Even minor changes in a PCB's layout can dramatically impact RF performance, affecting wireless range, battery life, and overall reliability. This guide is designed to provide both entry-level developers and seasoned engineers with a comprehensive checklist covering the foundational principles and common pitfalls of on-board antenna design for 2.4 GHz (Wi-Fi, Bluetooth) and Sub-GHz (433/868/915 MHz) frequencies.

2.4 GHz vs. Sub-GHz: The first consideration is size. Higher-frequency 2.4 GHz antennas (for Wi-Fi/Bluetooth) are compact (a quarter-wave in free space is about 31 mm), while Sub-GHz antennas are significantly larger (a quarter-wave in free space is approximately 75–170 mm). This difference fundamentally affects your choice of antenna type, placement, and overall board layout. On a PCB microstrip or coplanar structure, the actual physical length required will be shorter due to the higher effective dielectric constant of the material.

NextPCB's RF Expertise: Designing for these frequencies demands precision manufacturing, particularly with controlled impedance routing and specialized materials. NextPCB's capabilities in 50 Ω impedance-controlled fabrication, a wide selection of high-frequency substrates (Rogers, PTFE), high-precision assembly, and RF-aware Design for Manufacturability (DFM) support ensure your IoT PCB meets its performance goals. For instance, Rogers laminates offer superior dielectric loss and a more stable dielectric constant than standard FR-4, which is crucial for maintaining signal integrity at 2.4 GHz and beyond.

Table of Contents

1. Antenna Type Selection and Placement
2. Antenna Keep-Out Zone and Ground Clearance
3. Controlled Impedance Feed Line (50 Ω Routing)
4. Antenna Matching Network (Basics of Tuning)
5. Ground Planes and RF Layout Do's & Don'ts
6. Layout Validation and Testing
7. Common Pitfalls and How to Avoid Them
8. NextPCB Advantages for IoT RF PCB Designs
Conclusion: From Concept to Clear Signal

1. Antenna Type Selection and Placement

- Choose the Right Antenna:

Your first step is to select the right antenna for your application and expertise. You can choose between a PCB trace antenna (e.g., inverted-F, meander), a compact chip antenna, or an external whip antenna. For beginners, using a certified module's built-in antenna or a chip antenna with its reference layout is the safest and most reliable approach. Experienced designers might create a custom trace antenna to optimize size and performance for a specific product.

- Prioritize Edge Placement:

Always position the antenna at an edge or corner of the PCB. This placement allows the antenna to radiate freely into space, maximizing performance. For example, popular IoT module manufacturers like Espressif recommend placing the antenna portion of the module so it extends off the main board. This prevents the main PCB's ground plane from blocking the antenna's signal or detuning its frequency.

- Orientation and Feed Point:

The antenna's feed point, where it connects to the RF feed line, should be as close as possible to the main board's circuitry. The bulk of the antenna element should extend outward, away from other components. This minimizes the influence of the board's layout on the antenna's performance and makes tuning more predictable.

- Keep Distance from Metal:

Antennas are sensitive to nearby conductive materials. Keep them as far as possible from metal objects, large components, and especially batteries and displays. Metal can detune the antenna and significantly block the signal, drastically reducing range. For example, placing a LoRa module's antenna right next to a metal shielding can will severely degrade its signal.

2. Antenna Keep-Out Zone and Ground Clearance

Define a No-Copper Zone:

The keep-out zone is a fundamental requirement. It is a designated area on and under the antenna that must be free of all copper pours, traces, and components on every layer of the PCB. For a 2.4 GHz antenna, a clearance of at least 15 mm is a good starting point if the antenna is near the board's interior. This clearance ensures the antenna can radiate efficiently without being detuned by the PCB's ground plane.

How large of a keep-out zone is needed?

Answer: The required keep-out distance is closely tied to the specific chip, chip antenna, or module datasheet. Always follow the official guidelines from the manufacturer (for example, Espressif guidelines typically recommend a minimum of ≥15 mm).

All Layers Matter:

On multi-layer boards, it is critical to remove all ground and power planes from the area directly beneath the antenna. Any metal directly under a PCB antenna can shift its resonant frequency or dampen its efficiency.

Ground Plane Edges:

The main ground plane of the PCB should end just before the antenna keep-out region. For many PCB antenna designs, the ground plane is an integral part of the design, specifically supporting the feed point, but it must not extend under the radiating element itself. Ensure the ground plane edge is a clean, smooth line—avoid irregular shapes that can cause impedance bumps or disruptions to the current path.

Mechanical Clearance:

In addition to copper clearance, you must also ensure mechanical clearance. No tall components or the device's enclosure should intrude into the antenna's space. Even a plastic case or its paint can detune an antenna if it is too close. Always perform a range test with the device in its final housing to confirm the antenna still performs as expected.

3. Controlled Impedance Feed Line (50 Ω Routing)

Design for 50 Ω:

Almost all modern IoT antennas and RF front-ends are designed for a 50 Ω characteristic impedance. Therefore, the PCB trace from the RF chip or module to the antenna (or its connector) must be a 50 Ω impedance-controlled transmission line. This is not merely a design detail but a fundamental requirement for high-speed and high-frequency signals. Without a proper impedance match, the signal will reflect back to the source, causing signal degradation, random errors, and electromagnetic radiation.

How is the 50 Ω trace width calculated?

Answer: The precise trace width needed for a 50 Ω impedance depends on your PCB stack-up, including layer height, the dielectric constant (εr) of the material, and copper thickness. NextPCB provides an online PCB Impedance Calculator that helps designers quickly and accurately determine the correct trace geometry based on real stack-up information. This powerful tool simplifies complex calculations, allowing engineers to ensure the electrical integrity of their designs.

> Recommend reading: PCB Trace Width Calculation: High-Current Design & Thermal Analysis

Maintain a Continuous Reference Plane:

Route the RF trace over a solid, continuous ground plane with no breaks or gaps. A solid reference plane below the feed line ensures the impedance remains consistent and provides a low-inductance return path for the signal current. Routing the RF trace over a split or void in the ground plane is a common and critical mistake that can lead to unpredictable antenna behavior and EMI issues.

Minimize Discontinuities:

Keep the RF feed line as short and straight as possible. Every bend, via, or component pad can create a small impedance discontinuity. Use gentle curves or mitered bends instead of sharp 90° corners. If the feed line must change layers, use via stitching with ground vias placed near the signal via to ensure the return current path is maintained.

Manufacturer Control:

Partner with a manufacturer that specializes in impedance control. NextPCB supports controlled impedance fabrication and can establish target tolerances based on the stack-up and project requirements (with a common industry standard of ±10%), and can provide impedance testing to ensure your RF trace meets its 50 Ω target.

4. Antenna Matching Network (Basics of Tuning)

Include Matching Pads:

Even with a well-designed antenna, it is wise to include a small π-network or L-network footprint between the RF feed line and the antenna. This matching network allows you to fine-tune the antenna's impedance to 50 Ω after fabrication by adjusting the component values. NextPCB's high-precision PCBA services are equipped to handle the placement of these tiny components, including passive parts as small as 01005.

When to Match:

If your antenna's impedance is already close to 50 Ω, the matching network's components might be left unpopulated. However, if testing shows poor performance (high VSWR or low return loss), you can populate or adjust these capacitors and inductors. This is far easier and more cost-effective than changing the PCB layout in a new revision.

What is the principle of antenna matching?

Answer: Antenna matching is about maximizing the transfer of power from the transmitter to the antenna. An impedance mismatch causes power to be reflected back, reducing range and potentially damaging the power amplifier. By conjugately matching the antenna's impedance to 50 Ω, you minimize reflections and ensure all the RF energy is radiated efficiently.

ESD Protection:

For devices that are handled by users, consider adding an ESD protection diode at the antenna feed. RF ESD protectors can safely shunt electrostatic discharges to ground without adding significant capacitance that would interfere with the RF signal.

5. Ground Planes and RF Layout Do's & Don'ts

A Solid Ground is Your Friend:

For the RF section, use a solid ground plane (typically an internal or bottom layer) to provide a stable return path for signals. Stitch this ground plane with an abundance of vias near the antenna feed and matching network components. All RF ground pins should be connected to this plane with vias as close as possible to the pin, which reduces ground inductance and helps create a stable RF reference.

Avoid Interruptions:

Do not route any digital signals or cut slots in the ground plane near the antenna feed line. Any break in the ground plane forces the high-frequency return currents to detour, increasing loss and changing impedance. Similarly, never route the RF trace over a split or void in the ground plane, as this can lead to unpredictable behavior and significant EMI issues.

Component Placement:

Keep noisy components, such as oscillators, DC-DC converters, and high-speed MCU lines, far away from the antenna and its feed line. High-frequency noise from these components can couple into the antenna and its matching network, raising the noise floor and degrading performance. For example, Espressif's guidelines recommend isolating the antenna from high-speed clock lines and well-shielding any UART or SPI lines that must run near the RF section.

> Understanding the Differences Between CPU, MCU, MPU, SoC, DSP, ECU, GPU, and FPGA

Proper Via Usage:

Use plenty of ground vias ("via fence") along the edge of the ground plane adjacent to the antenna area. This helps contain the RF energy and can improve antenna efficiency, particularly for coplanar waveguide designs. However, do not place any vias or metal within the antenna's keep-out zone, as this will detune the antenna.

6. Layout Validation and Testing

Use Simulation Tools:

Before ordering your PCB, consider simulating the antenna and its RF section using specialized electromagnetic simulation software. At a minimum, use your PCB CAD tool's field solver to verify the 50 Ω trace impedance and check for any design rule violations in the RF areas.

How can I quickly check my design files?

Answer: NextPCB offers a powerful, free software tool called HQDFM, which allows you to perform a one-click analysis of your Gerber files to find hidden design hazards and get low-cost optimization solutions. This proactive approach can catch critical issues like insufficient clearances or manufacturability problems in the antenna pattern before you even submit your order.

Prototype Testing:

Once you have the physical boards, perform thorough RF tests. If you have access to a Vector Network Analyzer (VNA), measure the antenna's return loss (S11) to confirm that its resonance is centered in the desired frequency band. A well-tuned antenna should have an S11 dip (low reflection) in your target band. For teams without access to advanced equipment, a simple range test can compare the communication distance or received signal strength indicator (RSSI) against a known good design.

Iterate If Needed:

It is common for RF designs to require a board respin to achieve optimal performance. NextPCB's rapid prototyping services can deliver new boards in as little as 24-72 hours, with standard PCBA orders taking 5-20 business days. In cases where all components are ready, NextPCB can begin assembly in as fast as 24 hours. This agility is invaluable for refining antenna placement and tuning without missing your development schedule.

7. Common Pitfalls and How to Avoid Them

Placing the Antenna in the Middle of the PCB:

This classic mistake traps the antenna in copper, severely hurting its radiation. Always place the antenna at an edge or a corner with open space.

Insufficient Keep-Out Zone:

Pouring copper too close to or under the antenna will detune it and kill its performance. Always respect the keep-out guidelines (e.g., the 15 mm rule for 2.4 GHz) and remove all copper from the designated area on all layers.

Bad Reference Plane Management:

Routing the RF feed line over a split or cutout in the ground plane will cause an impedance mismatch. Always ensure a continuous reference plane for the entire length of the RF trace.

Skipping the Matching Network:

Omitting a matching network to save a few cents can result in an underperforming product. Always include the footprints for tuning components. It is much cheaper to tweak a component value than to respin a board to adjust the antenna geometry. NextPCB's ability to handle fine-pitch components down to 01005 makes this process reliable.

Interference from Other Circuits:

High-speed digital traces or power converters placed too close to the RF section can inject noise. Keep these noisy components far away from the antenna.

Using the Wrong Material:

While FR-4 is suitable for most applications, its properties can vary. For designs that require high consistency or minimal signal loss, using an RF substrate like Rogers or PTFE is the professional choice. NextPCB provides these options to match your material to your performance needs.

> Guide to PCB Materials: Properties, Selection, and Applications

8. NextPCB Advantages for IoT RF PCB Designs

By leveraging a manufacturing partner that understands RF, you can turn a good design into a great one. NextPCB provides a comprehensive suite of services that directly align with the best practices outlined in this guide:

Controlled Impedance & High-Frequency Materials:

NextPCB specializes in fabricating impedance-controlled PCBs with a standard tolerance that can be customized to your project (common industry standard is ±10%), ensuring signal integrity from design to production. They offer a wide range of materials, including Rogers, Taconic, and PTFE, which are crucial for low-loss, high-frequency applications.

Turnkey PCBA & Rapid Prototyping:

The company offers a seamless, end-to-end turnkey assembly service, handling everything from component sourcing, manufacturing, and assembly to testing. The regular Minimum Order Quantity (MOQ) is 5 pieces (with special prototype promotions available), making it accessible for any project size.

Online Quote System & Cost Transparency:

NextPCB's instant online quote system provides real-time pricing and estimated lead times for both PCB fabrication and PCBA. This transparency allows engineers to make informed trade-offs between cost and design choices, accelerating the procurement process. You can simply upload your Gerber and BOM files to get an automatic DFM analysis and a quote within an hour.

>Recommend reading: Custom PCB and PCBA Cost Analysis: How to Optimize Design and Reduce Expenses

Free DFM Tools & Professional Support:

NextPCB's free HQDFM software is a proactive tool that allows you to analyze your design files before submission, automatically identifying hidden hazards and offering optimization suggestions to reduce costs and improve quality. Their team of experienced engineers can also provide one-on-one consultation to help you choose the right stack-up and design practices.

Conclusion: From Concept to Clear Signal

Building a robust IoT device with a reliable antenna is part engineering, part detective work, and part collaboration. By diligently following this checklist, you can avoid common pitfalls and lay a strong foundation for your wireless hardware. Remember that your PCB manufacturer is a critical partner in this process. By leveraging NextPCB's advanced fabrication capabilities, RF-aware DFM support, and rapid prototyping services, you can confidently move from concept to a great product. Here's to clear signals and successful wireless projects!