Sub-GHz PCB Design for 433, 868 & 915 MHz

What Is a Sub-GHz PCB?

A sub-GHz PCB is a printed circuit board designed to operate at frequencies below 1 GHz—most commonly at 315, 433, 868 or 915 MHz. These frequencies serve long-range, low-power protocols such as LoRa, Sigfox, Z-Wave, TPMS, keyless entry and industrial telemetry. Compared to 2.4 GHz and higher bands, sub-GHz signals propagate farther for the same transmit power and penetrate buildings more readily, but the longer wavelengths make on-board antennas physically larger and ground-plane management more critical.

Why Sub-GHz Frequencies Still Matter

Despite the popularity of Wi-Fi and Bluetooth at 2.4 GHz, the sub-GHz ISM bands remain the backbone of battery-powered IoT. A 915 MHz link in free space enjoys roughly 8 dB less path loss than a 2.4 GHz link at the same distance (Friis equation), translating directly into longer range or lower transmit power. Regulatory duty-cycle limits in the 868 MHz European band (typically 1 % or 10 % depending on sub-band) and FCC Part 15.247 rules at 915 MHz shape protocol design, but the physics advantage is real.

Common applications include RFID, IoT and LoRa modules, smart metering, agricultural sensors, TPMS tire-pressure sensor boards, and alarm systems. Each has different range, size and cost constraints, but they all share the same set of PCB-level design challenges.

Stackup and Material Selection

Can You Use FR-4 at Sub-GHz?

Yes—and most designers do. At 433–915 MHz the dielectric loss of standard FR-4 (Dk ≈ 4.3–4.5, Df ≈ 0.018–0.025 at 1 GHz) is usually acceptable. A 50 Ω microstrip on 1.0 mm (39.4 mil) FR-4 with 1 oz copper is roughly 1.8 mm (71 mil) wide, which is easy to route and etch. The loss per unit length at 915 MHz on such a line is typically 0.15–0.25 dB/cm, and since sub-GHz traces are short (a few centimeters at most from transceiver to antenna), total insertion loss stays well below 1 dB.

Move to a low-loss laminate only if you have a specific reason: very tight link budget, a filter or balun that needs a tighter Dk tolerance (±2 % instead of FR-4’s ±10 %), or a hybrid board that also carries a millimeter-wave section. For pure sub-GHz work, spending on Rogers RO4350B (Dk 3.48, Df 0.0037) is rarely justified.

Recommended Layer Count

A two-layer board is the cost-optimal choice for simple sub-GHz radios. Place the RF section on the top layer with a continuous ground plane on layer 2 directly beneath it. If the board also carries digital buses, a four-layer stackup (signal–ground–power–signal) gives better isolation and a cleaner return path. The key rule: every RF trace must have an unbroken reference plane immediately adjacent.

Impedance-Controlled Trace Routing

The single most common failure mode on sub-GHz boards is a poorly controlled 50 Ω feed line between the transceiver output and the antenna or connector. At 915 MHz a quarter wavelength in FR-4 is about 38 mm (1.5 in), so even a 10 mm trace that drifts to 65 Ω creates a noticeable mismatch.

Practical Guidelines

• Trace width: Calculate for 50 Ω microstrip using your actual stackup (substrate height, Dk, copper weight). On 1.0 mm FR-4 with Dk 4.4 and 1 oz copper, expect roughly 1.8 mm width.
• Ground clearance: Keep the ground copper at least 3× the substrate height away from the microstrip edges to avoid altering the impedance. Do not pour copper fills right up to the trace edge unless they are stitched ground.
• Bends: Use 45° miters or curved bends. At sub-GHz the electrical effect of a right-angle bend is small, but mitering costs nothing and avoids acid traps during etching.
• Via stitching: Place ground vias every λ/20 or closer along both sides of the RF trace. At 915 MHz, λ/20 ≈ 16 mm, so a via every 10–15 mm is comfortable.
• Component pads: Land the matching-network pads (0402 or 0603) directly in-line with the 50 Ω trace. Stubs to side-mounted pads add parasitic inductance that shifts the match.

For foundational concepts on trace impedance and return paths, see our overview of antenna basics for PCB designers.

Antenna Options for Sub-GHz Boards

Antenna choice dominates the mechanical design of any sub-GHz PCB because the wavelengths are large: λ₀ at 433 MHz is 692 mm, at 868 MHz it is 345 mm, and at 915 MHz it is 328 mm. A quarter-wave monopole at 433 MHz is 173 mm—far too long for most enclosures. Designers therefore trade efficiency for size using one of several strategies.

PCB Trace Antennas

A meandered inverted-F antenna (IFA) or a monopole etched directly on the PCB is the lowest-cost option. The critical requirement is a ground plane of adequate size: for 868/915 MHz the ground plane should be at least 40 × 80 mm for reasonable efficiency. Shrinking the ground below λ/4 in any dimension shifts the resonant frequency and degrades gain rapidly. Our LoRa antenna design guide walks through the geometry in detail for 868 and 915 MHz.

Chip Antennas

Chip antennas (e.g., Johanson 0868AT43A0020, Taoglas PC81) save board area but demand strict adherence to the vendor’s recommended keep-out zone and ground-plane dimensions. Moving the antenna even 2 mm from the datasheet position can shift resonance by 20–30 MHz—enough to move you out of band. Always plan for a π-network matching section between the transceiver and the antenna feed point.

Choosing the Right Antenna

For help weighing chip, PCB-trace and external antenna trade-offs across IoT frequency bands, see our IoT antenna selection guide.

Matching Network Design

Nearly every sub-GHz radio IC (TI CC1120, Semtech SX1276, Silicon Labs EFR32) presents a non-50 Ω output impedance—often in the range of 15–30 + j10–j30 Ω. A two- or three-element LC matching network transforms this to 50 Ω at the antenna port.

Step-by-Step Approach

1. Get the IC impedance. Use the value from the datasheet’s Smith-chart plot at your exact frequency and supply voltage. TI’s SmartRF Studio provides measured impedance data for CC-series transceivers.
2. Design a target network. A simple L-network (series L, shunt C or vice-versa) often suffices. For narrowband sub-GHz links a two-element match gives Q ≈ 3–5, which is fine for the 26 MHz-wide 902–928 MHz ISM band.
3. Simulate with parasitics. A 0402 capacitor has roughly 0.4–0.6 nH of parasitic inductance; a 0402 inductor has 0.05–0.1 pF of parasitic capacitance. At 915 MHz these shift the match noticeably. Use the component vendor’s S-parameter models (Murata SimSurfing, Coilcraft S-parameter files).
4. Leave pads for tuning. Place a three-pad π-network footprint even if you only populate two components. The spare pad lets you add a shunt element during bench tuning without a board respin.
5. Verify on the bench. Measure S11 with a VNA at the antenna port. Target |S11| ≤ −10 dB (VSWR ≤ 1.93) across the operating band. For LoRa in EU868, that band is 863–870 MHz; for US915, it is 902–928 MHz.

Layout Pitfalls and How to Avoid Them

Ground-Plane Splits Under RF Traces

A slot or split in the ground plane beneath a 50 Ω trace forces the return current to detour, raising impedance and radiating. Route all non-RF signals around the RF section, not through it. If a digital trace must cross, do so on an adjacent layer and bridge the ground split with closely spaced vias.

Crystal and Clock Noise

Sub-GHz receivers have noise figures around 6–8 dB, and their sensitivity can reach −130 dBm (LoRa, SF12). A 32 MHz crystal oscillator’s third harmonic at 96 MHz or a switching regulator’s harmonics can land in-band. Keep the crystal within 5 mm of the IC but shield it from the antenna feed with a ground-via fence. Use a ferrite bead or LC filter on the power-supply rail entering the RF section.

Enclosure and Proximity Effects

Plastic housings with metallized paint, nearby batteries and LCD assemblies all detune a sub-GHz antenna. Budget 10–15 % frequency shift from free-space resonance and plan to retune the matching network after the first in-enclosure measurement. Slot the enclosure wall opposite the antenna if the housing is metallized.

Sub-GHz vs Higher-Frequency Design

Engineers familiar with 2.4 GHz or UWB PCB design at 3–10 GHz often underestimate how much the ground plane matters at sub-GHz. At 2.4 GHz a 25 × 25 mm ground plane is already a respectable fraction of a wavelength; at 433 MHz the same plane is electrically tiny and the antenna’s radiation resistance drops, hurting efficiency. Conversely, dielectric loss and surface roughness—dominant concerns at millimeter-wave frequencies—are negligible at sub-GHz, which is why FR-4 works well here.

Design Checklist for Sub-GHz PCBs

• Continuous ground plane under all RF traces—no splits, no routing channels.
• 50 Ω microstrip width calculated for the actual stackup Dk and height.
• Ground-plane area ≥ 40 × 80 mm for an on-board 868/915 MHz antenna (larger is better).
• π-network matching footprint between transceiver and antenna, with a spare pad.
• Ground-via fence around the RF section, pitch ≤ λ/20.
• Crystal oscillator placed close to the IC, shielded from the antenna feed.
• Switching-regulator inductor and its input capacitor kept away from the antenna.
• Antenna keep-out zone per vendor datasheet (chip antenna) or simulation (PCB trace antenna).
• Fabrication note specifying controlled impedance with target, tolerance (±10 % typical) and reference stackup.
• Plan for in-enclosure VNA measurement and matching-network retune.

Frequently Asked Questions

What is the best PCB material for a sub-GHz board?

Standard FR-4 (Dk ≈ 4.3–4.5, Df ≈ 0.020) is sufficient for most sub-GHz designs. Dielectric loss at 433–915 MHz is low enough that a few centimeters of microstrip adds well under 1 dB. Reserve low-loss laminates for hybrid boards that also carry microwave or millimeter-wave circuitry.

How big does the ground plane need to be at 915 MHz?

For a PCB trace antenna, aim for at least 40 × 80 mm of unbroken ground. Smaller planes shift the antenna resonance upward and reduce radiation efficiency. A chip antenna’s required ground dimensions are specified in its datasheet and are typically similar.

Can one PCB cover both 868 MHz and 915 MHz?

Yes. The two bands are only 47 MHz apart, so a single antenna with a bandwidth of roughly 60–70 MHz (easily achievable with a well-matched IFA or monopole) covers both. Use the matching network to center the return loss between 863 and 928 MHz. Populate different firmware or channel tables per region.

Do I need a balun for a sub-GHz transceiver?

It depends on the IC. Differential-output transceivers (e.g., CC1101, CC1120) need a balun or differential matching network to drive a single-ended 50 Ω antenna. Single-ended parts (e.g., SX1276) do not. Always check the reference design in the transceiver datasheet.

How do sub-GHz boards compare to 5G mmWave boards?

Sub-GHz boards are simpler in materials and fabrication but harder in antenna size management. Millimeter-wave boards at 28 GHz face the opposite trade-off: tiny antennas but extreme sensitivity to dielectric tolerance and surface finish. The two worlds rarely share a stackup.