900 MHz PCB Antenna Design: Complete Guide for US ISM & LoRa 915 MHz

Designing a 900 MHz PCB antenna for the US ISM band requires understanding the unique characteristics of the 902–928 MHz spectrum. When I transitioned from European 868 MHz designs to US 915 MHz projects, I expected everything to scale directly—after all, it’s only a 5% frequency difference. The antenna dimensions are indeed similar, but the regulatory environment is completely different. The US band is wider (26 MHz versus 7 MHz), allows significantly higher power (up to 1W versus 25mW), and has no duty cycle restrictions. These differences affect how you optimize your antenna design.

This guide covers practical 900 MHz PCB antenna design for US LoRa applications, Meshtastic nodes, Helium hotspots, and general ISM band devices. Whether you’re building an SX1262-based sensor, a long-range mesh network node, or an industrial IoT gateway, these dimensions and layout guidelines will help you achieve reliable performance. I’ll focus on proven antenna structures that work on standard FR4—meander monopoles, inverted-F antennas (IFA), and inverted-L antennas (ILA)—with specific dimensions for 915 MHz center frequency.

Understanding the US 900 MHz ISM Band

The US ISM band centered at 915 MHz offers significant advantages over the European 868 MHz band, particularly for LoRa and other spread-spectrum applications.

US 902–928 MHz Band Specifications

The wider bandwidth and higher power allowance make the US band particularly attractive for long-range applications. A well-designed 900 MHz PCB antenna can help you achieve multi-kilometer range with standard LoRa modules.

LoRaWAN US915 Channel Plan

Your antenna needs to cover 902–928 MHz with reasonable efficiency across the entire band. The 26 MHz bandwidth requirement means slightly wider-band antenna designs compared to the narrower European band.

900 MHz Wavelength Calculations

At 915 MHz, the quarter wavelength (82mm) is slightly shorter than at 868 MHz (86mm). This 5% difference means your 868 MHz antenna designs will resonate slightly low when used at 915 MHz—typically requiring trace shortening or matching network adjustment.

900 MHz PCB Antenna Types Compared

Several antenna topologies work well at 900 MHz. Silicon Labs’ AN847 application note documents nine different types specifically for the 915 MHz band.

Antenna Type Selection for 900 MHz

When to Use Each 900 MHz Antenna Type

For most 900 MHz PCB antenna applications, the meander monopole or IFA provides the best trade-off between size and performance. The wire monopole remains king for pure range but requires 82mm of straight length.

Meander Antenna Design for 915 MHz

Meander antennas compress the electrical quarter-wavelength into a serpentine pattern. At 915 MHz, dimensions are slightly smaller than at 868 MHz.

How 900 MHz Meander Antennas Work

The meander monopole folds a quarter-wave element back and forth. The total trace length must approximate the effective quarter wavelength on your PCB substrate (typically 49–60mm on FR4), while the physical footprint shrinks to 40–60% of a straight monopole.

Key design parameters:

• Total trace length: 50–65 mm typical
• Physical footprint: 25–45 mm length
• Bandwidth: 20–40 MHz (adequate for full US band)
• Efficiency: 70–90% depending on ground plane

Meander Antenna Dimensions for 915 MHz

Compact Meander (25 × 12 mm footprint):

Standard Meander (38 × 18 mm footprint):

TI Reference Meander (38 × 25 mm footprint):

Based on Texas Instruments DN024 design note:

The TI DN024 reference design is well-documented and achieves excellent performance. It’s worth studying even if you modify dimensions for your specific application.

IFA and ILA Design for 900 MHz

Inverted-F antennas (IFA) and Inverted-L antennas (ILA) route along the PCB edge, efficiently using board perimeter. These are popular for 900 MHz PCB antenna implementations in commercial products.

IFA vs ILA Comparison at 915 MHz

IFA Dimensions for 915 MHz

ILA Dimensions for 915 MHz

Ground Plane Requirements for 900 MHz

The ground plane is critical for any 900 MHz PCB antenna. It serves as the antenna’s counterpoise and significantly affects both impedance and radiation pattern.

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Minimum Ground Plane Dimensions

Ground Plane Design Rules

At 915 MHz, a ground plane of at least 25×45mm provides acceptable performance. Larger grounds improve efficiency and pattern consistency, but diminishing returns occur beyond about 50×80mm.

SX1276 and SX1262 US Band Integration

The Semtech SX1276 and SX1262 are the dominant LoRa transceivers for US 915 MHz applications. Both have straightforward RF interfaces.

LoRa Transceiver Specifications

Popular US 915 MHz LoRa Modules

Most commercial modules include internal matching optimized for 50Ω. When designing custom boards with bare SX1262/SX1276, always include Pi matching network footprints.

Matching Network Design for 915 MHz

Even well-designed antennas benefit from matching networks to optimize performance and compensate for enclosure effects.

Pi (CLC) Matching Network

LoRa Module ──[C1]──┬──[L1]──┬──[C2]── Antenna                    │        │                   GND      GND

Starting Component Values for 915 MHz

For a typical PCB antenna presenting slightly inductive impedance:

Component Selection Guidelines

High-Q components are essential. Using cheap multilayer ceramic inductors wastes 1–3 dB of link budget—significant when you’re trying to maximize LoRa range.

PCB Layout Guidelines for 900 MHz

Proper layout ensures your 900 MHz PCB antenna performs as designed and survives production variation.

Antenna Placement Rules

50Ω Microstrip Dimensions for 915 MHz

Layout Checklist

Meshtastic and Helium Antenna Considerations

Meshtastic mesh networking and Helium IoT mining have driven significant interest in optimized 900 MHz PCB antenna designs.

Meshtastic Device Requirements

For Meshtastic, antenna efficiency matters more than raw gain because devices are often battery-powered and portable. A well-matched PCB antenna often outperforms a poorly-matched external antenna.

Helium Hotspot Antenna Selection

Helium hotspots benefit from external antennas in most cases, but PCB antennas work for development and indoor testing.

Testing Your 900 MHz PCB Antenna

Proper testing validates your design before production.

Test Equipment

S11 (Return Loss) Targets

LoRa Range Testing Benchmarks

LoRa’s spread-spectrum modulation allows communication at very low signal levels. With SF12 and proper antenna design, communication at -130 dBm or lower is achievable.

Common 900 MHz PCB Antenna Mistakes

Mistake 1: Using 868 MHz Dimensions Without Adjustment

Problem: Copying European 868 MHz antenna directly for US 915 MHz. Effect: Antenna resonates 5% low, increased VSWR, reduced efficiency. Solution: Scale dimensions by ~5% shorter, or adjust matching network.

Mistake 2: Insufficient Bandwidth for US Band

Problem: Narrow-band antenna design that works at 915 MHz but fails at band edges. Effect: Poor performance on some LoRaWAN channels (902–928 MHz range). Solution: Design for > 30 MHz bandwidth to cover entire US ISM band.

Mistake 3: Ground Plane Too Small

Problem: Inadequate ground plane for 900 MHz operation. Effect: Pattern distortion, impedance variation, reduced gain. Solution: Minimum 25×45mm ground, larger preferred.

Mistake 4: Forgetting Inner Layer Copper

Problem: Ground or power plane copper under antenna on inner PCB layers. Effect: Severe capacitive loading, resonance shift of 50+ MHz possible. Solution: Check ALL layers in design review—create keep-out on every layer.

Mistake 5: Ignoring Enclosure Effects

Problem: Antenna tuned on bare PCB, then enclosed in plastic housing. Effect: Resonance shifts down 5–20 MHz, VSWR degrades. Solution: Final tuning with enclosure in place, use matching network.

Mistake 6: Battery Placement Under Antenna

Problem: Li-ion cell positioned beneath PCB antenna area. Effect: Significant detuning, pattern distortion. Solution: Position battery away from antenna, add ground shield if needed.

Useful Resources for 900 MHz Antenna Design

Application Notes and Design Guides

Design Tools

Reference Module Designs

Frequently Asked Questions

What’s the difference between 868 MHz and 915 MHz PCB antennas?

The physical dimensions are similar—quarter wavelength is 86mm at 868 MHz versus 82mm at 915 MHz, about 5% difference. However, the US 915 MHz band is much wider (26 MHz versus 7 MHz), so your 900 MHz PCB antenna needs broader bandwidth to cover all LoRaWAN channels. US regulations also allow significantly higher power (1W versus 25mW) with no duty cycle limits, which affects matching network design if you’re pushing maximum output. An 868 MHz antenna will work at 915 MHz but with degraded VSWR—typically 1.5–2.5 dB worse return loss. For best performance, design specifically for your target frequency.

Can I use the same antenna for 902 MHz and 928 MHz?

Yes, but you need adequate bandwidth. Design your antenna for approximately 915 MHz center frequency with at least 30–40 MHz bandwidth (S11 < -10 dB across 902–928 MHz). Most properly designed PCB antennas naturally achieve this bandwidth. If your antenna is too narrow-band, it may work well at 915 MHz but show 3–5 dB degradation at band edges where some LoRaWAN channels operate. During testing, verify S11 at 902, 915, and 928 MHz—not just the center frequency. Pi matching networks can broaden bandwidth if needed.

How much range improvement can I expect from a better antenna?

Antenna improvements follow the link budget equation. Every 3 dB improvement in antenna system (combination of gain and efficiency) roughly doubles your range in free space, or significantly improves reliability at fixed distance. Going from a -3 dBi chip antenna to a +2 dBi PCB antenna (5 dB improvement) can increase range by approximately 80% in ideal conditions. In real-world environments with obstructions, the improvement is often more dramatic because a better antenna can overcome marginal signal conditions. For Meshtastic and LoRa applications, investing in proper antenna design often yields more range improvement than increasing transmit power.

Do I need a matching network for my 900 MHz PCB antenna?

Almost always yes, at least during development. Include Pi matching network footprints in your design even if initial simulations suggest direct 50Ω match. Real-world factors—PCB manufacturing variation, enclosure effects, nearby components—inevitably shift impedance from ideal. Production antennas typically need matching adjustment after enclosure testing. You can populate with 0Ω series and leave shunts open initially, then add components as needed. For high-volume production, the matching network also allows compensation for PCB batch-to-batch variation without board respins.

What’s the minimum PCB size for a working 900 MHz antenna?

The minimum practical PCB size for an integrated 900 MHz PCB antenna is approximately 25mm × 45mm. This provides space for a compact meander antenna (25mm × 12mm) plus the minimum ground plane needed for the antenna to radiate effectively. Smaller boards require chip antennas or external wire antennas. The ground plane dimension matters more than total board size—a long narrow board (20mm × 60mm) often works better than a square board (35mm × 35mm) because the ground plane can extend adequately in at least one dimension. For Meshtastic and long-range LoRa applications, target 35mm × 55mm or larger for best results.

Conclusion

Designing a successful 900 MHz PCB antenna for the US ISM band requires attention to the specific characteristics of the 902–928 MHz spectrum. The wider bandwidth compared to European 868 MHz means your antenna needs broader frequency coverage, while the higher allowed power and lack of duty cycle limits make efficiency optimization more rewarding.

For new US 915 MHz designs, I recommend starting with the TI DN024 meander reference (38×25mm) if space permits—it’s well-documented and achieves excellent efficiency. For tighter spaces, an IFA along the board edge (50–65mm length) provides good performance with efficient use of PCB real estate. Either way, include Pi matching network footprints and plan for tuning after enclosure integration.

Test with a NanoVNA across the full 902–928 MHz band, not just at 915 MHz. Target S11 below -10 dB across the entire band with at least 30 MHz bandwidth. Compare LoRa RSSI readings against a known-good reference antenna to validate real-world performance.

The US 915 MHz band continues growing for LoRa, Meshtastic, and industrial IoT applications. With proper 900 MHz PCB antenna design, you can achieve 5+ kilometer range with standard LoRa modules—impressive performance from a few square centimeters of copper trace on FR4.