The 915 MHz ISM band (specifically 915–928 MHz in Australia) underpins much of today’s IoT and industrial telemetry landscape — powering systems such as LoRaWAN, Sigfox, and wireless sensor networks.
While transmit power and modulation schemes determine signal robustness, the antenna defines how efficiently that signal travels. Choosing the right antenna can mean the difference between a marginal link and a rock-solid connection.
This post explores the key antenna types for the 915 MHz ISM band — both omnidirectional and directional — explaining their structures, behavior, and ideal use cases, with a focus on real-world deployment in Australia.
1. Understanding the 915 MHz Context
At 915 MHz, the wavelength in free space is about 33 cm.
That means a quarter-wave antenna is roughly 8.2 cm long, and a half-wave antenna is about 16.4 cm.
This wavelength is short enough for compact antenna designs yet long enough to provide excellent propagation through vegetation, walls, and terrain — a major reason sub-GHz bands are favored for IoT.
2. Omnidirectional Antennas
Omnidirectional (omni) antennas radiate energy equally in all horizontal directions, offering 360° coverage. These are the go-to option for gateways, mobile nodes, or mesh networks, where signal direction is unpredictable.
2.1 Quarter-Wave Monopole
A simple metal rod or trace about one-quarter wavelength long, mounted above a ground plane (like a PCB or metal enclosure).
- Radiation Pattern: Doughnut-shaped, strongest to the sides.
- Gain: Around 2 dBi.
- Polarization: Linear, typically vertical.
- Use Case: Compact, inexpensive, and ideal for small IoT nodes — provided the ground plane is well designed.
2.2 Half-Wave Dipole
Two quarter-wave arms joined at a central feedpoint, requiring no ground plane.
- Radiation Pattern: Omnidirectional in the horizontal plane.
- Gain: About 2.15 dBi.
- Polarization: Linear (vertical or horizontal).
- Use Case: Common reference antenna for testing and calibration. Keep it clear of metal objects to maintain proper tuning.
2.3 Collinear (Multi-Element) Antenna
Several dipoles stacked vertically, separated by phasing sections to align signals in phase, resulting in a compressed vertical beam and greater horizontal range.
- Radiation Pattern: 360° horizontally, narrower vertically.
- Gain: Typically 3–9 dBi.
- Polarization: Vertical.
- Use Case: Ideal for base stations or gateways that need long horizontal coverage.
2.4 PCB / Chip Antennas
Printed directly on a circuit board or integrated as surface-mounted chips, these are optimized for small devices.
- Radiation Pattern: Nearly omnidirectional but influenced by nearby components or enclosures.
- Gain: –2 to +2 dBi.
- Polarization: Linear.
- Use Case: Space-constrained IoT designs. Follow layout guidelines precisely, and retune with a vector network analyzer (VNA) if enclosure materials change.
3. Directional Antennas
Directional antennas focus their radiation in one direction, increasing gain and range while minimizing interference. They are used in point-to-point and sectoral applications.
3.1 Yagi–Uda Antenna
Consists of a driven dipole, a reflector, and several directors on a boom.
- Radiation Pattern: Narrow front beam (~40–60° wide).
- Gain: 6–12 dBi.
- Polarization: Linear (depends on element orientation).
- Use Case: Excellent for rural or long-distance links. Requires careful alignment for optimal results.
3.2 Patch / Panel Antenna
A flat metal patch suspended above a ground plane, radiating through its edges. Multiple patches can form an array for higher gain.
- Radiation Pattern: Moderate beamwidth (60–90°).
- Gain: 8–16 dBi, depending on design.
- Polarization: Linear or circular.
- Use Case: Rugged and compact — great for fixed outdoor installations like LoRaWAN gateways or sector coverage.
3.3 Helical Antenna (Axial-Mode)
A wire wound into a helix shape, radiating circularly polarized energy when the circumference is about one wavelength.
- Radiation Pattern: Narrow beam (30–50°).
- Gain: 8–15 dBi.
- Polarization: Circular (right-hand or left-hand).
- Use Case: Perfect for drones or mobile systems where device orientation varies. Use matching polarization on both ends for best performance.
3.4 Parabolic Dish
A reflective dish that concentrates waves from a small feed antenna at its focal point.
- Radiation Pattern: Very narrow main beam (10–20° typical).
- Gain: 15–24 dBi.
- Polarization: Determined by feed.
- Use Case: High-gain, long-range point-to-point links. Requires accurate alignment and strong mounting against wind.
4. Polarization and Mounting Tips
Matching polarization between antennas is crucial. A mismatch (e.g., vertical vs. horizontal) can cause up to 20 dB loss.
Mounting Guidelines:
- Most IoT systems use vertical polarization.
- Keep antennas at least 8 cm from metal surfaces.
- Maintain line-of-sight for directional types.
- Use a solid ground plane for monopoles.
- Weatherproof outdoor connections to prevent corrosion.
5. Gain, EIRP, and Australian Compliance
Under the ACMA LIPD Class Licence 2015, 915 MHz ISM devices in Australia may emit up to 30 dBm EIRP (Effective Isotropic Radiated Power).
Example:
If your transmitter outputs 20 dBm and your antenna adds 6 dBi of gain, your EIRP is 26 dBm — safely under the limit.
Switching to a 12 dBi Yagi would push it to 32 dBm, so you’d need to lower the transmitter power to stay compliant.
For frequency and spectrum allocations, see the Australian Radiofrequency Spectrum Plan 2021.
6. Choosing the Right Antenna
| Application | Recommended Type | Reason |
|---|---|---|
| Urban LoRaWAN Gateway | 6–8 dBi collinear | Full 360° coverage for nearby nodes |
| Rural Gateway / Farm Telemetry | 9–12 dBi Yagi | Focused beam for long-distance sensors |
| Point-to-Point Backhaul | Panel or Yagi | High gain and clear directionality |
| Drones / Mobile Systems | Whip or Helical | Handles changing orientation |
| Dense Industrial Area | PCB / Low-gain Omni | Resilient to reflections and multipath |
7. Key Takeaways
- Omnidirectional antennas suit multi-node and mobile systems.
- Directional antennas excel at long-distance, fixed links.
- Always stay within EIRP limits and align polarization.
- Minimize cable losses — every 3 dB of loss halves your effective power.
- Use a VNA to confirm antenna tuning (VSWR below 2:1 near 915 MHz).
References
- ACMA Radiocommunications (LIPD) Class Licence 2015
- Australian Radiofrequency Spectrum Plan 2021
- Semtech LoRa Technology Overview
- LoRa Alliance Technical Resources
- ITU ISM Band Allocations