RF interference is affecting my node — how do I diagnose it
RF Interference Is Affecting My Node - How to Diagnose It
LoRa spread-spectrum modulation gives it excellent resistance to narrowband interference, but it is not immune. If your node is experiencing unexplained packet loss, poor RSSI from nearby nodes, or erratic behavior that does not correlate with distance or obstacles, RF interference may be the culprit.
Symptoms of interference
- High SNR variation: For instance, a link that normally shows around -5 dB SNR may suddenly drop to -15 dB or worse for no physical reason. (Baseline SNR varies from link to link; the diagnostic signal is the sudden, unexplained change, not the absolute number.) If SNR fluctuates wildly over minutes without any change in node position, an external RF source is likely corrupting packets.
- Unexpectedly poor RSSI from nearby nodes: RSSI substantially worse than your link budget predicts can indicate that wideband noise is raising the noise floor. Link-budget estimates carry large uncertainty, so treat a clear, persistent gap between predicted and observed RSSI - not a precise dB figure - as the warning sign.
- Time-of-day failure patterns: If failures cluster during business hours, when appliances run, or at regular intervals (e.g., every 30 minutes), a duty-cycling device such as a smart meter network is a likely source.
- One direction fails more than others: If nodes in one geographic direction consistently perform worse, a directional interference source may be oriented toward your antenna.
Diagnosis tools
The most effective way to see what is happening in the 902 - 928 MHz band is with a Software Defined Radio (SDR):
- Hardware: An RTL-SDR dongle (~$25 USD) with a 915 MHz antenna is sufficient. Higher-end options like HackRF or Airspy offer better sensitivity.
- Software: SDR# (Windows) or GQRX (Linux/macOS) - both are free. Open the spectrum analyzer view and zoom to 902 - 928 MHz.
- What to look for: Constant carriers (a vertical spike that never moves) indicate a narrowband source. Sweeping signals suggest frequency-hopping devices such as cordless phones. Periodic bursts at regular intervals suggest smart meter networks (ITRON, Sensus, etc.) which use this band extensively in North America.
Common interference sources in the 902 - 928 MHz band
- Baby monitors (some older analog models use 900 MHz, though most modern units operate at 2.4 GHz or on DECT at ~1.9 GHz)
- 900 MHz analog/digital cordless phones (older models - note these are not DECT, which is a separate ~1.9 GHz standard)
- Smart meter networks (AMI/AMR systems from utilities)
- Industrial wireless sensors and SCADA equipment
- Some older Wi-Fi extenders and video senders
Mitigation strategies
- Change frequency slot (channel) in Meshtastic: Meshtastic lets you select a different frequency slot within the ISM band while keeping the same modem preset. If a specific frequency is congested, moving to a different frequency slot shifts your center frequency away from the interference - this is the correct way to dodge a narrowband interferer. Important: the modem preset (LongFast vs. LongSlow vs. MedFast) sets the spreading factor and bandwidth (speed vs. range), not the center frequency. Changing the preset does not move you off an interfering frequency, and every other node in your mesh must use the identical region and preset to communicate - if you change your preset you will lose contact with all nodes still on the old preset. Change the frequency slot, not the preset, and keep your mesh on a common preset.
- Use a directional antenna: A Yagi or patch antenna pointed toward your intended mesh nodes, and away from the interference source, provides front-to-back rejection typically on the order of 10 - 20 dB, depending on the antenna design (a small patch may give less; a larger multi-element Yagi can give more).
- Relocate the antenna: Moving the antenna even a few meters can place a building or terrain feature between your node and the interference source, providing significant shielding.
- Reduce antenna height: Counter-intuitively, lowering an antenna can reduce pickup of distant interference while maintaining adequate coverage of nearby nodes.