SNR (Radio)

Signal-to-Noise Ratio (SNR) on a radio link is the difference, expressed in decibels, between the received signal power and the noise floor at the receiver. A positive SNR means the signal is louder than the surrounding noise; a negative SNR means the signal is actually buried beneath it. Where the received-signal-strength reading tells you how loud a signal is in absolute terms, SNR tells you how cleanly it stands out from the electrical hash around it — and for the mesh-radio links that sovereignty-minded builders care about, it is usually the number that decides whether a packet gets through.

What the noise floor is

Every receiver hears a baseline of unwanted energy: thermal noise in its own electronics, plus man-made interference from switching power supplies, LED drivers, motors, computers, and other transmitters sharing the band. That aggregate is the noise floor, and it varies enormously by location — a quiet rural hilltop may sit tens of decibels below an urban rooftop or, notably, a mining container full of high-current PSUs and fans. Because SNR is measured against this floor, the same transmitter and antenna can deliver a comfortable link in one place and an unusable one in another. Lowering local noise (moving the node away from switching supplies, using shielded cable, elevating the antenna) improves SNR just as surely as raising transmit power.

Why LoRa users watch SNR

For long-range, low-power mesh radio, SNR is often the deciding factor for whether a packet decodes. Typical LoRa SNR values run from about -20 dB to +10 dB. Remarkably, the chirp spread-spectrum modulation LoRa uses can demodulate signals roughly 7.5 to 20 dB below the noise floor, depending on spreading factor — a trait that lets a few milliwatts carry messages for kilometres. That capability is exactly why off-grid Bitcoiners reach for LoRa-based tools like Meshtastic to relay messages or coordinate without depending on carrier networks: the modulation is engineered to dig signals out of noise that would drown conventional narrowband radio.

Reading SNR in the field

A higher spreading factor improves SNR tolerance at the cost of slower data and longer airtime, which interacts with regional transmit limits governed by the radio duty cycle. When a hop is unreliable, compare SNR against signal strength and the diagnosis usually writes itself: a strong signal with poor SNR points to local interference at one end — find and kill the noise source; a weak signal with adequate SNR is a genuine range problem — raise the antenna, improve line of sight, or accept a slower, more robust spreading factor. Watching how SNR changes as you reposition an antenna even a metre at a time is the cheapest site survey there is.

A number worth logging

Mesh deployments live and die by their worst link. Logging per-hop SNR over days reveals patterns a single reading hides: noise that appears when a neighbour's equipment switches on, foliage attenuation after rain, or a marginal link that only fails at certain temperatures. For a network you actually depend on, those logs are your early-warning system.

Because SNR is a decibel figure, small numbers mean more than they look: 3 dB is a doubling of ratio, and 10 dB an order of magnitude. This is also where SNR fits the wider notion of a link budget — the running sum of transmit power, antenna gains, and path losses that determines what arrives at the receiver. You can spend budget on power, antennas, or height; SNR at the far end is the final scorecard telling you whether you spent enough, and the demodulation limit of your chosen spreading factor is the pass mark it is graded against.

SNR and received strength are read together, not in isolation. See RSSI for absolute received power, wavelength for the physics that shape propagation, and Radio Duty Cycle for the airtime rules that govern how aggressively you can transmit.