Radio Duty Cycle

Radio duty cycle is the proportion of time a transmitter is allowed to be actively sending on a given frequency, usually expressed as a percentage of a one-hour window. A 1% duty cycle means roughly 36 seconds of airtime per hour — everything else is enforced silence. It is primarily a regulatory limit, not a hardware one: regional authorities cap duty cycle in the shared, licence-free ISM bands so that no single device can hog the airwaves and everyone gets fair access to a finite, unlicensed resource. For anyone building an off-grid mesh, duty cycle is the invisible budget that every design decision ultimately spends against.

Why regulators impose it

Licence-free spectrum works on a simple social contract: you may transmit without asking permission, but you must be a polite neighbour. In bands where thousands of sensors, meters, alarms, and mesh radios coexist, an always-on transmitter would raise the noise floor for everyone and effectively jam the band. Duty-cycle rules (and their cousins, dwell-time and listen-before-talk requirements) keep the channel statistically quiet enough that short, bursty transmissions from many independent devices can all get through. The limit is per transmitter and per sub-band, and it applies regardless of how clever your protocol is.

Europe versus North America

The two big ISM band regimes handle fairness differently. In Europe the 868 MHz band imposes a strict 1% duty cycle on most sub-bands — about 36 seconds of transmit time per hour — with some sub-bands as tight as 0.1% and a few relaxed ones reaching 10%. North America's 915 MHz band takes another route: instead of a flat percentage, FCC rules limit dwell time per channel (on the order of 400 ms) and require frequency hopping across many channels. In practice the North American regime is considerably more permissive for chatty mesh traffic, which is one reason identical hardware behaves so differently between regions. Always flash your node's firmware to the correct regional setting; the radio parameters, hop patterns, and duty-cycle enforcement all follow from it.

Spreading factor eats the budget

For LoRa and Meshtastic operators, duty cycle is often the real ceiling on throughput — more so than raw bandwidth. Each step up in spreading factor roughly doubles a packet's time on air, so a message that costs a fraction of a second at a fast setting can burn several seconds at the long-range settings. Under a 1% regime, a handful of long-range packets can exhaust an entire hour's allowance. That forces the fundamental trade: range versus message rate. The disciplined answer is to run the lowest spreading factor the link's SNR will support and spend antenna height and gain — improving the link budget — instead of airtime.

Designing within the budget

Sovereign builders plan around the cap the same way a homesteader plans around a small solar array: measure the budget, then trim consumption. Keep payloads small and telemetry lean. Lower beacon and position-broadcast intervals — a node announcing itself every 15 minutes instead of every 3 spends a fifth of the airtime. Let routers and repeaters carry the long hops so edge nodes can run faster radio settings, and keep the hop limit tight, because every rebroadcast is airtime spent from someone's budget. Exceeding the legal duty cycle is both unlawful and antisocial: it degrades the very commons your mesh depends on. A mesh that respects the budget is also a mesh that scales — the fewer seconds each node wastes, the more nodes the band can carry, and the more resilient the network becomes when you actually need it.