How a Decades-Old Radio Rule Shapes a Modern Mesh Network

More importantly, radios using the two presets are not directly compatible with each other.

Why would a modern mesh network make a change that can separate new radios from an established network?

The answer takes us below the apps and mesh routing and into the radio theory behind Meshtastic—and into long-standing FCC rules governing how higher-power unlicensed devices use the 902–928 MHz band.

The 902–928 MHz Industrial, Scientific and Medical (ISM) band is attractive to systems such as Meshtastic because certain devices can operate there without every user obtaining an individual radio license.

But “license-free” does not mean unrestricted.

Many of these devices operate under Part 15, the portion of FCC regulations covering a wide range of low-power and unlicensed radio-frequency devices. Part 15 establishes technical conditions involving factors such as transmit power, emissions, and modulation.

The roots of the rules involved here extend back to the early era of unlicensed spread-spectrum radio. Over time, the FCC expanded that framework to accommodate digital modulation techniques in addition to traditional frequency-hopping systems.

The basic engineering idea is spectrum sharing. Relatively high-power unlicensed devices can operate in these bands, but they do so within technical limits intended to help many different systems coexist.

That regulatory framework existed long before LoRa or Meshtastic.

And that is important, because LoRa approaches spread spectrum differently from the frequency-hopping radios many operators may associate with the term.

One source of confusion in this discussion is the phrase spread spectrum.

A familiar form is frequency-hopping spread spectrum, or FHSS. Instead of remaining on one frequency, the transmitter changes among multiple frequencies according to a defined hopping pattern.

LoRa works differently.

It uses chirp spread spectrum (CSS), a modulation technique in which the transmitted signal sweeps through a range of frequencies within its configured channel bandwidth. That approach contributes to LoRa’s ability to recover weak signals and operate effectively in noisy RF environments.

But a chirp sweeping through a 250 kHz channel is not the same thing as a transmitter hopping among a sequence of separate channels. That distinction matters under FCC §15.247.

The rule provides different technical requirements for frequency-hopping systems and digitally modulated systems. For a system using the digital-modulation provisions in the 902–928 MHz band, the FCC specifies a minimum 6 dB bandwidth of 500 kHz. Systems meeting the applicable requirements can operate with conducted output power as high as 1 watt.

Now compare that requirement with Meshtastic.

The familiar LongFast preset uses a nominal 250 kHz LoRa bandwidth.

That does not mean that “250 kHz LoRa is illegal.” That statement would be much too broad. A 250 kHz LongFast configuration does not satisfy the 500 kHz minimum-bandwidth requirement for operation under this particular higher-power digital-modulation provision. Section 15.247 also provides a separate path for qualifying frequency-hopping systems, while other Part 15 provisions such as §15.249 permit operation under much more restrictive field-strength limits. Meshtastic’s normal LongFast preset does not implement the frequency-hopping behavior required to qualify through the FHSS path.

There is a small measurement nuance as well. The bandwidth selected in a LoRa modem and the FCC’s formal 6 dB bandwidth measurement are related, but they are not identical definitions. For understanding the Meshtastic issue, however, the important distinction remains 250 kHz versus the 500 kHz regulatory threshold.

Meshtastic’s LongTurbo preset moves the LoRa bandwidth from LongFast’s 250 kHz to 500 kHz.

But bandwidth is not the only setting that changes.

Both presets use spreading factor 11, but LongFast uses a 4/5 coding rate while LongTurbo uses 4/8. A higher coding-rate denominator adds more error-correction redundancy, which can improve resilience at the cost of additional overhead and lower raw efficiency.

That means LongTurbo is not simply LongFast with a wider signal.

There is a performance tradeoff as well. Increasing receiver bandwidth generally admits more noise, affecting the system’s link budget—the overall signal margin available between transmitter and receiver. A radio configuration can therefore gain bandwidth or data throughput while giving up some weak-signal performance.

For many Meshtastic users, though, the most obvious consequence is compatibility.

A receiver must be configured for the characteristics of the signal it is trying to decode. A LongFast radio expects different modem parameters from a LongTurbo radio. Even though both may be operating within the same 902–928 MHz band, they are effectively listening for different types of LoRa signals.

As a result, an established LongFast mesh can continue operating normally while a newly configured LongTurbo node nearby may not participate in it until its modem preset is changed.

That makes the default setting unusually important.

A software menu may say “LongFast” or “LongTurbo,” but behind those names are physical-layer decisions about bandwidth, error correction, receiver behavior and ultimately which other radios a node can communicate with.

The larger lesson extends well beyond Meshtastic.

Radio designers regularly have to balance three things: regulatory compliance, RF performance and interoperability. Changing one can affect the others.

Increasing bandwidth can alter receiver performance. Changing modem parameters can separate a radio from an existing network. And a setting that appears to be a minor software choice can determine how the transmitter actually occupies spectrum.

The age of the underlying rules does not automatically make them obsolete. They establish the technical conditions under which many different unlicensed devices can share spectrum, while newer radio technologies have to determine how their modulation methods fit within that framework.

That is what makes the LongFast and LongTurbo story more interesting than a simple Meshtastic configuration change.

Mesh routing, encryption and smartphone apps may be modern, but eventually every Meshtastic message becomes an RF signal occupying real bandwidth in shared spectrum.

And sometimes rules developed decades before a technology existed still help determine what that signal must look like.