The 915 MHz ISM Band

Nearly all LoRa mesh devices sold for North America operate in the 915 MHz ISM band (902 - 928 MHz). (A few 433 MHz LoRa devices also exist and are usable here, but they are uncommon.) Understanding what that means - and what the rules are - will help you choose the right hardware, set the right channels, and avoid interference with your neighbors.

What Is the ISM Band?

ISM stands for Industrial, Scientific, and Medical. These bands are designated internationally (ITU Radio Regulations) for industrial, scientific and medical RF applications. They are not, strictly, "set aside for unlicensed communications" - rather, in the US the FCC additionally allows unlicensed communications devices (like LoRa) to share them under Part 15 rules, on a non-interference, secondary basis. (Note: 902-928 MHz is an ISM band only in ITU Region 2, the Americas.) The trade-off is that these bands are open to many users, and everyone has to play nicely together by following power limits and other technical rules.

In the United States, the band used by LoRa spans 902 to 928 MHz (commonly referred to as the "900 MHz band" or "915 MHz band"). It is regulated by the FCC under Part 15 of the Code of Federal Regulations.

FCC Part 15 Power Limits

The FCC imposes strict limits on how much power you can transmit in this band:

• 1 watt (30 dBm) conducted power - this is the maximum power at the antenna connector of the radio.
• 4 watts (36 dBm) EIRP (Equivalent Isotropically Radiated Power) - this is the derived ceiling that results from the gain-reduction rule, not a separate flat allowance. The 1 W conducted limit assumes an antenna gain of up to 6 dBi. If your antenna gain exceeds 6 dBi, you must reduce conducted power dB-for-dB by the amount the gain exceeds 6 dBi (per 47 CFR 15.247(b)(4)). An antenna of 6 dBi or less requires no power reduction.

What does this mean in practice? Most LoRa modules transmit at 20 - 27 dBm (0.1 - 0.5 watts). A typical 3 dBi gain antenna is perfectly legal at full transmit power. A 10 dBi antenna (4 dB above 6 dBi) requires reducing conducted power by 4 dB, to 26 dBm - the reduction is keyed to antenna gain, not to any EIRP arithmetic. (For fixed point-to-point links the reduction is more lenient, 1 dB per 3 dB of gain above 6 dBi.) Almost no consumer LoRa hardware comes close to these limits, so for most users, this is a non-issue.

No License Required (With Caveats)

Because LoRa operates under Part 15, you do not need an amateur radio license or any other license to operate a Meshtastic or MeshCore node in the United States. This makes community mesh networks accessible to everyone, not just licensed ham radio operators.

However, Part 15 devices must accept all interference and must not cause harmful interference to any authorized radio service - whether in-band (including primary and government users of 902-928 MHz) or in adjacent bands. In practice, the 900 MHz band is busy with cordless phones, baby monitors, some Wi-Fi equipment, and other ISM devices - but LoRa's spread-spectrum nature makes it naturally robust against narrowband interference from these sources.

Duty Cycle Considerations

In the United States, digitally modulated systems in the 902 - 928 MHz band - the category that covers LoRa mesh devices - have no duty-cycle limit. (Frequency-hopping systems, by contrast, do have per-channel dwell-time limits under 15.247(a)(1)(i).) LoRa itself does not frequency-hop (it stays on one channel per packet), and the Part 15 rules permit continuous operation as long as power limits are respected.

That said, good network citizenship means keeping your transmit duty cycle low. If every node on a channel is transmitting constantly, collisions will degrade performance for everyone. Meshtastic and MeshCore both implement built-in duty cycle management and back-off algorithms to prevent nodes from saturating the channel.

Channel Selection and Frequency Hopping

Within the 902 - 928 MHz band, LoRa devices can use many different center frequencies (channels). Meshtastic does not use a single fixed frequency for its default LongFast preset; instead it computes the exact channel frequency from the selected region, preset (modem config), and channel name, dividing the band into numbered slots. For the US region, the LongFast primary slot lands near 906 - 907 MHz, but the precise value is derived by the firmware rather than being a hard-coded number - see the Meshtastic frequency-slot documentation. MeshCore's USA/Canada preset uses similar frequencies.

Frequency hopping (rapidly jumping between channels) is permitted and used by some competing technologies (like the older FHSS radios), but it is not required for LoRa and is not used by Meshtastic or MeshCore in their standard modes. Instead, they use a fixed channel, relying on LoRa's spread-spectrum nature to handle interference.

Channel selection matters when:

• You want to create a private channel separate from the public mesh.
• You want to avoid interference from other mesh users or industrial equipment.
• You are deploying multiple networks in the same area and need them to coexist.

What About Europe and Other Regions?

The 915 MHz band is specific to the Americas. In Europe, LoRa community mesh devices typically use the 868 MHz ISM band (863 - 870 MHz), regulated by the ETSI under different rules including duty-cycle limits on many sub-bands. Other regions have their own band plans:

Region	LoRa Band	Frequency Range	Key Rule
USA / Canada	915 MHz	902 - 928 MHz	1W conducted / 4W EIRP, no duty cycle limit
Europe / UK	868 MHz	863 - 870 MHz	25 mW ERP and 0.1-1% duty in most sub-bands; 500 mW ERP / 10% duty in 869.4-869.65 MHz (the Meshtastic EU_868 default)
Australia / NZ	915 MHz	915 - 928 MHz	1W EIRP
Asia (varies widely)	Varies by country	Varies	e.g. China 470-510 MHz, India 865-867 MHz, Japan/Korea ~920-923 MHz, Southeast Asia 920-925 MHz (AS923)

This is critical: a European 868 MHz LoRa device will not work on a US 915 MHz mesh, and vice versa. Always check that the hardware you buy is rated for your region's frequency band before purchasing. Hardware sold in the US is built for the 915 MHz band, but Meshtastic firmware ships with the region UNSET and will not transmit until you select your region (US) in the app on first setup. If you are buying from overseas vendors, double-check the product listing.

Some newer hardware (such as devices using the Semtech SX1262 chip) can be configured in software to cover both 868 MHz and 915 MHz, as the chip supports a wide frequency range. However, the antenna is typically tuned for one band or the other, so even if the chip can transmit on the wrong frequency, performance will be degraded.