The 915 MHz ISM Band

EveryNearly all LoRa mesh devicedevices you buysold for use in North America operatesoperate 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 frequencydesignated bandsinternationally (ITU Radio Regulations) for industrial, scientific and medical RF applications. They are not, strictly, "set aside by international agreement for unlicensed usecommunications" - meaningrather, youin dothe notUS needthe FCC additionally allows unlicensed communications devices (like LoRa) to share them under Part 15 rules, on a governmentnon-interference, licensesecondary tobasis. operate(Note: a902-928 radioMHz is an ISM band only in theseITU bands.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 ISM 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 accountsis forthe antennaderived gain.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 high-gain10 directionaldBi antenna (say,4 10dB above 6 dBi) would requirerequires reducing your transmitconducted power by 4 dB, to stay26 withindBm - the rules.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 from other users and cannotmust not cause harmful interference to licensedany servicesauthorized operatingradio 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 does- notthe category that covers LoRa mesh devices - have ano mandatory duty duty-cycle limitlimit. for frequency-(Frequency-hopping spreadsystems, spectrumby systems.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), butand the Part 15 rules still 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'sMeshtastic does not use a single fixed frequency for its default LongFast preset; instead it computes the exact channel frequency from the selected region, preset uses(modem 906.875config), MHzand aschannel itsname, dividing the band into numbered slots. For the US region, the LongFast primary channel.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 anduty-cycle enforced 1% duty cyclelimits 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,ERP and 0.1-1% duty cycle onin most sub-bandsbands; 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 (manyvaries countries)widely)	433Varies MHzby or 923 MHzcountry	Varies	Variese.g. significantlyChina by470-510 countryMHz, 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 almost always pre-configuredbuilt for the 915 MHz.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.