FCC Regulations and EIRP Reference

Operating LoRa mesh equipment in the United States requires compliance with FCC Part 15 rules. This page summarizes the relevant regulations and explains how to calculate whether your installation is within limits.

Disclaimer: This page is a general reference for community operators. It is not legal advice. For installations with high-gain antennas or unusual configurations, consult the FCC rules directly (47 CFR Part 15) or a licensed RF engineer.

The 915 MHz ISM band

LoRa mesh in North America operates in the 902–928 MHz band, designated as an ISM (Industrial, Scientific, and Medical) band. This band is available for unlicensed operation under FCC Part 15, Subpart C (Intentional Radiators).

Key rule: FCC Part 15.247 governs spread-spectrum operation in the 902–928 MHz band.

Power limits

Limit type	Value	In other units
Conducted output power	1 W (30 dBm)	Maximum power at the antenna port
EIRP (Effective Isotropic Radiated Power)	4 W (36 dBm)	Conducted power + antenna gain
EIRP with directional antenna	Special rules apply	See below

For fixed point-to-point links using directional antennas (Yagi, dish), FCC Part 15.247(b)(3) provides an additional 3 dBi of EIRP allowance for every 3 dBi of antenna gain above 6 dBi, up to a maximum gain limit. This exception enables long-range fixed links — but it does NOT apply to general area coverage nodes.

EIRP calculation

EIRP (dBm) = TX Power (dBm) + Antenna Gain (dBi) − Cable Loss (dB)

Example 1: Typical rooftop repeater (within limits)

Parameter	Value
TX power	27 dBm (500 mW)
Cable loss (3m LMR-200)	−0.3 dB
Antenna gain	+5 dBi
EIRP	27 − 0.3 + 5 = 31.7 dBm (below 36 dBm limit ✓)

Example 2: High-gain antenna requiring power reduction

Parameter	Value
TX power	30 dBm (1W maximum)
Cable loss	−0.5 dB
Antenna gain	+9 dBi
EIRP	30 − 0.5 + 9 = 38.5 dBm (exceeds 36 dBm — ILLEGAL)
Required TX power reduction	38.5 − 36 = 2.5 dB, so reduce TX to 27.5 dBm

Standard device compliance

Commercially sold LoRa boards (Heltec, T-Beam, RAK4631, etc.) are FCC-certified at their maximum power settings with the stock antenna. If you use the device as shipped with the included antenna or a comparable-gain replacement, you’re within the certification.

Custom installations — especially with high-gain external antennas or increased TX power settings — require you to verify EIRP compliance independently.

What happens if you exceed the limits?

FCC enforcement of Part 15 violations is uncommon for small-scale community mesh deployments, but the rules exist for good reason: excessive EIRP creates interference with other users of the band including industrial IoT systems, 900 MHz ISM devices, and legacy systems.

More practically: running higher power than necessary increases interference with nearby mesh nodes and doesn’t improve range as much as better antenna placement would. For most installations, 27–30 dBm TX with a 3–6 dBi antenna is the right operating point.

Canada (ISED) rules

Industry Canada (now Innovation, Science and Economic Development Canada — ISED) rules for 902–928 MHz operation are similar to FCC Part 15.247. The relevant standard is RSS-247. Conducted power limit is also 1W; EIRP rules align with FCC. Certified devices sold in both markets carry both FCC and IC certification numbers.

Frequency coordination

The 902–928 MHz band is shared with many other services and devices including:

• Other LoRa/LoRaWAN deployments
• 900 MHz WiFi (802.11ah/HaLow)
• DECT cordless phones (some models)
• ISM industrial/scientific equipment
• Some baby monitors and older wireless phones

The band is frequency-hopped or channel-selectable — if you experience unusual interference, try a different sub-channel frequency. Both MeshCore and Meshtastic allow channel frequency adjustment within the ISM band. Coordinate with other operators in your area to avoid overlapping on the same exact frequency.