MESHCOLA

For Meshtastic users and enthusiasts, understanding the legal power limits of our devices is essential for maximizing performance while staying compliant with federal regulations. Let’s take a closer look at what the FCC has to say about power limits for unlicensed radio transmitters, specifically under Title 47, Chapter I, Subchapter A, Part 15, Section 15.247, Paragraph (b)(3).

What is FCC Part 15.247(b)(3)?

FCC regulations in 47 CFR §15.247(b)(3) outline the maximum permissible power for devices operating in the unlicensed ISM band, which includes the 902-928 MHz frequency range that Meshtastic devices use in the U.S. The ISM band allows devices to operate without a license, provided they meet certain technical requirements, including restrictions on output power.

The Power Limits Defined

According to 47 CFR §15.247(b)(3), devices that employ digitally modulated systems (like Meshtastic with LoRa modulation) are subject to the following power limits:

• Maximum Output Power: The maximum allowable power output is 1 watt (or 30 dBm). This limit represents the maximum conducted power—the power delivered directly to the antenna, also known as the output of an intentional radiator. According to FCC Section 15.3(o), an intentional radiator is “a device that intentionally generates and emits radio frequency energy by radiation or induction.” In this context, a Meshtastic device qualifies as an intentional radiator since it is designed to emit RF energy for communication. It’s important to note that conducted power does not account for any gain from the antenna itself, which will increase the effective radiated power (ERP).

Understanding Effective Radiated Power (ERP)

In addition to understanding conducted power, Meshtastic users should also consider Effective Radiated Power (ERP), which factors in antenna gain. The ERP affects the overall range of your Meshtastic device, and the FCC imposes limits based on antenna characteristics to prevent excessive interference.

1. If using an antenna with 6 dBi gain or less, you’re typically allowed to operate at the maximum 1-watt conducted power.
2. If your antenna gain exceeds 6 dBi, the allowable transmitter power must be reduced to stay within FCC’s ERP limits. For instance, if using a 9 dBi antenna, you’d need to reduce the transmitter output power by 3 dBm to remain compliant.

These adjustments ensure that the ERP stays within legal bounds, even when higher-gain antennas are used.

Power and Antenna Gain Compliance Table: Ensuring Legal EIRP for Meshtastic Devices

The table below illustrates the relationship between radio output power, antenna gain, and effective radiated power (ERP) and effective isotropic radiated power (EIRP) for Meshtastic devices. To comply with FCC regulations, the EIRP must remain at or below 36 dBm. As antenna gain increases, the table shows corresponding reductions in radio output power to ensure compliance. Each row represents an example setup with antenna gains incremented from 2 dBi to 12 dBi, providing guidance on how to adjust output power based on your antenna choice.