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I would like to use my SDR to transmit arbitrary data.

Can I freely transmit in the ISM bands without any license, as long as I abide by the regulations? My understanding is that I can transmit at up to approximately 30 dBm for digitally-spread signals and -1 dBm for all others. Is this true even if I don't certify a product (i.e. I just use my SDR and don't sell anything?)

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  • \$\begingroup\$ See this for example. Spread spectrum covers a variety of techniques. Some methods of which are (in the US) reserved exclusively for military use. \$\endgroup\$ Commented Jun 3, 2023 at 0:17
  • \$\begingroup\$ How do you make sure your SDR doesn't unintentionally transmit outside the ISM band you chose, and how do you suppress harmonics generated in your power amplifier? \$\endgroup\$ Commented Jun 3, 2023 at 5:58
  • \$\begingroup\$ What's "the ISM bands"? This depends on where in the world you intend to use the product. You cannot transmit 30dBm anywhere except maybe in USA/Canada on the 902-928MHz band given that you use frequency hopping according to FCC Part 15 and given that you have a certified product. \$\endgroup\$
    – Lundin
    Commented Jun 12, 2023 at 8:08

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You should carefully read the regulations and draw your own conclusions, but here it my interpretation:

https://www.govinfo.gov/content/pkg/CFR-2016-title47-vol1/pdf/CFR-2016-title47-vol1.pdf

Part 15 starts on page 845

You can build up to 5 devices that conform to all technical requirements of part 15. The power limits vary by frequency and modulation and you have to be aware of whether the power requirements are power out or EIRP.

§ 15.23 Home-built devices. (a) Equipment authorization is not required for devices that are not marketed, are not constructed from a kit, and are built in quantities of five or less for personal use. (b) It is recognized that the individual builder of home-built equipment may not possess the means to perform the measurements for determining compliance with the regulations. In this case, the builder is expected to employ good engineering practices to meet the specified technical standards to the greatest extent practicable. The provisions of §15.5 apply to this equipment.

I doubt that using an off-the-shelf SDR would qualify as home built under this definition.

The spread-spectrum modulation does not have to be digital but does have to meet a bunch of requirements:

An application qualifies as a frequency hopping system under section 15.247 if: • The transmitter pseudo-randomly hops between frequencies that are separated from each other by at least the 20-dB bandwidth of the data channel, but not less than 25 kHz. • The 20-dB bandwidth of the hopping channel is not larger than 500 kHz. • If the 20-dB bandwidth of the hopping channel is less than 250 kHz, the system must use at least 50 hopping frequencies. The average time of occupancy at any frequency (dwell time) must not be larger than 0.4 seconds within any 20 second period. • If the bandwidth of the hopping channel is larger than 250 kHz, the system must use at least 25 hopping frequencies. The average time of occupancy at any frequency must not be larger than 0.4 seconds within any 10 second period.

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  • \$\begingroup\$ An aside: consider getting a Technician grade amateur radio license, they are pretty easy to get and would both provide some education on what is allowed under amateur radio rules and provide significant scope for experimentation (practically unlimited power, etc) \$\endgroup\$ Commented Jun 3, 2023 at 5:30
  • \$\begingroup\$ Note that FCC Part 15 still requires a certified product. License-free does not mean certification-free. EU is a bit more lax as you can self-certify under the RED directive, but on the other hand the EU license-free bands are more restricted in terms of output power. \$\endgroup\$
    – Lundin
    Commented Jun 12, 2023 at 8:10

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