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FCC Rules for Unlicensed Wireless Equipment operating in the ISM bands

The ISM radio bands were originally set aside for electromagnetic radiation produced by industrial, scientific and medical (ISM) equipment.  In the early 1990’s the Federal Communcations Commission (FCC) allowed using three of the ISM bands for unlicensed communication equipment.  These three ISM bands are:
  • 902 to 928 MHz
  • 2.400 to 2.4835 GHz
  • 5.725 to 5.875 GHz

In the United States the use of these bands for communication purposes is regulated by part 15 of the FCC rules.

Maximum Transmit Output Power in the ISM bands

Several of the FCC part 15 rules govern the transmit power permited in the ISM bands.  Here is a summary of those rules:

    1. Maximum transmitter output power, fed into the antenna, is 30 dBm (1 watt).

    2. Maximum Effective Isotropic Radiated Power (EIRP) is 36 dBm (4 watt).
      You can obtain the EIRP by simply adding the transmit output power, in dBm, to the antenna gain in dBi  (if there is loss in the cable feeding the antenna you may subtract that loss).

    3. If your equipment is used in a fixed point-to-point link, there are two exceptions to the maximum EIRP rule above:

      • In the 5.8 GHz band the rule is less restrictive. The maximum EIRP allowed is 53 dBm (30 dBm plus 23 dBi of antenna gain).
      • In the 2.4 GHz band you can increase the antenna gain to get an EIRP above 36 dBm but for every 3dBi increase of antenna gain you must reduce the transmit power by 1 dBm. The table below shows the combinations of allowed transmit power / antenna gain and the resulting EIRP.

      Transmit Power
   Antenna Gain 
30 6 36
29 9 38
28 12 40
27 15 42
26 18 44
25 21 46
24 24 48
23 27 50
22 30 52

The responsibility for staying within these power limits falls on the operator (or, if professionally installed, on the installer).

FCC Rules Applicable to ISM-Spread Spectrum Radios

The FCC regulates not only the “intentional radiation” of radio transmitters, but also the “unintentional radiation” of noise from all sorts of electrical equipment. The FCC regulations appear in title 47 of the United States Code of Federal Regulations (47CFR),  The FCC part 15 rules that affect operation of communication equipment in the ISM bands are listed below:

FCC 15.107 Conducted Emissions including AC Line
FCC 15.109 Unintentional radiated emission limits
FCC 15.205 Restricted bands of operation
FCC 15.207 Conducted emissions from intentional radiators
FCC 15.209 Radiated emission limits, general requirements
FCC 15.247 ISM Band Communication Equipment

(The FCC website is frequently reorganized, which breaks the links here. The links above are PDF files at the Government Printing Office (GPO) under 47CFR15. These tend to be much more stable.)


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