33-centimeter band

The 33-centimeter or 900 MHz band is a portion of the UHF radio spectrum internationally allocated to amateur radio on a secondary basis. It ranges from 902 to 928 MHz and is unique to ITU Region 2 (Americas). It is primarily used for very local communications as opposed to bands lower in frequency. However, very high antennas with high gain have shown 33 centimeters can provide good long-range communications almost equal to systems on lower frequencies such as the 70 centimeter band. The band is also used by industrial, scientific, and medical (ISM) equipment, as well as low-powered unlicensed devices. Amateur stations must accept harmful interference caused by ISM users but may receive protection from unlicensed devices.

The 900 MHz frequency is also used as a reference band e.g. to express the total power or impact of the electric field "E" - expressed in V/m - or the power density "S" - expressed in W/m² - of the overall cellular frequencies emission caused by all frequencies s.a. the four bands 850 / 900 / 1,800 / 1,900 MHz – which many GSM phones support and mobile phone operators use - used by all mobile phone operators at the same time to a certain space where e.g. humans are exposed to these frequencies over a certain span of time. More: Mobile phone radiation and health section.

In ITU Region 3, New Zealand domestically allocates 915 MHz to 928 MHz to amateurs. In Australia, this spectrum is allocated to radiolocation and scientific-medical services.

The 33–centimeter band has a somewhat short history, being one of the newest amateur radio bands. The Radio Regulations of the International Telecommunication Union allow amateur radio operations in the frequency range from 902.0 to 928.0 MHz.

In 1985, the Federal Communications Commission allocated the frequency band between 902 and 928 MHz to Part 18 ISM (industrial, scientific, and medical) devices. In that proceeding, the band was also allocated to the Amateur Radio Service on a secondary basis meaning amateurs could use the band as long as they accepted interference from, and did not cause interference to, primary users.

In the mid-1990s, many cordless phone manufacturers started producing phones that used the lower and upper ends of the 33–centimeter band for communication between the handset and base. These phones, which are regulated by Part 15 of the FCC's regulations, have made amateur use of the upper and lower end of the 33 centimeter band somewhat tricky due to the number of these phones used by the general population. Part 15 devices, by law, must accept interference from any licensed radio service with which they share frequencies.

In the early 1990s, wireless computer networking was just becoming affordable. Several competing technologies emerged that made use of the 902-928 MHz Part 15 band, such as:

The European equivalent is the SRD860 band. See also List of WLAN channels § 860/900 MHz (802.11ah).