In the Private Operational-Fixed Microwave Service, stations licensed or modified after July 1, 1976, must meet certain technical standards that became effective on that date. Among those standards, which are spelled out in Subpart C of Part 95 of FCC rules (in Title 47, Code of federal Regulations), are two categories of antenna standards, (see 47 CFR 94.75(b)).

These standards refer to each transmitting antenna licensed as a "station: on either end of a microwave (point-to-point) path. Category A standards apply to all stations operating in areas where certain microwave frequency bands are congested, or where there is a predictable risk of interference to other stations. Stations operating in other areas, where there is less risk of interference, may use category B antennas, but may have to change to category A (or better) standards if interference problems arise. Otherwise, exceptions to the standards are clearly stated in the rules (see footnotes at 47 CFR 94.75(b)).

The congested areas in which microwave stations must use category A antennas are listed below, arranged according to five different frequency bands: 952-960 MHz, 1850-1990 MHz, 2130-2150/2180-2200 MHz, 6525-6875 MHz (6 GHz), and 12 GHz. Congested areas vary from band to band, depending on types of operation, though some areas are congested in more than on band. Each band has its own list of areas, which are defined as rectangles bounded by lines of latitude and longitude near given reference points.

No congested areas have been identified for some microwave bands. The 2150-2160 MHz band was excluded because only omni-directional transmissions are authorized. The 2450-2500 MHz band is shared with mobile and radiolocation services and is not protected from interference from ISM devices, so frequency congestion is difficult to determine. For this reason, microwave stations operating in the 2450-2500 MHz band in any area may use category B antennas unless specific interference problems arise.

Stations operating in the 2500-2690 MHz band must comply with technical standards for stations in the Instructional Television Fixed service in Part 74 of FCC rules.

To identify these congested areas, the FCC staff analyzed the microwave data base and sorted stations according to frequency bands and geographical areas. They plotted the stations on a map of the United States divided into areas of about 1000 square miles (30 minutes of latitude by 30 minutes of longitude), then determined congestion based on such criteria as the number, average power, antenna sizes, and growth rates of existing stations in each of the different frequency bands. Taking all factors into consideration, the staff identified those areas that, in its judgement, would likely be congested.

This list is based on 1979 data. A revised list will be published as soon as an analysis of the current data base is complete.