Demand for bandwidth has been on the rise, pushing regulatory agencies such as the US Federal Communications Commission (FCC) to explore the use of millimeter-wave bands for commercial applications. Already, wireless local-area networks (WLANs) have been developed for millimeter-wave frequencies. In addition, many scientists¹⁻³ have reported on requirements for millimeter-wave equipment for cable-television (CATV), as well as terrestrial and satellite-broadcast systems. A 60-GHz CATV system, for example, would enable the development of very compact transmitters (Txs) and receivers (Rxs), and allow a television set to receive signals anywhere in a room without wired connections. But millimeter-wave signals do not propagate well through the inner walls of buildings, requiring that each room have at least one antenna to satisfy the technical requirements of WLAN systems, for example.

In a Notice of Proposed Rulemaking issued in June 2002,⁴ the FCC signaled its intention to evaluate the potential commercial use of portions of the so-called millimeter-wave spectrum. The affected bands are 71 to 76 GHz, 81 to 86 GHz, and 92 to 95 GHz (see table). This could be a boon to the deployment of high-speed WLANs and broadband-access systems for the Internet. These bands are currently restricted to government use, and are being used in radio astronomy, space-borne cloud radars, and military applications. In addition to their possible use for high-speed Internet and network access, the FCC believes that the spectrum could also be used for other applications, including passive imaging of airport runways and imaging systems that could be used to display hidden contraband, weapons, and non-metal objects.

The table provides an overview of the WARC-79 and current (2002) US allocations for 71-to-76-GHz, 81-to-86-GHz, and 92-to-95-GHz bands. In the table, satellite services in the 71-to-76-GHz and 92-to-95-GHz bands are to transmit in the earth-to-space (uplink) direction and satellite services in the 81-to-86-GHz band are to transmit in the space-to-earth (downlink) direction. Portions of this spectrum are also allocated to the broadcasting, radio location, radio-astronomy service, and amateur services.

To make commercial millimeter-wave systems a reality, however, practical, inexpensive antennas are needed. What follows is a description of an inexpensive antenna configuration for indoor use to meet the requirements of millimeter-wave WLANs. The main idea of a millimeter-wave antenna with highly shaped beam pattern is based on the earlier work of Kumar.⁵⁻⁷ These report and papers describe an X-band, right-hand-circularly-polarized (RHCP) shaped-beam telemetry antenna suitable for retransmitting the radar data back to an earth terminal. The antenna has been used by the European Space Agency (ESA) and Canadian Space Agency (CSA) for Earth Remote Sensing (ERS) satellites and RADARSAT, respectively. The main idea is to use a highly shaped beam-reflector antenna hanging from a room ceiling. To compensate for free-space attenuation at millimeter-wave frequencies, the reflector antenna produces a sec² θtype of radiation pattern in the elevation plane. The antenna provides very sharp cell (room) boundaries with negligible radiation outside the cell limits. A characteristic of sec² θ patterns is that the cell dimensions are scaled to the antenna height. This characteristic provides a simple means to control illumination of the walls at the edge of the cell to maintain an adequate compromise between multipath effects and the need for alternative paths in case of line-of-sight blockage.

Millimeter-wave applications such as WLANs require constant electromagnetic (EM) field intensity throughout the coverage area (the room). The fixed-terminal antenna is mounted near the ceiling at the centre of the room and is required to produce sec² θ illumination with a square region that extends from nadir (θ = 0) to (but excluding) the walls (0 < θ < q_max). The desired sec² θ characteristic compensates free-space attenuation at each θ direction, producing constant electric-field illumination at constant height everywhere within the cell limits.

The design of the reflector profile is based on geometrical optics (GO) and the uniform theory of diffraction (UTD) to produce the required shaped beam. Optimization of the different parameters that define the antenna reflector has been carried out through software developed by Kumar.⁵