WAMU broadcasts from a tower located on American University's campus near the corner of Nebraska and Massachusetts Avenues in upper Northwest Washington. The tower is a four-legged self-supporting 321-foot steel structure originally erected for TV Channel 7 in 1947. WAMU began broadcasting from the tower in 1961 and it was fully gifted to American University by the owners of Channel 7 in the early 1970s. The geographic coordinates are North Latitude 38-56-10 and West Longitude 077-05-33. Today, WAMU operates the tower on behalf of American University for the benefit of its listeners, other broadcasters, cellular tenants, and the greater community.
The tower was extensively renovated between 2019 and 2020, including replacement of the 113-foot mast that is buried in the top portion of the tower, bringing its total height (the location of its top-most aircraft warning beacon) to 419 feet. The 2019-2020 project added a new ERI “Cogwheel” model 1183 four-bay, three-around master panel antenna that broadcasts the WAMU signal, as well as that of multiple other FM radio stations who lease space from WAMU. It is the largest combined FM antenna system in Washington, DC and is similar to some of the most significant FM broadcast antennas in the world, notably those atop the Empire State Building. The antenna is fed by an ERI constant-impedance combiner system, which enables WAMU to share this massive antenna with other DC broadcasters. The tower mast also holds separate, smaller antennas for two other radio stations in addition the main Cogwheel antenna.
The center of radiation from WAMU's main ERI Cogwheel antenna is 512 feet above average terrain. WAMU also has an auxiliary antenna lower on the tower that can be used in case of a failure of the main antenna. Ground elevation in the Tenleytown area of Washington, the highest point in the District, is 410 feet above sea level. Television and FM reception requires line-of-site between the transmitting and receiving antennas, which is why most of the TV and FM transmission facilities in the Washington area are located in or near Tenleytown. Among the Tenleytown towers, WAMU’s tower is one of the shortest, though its engineers believe this lower height gives WAMU and the tenant FM stations who share the tower a strategic advantage: more of the power that they broadcast is truly received at ground level. This means that our signal is better able to penetrate thick buildings and navigate the concrete canyons of the District, while still having sufficient height to be heard well north of Baltimore and deep into Virginia. It’s notable that the stations that broadcast from the WAMU tower are regularly at the top of the radio ratings charts for DC.
WAMU creates its 88.5 signal using two liquid-cooled transmitters that function as a main and a backup. The outputs of these transmitters each provide approximately 30.8kW (30,800 watts) of power. If the main transmitter fails, the backup can be switched into the antenna with minimal interruption of service. Each transmitter has multiple power modules and a failure of any one only reduces the power by a small amount. WAMU’s choice to use “liquid cooling” for its transmitters is an energy-efficient “green” transmission technology, which enables the station to use less power for air conditioning by directly sending the heat of its powerful transmitters outdoors via a cooling liquid that is in constant circulation.
WAMU also transmits an HD Radio signal combined with the analog signal into its antenna. WAMU has been broadcasting with HD Radio since 2004, for the benefits of a clearer signal in some cases over the analog FM signal for those using a digital HD radio. It also enables WAMU to broadcast several other “stations” alongside the station heard in analog at 88.5FM. WAMU’s Bluegrass Country, for example, is heard on the HD-2 channel of its HD Radio signal. The digital signal is broadcast at -14dB relative to the analog FM carrier, for an additional output of approximately 1225 watts. While this is much less power than the main signal, it provides an HD Radio service area roughly equivalent to the service area of the analog FM system.
WAMU also maintains a separate backup transmission facility in Virginia, which is utilized in the event of failure at its main site or for planned maintenance. Because the signal broadcast by this backup site must “fit” within the signal area created by the main site on the AU campus, the site broadcasts at a lower power level to create the required smaller signal to fit within the confines of the main signal’s footprint. When operating from the backup site, more distant areas (such as Baltimore and north, and deep into Virginia) are no longer able to clearly hear the 88.5 signal. This is also true for in-building listening within and closer to the district. However, it is difficult to notice a difference between the sites when driving in the greater DC region. This backup site was integral during the 2019-2020 tower renovation project, when large portion’s of WAMU’s tower on the AU campus were disassembled and temporarily removed from service. It is now relatively infrequent that WAMU broadcasts from its backup site, though it remains available as a ‘hot standby’ in case of trouble or scheduled maintenance at the main site.