Origins

     Washington, D.C. dentist and wireless inventor Mahlan Loomis may have been the first user of an antenna with his experiments in the Blue Ridge Mountains west of the capital city in the late 1860s. Loomis' antenna consisted of a wire suspended beneath a high-flying kite. The real inception of modern radio antennas came decades later with Marconi's understanding of the need for a high aerial or antenna to aid in receiving wireless telegraphy signals over large distances. He and other wireless pioneers experimented with many often highly complex antenna designs before settling on the use of three or four wooden (later steel) transmission towers at a given transmitter site.

     The design of antenna structures rapidly improved in the years before and after World War I. The spread of broadcasting after 1920 and the use of higher frequencies for radio services prompted development of transmission antennas with greater efficiency. Radio receivers had improved by the late r 920s to the point that they no longer always required external antennas for effective operation, thus making them easier to use and less expensive. The first transmission antennas that could propagate signals in a given direction or pattern (dubbed directional antennas) were developed in the 1930s, allowing a station to transmit its signal to a more specific area and thus help avoid interference with other stations. World War II saw the development and refinement of microwave transmission and special antennas for that service. Much was learned about how the careful location of an antenna tower could have substantial impact on its efficiency. Post-war antenna improve­ments included substantial refinements in methods of both transmission and reception.

AM Radio 

     AM radio station antennas make use of the entire transmission tower as a radiating element. To achieve the most efficient radiation, an AM antenna tower's height should equal about half the length of the radio waves being transmitted. For example, an AM station on 833 kHz, where wavelengths are about 360 meters long, should ideally use an antenna tower that is some r 80 meters or about 590 feet tall (nearly the length of two football fields). Additionally, because AM makes use of ground waves to distribute its signal, the tower is usually merely the visible part of a complex system that extends under and through the ground. As good ground contact is essential for an efficient ground wave, AM antennas often feature an extensive web of copper cables radiating out from the tower location and buried just below ground level. AM antennas are sometimes located in damp or moist locales (such as swamps or marshes) to aid in ground wave conduction.

     Both to help reduce interference and to focus signals where most listeners are, the vast majority of the country's AM stations transmit a directional signal. Rather than the circular coverage pattern that would result from a standard antenna (assuming flat terrain), most AM stations use two or more antenna towers to transmit signals in a pattern away from another station, or away from a body of water or some other physical feature with few listeners, and toward population centers. Multiple AM towers for different stations are often clustered together in antenna "farms" to keep potential obstructions to airplanes at a minimum. In some locations, FM antenna towers and towers for television stations are located within the same antenna farms.

FM Radio

     Unlike AM antennas, where the entire tower can assist in radiation the signal, in FM broadcasting the antenna is a relatively small device mounted on top of the transmission tower. This is because the wavelengths in the VHF spectrum used by FM radio are far shorter than in the medium wave spectrum used by AM stations. An FM outlet licensed to operate at 100 MHz, where the wavelengths are about three meters long, only requires an antenna about 1.5 meters (about five feet) in length. Since FM stations rarely need a directional pattern, multiple FM antennas for a single station are uncommon. A taller transmission tower allows the antenna element to be mounted higher above average terrain, thus extending this line-of-sight service's coverage.

     FM antennas can be polarized in three different ways, each affecting how the signal will be received. Horizontal polarization was long the FM standard in the United States, but most stations now employ circular polarization for better service to car radio antennas of different types. A third type, vertical polarization, is generally restricted to public FM stations seeking to avoid adjacent channel interference with television channel six (which occupies 82-88 MHz, immediately below the lowest frequencies of the FM band).

Other Types: Shortwave and Microwave

      Shortwave radio station antennas are very complex because of the many different shortwave bands that may be in use during a typical broadcast period. Extensive antenna arrays of differ­ ent sizes, often covering a large ground area, help to sharpen reception.

     On many transmission towers across the country one can observe various ancillary dishes or horn-shaped antennas, sometimes mounted on the top of a tower, but often attached to its sides. These are nearly always for microwave transmission links, some used for voice or data links, and some for sending television signals for use by distant cable television systems.

     All radio antennas in the United States must be built and operated within strict licensing and tower marking requirements. A tower's location, height, and other characteristics are specifically defined in a station's license from the Federal Communications Commission. Generally speaking, a higher antenna will help to extend a station's coverage area, although this is more true for FM than AM services. All transmission towers must be painted in wide bands of red and white to make them more visible in low light conditions, and must be electrically highlighted at night. Jurisdiction over these requirements (as well as locating towers as far away from airports as possible) is shared with the Federal Aviation Administration.

Reception Antennas

     Antennas to receive AM or FM signals are rarely seen by consumers as they are increasingly built into radio receivers. Some home stereo systems, for example, come with a plastic-and­ wire loop antenna for improved AM reception indoors. Likewise an FM antenna, usually a long wire with a "T" shaped ending, can be attached for better indoor reception.

     With the inception of satellite communication in rhe 1960s, large dish-shaped antennas dubbed “earth stations" were built in several locations around the world. Often huge dishes up to 200 feet in diameter, they were designed to gather in weak satellite signals and boost them to an audible range. Also used to receive satellite-delivered audio signals are back-yard or building-top dishes typically called "television receive-only" ("TYRO") antennas. Direct-to-home reception of audio and video signals can now take place using small dishes of a foot or 18 inches in diameter.