Colocating AM Transmitter Facilities with Cellular Monopole Towers

Cellular monopole towers can be efficiently used for AM transmitter facilities. Slant or shunt-fed antennas have been used for years, but recently the technique has become an efficient way use the ubiquitous cellular monopole as an AM antenna.

Co-locating the AM transmission facility with the cellular monopole is somewhat ironic. In the early days of cellular phone service development, AM broadcasters often leased space at the top of their AM towers for cellular carriers that were anxious to accommodate the rapidly developing cell phone market. Broadcasters modified their common series fed towers with skirt fed modifications to allow the installation of coaxial and power feeds across the antenna base to the cellular antennas mounted at the top of the AM tower.

Meanwhile, AM broadcasting has been in survival mode. Long-established AM transmitting sites are being displaced as a result of increasing real estate values and environmental restrictions. The AM broadcaster is also dealing with competition from online streaming and higher quality FM broadcasting. In the last decade, the number of licensed AM stations has declined by 188, while FM stations have increased by 1,242 according to FCC statistics.

Various means of relocation of AM antenna facilities have been developed with the most popular to co-locate with existing AM stations, with two or more stations sharing the same transmitter site where the common location will provide adequate service.
The technique of using a slant-fed or shunt-excited antenna was developed decades ago in the early development of AM radio. Today, with several thousand cellular monopole installations, many of them ranging from 100 to 200 feet high, many have the potential to serve as AM antennas.

Several years ago, Ben Dawson of Hatfield and Dawson Engineers in Seattle, published a paper titled ?The Slant Wire Shunt Fed Monopole: A Neglected but Invaluable Technique?. In his paper, Dawson concludes that the slant wire feeds are simpler electrically than other ways of feeding grounded-based structures. The slant wire technique also imposes far less structural load and is less susceptible to weather-related damage in hostile climate conditions. Dawson also concludes the shunt-fed monopole provides convenient impedance matching, good bandwidth, and efficient radiation patterns.

James Dalke (the author) is implementing the Hatfield and Dawson designed slant-wire-fed cellular monopole for station KARR-AM, 1460 kHz, in Kirkland, WA. The new transmitter facilities are installed with a slant feed on a 125-foot cell tower. The station is licensed to operate with 740 Watts daytime and provides good coverage for the Kirkland area.

While the AM coverage from the new site is significantly less than the original site abandoned in 2011, KARR has an FM translator associated with the AM license under the FCC?s AM revitalization rules. The translator operates on 98.5 MHz with 250 Watts and provides a good signal for much of the Seattle market area that was covered by the original AM facility operating at 5 kW.

James A. Dalke | Dalke Broadcast Services, Inc. | Bellevue, WA
Stephen S. Lockwood | Hatfield and Dawson Consulting | Seattle, WA