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Planning for a digital future
This past summer American Tower updated its master antenna installation in St. Louis. The tower site, originally completed in 1986 by EZ Communications, was built to host six class C and C1 St. Louis FM stations. With a center of radiation at 1,120 feet HAAT, the master antenna provided excellent coverage of the St. Louis market and the surrounding area. The Harris CBR antenna performed extremely well for 14 years. In 2000, while in the process of adding two more stations to the system, the antenna suffered a major failure and required a complete rebuild. The antenna was repaired and the two stations were successfully added into the combiner and antenna.
At that time, a new monitoring system was custom designed and built by QEI to monitor and control each of the eight stations transmitting from the antenna in 2000. Considering the amount of power going into the antenna, the monitoring and control of the transmitters is critical to protect the transmission system in the event of a failure somewhere in the combiners, feedline, hybrids or antenna. The total combined TPO into the combiner system was well more than 240kW to produce an effective radiated power of more than 750kW. The rebuilt and modified antenna and combiner system continued to operate until the decision was made to upgrade the antenna again.
In 2005 American Tower, the present owner, was approached by current and prospective tenants to consider modifying the aging antenna and combiners to transmit HD Radio. A major broadcast group in St. Louis was also interested in adding two more stations to the antenna system.
American Tower turned to Dielectric to develop a solution to address the growing needs in St. Louis. A high priority was the inclusion of digital IBOC signals as well as the additional analog signals. Initially, modifications to the combiner system were considered. A four-port hybrid could be added to the existing systems feeding a pair of crossed dipoles. This would space combine the analog and digital signals into isolated right-hand and left-hand polarized radiation. A second feed system would be added to inject the digital signal.
This approach, although seemingly logical and cost-effective, posed serious problems due to poor isolation caused by cross coupling within the antenna and mutual coupling between antenna bays. Poor isolation would require high power RF circulators that were not realistically available. It also forced the analog or digital VSWR to be optimized at the expense of the other. Another solution was needed.
After 10 months of development Dielectric introduced a unique radiator element design for broadband multi-channel applications. The symmetrical element design almost eliminates cross coupling and minimizes interbay coupling. This allows for high isolation and excellent VSWR for the analog and the digital signals. The new design consists of pairs of crossed right-angled equilateral dipoles referred to as Transverse Quadrilateral Technology (TQT).
The Dielectric HDFMVee antenna is designed to accommodate analog broadcast signals and their accompanying digital IBOC signals for HD Radio. The primary design challenge was the power level required for the St. Louis installation. The HDFMVee installed in St. Louis is the only multi-channel non-directional antenna in the country to accommodate 10 class C FM analog and digital IBOC signals. The antenna is built into its own mast. The top section of the tower was removed along with the old antenna and the new antenna was installed in its place.
The antenna consists of eight bays at eight levels with three around at each level. The antenna is split into an upper and lower section for redundancy; the upper or lower antenna can be fed separately for maintenance or emergency operation. Each section is fed with a 6 ⅛” feedline for the analog signal and a 3” line for the digital signal. RF switches in the combiner room allow for the upper or lower antenna, or both (normal operation), to be fed from the combiner system.
Installation of the new antenna commenced on July 22. All eight stations were required to operate at 25 percent of their normal power into the upper half of the antenna while the lower half feed line was removed and other preparations to the tower were made for the removal and replacement of the old antenna. On Aug. 20 the stations switched to their auxiliary antennas. All but one of the stations have off-site auxiliary sites. The one station used an auxiliary antenna on the same tower. This station was required to lower power and shut off as needed to accommodate any work in the vicinity of its antenna.
On Sept. 26 testing began on the new antenna. Each transmitter was energized into the system; one by one at half power into the lower and then the upper antennas. When it was determined that everything was normal, each transmitter was increased to full power into the antenna. After this initial testing some final installation work near the antenna had to be completed, so the stations switched back to their auxiliary sites.
Full power operation into the HDFMVee was initiated on Sept. 29, 2006. All 10 of the analog signals are now broadcasting from the site. At the end of November only two stations are transmitting an IBOC signals, but more stations are installing equipment. So far the antenna and combiner system have performed flawlessly, and all pertinent measurements have been superior to the original antenna and combiner. The new antenna has greater gain that reduces the required TPO from the analog transmitters.
Rice is engineering director of Bonneville International, St. Louis.
In May 1988, Broadcast Engineering featured an article that discussed FM combiner systems. The new (at the time) St. Louis master antenna project was featured in that article.
An explanation of the technology is available online at beradio.com.
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