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Trends in Technology: AM Radio
Stephen Lockwood of H&D recommended Bobby Cox and James Banks of Kintronic Labs of Bluff City, TN, to design and build the Phasors and ATUs that would be used to generate the appropriate directional patterns, as well as the diplexer and filters that would allow the two stations to share towers at the new transmitter site.
The new location presented some challenges to Andy and the engineering team, since it is located in a river flood plain, frequently visited by salty marine fog and high winds. All outdoor components (ATUs and filter/diplexers) built by Kintronic use unistrut framing, thick aluminum walls, weatherproofing, and grounded doors. If the river floods, the water can be up to 16’ deep, not counting wave action. “We designed the entire plant to operate in a worst-case flood condition,” said Andy. “Everything had to be elevated, and we put all our power, control, and RF lines in underground conduit in case we ever had to replace it or add something else in the future.”
The KRKO and KKXA engineering team didn’t just give consideration to the environmental aspects though; other parts of the facility were also hardened so that the stations would be able to continue operating through other emergency situations. They use a Burk ARC Plus with both telephone and IP interfaces so the site can be easily and immediately reached remotely. “We have alarmed nearly everything humanly possible from fuel level to activation of our main surge clamp to generator telemetry to tower light telemetry to building temperature,” said Andy. STL switching is automated, with three different sources. A licensed 11GHz microwave system provides the main STL audio source, along with Internet connectivity, telephone extensions, and other TV, FM and AM audio sources to the transmitter site, where a basic on-air and production facility is also ready for use. The duplex nature of such a link allows for security devices and video cameras to be monitored remotely as well. Four HVAC units are onsite, though only two are needed to provide the necessary cooling (except during summer heat waves that do happen, even in the great northwest). Onsite generator power is tested weekly; the site can run 14 days, with both transmitters at full-power, should there be a long-term power outage.
One problem that can occur in a potential diplexing situation is that the partners are close in frequency. Just how close can they be? Phasetek of Quakertown, PA, completed a diplex recently in which the stations were separated by just 80kHz (WSKY (1230kHz) and WISE (1310kHz) of Asheville, NC). What are the special design considerations in a case such as this? “With close frequencies, careful choice of filtering circuits must be made to provide adequate attenuation and reasonable bandwidth,” said Kurt Gorman, president of Phasetek. “I have developed a ‘pi’ filter circuit that does a good job for this. Typically on close frequencies, it is better not to ‘brute force’ the filtering. Placing filtering, where possible, at 50Ω points is better.”
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