Entercom, Shively Report IBOC Combiner Experiments

Participants of a test in Seattle say they’ve found an approach to implementing IBOC at combined facilities that could save stations money. The test is happening at the Cougar Mountain auxiliary transmission facility of Entercom Communications and is based on a proposal made by Clay Freinwald, Entercom Seattle’s senior facilities engineer, in conversations with Bob Surette, RF manager for Shively Labs. Earlier this month, engineers from Entercom, Shively, Broadcast Electronics and Ibiquity met to test the feasibility of the technique for feeding digital IBOC signals through existing balanced combiner systems. “Freinwald’s approach involves feeding the digital signals through an isolator into the combiner in the same manner as the analog signals, but through the opposite or normally terminated leg of the input hybrid,” according to a statement from Shively. “The digital signals are then combined and travel down the broadband line in opposite directions. The digital signals exit the combiner through the analog wideband input, where a transmission line feeding a separate ‘digital’ antenna replaced the standard wideband dummy load.”
Already in place was a Shively 6014 six-bay broadband panel antenna configured with dual inputs, making it possible to feed analog to the top half of the antenna and digital to the bottom half, simultaneously, while maintaining analog transmitter power. “This not only enabled the digital signal to be tested in the presence of multiple high-power analog signals, but also meant a minimum of downtime while the system was reconfigured for each night’s testing,” Shively stated. This was a requirement for Entercom because the eight stations on the system needed to maintain operational readiness while routine maintenance was being performed at the main broadcast site on West Tiger Mountain.
According to Surette, these tests prove the feasibility of broadcasting analog and IBOC digital signals over the same equipment without combining them in a high-level injector type system and without causing interference. Also, he said, the 90-percent digital and 10-percent analog signal losses associated with normal high-level IBOC combining are eliminated. The Cougar Mountain test is being conducted on Entercom’s Shively Labs Model 2540 combiner using a BE Fsi-10 IBOC signal generator, BE-Fxi-60 digital FM exciter coupled into a broadband FM-1C1 transmitter and Ibiquity DAB encoder and processing equipment. To keep the DAB signals within the Ibiquity spectral mask, the RF output of the DAB package was fed into a small Shively IBOC filter before coupling into the existing combiner module.