Shively and Entercom Seattle Partner on IBOC Tests
Aug 1, 2002 12:00 PM
Seattle - Aug 7, 2002 - Following discussions at NAB2002, Bob Surette, RF manager of Shively Labs and Clay Freinwald, senior facilities engineer of Entercom Seattle, got together at NAB 2002, Freinwald proposed a radical approach to creating a high-level combined hybrid IBOC signal. Entercom Seattle has a multi-station combiner on Cougar Mountain near Seattle. Freinwald's idea was to use the existing combiner equipment to create the analog/digital transmission. Surette applied his knowledge of IBOC and analog combining systems and devised what could be another technique to simplify a station's IBOC implementation.
The EMR circulator connected to the normally terminated port on the combiner input hybrid.
Following the theoretical research, a real-world test was set up. Entercom�s Cougar Mountain was the likely candidate for the test not only because of Freinwald's involvement, but also because the transmission facilities include a state-of-the-art, highly versatile combiner and antenna system. Add to this the challenging broadcast environment that exists in this area.
Early in August 2002, a team of broadcast engineers from Entercom, Shively Labs, Broadcast Electronics, and Ibiquity assembled at the transmission facility to test the feasibility of the new technique. Instead of adding an additional combiner network for the digital signals, the plan was to feed the digital signals through the existing balanced combiner systems through an isolator into the combiner in the same manner as the analog signals, but through the normally terminated leg of the combiner's input hybrid. 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.
Shively senior RF techician Rob Liebe at work up on the combiner mezzanine.
In the case of the Cougar Mountain site, the Shively 6014 six-bay broadband panel antenna is configured with dual inputs, making it possible to simultaneously feed analog to the top half of the antenna and digital to the bottom half, while maintaining full 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. This was a fundamental requirement for Entercom since the eight stations on the system needed to maintain operational readiness while routine maintenance is being performed at the main broadcast site on West Tiger Mountain.
"The tests prove that it is feasible to broadcast analog and IBOC digital signals over the same equipment without combining them in a high-level injector-type system - and without causing interference," said Surette. "In practical terms, this process allows stations operating on combiners to forego many of the start-up and operating costs of IBOC implementation. Since the digital and analog signals are never combined, the losses associated with injecting the digital onto the analog carrier are eliminated. The 90 percent digital and 10 percent analog signal losses associated with normal high-level IBOC combining are eliminated. For multi-station sites where ac power, air conditioning and space increases make standard high-level combining especially expensive, this technique provides a real alternative."
The Shively antenna patch, showing the RF connection from the IBOC feed to the lower half of the Shively antenna.
The Cougar Mountain test is being conducted on Entercom�s Shively Labs Model 2540 combiner using a Broadcast Electronics Fsi-10 IBOC signal generator, BE-Fxi-60 digital FM exciter coupled into a broadband FM-1C1 transmitter and an 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.
"I am very pleased with the results of this proof-of-concept test and I want to thank all the participants, " said Marty Hadfield, Entercom�s vice president of engineering. "Stations implementing this novel use of existing multi-station balanced FM combining equipment will realize significant cost savings over the options if installing a separate, multi-station IBOC combiner at their site or the alternatives of expensive high- and low-level combining techniques." Hadfield continued, "This test was a success when the Ibiquity DAB signal from the Broadcast Electronics transmitter package connected [using a conventional low-power EMR RF isolator] through the normally terminated ports of the Shively balanced combiner modules and into half of the Shively panel antenna [effectively to a separate antenna]. In this particular test configuration, the analog transmitter is a vintage Collins 831G-2 providing 10kW output power. The DAB package is running an average transmitter power output of 100W. The center of radiation difference between the two halves of the split-fed Shively panel antenna is just 10 meters, and they exhibit comparable gain factors and envelope patterns, thus providing reasonable assurances that the analog and digital signals maintain their complementary carrier levels throughout the coverage area."
The test participants include:
Robert Liebe, senior RF technician
John Abdnour, RF national accounts manager
Richard Hinkle, director of engineering, RF Products
Jay Linderer, principle engineer
Tom Pierson, Entercom Seattle market chief engineer
Clay Freinwald, Entercom Seattle senior facilities manager
John Price, assistant to VP Engineering, Entercom
Information provided by Shively Labs.