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
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
The Shively antenna patch, showing the
RF connection from the IBOC feed to the lower half of the Shively
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.