Clear Channel Chicago, owner of seven stations serving the city and surrounding area, recently completed its FM transmission build-out at the Sears Tower. WGCI-FM had been operating in a valley between tall buildings, and a new building was about to be erected across the street from our 600-foot high antenna. The site that offered the fewest restrictions to adjacent-channel interference was the Sears Tower.
The first of 11 Harris Z-Series FM transmitters (a Z10 transmitter) was installed in December 2000 when WGCI-FM moved its main transmitter site to the Sears Tower. One antenna aperture was available within the FM area of the building's east antenna mast, but we chose to diplex WGCI with another co-owned station already operating from the site. The cost was considerably less than installing a new antenna, and the power-handling capacity of the antenna system was roughly 33 percent greater than our proposed total input power.
WKSC-FM was the next station to receive a Z10 transmitter. The station had been operating with a pair of alternate-main Harris FM25K transmitters at 10.5kW TPO. Once the WKSC Z10 was installed a third Z10 was purchased to operate as a frequency-agile transmitter between the two stations. WLIT-FM replaced its single-tube transmitter with a Z10 in 2002. Because all the Clear Channel Chicago FM stations are backed up with auxiliary sites, another new Z10 was commissioned alongside the WLIT transmitter as an auxiliary site transmitter for WNUA-FM.
WNUA's main site is located in the John Hancock Center across town. Z10s were installed here for this station and WVAZ-FM's main transmission site. Our eighth Z10 was installed in October 2005 at the Prudential WKSC-FM auxiliary transmitter site.
Performance at a glance
Hot-swappable PA modules
Easy interface for second exciter
Frequency agile over entire FM band
N+1 version available to instantly switch frequency
Back at the Sears Tower, the challenge arose to install HD Radio transmission equipment. WLIT, WKSC and WGCI are co-located, with WKSC and WGCI's transmitters in one room and WLIT in an adjacent room. Several options existed, including the installation of a separate community HD Radio antenna and small combiner. Costs and timeliness were large factors in discouraging the space combining method. The favored option became Harris' split-level combining system, also known as mid-level combining.
Split-level combining uses our existing Harris Digit exciter — soon to be a Flexstar HDX-FM exciter — in the Z10. The output is split between the Z10 IPA input and the second power amplifier/HD Radio transmitter, a Harris Z6HD. The majority of the exciter's output continues to be routed to the Z10 IPA input, and a small sample of the RF is sent to the Z6's IP2D linear maximization input. The outputs of the Z6 and Z10 are comined through a 6dB hybrid combiner.
The transmitters for WGCI and WKSC.
Many installation options exist to make HD Radio work. Because each of our five FM stations have auxiliary transmitter sites, we did not install RF bypass switching. Such switching normally connects a transmitter directly into the antenna should a Z6 or Z10 transmitter or the 6dB hybrid fail. We also opted not to use the split-level controller, which can control the exciter muting logic from the controller (rather than the traditional transmitter controller). This decision kept the exciter RF output path as simple as possible, installing the RF splitter in the Z10 cabinet. Only one RF cable was required to connect the Z10's Digit exciter output to the Z6 IP2D input.
Space was another consideration; we ran out of room in the previous configuration. What was a wall and hallway soon was converted to transmitter floor space. The three Z6HDs transmitters plus two additional equipment racks were placed within the wall. The hallway that faced the front of these transmitters was converted to interior space by relocating a doorway. We cannot see both split-level transmitters at the same time, which is sometimes a nuisance. The one requirement we always insist on when installing a Z-Series transmitter is clearance to roll out the power supply so we can have access to all components found in the supply. Supply failures are rare, but they do occur. Not being able to physically reach all of the supply components for future maintenance would prove frustrating, at the least.
For the reject load resistors, all three stations use the Bird model 8922 Termaline RF coaxial termination, a 5kW, oil-cooled load with a 1⅝ EIA flange. If the Z6 transmitter should fail in the split-level arrangement, the loads will dissipate about 3kW of heat. This could cause damage to the load resistor and dielectric silicone fluid seals if we didn't use forced airflow around the unit. These loads have high-limit temperature, snap-action thermometers to activate alarms or shut down transmission equipment connected to the load.
The three split-level systems have been operating problem-free since mid-December 2005.
Boehm is the transmitter supervisor for Clear Channel Chicago.
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