Repacking the television spectrum following the commission’s incentive auction is arguably the most complex and engineering-intensive undertaking ever performed within our broadcast industry. While its focus is on what amounts to the reconstruction of a large fraction of the television transmission plants, its effects are more far-reaching. Due to myriad other requirements, leasing issues and structural concerns, many FM stations will be directly affected; to a lesser degree so too will be AM stations on whose radiators FM auxiliary facilities have been, or will be, constructed.
Probably nowhere is the impact to FM operators more acute than at the large multi-tenant sites — buildings and mountaintops such as Willis Tower, Mount Wilson and the subject of this article, the Empire State Building.
There, in order to minimize interruption to FM operations during repack activities, Electronics Research Inc. designed and installed a 19-channel FM combiner and antenna system. Not only did ERI accommodate all of the respective analog carriers, it also designed the system to run IBOC for each of the stations at –10 dBc.
Nicholas Paulin, product engineering manager at ERI, will present an overview of the project on Sunday April 8 at 2:30 p.m. during the Broadcast Engineering and Information Technology Conference.
I had the opportunity of chatting with Nick about the project and discussing the challenges he and his team faced in bringing the system to fruition.
TWISTS AND TURNS
There is an old adage about location, location, location; it was no exception here. Being at one of America’s tallest buildings provides certain advantages. But as seasoned readers of my own musings know, I have a certain conceptual fondness for the lack of a free lunch. That certainly is the case with this system.
A relatively short transmission line run from the combiner to the antenna is all that is necessary due to its building location. However, transmission line runs in buildings typically are tortuous, requiring many elbows. In this system, more than 20 elbows are spread over the two 6-inch rigid runs. Additionally, due to corridor widths and corners, standard transmission line lengths would not fit into the building for installation. Thus, shortened section lengths were used, which required consideration of flange reflection pileup issues. Here ERI used sticks 8 feet in length, which gave good performance, and addressed installation concerns.
Normally complicated line runs can be a challenge to phase properly. In this project, ERI used two separate combiners. One provides right-hand circular, while the other provides left-hand circular. Because of this arrangement, phasing the two lines was not necessary.
In the construction of the two combiners, standard cavities were utilized. Due to the high voltages associated with the IBOC operation combined with physical dimension limitations, a new hybrid design was used in this installation. This design has a more compact footprint while at the same time increasing the voltage rating.
The compact footprint for the hybrids was crucial due to the room size. The dimensions of the combiner room are 15 by 25 feet, which required the cavities being laid horizontally in a two-tier type of configuration. Additionally, the system was height-limited due to existing ductwork in the ceiling. After installation, the vertical clearance was only a few inches.
Since the combiner location is within the building, it was possible to perform most of the installation tasks on that portion of the system during daytime hours. The installation time for the combiner spanned approximately two weeks.
On the antenna side of the system, work was performed mostly during nighttime hours due to RFR considerations. There, due to the shorter work window and weather considerations, the installation took around a month to complete. Before the installation of the antenna was performed, structural reinforcement was required.
The antenna is the first of its kind and utilizes a solid ground plane, rather than a screen or grating type. The system design specifications called for the ability to be able to climb through and work behind the array while it was active. Although a mesh ground plane would have limited the RF level behind the elements, the solid ground plane will prevent the protrusion of hands and feet into areas where they would experience high RF levels. In addition to the concerns behind the antenna, the potential for high RF levels at publicly accessible areas of the building was considered.
Additionally, the antenna, which comprises three layers, four around, utilizes a comb filter. This type of filter is created by the deliberate introduction of a delayed version of a signal to itself. This causes interference patterns to be created, which have the appearance of a comb, and are in this instance used to help smooth out the antenna radiation pattern. Given the central location of the site relative to the market, less scalloping in the pattern would be desirable.
This is a really cool system and one where necessity bred substantial advancement to our art. I know where I will be Sunday afternoon at the show.