RFS brings DTV to LA viewers
Aug 1, 2004 12:00 PM
 Dual broadband panel arrays from RFS provide sculpted digital/analog signals for four Los Angeles broadcasters at Mount Wilson. |
The advent of digital television and simulcast digital/analog services has changed the face of global broadcasting. With real estate often at a premium, the escalating potential for interference and the considerable cost of deploying new infrastructure, many broadcasters are moving toward multiservice systems.
For four Los Angeles broadcasters (KDOC-TV, KJLA-TV, KOCE-TV and KXLA-TV) seeking to add DTV to existing analog services, these considerations ultimately have led to the deployment of a shared broadcast facility at Mount Wilson. The solution incorporates a dual broadband panel antenna/combiner system from Radio Frequency Systems (RFS).
A shared system was imperative. Not only is Mount Wilson a highly congested site, but the existing analog services being moved there coupled with the new digital services—the whole involving adjacent channels—needed to be broadcast from the same location to prevent interference. A shared system also offered the advantage of economy of scale. For channels 32, 44, 48, 49, 50, 51 and 56 to be broadcast, dual antenna/combiner subsystems were needed, for several reasons. The first of these was to simplify the combiner system, eliminating the need for adjacent channel combining. Dual subsystems also allow greater flexibility for main/standby services, as well as accommodating the stations’ individual pattern-tailoring requirements.
Sculpting the signal
The dual, 10-level broadband panel arrays deployed at Mount Wilson were designed by RFS in close collaboration with broadcast consultant Merrill Weiss. Early in the project, it was decided to use panels arrayed on three faces of a five-sided column, with panels omitted on the two northern faces because coverage was not required over the nearby mountains to the north. Using sophisticated computer modeling techniques, the design team assessed the effects of electrically tilting the three faces individually, coupled with power distribution and phasing adjustments, to determine the optimum pattern for each antenna.
This antenna pattern optimization process involves numerous variables. The act of changing the beam-tilt on individual faces leads to “transition regions” in the pattern that require careful analysis during the design process. Also, the effect of signal phasing on the pattern is tightly integrated — such that a change of phasing on one face has cascaded effects on other antenna parameters. Finally, due to the broadband nature of the antenna (580- to 756MHz), any adjustments for one frequency lead to follow-on effects across the bandwidth, so that the design is four-dimensional.
The process of ensuring optimum coverage for each of the services — and particularly, that the signal restrictions over Mexico didn’t degrade the performance of those channels not requiring it — involved many iterations of the key design variables. Interference issues and the challenge of achieving the required gain within specified power limitations also were taken into account, with the ultimate result being two, separate stacked panel arrays (each capable of handling up to 195kW total average power input) that meet the stringent performance requirements of all four broadcasters.
Crucial combining
A pair of parallel RFS directional waveguide combiner chains support the dual broadband arrays. The channel combiner subsystems comprise five directional waveguide filters and one blank section to allow for the introduction of additional channels. The system is designed to accommodate future channel reallocation; its compact nature offers space in the building for two additional combiner systems if required for future expansion.
In order to accommodate the high-transmitted powers of analog services on channels 50 and 56, the company developed a new full-wavelength directional waveguide combiner. The combiner incorporates resonators a full wavelength in height (instead of half wavelength), providing twice as much surface area to dissipate the greater heat generated by losses in the high-power, higher-frequency channels. Because of this, the system does not require forced-air cooling to ensure that the operating combiner does not exceed the design temperature rise. Complementing the combiner system is a network of rigid transmission lines linking the transmitters, mask filters, combiners and flexible coaxial feeders, which are each in different locations owing to the crowding at the site. During the design phase, the team took care to minimize reflections that might otherwise have occurred due to the number of components in the rigid feed system. To do this, they developed broadband elbows that were tuned to optimize system performance. In addition, eight 5-inch RFS HELIFLEX flexible coaxial transmission lines were installed to feed the panel array — four for each sub-array.
 The panel arrays are supported by a pair of parallel RFS directional waveguide combiner chains, including a new ‘full-wavelength’ directional waveguide combiner. |
The net result in the performance of the transmission line system was reflected power so low across all channels that several transmitters’ reflected power indicators did not even move when the transmitters were energized.
Structural challenges
Owing to the congested nature and potential seismic activity of the site, installation of the combiner proved a challenge. A combiner room was built as a bridge suspended over an existing building, and the entire combiner and separate digital-mask filter systems were bolted onto steel frameworks suspended from groups of four vertical steel members. Seismic horizontal ties connect the steel frameworks to the building structure and prevent them from excessive swinging during seismic activity.
The internal cavity of the antenna column also needed to be expanded in order to maintain human access, as well as contain a large volume of equipment — including the eight flexible feeders, branch feeders and power dividers used for pattern sculpting. The result was an asymmetrical crosssection, which led to issues with the antenna/tower interface. This was solved in collaboration with the tower designer through the fast-track development of a unique multidimensional antenna-clamping mechanism.
An additional design consideration was the minimization of tower harmonics due to wind-induced vibrations. To provide dynamic stability to the antenna structure (two RFS panel arrays plus a third antenna mounted on top of these), the team introduced a tuned liquid damper at the top of the 20-level, panel-antenna column comprised of stainless-steel tanks filled with a specifically calculated volume of ethylene glycol that moves against the modes of vibration, potentially reducing the magnitude of oscillations by a factor of 20.
Final result
Conceived and designed over a period of more than four years, the final RFS combiner/antenna system at Mount Wilson can accommodate a total of 12 digital or analog services from channel 32 to 56. The combiner chains were installed in the first half of 2003, followed by the raising of the two stacked panel arrays that October, and rigid line optimization in early 2004. Currently configured for nine channels (including two standby services), the first services went on-air in April 2004 with the others joining in the following months. After extensive theoretical design and modeling, the physical realization of the individual components, and the ultimate installation and commissioning, the RF broadcast system has met all performance objectives. Mike Dallimore is vice president, broadcast and defense systems, Radio Frequency Systems.
