When the DTV rules were first announced, or even offered for comment, broadcasters started to worry about how a second channel could be accommodated within the framework of their existing facilities. Transmitter space could be handled by, at worst, adding on to buildings, but antennas and lines were often a problem, especially on older towers that were built to different specifications than today's towers. For stations that were tenants on sites where space was a problem, either regarding the building or the towers, the problems quickly became real and difficult.
The layout of the Sears Tower
In Chicago, D.L. Markley and Associates was hired to work with the Sears Tower and broadcasters to determine what their needs would be and find a way to accommodate new hardware. The building operations manager determined the transmitter space needs and came up with a plan that gave each station the amount of space it needed — which wasn't necessarily what each station wanted, but it worked.
It was quickly determined that while the transmitters would fit, there simply wasn't enough vertical real estate to get everyone on the air while all the analog antennas were still in place.
I'm sure that everyone has seen the antenna installation on the Sears Tower. Just watch an NFL game from the aesthetically challenged Soldier Field Stadium. Shots often include the Sears Tower, clearly displaying the antenna layout.
The original structure consists of two towers, totally enclosed in radomes, mounted on top of two huge steel cylinders. The walls of the cylinders have not been penetrated by anything other than small openings for cables to maintain structural integrity.
The towers are in three levels, with each level being smaller than the one below it. Each level does not have more than one full-size UHF antenna, with some VHF activity on the lower level. The west cylinder has a couple of VHF antennas mounted on its face under a large fiberglass bubble. Those two antennas, now used as auxiliary, were once used for some of the early measurements and data gathering concerning circular polarization for VHF TV.
Other than the second space down from the top of the east tower, there wasn't room for anything new. Changes would either have to be made by cooperation between the existing tenants or something new would have to be added to the installation. In the final analysis, both paths were taken.
Resolving an issue of space
With little significant objection, the tenants worked out their problems. This involved using some new space, to be discussed in a bit, and reconfiguring an existing antenna.
The antenna at the top of the west tower was a big unit designed for 5MW ERP. Through a series of maneuvers, both after and prior to the DTV problem, that antenna was reconfigured. It was removed and replaced by three DTV antennas, lovingly referred to as the triple-stack.
The new technology and much lower power requirements for DTV enabled three channels on the antenna space. The antennas were directional, which is the norm for the UHF stations on the building. To the northeast lies Lake Michigan. The stations run out of steam in that direction before hitting the other side, so the use of the directional antenna is obvious.
The building was originally designed to accommodate broadcasters with a tower/cylinder combination. Like most designs, that worked for a long time — until DTV came along. In trying to work out a solution to the real estate problem, the structural engineer on the project mentioned that there were four load platforms on the roof, located between the cylinders and the building corners. The structures could hold a fairly good load for future uses, whatever those might be.
At first, not much thought was given to those platforms as they weren't thought to have the necessary mechanical loading abilities to hold a self-supporting structure. Anything mounted on those pads on the east side would have to at least get the top antenna near or above the cylinders for good service. The same applied in part to anything on the northwest pad due to shadowing from the west cylinder.
Eventually we decided to install monopoles on the pads, which could be anchored by a couple of braces to the cylinders. That greatly reduced the horizontal loading problem for the rooftop pads. The vertical loads were acceptable. Due to the shadowing that would occur over parts of the market area, we used the two east monopoles for auxiliary service only with multiplex panel antennas.
Next came the juggling act. We contacted the manufacturers of these antennas to determine the specifications of their units that could handle the projected power and pattern. That data then went to the structural engineer for analysis and approval from a loading point of view, which then was used as a filter for the proposed antennas. Finally, one was selected and was used on both the northeast and southeast monopoles.
In a similar fashion, a high-power panel antenna with the same pattern was installed in the last available space on the east tower. That antenna could accommodate two stations at 5MW each or several lower power antennas. It was installed easily as it was not set in place in one unit but was actually constructed on the existing tower. After that, the only problem was getting all those signals to their respective antennas. The numbers included three signals into the new main, five into one auxiliary and seven into the other.
Handling all those signals wasn't easy. High-power multiplexers are traditionally large structures. However, the space for those units was not big. Therefore, we needed multiplexers that did not take up much space. The search quickly settled on manifold technology. UHF use at high power is normally done by using a waveguide base with waveguide filters on directional couplers installed into that base.
All the stations were accommodated, and there is some growth potential. Although, to do so the new multiplexers may have to be broken up into two pieces, with the new sections being connected to the old by cables.
So far, we have discussed this antenna project as though no problems were encountered. That wasn't the case. There were enough problems — some terribly significant — to fill another article. Look for the next article in this series in the March issue of Broadcast Engineering magazine.
Don L. Markley is president of D.L. Markley and Associates.
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