We Like Our Precast Transmitter Shelter
     

Back in the 1960s and 1970s, we heard a lot about “space age” products. A lot of this was advertising hype, but some was the real deal. The race to space, to put a man on the moon, to send unmanned vessels to other parts of our solar system and beyond, and to get a viable orbital transport up and running resulted in a lot of new technologies.

These found their way into our daily lives in everything from car wax to the integrated circuits that spawned the “digital age.” We have all benefited from these advances.

We don’t hear about it as much, but another business more recently has driven a significant number of technology advancements. The wireless industry, still growing almost exponentially in some regions, has spun off any number of advancements that we in the broadcast world enjoy.

The electrical system comes pre-configured to the customer’s specifications, in this case a three-phase disconnect for a 30 kW FM transmitter.

One of these “wireless age” components is the rapidly deployable precast equipment shelter.

For most of my career, transmitter buildings were constructed on-site from traditional building materials — wood and masonry of one type or another. My personal favorite was joisted masonry construction: concrete block walls topped with closely-spaced roof joists covered with a wood or metal roof deck. This provided for a sturdy, secure, well-insulated and durable shelter for transmitter equipment.

Ten or so years ago, in the early 2000s, the building environment changed. That was about the time that the cellular network buildout began to level off. Companies that had for many years stayed very busy providing the wireless networks with prefabricated equipment shelters began finding themselves with time and materials on their hands. They began looking for new markets, and broadcast was one of the industries that they targeted.

Precast shelters are shipped on ‘low boy’ flatbed trailers and unloaded with a crane.

It was about that time when I found myself in need of several transmitter buildings around our company. I initially started down the old path of building joisted masonry structures. When I priced those against some of the proposals I had received from precast equipment shelter manufacturers, I quickly concluded that I could get a much better shelter more quickly and for a lot less money than anything I could do using standard building techniques on site.

THE ICE CHALLENGE

One of the challenges that we have long faced with our transmitter buildings, particularly in FM applications, is ice fall protection. Transmitter buildings typically are placed close to the tower base to minimize line losses, and it is that placement that puts those buildings right in the fall zone for large chunks of ice that fall off the tower. Joisted wood and metal roofs are not very good at deflecting such missiles. At best, they often end up with holes in the roofing membrane and have resultant leaks. Sometimes, ice bombs do a lot more damage than that.

With their concrete lids, precast shelters really shine in the area of ice fall immunity. In my first application, the new shelter would sit at the base of a 1,380-foot tower in Alabama, a place where we do get some ice storms from time to time. I knew that a conventionally-constructed building would have to be protected with a galvanized steel ice shield, which would really run up the costs and make re-roofing a royal pain down the road. That, more than any other factor, is what convinced me to try a precast shelter that first time. Since then, I have not even considered going back to conventional construction.

BUILT TO SPEC

Precast equipment shelters typically are ordered with all the electrical, HVAC, security, cable ladders and other facilities built right in. The customer specifies all this and the factory builds it all. The idea, which again came from the wireless industry, is rapid deployment — with all these items taken care of at the factory, the site can be up and operating within hours of the shelter’s arrival.

Fine-tuning the placement on the foundation slab.

I have found this to be a huge convenience and time-saver. Our people don’t have to deal with or coordinate the schedules of all the subcontractors. The electrician is not getting in the way of the mechanical contractor and vice versa. In fact, you only need a general contractor to construct a mounting pad (you can set most precast shelters on a bed of gravel if you wish) and an electrician to tie in the power from the utility. Once those two are done, the building is all yours to begin installing equipment.

The electrical specifications include what circuits are needed for what. For example, you might specify two 150-amp three-phase circuits for the main and aux transmitters. These would be provided in the panel and conduit would be installed to a junction box on the ceiling above the specified locations of the power entries for the two transmitters. A drop of Liquid Tight conduit, complete with connector, would be provided for the final connection to the equipment. Ditto for the electrical circuits for the equipment racks and other equipment.

A generator transfer switch (manual or automatic) can be specified, or provision for a vendor-supplied switch can be ordered. A generator receptacle also can be specified for connecting a portable (trailer-mounted or rental) generator if a permanent pad-mounted generator will not be provided.

Another great option available with most precast shelters is a “halo” ground system. This consists of an AWG #2 wire that runs around the perimeter of the inside of the shelter about six inches below the ceiling (like a halo, thus the name) with drops to the various pieces of equipment, cable ladder, transmission lines and grounding blocks. Wireless operators have figured out how to protect their stuff from lightning damage, and the “halo” ring is a big part of that.

Broadcasters have not been quick to employ cable ladders at their transmitter sites, but again, this “wireless age” innovation is making inroads. Precast equipment shelter manufacturers routinely provide overhead cable ladders to the customer’s specifications, and this is another huge time-saver. Control, audio, AES, monitoring and other cabling can be installed quickly and neatly, and routed between transmitters and equipment racks, between equipment racks and telco board and between microwave racks and “waveguide ports” (industry-speak for transmission line egress ports). I, too, was slow to come around to this way of thinking, but now I’m a believer.

Cable ladders, ‘halo’ ground, telco board and high/low temp alarm thermostats are available options.

There are many HVAC options available. Typically, two self-contained units are provided on the end of the shelter in a lead-lag configuration, where one unit runs for a week and then the other unit takes the load for a week. If either unit fails, the other takes over and an alarm contact is closed. Mechanical thermostats often are provided for settable high- and low-temperature alarm contacts. You can specify anything special that you may want, such as exhaust ports for transmitter cooling air, make-up air intakes and emergency ventilation systems.

One important consideration is the width of the door. Single doors are available for up to about 48-inch widths and beyond that a double-door must be specified. Some manufacturers also offer doors with more resistance to bullet penetration, usually specified as a particular caliber fired from a certain distance. That is certainly something to think about in a lot of installations where vandals might find a steel door to a transmitter building an irresistible target.

UP TO CODE

Precast manufacturers have engineers on staff who are licensed in virtually all states; they can provide sealed drawings acceptable in just about any jurisdiction. This makes the permitting process straightforward. My experience has been that plan checkers at local building departments are well-acquainted with precast shelters and in fact prefer them to on-site construction.

Precast shelters are delivered on flatbed trucks. It’s up to the customer to ensure a clear path for the truck to the installation site and provide a crane and operator for unloading and placement. I have used the general contractor for the crane work in every case to handle the site details, and I have found that local crane operators tend to be fairly well-acquainted with precast buildings. All the ones I have used have been capable and well-versed in their unloading and placement.

I have now installed five precast equipment shelters and am now planning for a sixth, the latest an oversized 12-by-30-foot building for a 50 kW AM that will be shipped in two pieces and joined on site.

As far as I’m concerned, this particular product of the “wireless age” is an excellent fit for broadcast transmitter site applications. I intend to use precast shelters for all my future projects.

W.C. “Cris” Alexander is director of engineering at Crawford Broadcasting and a past recipient of SBE’s Broadcast Engineer of the Year award.


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