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