Outdoor enclosures
Dec 1, 2010 12:00 PM, By Steve Rossiter
Custom prebuilt enclosures provide diverse options for RF installations.
Protection from external elements, including dirt, moisture and corrosive materials, is pertinent for ensuring long life and continued operation of the many components that ultimately generate the terrestrial signal.
The ongoing spectrum debate has shed new light on a fact that the broadcast industry has always known: The terrestrial broadcast model provides the most efficient means of delivering broadcast and multimedia content from one site to many consumers.
The increasing number of terrestrial multichannel DTV and mobile television broadcasts means that most local viewers of free over-the-air television can receive more content today than ever before. This proves that local broadcasters are recognizing and taking advantage of their most prized commodities — their wireless transmission capabilities and allotted spectrum — in new and innovative ways.
Broadcasters are also exploring and deploying new ways to transmit content. More high-power broadcasters today are adding repeater sites to effectively blanket their markets following the digital transition. Meanwhile, low-power and mobile broadcasters may rely on shared sites or single-frequency networks with multiple transmitters.
All of these factors have influenced a growing number — and diverse array — of broadcasters to explore new locations not traditionally used for terrestrial broadcast services. Such locations, including rooftops and shared cellular towers, offer a variety of benefits, including easy deployments and open real estate. However, cost of deployment remains a drawback.
Custom-built outdoor enclosures, constructed of steel or aluminum, offer a cost-effective and technically-sound alternative for broadcasting from nontraditional locations. Prebuilt enclosures vastly reduce the square footage compared with traditional structures. Installation costs are also minimized: Enclosures are transported to the site and dropped onto a small concrete foundation.
From a technical standpoint, properly designed enclosures also offer everything required to support terrestrial transmissions. This includes HVAC and electrical systems, the transmitter and associated hardware, filtering, and antenna systems. Quality enclosures also implement design characteristics that meet the most pertinent industry standards for construction and equipment protection, ensuring that the broadcaster's investment in technology is secure and well-protected.
Design specifications
The choice of steel or aluminum will mainly align with the size and application of the enclosure. Steel is thinner but far heavier, rendering rooftop installations difficult at best. Cost is also an issue, making ground-level installations at cell sites expensive. Steel enclosures make more sense for pole-mount installations on towers housing very small, low-power transmitters. These are more common for low-power gap fillers in multisite transmission systems. Stainless steel enclosures can also be used in highly corrosive environments.
Building out the infrastructure begins with identifying electrical requirements and ensuring compliance to various safety and construction standards. Electrical systems are specified for single-phase, split-phase or three-phase depending on availability; single- and split-phase are most common given the site seclusion of most enclosure installations.
The electrical system is often built out within a small cabinet or AC distribution box affixed to the side of the enclosure. The AC distribution panel lives within the cabinet to accommodate the appropriate breakers. The electrical system is then specified and built to meet the appropriate UL standards, typically UL60950, an American National Standard detailing Safety of Information Technology.
Protection from outside elements is another critical design consideration. Transmitter outdoor enclosures should meet NEMA 4X-rating, UL508A listings and partial Telcordia specifications. Outdoor enclosures protect equipment from external dirt, moisture and corrosive materials. NEMA 4X and UL508A standards represent compliance for the construction of industrial control panel enclosures built for installations in hazardous and environmentally-diverse locations.
The air-conditioning system should be a closed loop system that is not open or vented to the outside. This keeps external moisture, dust and dirt ingression out of the enclosure. Watertight and airtight seals around entry points for transmission line and other cables will further assist in keeping dirt and moisture outside.
Telcordia GR-487-CORE specifications should be met to support noise pollution standards as well as specifications for wind resistance, and more specifically, to prevent significant leakage from wind-driven rain. This standard provides criteria for analyzing electronic equipment cabinets used in outside plant environments and applications, noting acoustic noise and environmental vibrations. It ensures that transmission systems outfitted within compliant enclosures in urban or congested areas, like suburban neighborhoods, will pass noise tests with the neighbors. Enclosures built off-site and delivered by vehicle should also pass Telcordia standards such as GR-63-CORE, which addresses transportation vibration and shock — ensuring the enclosure reaches the site in one piece.
Equipment cooling
Proper insulation, typically an R rating of 6.5 or better, will keep heat from the sun and ambient sources to a minimum. R6.5 insulation is a 1in, foam-based, rigid foil-faced insulation that protects equipment from external heat sources. Still, even with modern advancements in cooling efficiency, transmission systems will require environmental control. An efficient HVAC system and airflow strategy is required to keep equipment at the proper operating temperature and costs low. Heating the enclosure may also be required for cold environments, and cold startup of electrical equipment, transmitters and RF components.
HVAC is perhaps the most challenging requirement. The first step is to ensure compliance with EPA standards for environmentally safe refrigerants as mandated by U.S. EPA Title VI of the Clean Air Act, governmental regulations. The mandate, as of Jan. 1, 2010, states:
The Montreal Protocol requires the U.S. to reduce its consumption of HCFCs by 75% below the U.S. baseline. Allowance holders may only produce or import HCFC-22 (R22) to service existing equipment. Virgin R-22 may not be used in new equipment. As a result, heating, ventilation and air-conditioning (HVAC) system manufacturers may not produce new air conditioners and heat pumps containing R-22 after Jan. 1, 2010.
With the differences in the types of refrigerants, there are differences in the way HVAC systems react in adverse environments. This requires changes in the way systems are designed. Newer environmentally-friendly refrigerants such as 407C have replaced older R22 refrigerants that had adverse effects on the ozone layer. These newer refrigerants are also adequate for use in enclosures shipped overseas.
The size of the cooling system is based on the size of the structure and its estimated heat load. This will help to determine the appropriate airflow based on the directed airflow of the equipment going inside. Most of the hot air from the transmitter is expelled at the rear of the transmitter and remains in the enclosure; this is because air is moving through the front to the back. Directing a portion of the cool air from the HVAC system to the rear of the transmitter will create an appropriate cold and hot air mixture. Turbulence in that area mixes the hot and cold air that is pertinent to maintaining an efficient and reliable transmission system.
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