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.
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.
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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The movement of air inside the enclosure should be in a circular pattern. The HVAC unit pulls air from the top of the enclosure and blows cool air into the center. An air deflector will keep the cool air from going directly back into the HVAC unit and also direct the airflow to the bottom front of the enclosure. This creates a circulation pattern from bottom front to top front and onto the top back of the enclosure. The large circular direction eliminates hot spots in the enclosure (good point).
Note that efficient transmitter designs will reduce heat load. Transmitter cooling with internal fan systems that direct airflow in one direction, from the front of the enclosure across the PA modules and through to the rear of the transmitter cabinet, helps the circulation of the air in the enclosure. Fans moving in multiple directions within the transmitter will upset the airflow throughout the enclosure.
A “smoke test” is a good way to confirm there is proper airflow within the enclosure. Strike a match, blow out the flame, and hold it inside the cabinet. The smoke will circulate in the same direction as the airflow, identifying “dead” areas where air is not moving. This will supply information needed to make the appropriate HVAC system adjustments and improve circulation.
RF and antenna systems
Most outdoor enclosures will be spacious enough to accommodate the transmitter and associated hardware without issue. Depending on the operation, additional hardware might include satellite or microwave receivers, wireless receivers (for wireless network connections), media servers, routers and computers. A variety of monitoring equipment can monitor temperatures inside the cabinet and alert engineering staff of open doors or intrusions. A UPS system is also necessary, often coupled with an IP-controlled power distribution unit to remotely cycle power on specific components.
Digital broadcasts today require a mask filter to minimize out-of-band interference. In UHF systems, ceramic 12-pole filters are commonly used to minimize the system footprint. This design features ceramic resonators with small cavities on the inside of the filter as opposed to using traditional, larger air cavities. VHF applications will require traditional air cavity mask filters, making outdoor enclosure systems for VHF broadcasts far more challenging in terms of cost and footprint.
AC line filtering is also useful. This removes conducted AC emissions such as harmonics and other undesirable artifacts from the AC line — a requirement for minimizing interference with nearby consumer radios and TV sets. Surge suppressors will minimize electrical spikes for lines coming in the enclosure. Examples include GPS and satellite antenna cables.
Transmission line out to the antenna and tower will usually be of the 1-5/8in to 3in variety, in rigid or flexible style. Rigid line is best used up to the point it leaves the enclosure given the limited space inside. Attaching a standard rigid EIA flange at the exit point will give broadcasters the option of continuing with rigid line or switching to a flexible-style solution. Dehydrators for transmission line and antenna pressurization can assist inside the enclosure.
Antenna mounting systems vary, so it's best to provide a few options atop the enclosure. An ideal mounting system includes three separate pipes, about 18in long and 1in in diameter to support antennas in the range of 5lbs to 6lbs. An antenna shelf provides enough space for comfortable separation between GPS, Yagi, wireless WAN/LAN and other antenna choices.
GPS-capable antennas are especially ideal for single-frequency networks, effectively locking in exciters across multiple enclosures in the same market and allowing for a perfectly timed, synchronized transmission network. Small wireless antennas, such as G3-style models, can also assist with monitoring procedures over cellular communications — especially useful if a local T1 line is not available to the site.
The antenna shelf provides mounting for an ice bridge to protect the antenna systems' associated transmission line and cabling. Multiple predrilled holes into the antenna shelf will support multiple ice bridge mounting options, designed to protect both the antennas and the associated transmission line from snow, ice and other elements that can settle on top of the systems or fall from the tower, potentially causing service problems.
It should be noted that transmission systems for virtually any standard can live inside these enclosures. It doesn't matter if the system is ATSC, ATSC-MDTV, DVB-T, DVB-H, ISDB-T, FM or DAB/DMB; these enclosures are perfectly capable of supporting mobile and terrestrial broadcasts for main and repeater systems at the site of your choosing.
Steve Rossiter is mobile applications engineer for Harris Broadcast Communications.
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