Outdoor enclosures
Dec 1, 2010 12:00 PM, By Steve Rossiter
Custom prebuilt enclosures provide diverse options for RF installations.
This diagram breaks down the design of an outdoor enclosure featuring a Harris Maxiva UAX air-cooled transmitter. Note the antenna mounting options on the roof and how the AC distribution cycle pulls air from the top of the enclosure, creating a continuous airflow pattern that maintains operational efficiency.
Select figure to enlarge.
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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