Public safety: Page 4 of 5

Physical construction and design considerations

Donor antenna(s) and placement

  • Close coordination with the building owner is key for this consideration. The antenna location should be chosen to minimize coax cable run distances while having good line of site to the transmitting station. Ideally it should be near a building edge to avoid transmitting signal across a rooftop. Signals transmitted across rooftops can leak into floors below, causing an RF feedback loop which can degrade or disrupt system performance. Suitable roof penetration and waterproofing techniques are required. Existing penetrations are ideal. It is best to coordinate with the owner’s roofing contractor if a new penetration is required, especially in the case of rubberized roofing. Not doing so could void the owner’s roof warranty.
  • Another consideration is antenna height. At issue are visual aesthetics, PIM (Passive Intermodulation Interference) performance, and wind loading.
  • In general property owners prefer that donor antennas are not obvious. RF neutral screens and other camouflage techniques can be used to minimize this concern.
  • PIM interference can be created by locating an antenna too close to metal structures of metallic roof flashing along building edges. Extra antenna height and careful placement can reduce or avoid this problem.
  • Finally, antenna masts and ballast suitable for the required height are important. Taller masts must be sturdier and use more ballast to handle added wind loading. These must be matched to roof load bearing ability and antenna type.
  • Cables must be properly protected to avoid cable or roof damage. Best practice is to route donor coax through PVC, raised on sleepers to avoid roof damage.
  • Proper grounding and lightning arrestors are required per code.

Cable runs

  • After the outside surveying is complete, the next order of business is identifying a vertical chase that will get the cable runs from floor to floor. Once this has been identified, a network closet/IT room where the booster equipment can be parked must be located. Ideally, you would want the two elements; the vertical chase and the network closet/IT room to be as close to each other as possible – if not one and the same.
  • A walk through the facility should allow the DAS designer to begin to see potential cable runs in certain locations more so than others. Certain areas should jump out that would be better locations for internal antennas. These areas should be hallway juncture points and areas that are in need of strong coverage (e.g., manager’s office, security office, etc.) consistently. This would mandate a dedicated internal antenna within close proximity.
  • Another item to look for is the method of transport for the cabling. Are there dedicated cable trays? Is conduit required? Does the local fire code mandate plenum ratings on the cable? If fiber is the method of delivering RF, is there any dark fiber available to use? If so, what type is it? Is the fiber of the single mode or multimode variety?
  • What do the ceilings look like? False ceilings? Hard lid ceilings? A mix? The amount of labor to get through different ceiling types will vary as will the time/cost. Ceiling types will have a huge impact on which antenna to use.
  • In some cases, the end customer may want the antennas out of view. Examples of what the antenna looks like should be presented to the customer for approval from a cosmetic perspective. How high are the ceilings? Will a hydraulic lift be required to gain access for antenna installation? These are items that need to be considered when doing the site walk.
  • An area where core drilling is required is an important cost/time consideration that can be identified during a thorough site survey. Firewall locations need to be identified as they require special prep work for penetrations and pulling cable from one side to the other. 

Wall construction and attenuation factors
The building materials used in the construction of the building and walls should be scrutinized closely. What is the makeup of the walls? Drywall, sheetrock, cement blocks, brick? Is there any metal? In hospital environments, lead will be present in the walls near radiology units. What about insulation or ductwork? Metallic backing on certain types of insulation will strongly attenuate RF signals from propagating. Metal duct work will also have an impact on a RF signal.

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