Thursday, April 18, 2024


A first responder in-building distributed antenna system, DAS; has three parts….. passive, active and signal source

Das DefiinedMost municipalities in the US have building codes that mandate the installation of an in-building public safety radio distributed antenna system commonly referred to as a DAS. At emergency events twenty years ago most of the time only the on- scene commanders and their lieutenants were in continual voice contact via land mobile radios. Today, most if not all, fire fighters, police and emergency medical personnel have handheld radios and use them frequently. To control the county’s voice communication system each 911 center houses very sophisticated ‘trunking systems’. These impressive arrays of computers and radios guarantee access into the system and then manage hundreds of simultaneous conversations on the system without delay or conflict. Working in unison, the 911 center, outdoor repeater/tower system, land mobile radios and the in-building DAS have made conducting emergency operations much less dangerous for the first responders and are a critical factor in producing better outcomes.
Although the term DAS is used by commercial real estate developers and construction folks during the process of any new construction, many times the term is not really understood. It is just another acronym, another check list item, and another add-on building cost.   Commercial real estate and construction personnel don’t need a heavy technical knowledge, but a good conceptual understanding is most useful.
A first responder in-building distributed antenna system, DAS, has three parts passive, active and signal source.

  1. The Passive Part….. This is the actual Distributed Antenna System It is the basic DAS infrastructure analogous to the pipes in a hi-rise plumbing system. It carries and evenly distributes the emergency frequency signal bi-directionally throughout the structure just as pipes distribute and return water throughout the building. The antennas typically cover 5-15K square feet depending on wall configuration and other structural obstacles. Using a CAD program specifically purposed for RF design, the antennas are strategically placed through-out the structure to ensure even coverage at proper signal strength levels. They, and their connecting cables, are considered the PASSIVE part of the DAS.
  2. The Active Part ….. This system component is called the Head-End. It is analogues to the pumps that push the water through the pipes in the plumbing system analogy. The Head End takes in the signal, conditions it, amplifies it and changes it into light, for fiber cable distribution, or RF energy for coaxial cable distribution. It then drives the signal throughout the structure via the DAS. The Head End, like the water pumps, is considered the ACTIVE system component.
  3. The Signal Source….The emergency frequency signal is acquired by placing a relatively inexpensive directional ‘donor’ antenna on the roof and aiming it at the closest county repeater tower/site. The donor antenna captures the signal from the free air. This is referred to “off-air capture” or OTAR, over the air. Via coax cable, the newly acquired signal makes its way from the roof to the head-end for processing. It is then routed to the bidirectional in-door antennas. The first responder receives a voice communication on his hand held radio from the antennas and then transmits back his response to all of those in the system on that channel/talk group by simply reversing the path. All this takes only a second or two and is precisely controlled by the trunked system computers back at the 911 center.

So the signal is captured on the roof, amplified at the head end, and bi-directionally distributed throughout the structure via the DAS. The signal source and the active and passive part as defined are the three components of a first responder in-building public safety distributed antenna system.
The concept is pretty straightforward, but the design and implementation must be done correctly for the system to work reliably. Many times these systems are a first responder’s life-line so it is very important to retain an experienced and reputable systems integrator.  The lowest bid should not be the sole determining factor for awarding a contract. See CHOOSING THE RIGHT INSTALLER FOR AN IN-BUILDING PUBLIC SAFETY RADIO PROJECT in the September 21 weekly.
If you found this article interesting click here for more.
For additional information contact Bob Butchko at or 973.244.5868 x104.

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