Fire department portable radios are frequently unreliable inside buildings and other structures such as tunnels. Construction materials, earth, and changes in the radio frequency environment can greatly reduce the strength of radio signals. If a firefighter inside is unable to transmit or receive, he or she must relocate closer to an exterior opening, move to a different floor, use an alternate means of communication, or resort to runners or direct voice communications. Cell phone signals are affected by the same factors as radio signals. Land line phones will allow firefighters to communicate with dispatchers, but not other units; they may also be affected by the incident occurring in the building. All of these factors may delay operations, and create greater challenges in maintaining crew integrity (Figure 7.3).
New technology can improve signal transmission within buildings and structures through fixed communications infrastructures. Passive approaches simply provide a conduit to assist in the transmission of signals. However, active methods involve powered devices to amplify and retransmit signals.
For example, the “passive antenna system” includes both an internal and an external antenna, connected with a short coaxial cable. A “radiating cable,” also known as a “leaky coax” is a network of coaxial cables with slots in the outer conductor that create a continuous antenna effect.
Increasing in popularity is an active signal transmission method involving a signal booster also known as a “Bi-Directional Amplifier,” or simply BDA. These powered devices amplify signals between an external antenna and one or more internal antennae. Both reception and transmission are amplified messages on portable radios within the building. A network of antennae placed at strategic locations or a leaky coaxial cable distribute signals throughout the coverage area.
Some installations combine passive and active approaches. Passive antennae generally work well in small, well-defined areas. BDAs function well in larger, diverse areas that need a coverage solution.
Some jurisdictions have adopted laws or ordinances concerning public safety radio communications. In others, designers should consider specifying a study to determine the possible need for retransmission devices. Installing stationary communications infrastructures in high-rise buildings and tunnels is one way to resolve communication problems like those encountered by the Fire Department of New York on September 11, 2001.
Without laws requiring this equipment, cost considerations may discourage owners from voluntary installations. However, owners of high-rises or other target hazards may be swayed by increases in property value or improved safety for tenants. Alternatively, in high-rise buildings where firefighter communication systems are required, a code official may permit the substitution of fixed communications infrastructures as discussed in the section, Fire Department Communications Systems, on page 57. Perhaps, in the future, insurance companies will offer lower premiums for buildings containing such installations.
Many local communications ordinances currently in effect in the U.S. contain specific requirements for system performance. These include signal strength, area coverage, reliability, secondary power supply, interference filters, acceptance testing upon completion, and ongoing periodic testing.