A nice summary of approaches and uses
The objective of limited area broadcasting is different from conventional commercial broadcast applications. Instead of trying to cover the maximum area possible, limited areas are chosen and a broadcast signal is focused on them. In the United States the FCC has defined limited area broadcast methods and limits under Part 15 of its Rules, which do not require either a license or notification for operation.
1.1 Carrier Current
Carrier Current broadcast systems utilize an impedance-matching Transmitter Coupling Unit to impress a Radio Frequency (RF) signal on the AC wiring of a building. This method essentially turns the AC power wiring of a building into an antenna which covers the interior of the building and some of the surrounding area. A typical system would consist of a transmitter and coupling unit in each building which required coverage (although electrical tricks can sometimes reduce the amount of equipment needed) and a Distribution Amplifier to send audio to the transmitters via telephone lines. LPB and Radio Systems both manufacture equipment geared to Carrier Current.
Applications - Carrier current systems are employed in a variety of locations, most frequently in educational settings. More than 1500 colleges and high schools in the United States and other countries have installed carrier current systems. The relative ease of installation, maintenance and use of carrier current equipment has made it very appealing for schools. A constantly changing student body and lack of professional engineering skills do not limit the operation of carrier current equipment!
The Radio Reading Services for the Blind distribute signal to individual and group housing around the United States using an FM subcarrier and a subcarrier receiver to supply audio to carrier current transmitters.
These transmitters permit the blind to listen to book readings and other services with a common AM receiver. There are more than 100 of these systems in operation in the US.
Carrier current systems also operate in government administrative buildings (in 2 US states) in order to rebroadcast speeches from the Senate meeting rooms. Local social groups have developed carrier current systems for urban neighborhoods and churches in a variety of major cities. Many auto racing tracks have carrier current systems broadcasting race information to fans (information which might otherwise be drowned by engine noise). The possibilities are fairly unlimited for small carrier current systems.
Carrier current systems traditionally receive audio via a twisted pair telephone line. A recent development is a fiber optic interface for carrier current interlinking. Using fiber optic lines the modulator has been removed and modulated RF is fed directly to the linear amplifiers. These systems are able to achieve a greater audio bandwidth thanks to the use of fiber optics, and eliminate any potential for beat note interference.
1.2 Radiating Coaxial Cable Using AM or FM Stereo
To cover a fixed area “leaky” or radiating coax can be employed. A specially woven outer braid creates a leakage pattern which follows the entire length of the cable. Cable systems can carry either AM or FM Stereo, or both through the use of filters.
The "NF" series of radiating coaxial cable is available in two models : NF-1D (0.29") and NF-2D (0.42") with a variety of polyvinylchloride (PVC) jackets. Jackets include UV resistant with reinforcing wire for aerial suspension (-S) and standard.
AM systems can be designed to run approximately 3500ft of NF-2D cable or 1300ft of NF-1D in a single end-fed length.
Double that length can be made with the addition of a splitter. Linear amplifiers can be added into the system to extend it without altering the RF signal. Typical reception distance is approximately 75-100ft on either side of the cable. The longest radiating coaxial cable system in the world is along the Atlantic City Expressway in New Jersey covering 22 miles.
FM Stereo systems can operate with 100ft of NF-1D or 400ft of NF-2D in a single end-fed run. Again, double the distance using an RF splitter as well. Linear amplifiers are available to extend these distances as well. Typical reception distances are from 100-200ft on either side of the cable in stereo and about triple the distance in FM mono.
For both AM and FM applications requiring extremely long lengths of cable without the use of linear amplifiers, use Andrew Radiax¨ radiating coaxial cable. A single 30 watt AM transmitter can feed 11000ft of Andrew Radiax¨ in a single end-fed run.
Applications - The designs for radiating cable systems are limitless, and ever-increasing. Common systems include amusement park entry roads (for driver information), commercial information along roadways in front of individual stores or shopping centers, language translation systems, hearing-impaired systems, health club systems, etc.
Several major amusement parks and tourist attractions in the United States use radiating cable systems along their entry and exit roads to inform motorists of the available activities, hours of operation, parking and weather information. One park includes a drive-thru safari in which the animals roam free around passing cars. Each animal area has a separate message zone broadcast to the vehicles, providing information on the habitat and animals as they pass. Careful engineering design and installation prevent interference between transmitters and message zones.
The Reverend Billy Graham used up to seven radiating cable systems to provide simultaneous language translation services to attendees of his Crusade worldwide. Audio is provided by a translator and patrons purchase inexpensive walkman-type receivers with headphones which allow them to listen to the service in their native language. Many other religious organizations also use these systems for simultaneous translation duties worldwide. These systems can be configured for multiple channels in AM and FM packages.
Nearly 70 sports arenas in the United States have a radiating coaxial cable system suspended below the ceiling to rebroadcast the play-by-play for the game. Sports arena systems rebroadcast a local radio station, which has poor reception in the stadium, but has licensed the broadcast rights for the local team. These systems are growing rapidly in popularity as they are also very useful to the hearing-impaired, who are able to tune in the event and adjust the headphone volume to their necessary levels.
Radio-Aide systems were developed for churches requiring either translation or additional services for the hearing-impaired. These are radiating cable systems for coverage of a main worship room and/or a large outdoor area designed to reach all of the seating in the designated area and permit all worshippers access. An extension of these systems has been a parking lot system, which provides church service rebroadcasts for cars, typically for the infirm or families with small children.
A common application for FM radiating cable systems is in health clubs and gymnasiums. Many of the larger exercise rooms have multiple TV or video programs. Using FM transmission on multiple frequencies isolates the audio program of the videos so that only those wishing to hear them may do so (with a pocket radio) and those who are not following a video program are not disturbed by the excessive noise generated by the multiple audio programs. These systems are also used around indoor jogging paths to provide music and encouragement to runners.
Currently, FM coaxial cable systems are in use at more than 50 US schools. These systems allow a college to operate an FM Stereo radio station without having a license from the FCC, and to contain the signal within the school property. These stations, like carrier current, provide the students with a broadcast experience and an outlet for their own music, which might not otherwise be available in their market.
Applications - TIS systems at airports provide incoming drivers with information on parking, passenger drop-off and pick-up, airline terminal locations and other information well outside the airport, this reduces the number of confused and misdirected drivers, thereby allowing for smoother operation of airport roadways and a more efficient flow of traffic. Use of multiple frequencies at an airport would permit specific flight information to be provided as well, if desired.
Although these systems are typically used with only 10 watts of input power, exceptions have been licensed for emergencies. In areas of the United States which are regularly subjected to severe weather conditions (hurricanes, flooding, etc.) there are a number of 100 watt systems which operate only in case of an emergency (people are alerted to tune in via a loud siren). These systems are also in use around some of the nuclear power plants and other hazardous material sites in the US.
1.4 Tunnel and other Rebroadcasting Systems
Limited area broadcasting is used to rebroadcast local stations in motor vehicle tunnels throughout the world. These transmitters operate in tunnels in Europe and Asia, as well as the United States. These systems are used either to rebroadcast local radio stations, or emergency information to vehicles in the tunnels.
These systems have also designed systems for rebroadcasting in underground environments and/or construction areas with heavy RF shielding which would otherwise prohibit the use of radio communications. One of these systems will be installed in July 1997 in Taipei, Taiwan in an underground shopping mall. The system rebroadcasts ten FM radio stations and includes emergency override communications in the event that they will be required.
1.5 Free Radiate FM
The FM band covers 87.9MHz to 107. The Part 15 field strength regulations will permit this system the same coverage field as the radiating cable systems - 250µV/m at 3 meters.
This typically means a 200 foot radius with a mono signal, less with stereo. As with any low power system, the quality of the receiver and the availability of a good frequency will influence the coverage of the systems.
Applications - Free Radiate FM has a wide application base due to its ability to operate as both a fixed and mobile transmission device. It can be used in either fixed or mobile applications. In standard fixed-site mode, it will provide opportunities for use in Health Clubs, Language Translation, Hearing-Assistance, Real Estate, Business Advertising and many other areas.
In the portable mode, it can be used with mobile business advertising such as tree surgeons, electricians, plumbers and any other business that relies on signage on a vehicle for attracting customers. Additional possibilities include use on highway construction or emergency vehicles.