More than a score of libraries now use radio for wireless transmission of data from branches or bookmobiles to the central site. Data radio telecommunica-tions operate online in real time; that is the data entered on a terminal is transferred directly to the computer, and search results are immediately displayed. The principal advantage of data radio: extremely low operating cost because there is no telco or other carrier sending monthly statements.
While telephone modems use frequencies in the thousands of cycles per second, radio modems use frequencies in the millions of cycles. In the U.S., the FCC regulates the use of radio frequencies by subdividing the available spectrum of frequencies into small segments. This subdivision makes it possible for two users, transmitting in close proximity, to stay out of each other's way. Any library or individual who wants to use a radio frequency must apply for a license from the FCC. License applications typically take about three months to process. Mobile data radio's transmission range is typically around 20 miles over flat terrain.
Since data radio uses FM radio waves, it is line-of-sight dependent. Geographic features such as hills and tall buildings can affect transmission, as can weather and other atmospheric conditions. It also is necessary to have considerable special equipment: amplifiers, antennae, towers, repeaters and boosters, etc.
An engineering study called a "link budget" is usually conducted to determine the feasibility, specifications, and cost of a data radio system. Link budget studies typically include specifications for tower height, antenna gain (related to signal strength), and the need for repeaters and other boosters. These studies are usually performed by local telecommunications contractors. Link budget studies vary widely in cost, from around $1,500 to $5,000 and up.
A single channel data radio node costs about $4,000. The simplest data radio system requires two nodes. The antennae cost at least $1,000 each (although bookmobile antennae cost as little as $600). Antennae usually have to be mounted on towers ranging from 30 to 100 feet in height, typically costing $2,000 to $6,000. Therefore, the simplest complete single channel system would cost $12,000 or more without taking into consideration in-building cabling and special contractor fees. The cable used between the data radio and the antennae has to have less than one dB (decibel) of signal loss over its entire distance. In addition, local contractors often have miscellaneous installation costs. It is important to find a contractor with substantial "low-power" installation experience.