A selective signaling paging system allowed an individual to receive messages only when his pager was alerted. Motorola began installing systems using this emerging technology in 1955.
A need for quieter communications
Before paging systems were invented, communications within a large building such as a factory, hotel or hospital used a public address system. Speakers were placed where messages needed to be heard. For example, in a hospital when a doctor was needed in the operating room, the message went out to the entire hospital, and everyone—patients, doctors, nurses, other staff—within hearing range of a speaker heard it.
Radio paging systems were intended to replace noisy public address systems within a building. But even early systems broadcast messages to everyone’s pager on the system.
Motorola’s selective signaling paging system
To alert only one receiver, Motorola engineers worked on a technology called selective signaling. Lead engineer Daniel E. Noble invented the Vibrasender electromechanical reed that transmitted a pure tone, and the companion Vibrasponder relay reed that vibrated when it received a signal on its frequency. Because each pager’s Vibrasponder was tuned to a different frequency, the user received only the message that was directed to him. When the pager’s alert tone sounded, the user pressed a “push-to-listen” button and waited to hear his message.
The message was sent from a selector console where a receptionist or switchboard operator pushed the button assigned to the pager she wanted and spoke into a desk microphone. Cabling connected the console to an FM transmitter 25-50 feet away, which contained a Vibrasender.
Radio coverage inside a large factory or hospital complex challenged the engineers. At first, the installation process required antenna wire to be strung throughout the building. Motorola engineers tested a system in an AC Sparkplug plant in Flint, Michigan, USA. Engineer William J. Weisz recalled, “We wired the whole plant, crawling around the ceilings and so forth."
A working system
In 1955, Motorola installed a radio paging system in the Mount Sinai Hospital complex in New York City. The installation required 30,000 feet (9,144 meters) of antenna wire that connected 20 buildings and provided 100% coverage inside the buildings. A transmitter, eleven operator positions and 175 Handie-Talkie pagers completed the system.
Even though there were limitations with the low frequency system (50-100 KHz), Motorola engineer John F. Mitchell remembered that those who tried it wouldn't give it up: "[If] you went to Mount Sinai Hospital and tried to take the first pager back, they wouldn't give it to you. It was a concept they didn't want to give up.”