This invention relates to a repeater system for enabling fixed or mobile radio stations to communicate with each other and, in particular, to a repeater system having reduced power loss.
A repeater is the combination of a receiver and a transmitter in a single unit for receiving a signal at one frequency and simultaneously re-transmitting the same signal on a second frequency. Depending upon application, the transmitted frequency may be relatively close to the received frequency, e.g. 600 khz., or greatly displaced from the received signal. Depending upon application, frequency, and government regulation, the transmitter in a repeater may be relatively powerful, hundreds of watts, or may be rated at just a few watts.
Commercial two-way radio communication has evolved into two different techniques for mobile operation, cellular and specialized mobile radio (SMR) or dispatch service. Cellular systems use several repeaters dispersed in a geographic area and operating at low power to keep propagation relatively short, e.g. within a radius of less than ten miles. The local area covered by each repeater overlaps the local areas covered by neighboring repeaters, forming overlapping "cells" of coverage. A subscriber traveling from one cell to another cell is automatically switched from one repeater to another by a computer coupled to the repeaters by microwave link, optical fiber, or wire.
SMR uses a relatively powerful repeater, compared to cellular systems, and the repeater is usually located at the highest available elevation in a geographic area. The repeater is coupled to an omni-directional antenna to cover the entire geographic area, enabling dispatchers to communicate with a fleet of mobile subscribers in the geographic area and enabling the subscribers to communicate with each other.
At an antenna site, a plurality of repeaters are located in a suitable structure and are coupled to one or more antennas on a tower several yards away. Each repeater operates on a particular pair of frequencies and is a complete receiver/transmitter in itself. A tower may be one hundred to five hundred feet high. The cable coupling the antenna to the repeaters, even low loss cable, has an attenuation of at least three db (decibels) per one hundred feet. As a result, the power actually reaching the antenna is considerably less than that produced by the final amplifier in each repeater.
It is known in the art to locate a Class C amplifier in an antenna tower to reduce cable losses. Adding such an amplifier to a repeater system is not possible because of the plurality of channels in use and the relatively high intermodulation distortion of Class C amplifiers.
In view of the foregoing, it is therefore an object of the invention to reduce power loss in repeaters.
Another object of the invention is to increase the effective radiated power of repeaters in a multi-channel communication system.
A further object of the invention is to reduce the cost of communication systems using a plurality of repeaters.