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  • test09

    Understanding a bit about how FM repeaters are designed and how they work will improve your understanding of their on-air operations. Repeaters are not very different from other transceivers, but a few specialized components have been added to affect the repeat function via frequency pairs and to automate the repeater control. Let’s take a surface glance under the hood of a typical FM repeater.
    Duplexer: Inbound RF signals from a transmitting station are received by the repeater antenna and directed to a component called the duplexer. A duplexer is a device that allows a single antenna to be used simultaneously for transmitting and receiving two different frequencies. Why is this needed, you ask? Necessarily, a repeater must receive and transmit at the same time, albeit on two different frequencies. When a high power transmitter and a receiver of great sensitivity operate in the same band near one another in frequency, the transmitter’s signal will overwhelm the receiver even though the tuned frequencies for transmit and receive are not identical. A typical receiver system cannot adequately reject, or filter out, the powerful blast of the transmitter. As a result, the repeater’s strong re-transmitted signal will mask the weaker inbound signal from a distant station, making the repeater useless… unless a duplexer is introduced. A duplexer is a set of resonant circuits that serve as a very sharp and effective RF filter. This filtering circuit applied to the receiver side of the repeater keeps the frequency of the transmitter out of the receiver – the transmit frequency is rejected and not allowed to proceed to the receiver, but the normal receive frequency of the repeater is allowed to pass. Further, the duplexer rejects the normal receive frequency on the transmit side, requiring that all received signal strength is directed to the receive side path, while allowing the transmit signal to pass on to the antenna unimpeded. An alternative to using a duplexer in a repeater is to arrange separate antennas for transmit and receive that are physically separated by a substantial distance so that the strong transmitted signal does not overload the receiver. Because of the physical challenge and long feedlines this arrangement presents, the more common scenario is use of a duplexer with a single antenna. Duplexers are typically constructed as cylindrical cavities of highly conductive material, such as silver, and usually at least ¼ wavelength long for the repeater frequencies used. The cavity and associated components are designed to be resonant circuits of very high Q, meaning that only a very narrow range of frequencies will be resonant and passed through the circuit with good efficiency. In this way, the duplexer cavities serve as very sharp filters that may be tuned to pass only a narrow band of frequencies in the receive band or the transmit band of the repeater. A repeater utilizes automatic control since no control operator is in constant attendance of it functions.
    A repeater controller is an electronic device that is functionally interposed between the receiver and transmitter of the repeater to orchestrate repeater functions. The controller accepts the audio signals output by the receiver and routes them to the input of the transmitter. The controller automatically activates the transmitter when signals are received for re-transmission, it initiates the repeater station identification transmissions with proper regularity, and it stores data or audio for transmission of the station call sign, courtesy tones, and other information. The controller may also detect special tones or other signals transmitted to the receiver remotely by the station operator for the purpose of implementing or changing control functions. For instance, a repeater operator may transmit sequences of DTMF tones to the receiver to initiate a phone patch, a radio connection to a conventional telephone land line. Other control functions may change the repeater’s power output, the courtesy tones, the duration of the hang time over which the repeater continues to transmit following the termination of a received signal, or other functions.
    IC-9100, IC-7100, AT-180, Yeasu G-5500, GS-232B, JT222M, HRI-200 Wires-X, Astron RM-60M, DR-1X, SCOM 7330, Wouxun KG-UV950P, TYT TH-9000D, Motorola CDM 1250, Stepper IR DB11 Antenna