The KM6XU 51.800 MHz repeater consists of the following components:

  • Two GE Mastr II low-band VHF mobile radios.
  • A GLB GASFET tuned pre-amp.
  • A Wacom WP-609 full-size low-band four cavity duplexer.
  • A Micro Communications Concepts RC-1000 repeater controller.
  • A Communication Specialties TS-32P CTCSS encode/decode board.
  • A Diamond V-2000A tri-band antenna.
  • An Astron RS-20A-BB power supply and a large marine battery.
  • Forty feet of Belden 9913 coax.

    The details:

    The repeater's input frequency is 51.300 MHz; its output is 51.800 MHz, and a PL tone of 114.8 Hz is used to activate the repeater. A 114.8 Hz tone is always present on the repeater's output. The repeater presently resides at 1880 feet AMSL, approximately three miles northwest of Mount Jackson in Sonoma County, and is coordinated with NARCC (Northern Amateur Relay Council of California).

    Purists will note that only one of the radios could have been used in the building of this repeater. Two radios were used not only for simplicity's sake, but in order to gain added isolation and to avoid stressing the radio's internal ten volt regulated supply by running the receiver (250 mA) and the active exciter (400 mA) simultaneously during normal repeater operation. Properly tuned, the transmitter's output can be increased to full power (137 watts) with little desensitization. The added isolation was determined to be necessary after discovering that this transmitter design produces a few spurs, one of which is about about 500 KHz below the repeater's transmit frequency. The currently selected transmitter power of 20 watts minimizes the size of this spur which grows larger with either an increase or a decrease in transmitter output power.

    The addition of the GLB resulted in an extremely sensitive receiver with a 12 dB SINAD on the order of .12 microvolts. Desense is almost nil. The GLB contains four micro-sized helical resonators and a very nice GASFET transistor.

    The four cavity duplexer consists of two band pass and two band reject cavities. The transmitter is connected to one side of a "T" adapter via a 51.800 MHz band pass and a 51.300 MHz band reject cavity. The receiver is connected to the other side of the "T" via a 51.300 MHz band pass and a 51.800 MHz band reject cavity. The center of the "T" is connected to the coax which goes to the antenna. A test was performed in which the transmitter was run at full power for one hour with no detectable rise in cavity temperature. The cavities are full size and stand about five and a half feet tall. They are made from InVar®, a steel alloy with a very low thermal coefficient of expansion. This keeps the cavities' effective notch and pass frequencies right on the mark despite the significant changes in temperature that occur at their current location in an uninsulated building. Click here to see the printout of the cavities' bench test on a nearby frequency.

    While not high-tech, the RC-1000 repeater controller has worked well, and has only had to be cleared twice since 1994. Both of these incidents were the result of configuration efforts gone awry; the RC-1000 has otherwise performed without fault through large ranges of temperatures, humidity (don't ask!), power supply voltages, and stray RF.

    The RC-1000 is directly connected to the Communications Specialties CTCSS board. While a PL tone of 114.8 Hz is used to activate the repeater, those without PL who have DTMF capabilities can enter "*31" which will temporarily take the repeater out of PL until 30 seconds of inactivity has elapsed. A 114.8 Hz tone is always present on the output for those who wish to run full squelch.

    The repeater's antenna is about 30 feet above the surrounding terrain (which suffered some amount of deforestation for the sake of the antenna's radiation pattern), and because the repeater is on the south side of a mountain top, its coverage is from ENE to NW. The antenna is omni-directional with neither uptilt nor downtilt.

    The Astron RS-20A-BB power supply is a proven design in mountain top use. It is capable of being run continuously at sixteen amps, and it also trickle charges the backup battery that comes online in the event of an AC power failure. The repeater draws about 650 milli-amps in standby mode, and six amps when it is active. The Sears Die-Hard marine battery has 71 amp-hours of storage capacity, thus with two hours of repeater activity per day, the battery should be good for at least two days of service.

    Local users have been as far away as San Jose (good copy), Yosemite (poor copy), and the hills east of Fresno (poor copy). DX users have entered the machine from Portland (Oregon), Kingman (Arizona), and several places in southern California. The repeater is most at home serving a few people who have commutes of significant duration to obsure locations within Sonoma county. There is also an occasional weekend net conducted by Steve, KD6FYK, to ensure the repeater's continued operation.

    Special thanks goes to Alan Judd, N6VUD, for his help with virtually all phases of this project including much of its maintenance. I would also like to thank Jan Hobbel, NI6B, for helping me initially get off the ground, Mikey Gimblett, KA6AQS (now a silent key), for being a fountain of free but invaluable advice, Steve Whitelaw, KD6FYK, for keeping a sharp eye on the machine, and Scott Roberts (still "callsign-challenged"), who has been very resourceful in performing a couple of field repairs. Thanks to their combined help, the repeater has been very reliable since 1994.