MFARS Moray Firth Amateur Radio Society

2013 © Moray Firth ARS Webmaster| Constitution

GB3SS Technical Specification


Originally based on the GB3US/G3RKL logic and a "surgically separated" Uniden transceiver in a homemade chassis, the repeater was overhauled in early 2005 and now consists of Tait T830 series RX, TX and PA in a Tait T800 rack chassis. This ex-PMR system offers better performance and is physically much sturdier. The new logic controller is based on the HOZ Logic design used on several UK repeaters.

The Tait T830 series system consists of a synthesised receiver, synthesised exciter and rugged power amplifier, housed in bays in a rack chassis. The receiver and exciter have configurable signal processing, including filtering and compression. The power amplifier is capable of 50W continuous into poor loads, with over-temperature and high-SWR shutdown and alarms.

The logic controller is housed in a spare bay in the Tait chassis, with the front panel designed to match the Tait modules. All modules slide out of the chassis from the front for easy service. The logic and PA have "umbilical" cables connecting to the chassis wiring so they can be pulled out and adjusted while the repeater is in operation.

The transmit/receive duplexing is performed by a six-section cavity filter duplexer (3 RX, 3 TX). These filters were constructed by club members to a design by W1GAN in QST magazine and the ARRL handbook. An additional single cavity and a 3-stage helical filter in the RX line provide rejection of VHF paging transmitters (whose antennas are mounted adjacent to the GB3SS antennas).

The repeater unit is housed in a rack cabinet along with the cavity filters and a purpose-built power supply with capacity to spare. The cabinet shares one of several crowded rooms in a blockhouse with everything from microwave data links to broadcast radio.

The antenna system comprises two stacked (collinear) folded dipoles mounted at about 30m above ground on a shared (and very crowded!) pylon. The antennas are connected to the duplexer by about 45m of LDF4-50 heliax cable.

GB3SS has no battery backup, since the mains power supply to the site appears to be very reliable. The logic has provision for monitoring mains/battery power and backup could easily be added to the system should it be needed.

The GB3SS logic is a slightly modified version of the "HOZ" logic used on GB3GD, GB3LD and other repeaters. The logic was designed by GD4HOZ (hence the name) for the Lake District group and has been in use in the UK since 1999. The design is freely available and not particularly expensive to build. The changes for GB3SS are mostly customisation of timings and callsign, with some changes to the remote control features and toneburst access. New features can be added by modifying the program code.

The logic is based on a Microchip PIC microcontroller programmed in assembler. Audio routing, processing and tone generation (CTCSS, beacon and "pip" tones) are performed by a single chip - the FX828 Selcall/CTCSS processor from CML. CTCSS decoding is performed by a FX365C also from CML, and the HT9170B/MT8870 provides DTMF detection (for remote control and configuration). The logic status is indicated by 3 LEDs and an alpha-numeric LCD panel.

The logic features full CTCSS encode and decode in addition to the standard toneburst access, includes alignment tests (test tone generation) to make setup easy, and generates different acknowledgement pips to indicate different conditions. Callsign beacons are transmitted every 5 minutes, with a longer beacon including locator every 25 minutes. There is also a special "Raynet" mode which turns the system into a simple carrier-operated repeater with no access requirements or timeout, which can be used for public event or emergency support.

For more information on the HOZ logic design, visit the Lake District Repeater Group web site or join the HozLogic Yahoo group where you can download the design documentation.